This Appendix describes many of the basic measurements made on
RAF aircraft and, where appropriate, includes the equations used by
the RAF's NIMBUS software to calculate the derived measurements--those
resulting from the use of one or more other measurements (raw and/or
derived).
RAF has retired its GENPRO processor, the software program
previously used to produce its final production output data sets.
Variable names shown below in italics denote obsolete variable
names that were used with GENPRO (or by a retired instrument).
These names are included here to help those who are using older,
archived RAF data sets.
The RAF output data set includes a group of special
measurements which are used as references (aka "dependencies")
for other calculations. A reference measurement, chosen from a
group of redundant measurements, usually has a variable name
ending with the letter(s) X or XC, but some do not follow that
convention:
base_time,
time_offset,
ATTACK,
SSLIP and
EDPC.
(See their individual descriptions below.)
To make this document more compatible with the standard
ASCII character set, Greek letters and special mathematical
symbols have been avoided. As a result, some of the equations may
appear a bit obscure. The following symbols and conventions are
used:
- VARIABLES RELATED TO TIME
The data in output files are referenced to time. Specifically,
the aircraft data system (ADS) records time as Coordinated Universal
Time (UTC). (Some previous data sets have used local time instead.)
Normally the system time and date are synchronized to the Global
Positioning System (GPS) at the beginning of each flight. (For ADS-3
data acquisition systems, time is continuously synchronized with the GPS.)
- Time (s) - Time
- This is the reference "time of measurement"
for the output data files (data system versions after ADS-2).
It is output at 1 sample per second (sps) with an
initial value of zero. Since it increments each second, it can also
be thought of as a record counter. Use this measurement and the
reference time found in the variable's "units" attribute to
obtain the time for each data record.
Example attribute: Time:units = "seconds since 2006-04-26 12:55:00 +0000" ;
For code examples on extracting "Time" see:
http://www.eol.ucar.edu/raf/Software/TimeExamp.html
- Reference Start Time (s) - base_time
- This is the reference time for the netCDF output data files (data
system versions before ADS-3). It represents the time of the
first data record. Its format is Unix time (elapsed seconds after
midnight 1 January 1970). Use this measurement and add
time_offset
to obtain the time for each data record. (Note: base_time is a
single scalar, not a "record" variable.)
- Time Offset from Reference Start Time (s) - time_offset
- This is the time offset from base_time of each data record used for
the netCDF output files (data system versions before ADS-3). Since
it starts at zero (0) and increments each second, it can also be thought
of as a record counter. Use this measurement and add
base_time to obtain the time for each data record.
- Raw Tape Time (hour, minute, s) - HOUR, MINUTE, SECOND
- These three time variables are recorded directly from the
aircraft's data system (versions before ADS-3).
- Date (m, d, y) - MONTH, DAY, YEAR
- These three variables represent the date when the aircraft's data
system (versions before ADS-3) began recording data. They are
repeated as 1 sps variables but are NOT incremented if the time rolls
over to the next day. (Use
base_time and
time_offset for reference timing.)
Solar Angle Calculations
The calculations described in this group are used primarily for deriving
the Corrected Short Wave irradiance
(SWTC)
but can be used by themselves or in conjunction with other measurements
that need them.
- Solar Declination Angle (radians) - SOLDE
- This is a complex calculation of the astronomical measurement of solar
declination angle, the angular distance of the sun north or south of the
earth's equator. (Positive values are north.) It also makes a complex
calculation of solar hour angle that takes leap year into account. The
calculations were adapted by Ron Ruth from a FORTRAN program used by Lutz
Bannehr.
g = -0.031271 - (4.53963e-7 time) + theta
el = 4.900968 + (3.67474e-7 time) +
{[0.033434 - (2.3e-9 time)] sin(g)} +
[0.000349 sin(2 g) + theta]
sel = sin(el);
eps = 0.409140 - (6.2149e-9 time)
SOLDE = asin {sel sin(eps)}
where:
time = day number (corrected for leap year) since 1 January 1980
theta = coarse solar time (radians)
g = equation-of-time term for calculating declination (radians)
el = equation-of-time term for calculating declination (radians)
eps = equation-of-time term for calculating declination (radians)
sin = trigonometric sine
asin = trigonometric arc sine
- Solar Zenith Angle (radians) - SOLZE
- This is the astronomical measurement of solar zenith angle, the
angle from zenith to the sun, complementary to the sun's elevation angle.
SOLZE = asin {sin(lat) sin(SOLDE) + cos(lat) cos(SOLDE) cos(lha)}
where:
asin = trigonometric arc sine
sin = trigonometric sine
cos = trigonometric cosine
lat = latitude (radians)
lha = local hour angle (radians)
- Solar Azimuth Angle (radians) - SOLAZ
- This is the astronomical measurement of solar azimuth angle, the
angular distance between due south and the projection of the line of sight
to the sun on the ground. A positive solar azimuth angle indicates a
position east of south (i.e., morning).
SOLAZ = asin {cos(SOLDE) sin(lha) / cos(SOLZE)}
where:
asin = trigonometric arc sine
cos = trigonometric cosine
sin = trigonometric sine
lha = local hour angle (radians)
- Solar Elevation Angle (radians) - SOLEL
- This is the astronomical measurement of solar elevation angle, describing
how high the sun appears in the sky. The angle is measured between an
imaginary line between the observer and the sun and the horizontal plane on
which the observer is standing. The altitude angle is negative when the sun
drops below the horizon.
SOLEL = Pi/2 - SOLZE
where:
Pi = 3.1415926536...
- VARIABLES OBTAINED FROM THE INERTIAL REFERENCE SYSTEM
A Honeywell YG1854 Laseref SM Inertial Reference System
(IRS) is used to obtain aircraft position, velocity, acceleration
and attitude information. All output data from the IRS come via
a serial digital bit stream (the ARINC digital bus) to the ADS
(Aircraft Data System).
Raw IRS Variables
- Latitude (deg) - LAT
- This is the aircraft latitude output from the IRS at 5
samples per second (sps). Positive values are north. The
resolution is 0.00034 degrees (deg).
- Longitude (deg) - LON
- This is the aircraft longitude output from the IRS at 5 sps.
Positive values are east. The resolution is 0.00034 deg.
- Aircraft True Heading (deg) - THDG
- This is the aircraft's true heading output from the IRS at
25 sps. The resolution is 0.00034 deg.
- Aircraft Pitch Attitude Angle (deg) - PITCH
- This is the output of the aircraft's pitch angle from the
IRS at 50 sps. Positive values are up. The resolution is
0.00034 deg.
- Aircraft Roll Attitude Angle (deg) - ROLL
- This is the output of the aircraft's roll angle from the IRS
at 50 sps. Positive angles occur with the right wing down
(clockwise when facing forward in the aircraft). The resolution
is 0.00034 deg.
- Aircraft Vertical Acceleration (M/s2) - ACINS
- This is the output from the IRS vertical accelerometer with
its internal drift removed via pressure damping. The sample rate
is 50 sps with a resolution of 0.0024 M/s2.
- Computed Aircraft Vertical Velocity (M/s) - VSPD
- This is the pressure-damped output from the integrated,
vertical component of the IRS accelerometers. The damping
effectively removes accelerometer drift. The sample rate is 50
sps with a resolution of 0.00016 M/s (meters per second). (A
related variable, WP3, is described below.)
- Pressure-Damped Aircraft Vertical Velocity (M/s) - WP3
- This is a derived output incorporating a third-order damping
feedback loop to remove the drift from the inertial system's
vertical accelerometer (ACINS or VZI) using Pressure Altitude
(PALT) as a long-term, stable reference. Positive values are up.
(The Honeywell IRS also puts out its own version of this
measurement, VSPD. WP3 is calculated by the data-processing
software.)
- Inertial Altitude (M) - ALT
- This is the output of aircraft altitude by pressure-damping
the integrated aircraft vertical velocity. The sample rate is 25
sps with a resolution of 0.038 M.
- Aircraft Ground Speed (M/s) - GSF
- This is the output from the IRS (10 sps) giving the scalar
magnitude of the aircraft's ground speed. The resolution is
0.0020 M/s.
- Aircraft Ground Speed East Component (M/s) - VEW
- This is the output from the IRS (10 sps) of the east
component of ground speed. The resolution is 0.0020 M/s.
- Aircraft Ground Speed North Component (M/s) - VNS
- This is the output from the IRS (10 sps) of the north
component of ground speed. The resolution is 0.0020 M/s.
Esoteric Raw IRS Variables
The following raw IRS variables are not normally output, but
their values are recorded by the ADS.
- Raw Lateral Body Acceleration (M/s2) - BLATA
- This is the raw output from the IRS lateral accelerometer.
Positive values are toward the front, parallel to the aircraft
center line. The sample rate is 50 sps with a resolution of
0.0024 M/s2.
- Raw Longitudinal Body Acceleration (M/s2) - BLONA
- This is the raw output from the IRS longitudinal
accelerometer. Positive values are toward the right, normal to
the aircraft center line. The sample rate is 50 sps with a
resolution of 0.0024 M/s2.
- Raw Normal Body Acceleration (M/s2) - BNORMA
- This is the raw output from the IRS vertical accelerometer.
Positive values are up, normal to the aircraft center line. The
sample rate is 50 sps with a resolution of 0.0024 M/s2.
- Raw Body Pitch Rate (deg/s) - BPITCHR
- This is the raw output of the IRS pitch rate gyro. Positive
values are up, normal to the aircraft's lateral axis. The sample
rate is 50 sps with a resolution of 0.0039 deg/s.
- Raw Body Roll Rate (deg/s) - BROLLR
- This is the raw output of the IRS roll rate gyro. Positive
values are right wing down, normal to the aircraft center line.
The sample rate is 50 sps with a resolution of 0.0039 deg/s.
- Raw Body Yaw Rate (deg/s) - BYAWR
- This is the raw output of the IRS yaw rate gyro. Positive
values are nose turning to the right, normal to the aircraft's
vertical axis. The sample rate is 50 sps with a resolution of
0.0039 deg/s.
- VARIABLES OBTAINED FROM THE GLOBAL POSITIONING SYSTEM (GPS)
GPS variables aboard RAF aircraft are gathered by a Trimble
TANS-III GPS receiver. It has the ability to track up to 6
satellites at a time but needs only 4 to provide 3-dimensional
position and velocity data (3 satellites for 2-dimensions). The
accuracy of the position measurements is stated to be 25 meters
(horizontal) and 35 meters (vertical) under "steady-state
conditions." Likewise, velocity measurements are within 0.2 M/s
for all axes. Measurement resolution is that of 4-byte IEEE
format (about 6 significant digits). All variables are output
at a sample rate of 1 sps.
Note: The GPS signals at one time suffered from "selective
availability," a US DOD term for a dithered signal that degrades GPS
absolute accuracy to 100 meters. This was especially noticeable in
the altitude measurement, so GALT normally was not useful. As of
1 May 2000, selective availability was deactivated to allow everyone
to obtain better position measurements. (See the
Interagency GPS Executive Board
web site for more information.)
- GPS Latitude (deg) - GLAT
- This is the aircraft latitude output from the GPS. Positive
values are north.
- GPS Longitude (deg) - GLON
- This is the aircraft longitude output from the GPS.
Positive values are east.
- GPS Ground Speed (M/s) - GGSF, GGSPD
- This is the aircraft ground speed output from the GPS.
- GPS Ground Speed Vector East Component (M/s) - GVEW
- This is the GPS output of the aircraft east component of
ground speed.
- GPS Ground Speed Vector North Component (M/s) - GVNS
- This is the GPS output of the aircraft north component of
ground speed.
- GPS-Computed Aircraft Vertical Velocity (M/s) - GVZI
- This is the aircraft vertical velocity output from the GPS.
Positive values are up.
- GPS Altitude (M) - GALT
- This is the aircraft altitude output from the GPS. The
measurement is with respect to a geopotential surface (MSL)
defined by the GPS's built-in earth model. Positive values are up.
- GPS Mode (none) - GMODE
- This is the output of the GPS receiver's mode of operation.
Its normal value is 4, indicating automatic (not manual) mode and
the receiver is operating in 4-satellite (as opposed to fewer) mode.
- GPS Status (none) - GSTAT
- This is the output of the GPS receiver's status. A zero
value (0) indicates that the receiver is operating normally and
is doing position fixes. Among other things GSTAT indicates when
fewer than 4 satellites are being received and when full
measurement accuracy is being compromised.
- COMBINED OUTPUT FROM THE INERTIAL REFERENCE SYSTEM (IRS) AND
THE GLOBAL POSITIONING SYSTEM (GPS).
The method for obtaining a more accurate mean aircraft
position and ground speed (using the GPS to correct the IRS
Measurements) was implemented by Al Cooper (NCAR/EOL/RAF) and Dick
Friesen (NCAR/EOL/RAF). The "smoothly" continuous data of the
inertial navigation system (which can slowly accumulate large
errors over time) is combined with the highly-accurate but
"noisy" data from the GPS. This was accomplished by using a
complementary filter that high-passes the IRS data and low-passes
the GPS data. After filtering, the two data streams are summed.
At this time, RAF opines that these corrected variables are
the best available when the GPS is functioning. The GPS can suffer
from noise spikes and "dropouts" due to aircraft maneuvering, poor
satellite geometry and, to a lesser degree, satellite malfunction.
When the GPS data are lost, the algorithm in use slowly reverts from
the last "best" measurements to the pure IRS measurements
If a GPS dropout is short in duration (usually a few seconds
to a minute), accuracy is not affected. However, longer dropouts
will result in accuracies approaching that of the IRS. Without
the GPS or a ground reference, the IRS error cannot be determined
empirically, and one should assume that it is within the
manufacturer's specification (1 nautical mile of error per hour
of flight, 90% CEP). When the GPS is active, RAF estimates that
the correction algorithm produces a position with an error less
than 100 meters. Due to the nature of the algorithm, the error
will increase from the 100 meters to the IRS specification in
about one-half hour. In other words, if there is a continuous
GPS dropout that lasts one-half hour or longer, the corrected
position will revert to the full IRS position.
One can determine if the GPS is functioning by examining the
GPS status word GSTAT) or the GPS variables
directly. (Spikes or "flat-lines" in the data indicate a GPS
dropout which happens most frequently during a turn.)
- GPS-Corrected Inertial Latitude (deg) - LATC
GPS-Corrected Inertial Longitude (deg) - LONC
- Combined GPS and IRS output of latitude and longitude from
the experimental correction algorithm. Positive values are north
and east, respectively.
- GPS-Corrected Inertial Ground Speed Vector, East Component (M/s) - VEWC
GPS-Corrected Inertial Ground Speed Vector, North Component (M/s) - VNSC
- Combined GPS and IRS output of the east and north components
of ground speed from the experimental correction algorithm.
Positive values are east and north, respectively.
- ALTITUDE AND POSITION VARIABLES
- Altitude, Reference (MSL) - ALTX, GALTC
- This is the derived altitude above the geopotential surface
obtained primarily from
GALT, the GPS Altitude, with help from another
Reference altitude, typically
ALT, the Inertial Altitude.
The GPS signals, at times, are degraded, most often during
aircraft turns. Two GPS status measurements are used to detect this, but
sometimes the information comes too late. A 10-second running average is
calculated of the difference between the GPS altitude and the Reference
altitude. When the sample-to-sample altitude difference changes more than
50 meters or when the GPS status detects a degraded signal, the altitude
from this measurement changes from GPS altitude to an adjusted Reference
altitude (Reference altitude plus the running difference average). When
the GPS altitude is again "good" and to avoid a sudden
altitude transition, the output linearly "walks" back
to the GPS altitude over the next 10 seconds.
- NACA Pressure Altitude (M) - PALT
- This is the derived altitude above the geopotential surface
obtained from a barometric (static) pressure measurement using the
NACA standard atmosphere. An altimeter setting of 1013.246 mbar
normally is used in the calculation, but a local altimeter setting
can be used, if available.
PALT = (Tref/gamma) [1.0 -(Ps/Pref)x]
where:
Tref = reference temperature for the standard atmosphere
(288.15K)
gamma = assumed standard lapse rate = 0.0065 K/M
Ps = measured static pressure, mbar
Pref = reference pressure for the standard atmosphere
(normally 1013.246 mbar)
x = Ro gamma/(Mw g) = R gamma/g = 0.190284
Ro = universal gas constant
Mw = molecular weight of dry air, g
g = acceleration of gravity, M/s2
R = gas constant for dry air
- Geometric Radio Altitude (M) - HGM
- This is the output from a radio altimeter in meters above
ground. The maximum range is 762M (about 2,500 ft). The
instrument changes in accuracy at an altitude of 152M. The
estimated error from 152M to 762M is 7%, while the estimated
error for altitudes below 152M is 1.5M or 5%, whichever is
greater.
- Geometric Radar Altitude (Extended Range) (APN-159) (M) - HGME
- There are two outputs from an APN-159 radar altimeter, one
with coarse resolution (CHGME) and one with fine resolution
(HGME). Both raw outputs cycle through the range 0-360 degrees,
where one cycle corresponds to 4,000 feet for HGME and to 100,000
feet for CHGME. To resolve the ambiguity arising from these
cycles, 4,000-foot increments are added to HGME to maintain
agreement with CHGME. This preserves the fine resolution of HGME
(1.86 M) throughout the altitude range of the APN-159.
- Geometric Radar Altitude (Extended Range) (APN-232) (M) - HGM232
- This is the output from an APN-232 radar altimeter.
- Pressure-Damped Inertial Altitude (M) - HI3
- This is the aircraft altitude obtained from the
twice-integrated INS acceleration (ACINS), damped to obtain
long-term agreement with PALT.
- Distance East/North of a Reference (kM) - DEI/DNI
- These are derived outputs obtained by subtracting a fixed
reference position from the current position (one of the
measurements of latitude and longitude). The reference position
can be either the starting location of the flight or a user-defined
reference point.
DEI = (LON - LONref) x SFref x cos(LAT)
DNI = (LAT - LATref) x SFref
where:
LONref = reference longitude (deg)
LATref = reference latitude (deg)
cos = cosine
SFref = reference scale factor (111.17 kM per degree)
The accuracy of these values is dependent on the accuracy of the
source latitude and longitude measurements.
- Radial Azimuth/Distance from Fixed Reference (deg) - FXAZIM, FXDIST
- These are derived calculations of the radial azimuth and
distance from a fixed reference position (a user-specified
reference latitude and longitude) to the aircraft's location.
It is calculated by rectangular-to-polar conversion of DEI/DNI.
- AIRCRAFT AND METEOROLOGICAL STATE VARIABLES
Meteorological and aircraft state measurements are made at
various locations on an aircraft. Generally, the location is
coded into the variable's name in the output product. In this
appendix, locations will be represented by x. In measurements
from RAF aircraft, x may represent the following:
x = B bottom (or bottom-most, if redundant sensors
are used); (obsolete notation: boom)
x = F fuselage
x = G gust probe (obsolete)
x = R radome
x = T top (or top-most, if redundant sensors are
used)
x = W wing
x = X reference measurement used for derived
calculations
xx = xD a digital (as opposed to analog) sensor
xx = xH a deiced (heated) sensor
xx = xL left side (qualifies generic location)
xx = xR right side (qualifies generic location)
xx = RF reverse-flow (temperature sensor)
The present radome measurements all have pressure sensors
located in the nose area of the aircraft connected to the radome
by semi-rigid tubing.
RAF now uses only the radome as the reference system to
measure airspeed incidence angles. (A Rosemount Model 858AJ gust
probe is occasionally used for specialized measurements.)
- Attack Differential Pressure (mbar) - ADIF, ADIFR
- This is the differential pressure in the vertical plane of
the aircraft's flow-angle sensor system. The sensor is either a
Rosemount Model 858AJ flow-angle sensor (ADIF) or a nose radome
(ADIFR). The differential pressure is measured with a Rosemount
Model 1221 differential pressure transducer.
- Sideslip Differential Pressure (mbar) - BDIF, BDIFR
- This is the differential pressure in the horizontal plane of
the aircraft's flow-angle sensor system. The sensor is either a
Rosemount Model 858AJ (BDIF) or a nose radome (BDIFR). The
differential pressure is measured with a Rosemount Model 1221
differential pressure transducer.
- Reference Attack Angle (deg) - ATTACK
- This is a derived output of aircraft angle of attack chosen
as the reference from one of the other attack angle measurements
within the data set. It is used where attack angle is needed for
other derived calculations (e.g., wind measurements).
- Attack Angle (Differential Pressure) (deg) - AKRD
- This is a derived output of the aircraft's angle of attack
obtained from ADIFR (the radome's vertical differential pressure)
and a dynamic pressure using a sensitivity function that has been
empirically determined for each aircraft.
- Attack Angle (Differential Pressure) (deg) - AKDF
- This is a derived output of the aircraft's angle of attack
obtained from ADIF (the vertical differential pressure measured
by a Rosemount 858AJ flow-angle sensor) and a dynamic pressure
using an empirically-determined sensitivity function. For the
Rosemount 858AJ, the sensitivity function (GR) is a constant
0.079 for
Mach
numbers less than 0.515. For larger
Mach numbers the sensitivity decreases (a function of Mach number).
GR = 0.079 (for Mach <= 0.515)
GR = 0.086577797 -0.03560256 MACH +0.00006143 MACH MACH
where:
MACH = Aircraft Mach Number
The angle of attack is calculated using the following
equation:
AKDF (858AJ) = ADIF/(GR Qc)
where:
GR = sensitivity function
Qc = dynamic pressure, mbar
- Reference Sideslip Angle (deg) - SSLIP
- This is a derived output of sideslip angle chosen as the
reference from one of the other sideslip angle measurements
within the data set. It is used where sideslip angle is needed
for other derived calculations (e.g., wind measurements).
- Sideslip Angle (Differential Pressure) (deg) - SSRD
- This is a derived output of the aircraft's sideslip angle
obtained from BDIFR (the radome's horizontal differential
pressure) and a dynamic pressure using a sensitivity function
that has been empirically determined for each aircraft.
- Sideslip Angle (Differential Pressure) (deg) - SSDF
- This is a derived output of the aircraft's sideslip angle
obtained from BDIF (the horizontal differential pressure measured
by a Rosemount 858AJ flow-angle sensor) and a dynamic pressure
using an empirically-determined sensitivity function. For the
Rosemount 858AJ, the sensitivity function (GR) is a constant
0.079 for
Mach
numbers less than 0.515. For larger
Mach numbers the sensitivity decreases (a function of Mach number).
GR = 0.079 (for Mach <= 0.515)
GR = 0.086577797 -0.03560256 MACH +0.00006143 MACH MACH
where:
MACH = Aircraft Mach Number
The sideslip angle is calculated using the following equation:
SSDF (858AJ) = BDIF/(GR Qc)
where:
GR = sensitivity function
Qc = dynamic pressure, mbar
- Cabin Pressure (mbar) - PCAB
- Output of a Rosemount Model 1201 absolute pressure
transducer open to the interior of the aircraft's cabin.
- Static Pressure (mbar) - PSx, PSxC
- This is the output from a calibrated absolute (barometric) transducer
at location x. PSx is the measured static pressure that is affected
by local flow-field distortion. The PSxC value is a static pressure
corrected for local flow-field distortion.
(See Bulletin No. 21.)
For output variables PSFD and PSFDC, the letter "D" indicates the type
of pressure transducer used: a Rosemount Model 1501 digital absolute
pressure transducer. For output variable PSFRD, the letters "RD" indicate
the type of pressure transducer used: a Ruska Model 7885-1B digital
absolute pressure transducer. Other static pressure measurements are
made with a Rosemount Model 1201 absolute pressure transducer.
- Dynamic Pressure (mbar) - QCx, QCxC
- This is the output from a calibrated differential pressure
transducer. The measurement is the difference between a pitot
(total) pressure at location x and a static pressure. The QCxC
value is corrected for local flow-field distortion. A Rosemount
Model 1221 differential pressure transducer is used for all
direct dynamic pressure measurements.
- Total Temperature (C) - TTx, TTxH
- This is the output of the recovery temperature from a
calibrated temperature sensor at location x. (For Rosemount
temperature probes, the recovery temperature is a close
approximation to the total temperature.) In the standard output,
the total temperature (and ambient temperature) variable name
also conveys the sensor type. The variable named TTx is obtained
with a Rosemount Model 102 non-deiced temperature sensor. The
variable named TTxH is obtained with a Rosemount Model 102 deiced
(heated) temperature sensor.
- Dew/Frost Point (C) - DPx
- This is the output from an EG&G Model 137, a General Eastern
Model 1011B or Buck Model 1011C dew-point hygrometer. Below 0C the
instrument is assumed to be measuring the frost point.
- Corrected Dew Point (C) - DPxC
- This measurement is the corrected dew point with respect to
a plane water surface. Below 0C the measured frost point is
adjusted to dew point. The difference between the dew point and
the frost point is derived from the Goff-Gratch (1946) equations
for water vapor pressure over plane surfaces of water and ice.
The accuracy of the conversion (one sigma) is 0.02° C over
a range of 0C to -80C.
xDPC = 0.009109 +DPx (1.134055 +0.001038 DPx)
For instruments that primarily measure vapor density (RHO), e.g., the
Lyman-alpha and Laser hygrometers, the dew point is calculated by the
following expression:
Z = log((ATX+273.15)*RHO/1322.3)
DPxx = 273.0*Z/(22.51-Z)
where: log = natural logaritm
Z = intermediate calculation
ATX = reference ambient temperature (C)
RHO = vapor density (g/M3)
- Cryogenic Hygrometer Inlet Pressure (mbar) - CRHP
Raw Cryogenic Frost Point Temperature (C) - VCRH
Corrected Cryogenic Frost Point Temperature (C) - FPCRC
Corrected Cryogenic Dew Point Temperature (C) - DPCRC
- The output voltage from NCAR's cryogenic hygrometer is
converted to a temperature (VCRH) using a third-order calibration
equation. This temperature is the frost point of the air inside
the hygrometer's measurement chamber and not the frost point of
the ambient air. (Outside air flows through the hygrometer which
is mounted inside the aircraft's cabin.) By using the
Goff-Gratch (1946) equation for saturation vapor pressure, the
chamber's vapor pressure (with respect to a plane ice surface)
can be calculated:
eic = 6.1071 (10{-9.09718[273.16/(VCRH+273.16)-1.0]
-3.56654 log[273.16/(VCRH+273.16)]
+0.876793[1.0-(VCRH+273.16)/273.16]})
where:
eic = chamber ice vapor pressure (mbar)
log = common logarithm (base 10)
The pressure measurement inside the cavity (CRHP) is used to
correct the measurement chamber's vapor pressure to an ambient
value. The ambient vapor pressure (with respect to a plane ice
surface) is obtained using the following equation which includes
the enhancement factor (See
Appendix C.):
eia = eic (Ps/Pcr) fi
where:
eia = ambient ice vapor pressure (mbar)
Ps = ambient static (barometric) pressure (mbar)
Pcr = static pressure in cryogenic sampling chamber (mbar)
fi = enhancement factor over ice = 1.0003 +4.18x10-6 Ps
The corrected (ambient) frost point (FPCRC) and dew point (DPCRC)
are then calculated by recursively solving for the roots of the
Goff-Gratch equation using Newton's method (Hornbeck, 1975) with
ambient vapor pressure as the input. Since the instrument is
unstable at raw frost point temperatures above -15C, the output
of FPCRC, DPCRC and other derived measurements, such as mixing
ratio, are not defined above this value.
- Voltage Output From the Lyman-alpha Sensor (Vdc) - VLA, VLA1
- This value is the raw output voltage from the Lyman-alpha
absorption hygrometer, designed and built at NCAR to provide
fast-response, high-resolution water vapor density measurements.
(If a second sensor is used, a 1 is added to its name.)
- THERMODYNAMIC VARIABLES
- Ambient Temperature (C) - ATx, ATxH
- The ambient temperature variable name, ATx, conveys the same
information regarding sensor type and location as the variable
name used with total (recovery) temperature. (See discussion in
Aircraft and Meteorological State Variables
above.)
The ambient temperature (also known as the static air
temperature) is calculated from the measured recovery
temperature, which includes dynamic heating effects, using
conservation of energy for a perfect gas undergoing an adiabatic
(in this case, deceleration) process.
From conservation of energy:
Ua2/2 + cp Ta = Ut2/2 + cp Tt
where:
Ua = aircraft true airspeed, M/s
Ut = 0 for a perfect adiabatic deceleration at the sensing
element, M/s
Ta = the ambient temperature, i.e., the temperature of the
air at rest, K
Tt = the total temperature, i.e., the temperature of the air
at rest plus the temperature increase due to an
adiabatic deceleration of the air at the temperature
sensing element, K
cp = specific heat at constant pressure for dry air
Solving for ambient temperature, the above equation becomes:
Ta = Tt - Ua2/(2cp)
In general, most aircraft temperature sensors do not measure
the total temperature, Tt, exactly, but measure the temperature
of the air immediately in contact with the sensing element. This
air will not have undergone an adiabatic deceleration to zero
velocity and, hence, will have a temperature somewhat less than
Tt. This temperature is the measured or "recovery" temperature
Tr. The ratio of the actual temperature difference attained,
Tr-Tt, to the adiabatic temperature difference
Ua2/2cp is the recovery factor, r:
Tr - Ta Tr - Ta
r = ---------- = --------
Ua2/(2cp) Tt - Ta
where:
Tr = measured (recovery) temperature, K.
Thus, the ambient temperature becomes:
Ta = Tr - rUa2/(2cp)
The recovery factor, r, can also be expressed in terms of
the "fraction of the airspeed" in thermal contact with the
sensing element. By substituting fUa for Ut
and Tr for Tt in the
energy conservation equation above, the following result is obtained:
Tr - Ta Tr - Ta
r = (1 - f2) = ---------- = --------
Ua2/(2cp) Tt - Ta
where:
f = the "fraction of the airspeed" in direct contact with
the sensing element.
The Rosemount temperature sensors used on RAF aircraft are
designed to decelerate the air adiabatically to zero velocity.
In fact they do not, but they come very close. Recovery factors
determined from wind tunnel testing for the Rosemount sensors are
0.95 (non-deiced model) and 0.98 (deiced model). The recovery
factor determined for the NCAR reverse-flow sensor is 0.6. The
recovery factor for the, now retired, NCAR fast-response
(K-probe) temperature sensor was 0.8.
As can be seen in the above equation, the true airspeed,
Ua, is required to calculate the ambient temperature.
Conversely, the ambient temperature is required to calculate the
true airspeed. The
Mach
number is used along with the recovery
temperature to effectively solve two equations with two unknowns
and obtain both the ambient temperature and the true airspeed.
The Mach number is a dimensionless quantity that is not dependent
upon temperature but is a function of measured dynamic and static
pressure. (Mach 1 is the speed of sound.)
The above explanation can be expressed in terms of Mach
number as follows:
S = (gamma R Ta)0.5
Ua2 = M2 S2 = (gamma R Ta) M2
where:
S = speed of sound, M/s
gamma = ratio of specific heat at constant pressure to
specific heat at constant volume (cp/cv)
R = gas constant for dry air
Ta = ambient temperature, K
M = Mach number
Thus:
Ta = Tr - r(gamma R Ta M2)/2cp
Solving for Ta:
Ta = Tr/[1. + r M2 (gamma - 1.)/2]
ATx = Ta -273.16
- Aircraft True Airspeed (M/s) - TASx
- This is a derived variable based upon a Mach number
calculated from both the dynamic pressure at location x and
static pressure. As in the case with ambient temperature, this
expression is obtained from conservation of energy for a perfect
gas undergoing an adiabatic process.
For a perfect gas:
S = (gamma R Ta)0.5
where:
S = speed of sound, M/s
gamma = ratio of specific heat at constant pressure to
specific heat at constant volume (cp/cv)
R = gas constant for dry air
Ta = ambient temperature, K
Thus:
TASx = Ua = M (gamma R Ta)0.5
where:
M = Mach number = {2(cv/R)[(Qc/Ps +1.)R/cp -1.]}0.5
Qc = dynamic pressure, mbar
Ps = static pressure, mbar
Substituting the recovery temperature for the ambient temperature
(See discussion of
Ambient Temperature
above.) yields the following:
Ua2 = {gamma R Tr/[1. + r M2 (gamma - 1.)/2]} M2
where:
Ua = true airspeed, TASx, M/s
r = recovery factor
Tr = "recovery" temperature, K
Simplifying the above yields:
TASx = Ua = {gamma R Tr/[1/M2 + r(gamma - 1.)/2]}0.5
- Aircraft True Airspeed (Humidity Corrected) (M/s) - TASHC
- This is a derived measurement of true airspeed that takes
into account the deviations of moist air from perfect gas
behavior which manifests itself by slightly altering the value of
the specific heat at constant pressure, cp. (See List,
1971, pp 295, 331-339 and Khelif, et al., 1999) The present form of
this equation (Khelif's) is to add a moisture correction to an extant
true airspeed.
TASHC = TASx * (1.0 + 0.000304 * sphum)
where:
sphum = specific humidity, g/kg
- Water Vapor Pressure (mbar) - EDPC
- This is a derived intermediate variable used in the
calculation of several derived thermodynamic variables. The
vapor pressure is obtained from the Goff-Gratch Formulation (1946).
Since this formulation is for pure water and not moist air, the
enhancement factor also is incorporated into this calculation during
production processing. (See
Appendix C.)
Following are the formulae for vapor pressure with respect to a plane
water surface and with respect to a plane ice surface. (Which function
to apply is dependent upon temperature.)
A. T >= 273.15 K (vapor pressure with respect to plane
water surface):
ew = 10{23.832241-5.02808 log[T] -1.3816x10-7 [10(11.334 -0.0303998 T)]
+8.1328x10-2 [10(3.49149 -1302.8844/T)] -2949.076/T}
B. T < 273.15 K (vapor pressure with respect to plane
ice surface):
ei = 10[3.56654 log(T) -0.0032098 T -2484.956/T +2.0702294]
where:
T = ambient temperature (saturation vapor pressure),
dew point or frost point, K
log = common logarithm (base 10)
- Potential Temperature (K) - THETA
- This is a derived variable from the definition of potential
temperature.
THETA = Ta(1000/Ps)R/cp
where:
Ta = ambient temperature, K
Ps = static pressure, mbar
R = gas constant for dry air
cp = specific heat at constant pressure for dry air
- Equivalent Potential Temperature (K) - THETAE
- This is a derived variable obtained by the method of Bolton (1980).
THETAE = THETA {[(3.376/Tlcl) -0.00254]
[q (1.0 +0.00081 q)]}
where:
Tlcl = temperature at the lifting condensation level, K
Tlcl = {[2840./(3.5 ln{Ta} -ln{ew} -4.805)] +55.}
ln = natural logarithm (base e)
Ta = ambient temperature, K
ew = water vapor pressure, mbar
q = mixing ratio, g/kg
- Virtual Temperature (C) - TVIR
- The virtual temperature is the temperature of dry air having
the same pressure and density as the air being sampled. It is a
measure of the effect of water vapor on air density. The
calculation of virtual temperature in RAF output products is
taken from page 295 of the Smithsonian Meteorological Tables (1958).
Tvir = [Ta (1.0 +1.6078 q)/(1.0 +q)] -273.16
where:
Tvir = virtual temperature, C
Ta = ambient temperature, K
1.6078 = the ratio of the molecular weight of dry air
to that of water vapor
q = specific humidity, g/g
- Virtual Potential Temperature (K) - THETAV
- Derived output of potential temperature using virtual temperature
as a reference; otherwise it is the same as the derivation of THETA.
THETAV = (Tvir +273.16)(1000/Ps)R/cp
where:
Tvir = virtual temperature, C
Ps = static pressure, mbar
R = gas constant for dry air
cp = specific heat at constant pressure for dry air
- Relative Humidity (per cent) - RHUM
- Derived output of relative humidity from definition:
RHUM = 100. ew/ews
where:
ew = atmospheric water vapor pressure, mbar
ews = saturation water vapor pressure, mbar
- Absolute Humidity (Vapor Density) (g/M3) - RHOx
- Derived output of absolute humidity (water vapor density)
computed from its standard definition (equation of state).
RHO = 106 ew Mw/(Ro Ta) (multiplied by 106 to give g/M3)
RHO = 216.68 ew/Ta
where:
ew = water vapor pressure over a plane water surface, mbar
Mw = molecular weight of water
RO = universal gas constant
Td = dew point temperature, K
Ta = ambient temperature, K
This variable is calculated for a number of moisture sensors.
- Specific Humidity (g/kg) - SPHUM
- Derived output of specific humidity from definition:
SPHUM = 622. ew/(Ps -0.378 ew)
where:
ew = atmospheric water vapor pressure, mbar
Ps = static pressure, mbar
622 = 1,000 times the ratio of the molecular weight of
water vapor to that of dry air.
- Mixing Ratio (g/kg) - MR, MRCR, MRLA, MRLA1, MRLH
- A derived variable that is expressed in terms of grams of
water vapor per kilogram of dry air. It differs from specific
humidity in that it is related to dry air mass rather than the
total of dry air plus water vapor. The source instrument for
this measurement can be any of the hygrometers (MR from
thermoelectric, MRCR from cryogenic, MRLA/MRLA1 from Lyman
alpha, MRLH from Laser Hygrometer).
MR = 622. ew/(Ps -ew)
where:
ew = water vapor pressure, mbar
Ps = static pressure, mbar
622 = 1,000 times the ratio of the molecular weight of
water vapor to that of dry air.
- Calculated Surface Pressure (mbar) - PSURF
- This value is a calculated surface pressure obtained from
HGM, TVIR, PSFDC, and MR using the thickness equation. The
average temperature for the layer is obtained by using HGM and a
dry-adiabatic lapse rate. Due to the assumptions made in the
calculation of this variable, the result is only valid for flight
in a well-mixed surface layer or in other conditions in which the
temperature lapse rate matches the dry-adiabatic lapse rate.
PSURF = Ps exp[g/R (HGM/Tm)]
where:
Ps = static pressure, mbar
exp = exponentiation (natural antilogarithm, e = 2.71828...)
g = acceleration of gravity, M/s2
R = gas constant for dry air
HGM = radio altitude, M
Tm = mean temperature of the layer, K
= (Tvir+273.16) + 0.5 HGM (g/cp)
Tvir = virtual temperature, C
cp = specific heat at constant pressure for dry air
- WINDS
Bulletin No. 23
details the manner in which the wind components are obtained, both with
respect to the earth (UI, VI, WI, WS and WD) and with respect to the
aircraft (UX and VY). In the data processing, a separate function
(GUSTO in GENPRO, gust in NIMBUS) is used to derive these wind
components. That function uses the Inertial Reference System
measurements as well as aircraft true airspeed, aircraft angle of
attack, and aircraft sideslip angle. The wind components calculated
in GUSTO/gust are used to derive two additional components: wind speed
and wind direction.
- Wind Vector East Component (M/s) - UI
Wind Vector North Component (M/s) - VI
Wind Vector Vertical Gust Component (M/s) - WI
- These measurements comprise the three-dimensional wind
vector with respect to the earth. UI is the east-west component
with positive values toward the east. VI is the north-south
component with positive values toward the north. WI is the
vertical component with positive values toward the zenith.
- Wind Speed (M/s) - WS
Wind Direction (deg) - WD
- These variables are obtained in a straightforward manner
from UI and VI.
WS = (UI2 + VI2)0.5
WD = atan4(UI/VI) + 180
where:
UI = easterly component of the horizontal wind
measurement with respect to the earth
VI = northerly component of the horizontal wind
measurement with respect to the earth
atan4 = 4-quadrant arc-tangent function with output in degrees
- Wind Vector Longitudinal Component (M/s) - UX
Wind Vector Lateral Component (M/s) - VY
- These measurements comprise the horizontal wind vector with
respect to the aircraft. The variable UX is the component
parallel to the longitudinal axis; positive is toward the nose.
The variable VY is the component normal to the longitudinal axis;
positive is toward the left wing.
- GPS-Corrected Wind Vector, East Component (M/s) - UIC
GPS-Corrected Wind Vector, North Component (M/s) - VIC
- Combined GPS and IRS output of the east-west and north-south
components of the three-dimensional wind computed using the
experimental correction algorithm. Positive values are east
(UIC) and north (VIC).
- GPS-Corrected Wind Vector, Vertical Component (M/s) - WIC
- This variable is actually misnamed. The GPS-correction
algorithm does no vertical corrections, so this measurement does
the same calculation as WI with the option of using a different
vertical airplane velocity. Positive values are toward the zenith.
- GPS-Corrected Wind Speed (M/s) - WSC
GPS-Corrected Wind Direction (deg) - WDC
- These variables are obtained in a straightforward manner from UIC
and VIC.
WSC = (UI2 + VI2)0.5
WDC = atan4 [(-UI)/(-VI)] + 180
where:
UIC = GPS-Corrected easterly component of the horizontal wind
measurement with respect to the earth
VIC = GPS-Corrected northerly component of the horizontal wind
measurement with respect to the earth
atan4 = 4-quadrant arc-tangent converted to degrees
- GPS-Corrected Wind Vector, Longitudinal Component (M/s) - UXC
GPS-Corrected Wind Vector, Lateral Component (M/s) - VYC
- Combined GPS and IRS output of the longitudinal and lateral
components of the three-dimensional wind computed using the
experimental correction algorithm. Positive values are toward
the front of the aircraft and toward the left wing, respectively.
- RADIATION VARIABLES
- Radiometric (Surface or Sky/Cloud-Base) Temperature (C) - RSTx
- This is the output of equivalent black body temperature
obtained from one of two infrared radiometers. The x implies
either that the instrument is mounted on the bottom (B) or top
(T) of the aircraft. Both of these instruments are calibrated
using a black-body source manufactured by Eppley.
1) a narrow bandwidth, narrow field-of-view (2°) Heimann
Model KT-19.85 precision radiation thermometer. The spectral
bandwidth available is 9.6 to 11.5 µM.
2) a narrow bandwidth, narrow field-of-view (2°) Barnes
Engineering Model PRT-5 precision radiation thermometer. (This
instrument is now retired. The spectral bandwidth available was
either 8 to 14 µM or 9.5 to 11.5 µM. (Its cavity
temperature was monitored. The output variable was either TCAVB
or TCAVT.)
- Radiometer Sensor Head Temperature (C) - TRSTB
- This is the temperature of the Heimann radiometer's sensing head,
usually from RSTB, the primary down-looking instrument.
- Raw Pyrgeometer Output (W/M2) - IRx
- This is a calibrated thermopile output of long-wave, terrestrial
radiation from a pyrgeometer manufactured by Eppley Laboratory, Inc.
The pyrgeometer uses a coated glass hemisphere that transmits radiation
in a bandwidth between 3.5 µM and 50 µM. It is calibrated
at RAF according to procedures specified by Albrecht and Cox (1977).
The pyrgeometers are usually flown in pairs, one up-looking and one
down-looking. The x implies either bottom (B) or top (T).
- Corrected Infrared Irradiance (W/M2) - IRxC
- This is the derived output of infrared irradiance using the
pyrgeometer's thermopile output, the measured dome and sink
temperatures (DTx and STx), Boltzmann's constant, the emissivity
of the thermopile and a pyrgeometer-dependent, derived constant.
IRxC = IRx - DSCOR + TCOR
where:
DSCOR = difference in the dome and sink radiation, W/M2
= xk B (Td4 - Ts4)
TCOR = radiation from the thermopile, W/M2
= e B Ts4
xk = empirically-derived constant dependent on the dome type
B = Boltzmann's constant
Td = dome temperature, K
Ts = sink temperature, K
e = emissivity of the thermopile
- Shortwave Irradiance (W/M2) - SWx
- This is a calibrated thermopile output of short-wave radiation
from pyranometers manufactured by Eppley Laboratory, Inc. Typically, the
pyranometer uses UG295 glass hemispheres which give the widest coverage of
the solar spectrum (0.285 µM to 2.8 µM). Different bandwidths
can be obtained by use of different glass domes available from RAF upon
request. (See
Bulletin No. 25.)
The pyranometers are usually flown in pairs, one up-looking and one
down-looking. They are calibrated periodically at the NOAA Solar
Radiation Facility in Boulder, Colorado. The x implies either bottom
(B) or top (T).
- Corrected Incoming Shortwave Irradiance (W/M2) - SWTC
- This is the derived output of incoming (down-welling) shortwave
irradiance, taking into account both
solar position
(sun angle) and modest variations in aircraft attitude (at present, less
than 6 degrees in pitch and/or roll). (For more information, refer to
Bulletin No. 25.)
- Ultraviolet Irradiance (W/M2) - UVx
- This is the calibrated output from a UV radiometer/photometer
manufactured by Eppley Laboratories, Inc. The bandwidth of the
instruments available from RAF is from 0.295 µM to 0.385
µM. They are periodically returned to the Eppley Laboratories
for recalibration.
- CLOUD PHYSICS VARIABLES
- Raw Output PMS/CSIRO (KING) Liquid Water Content (W) - PLWC, PLWC1
- This is the output of a PMS/CSIRO (KING) liquid water probe (Watts).
It measures the power required to maintain a constant temperature through
a heated element as that element is cooled by convection and evaporation
of impinging liquid water. The convective heat losses are determined by
calibration in dry air over a range of airspeeds and temperatures.
- Corrected PMS/CSIRO (KING) Liquid Water Content (g/M3) - PLWCC, PLWCC1
- This is the corrected liquid water content obtained from relating the
power consumption (required to maintain a constant temperature) to liquid
water content, taking into account the effect of convective heat losses.
P = PDRY + PWET
= Nu Pi l k (Ts -Ta) + l d [L +clw(Ts -Ta)] Ua LWC
where:
P = total power dissipated by the probe
PDRY = power dissipated by the cooling effect of dry air
alone flowing over probe element
PWET = power needed to heat and vaporize the liquid water
that hits the probe element
Nu = Nusselt Number (relating conduction heat loss to
the total heat loss for dry air--a function of air
velocity and density)
Pi = 3.1415926536...
l = length of the master element, M
k = thermal conductivity of dry air
Ts = sensor temperature, K
Ta = ambient temperature, K
d = diameter of the master element, M
L = latent heat of vaporization of water
clw = specific heat of water
Ua = true airspeed, M/s
LWC = liquid water content, g/M3
- Raw Output Rosemount Icing Detector (Vdc) - RICE
- This is the output from a Rosemount 871F ice-accretion probe. It
consists of a rod set in vibration by a piezoelectric crystal. The
oscillation frequency of the probe changes with ice loading. When the
probe loads to a certain point, the rod is heated to remove the ice.
Its output voltage is related to the mass of the accreted ice.
- Derived Supercooled Liquid Water Content (g/M3) - SCLWC
- This variable is the supercooled liquid water content obtained from
the change in accreted mass on the Rosemount 871F ice-accretion probe
over one second. Note that the output is not valid during the probe
deicing cycle. This cycle is apparent in the RICE output (a spike,
followed by a decrease to near zero). Supercooled liquid water content
is determined by first calculating a water drop impingement rate which
is a function of the effective surface area, the collection efficiency,
the true airspeed, and the supercooled liquid water content.
The impingement rate obtained is equated to the accreted mass of
ice collected by the probe in one second (empirical voltage/mass
relationship). Solving the resulting equation for supercooled
water yields:
SCLWC = k (Dm/Dt)/Ua
where:
k = constant that is dependent on the effective surface
area of the probe, M2
Dm = mass of ice accreting on the probe, g
Dt = time interval (normally 1 second)
Ua = true airspeed, M/s.
- CLOUD PHYSICS VARIABLES FROM PMS 1-D AND 2-D PROBES
Following is some general information on the various PMS-1D probes
that RAF uses and the unique way they are identified.
Four- and five-character variable names shown in this section are
generic. The actual names appearing in NIMBUS-generated production output
data sets have appended to them an underscore (_) and three more characters
which indicate a probe's specific aircraft mounting location. For example,
AFSSP_RPI is the Total Accumulation from an FSSP-100 probe mounted on the
inboard, right-side pod. The codes presently in use are:
Code Location Aircraft Tail #
OBL Outboard Left C-130Q N130AR
IBL Inboard Left C-130Q N130AR
OBR Outboard Right C-130Q N130AR
IBR Inboard Right C-130Q N130AR
LPO Left Pod Outboard C-130Q N130AR
LPI Left Pod Inboard C-130Q N130AR
LPC Left Pod Center C-130Q N130AR
RPO Right Pod Outboard C-130Q N130AR
RPI Right Pod Inboard C-130Q N130AR
RPC Right Pod Center C-130Q N130AR
OBL Left Wing Electra N308D
IBL Left Pylon Electra N308D
WDL Window Left Electra N308D
OBR Right Wing Electra N308D
IBR Right Pylon Electra N308D
WDR Window Right Electra N308D
The probe type also is coded into each variable's name, sometimes using
four characters, sometimes only one: FSSP-100 (FSSP or F), FSSP-300 (F300
or 3), PCAS (PCAS or P), OAP-200X (200X or X), OAP-260X (260X or 6) and
OAP-200Y (200Y or Y). Prefix letters are used to identify the type of
measurement (A=accumulation per channel, C=concentration per channel,
CONC=Concentration from all channels, DBAR=Mean Diameter, DISP=Dispersion,
PLWC=Liquid Water Content, DBZ=Reflectivity Factor).
Generic
Name Probe Channels Usable Diameter Range Bin Width
FSSP F FSSP-100 0-15 1-15 (See FRNG below.)
F300 3 FSSP-300 0-31 1-31 0.3 - 20.0 µM varies
PCAS P PCAS 0-15 1-15 0.1 - 3.0 µM varies
200X X OAP-200X 0-15 1-15 40. - 280 µM 10 µM
260X 6 OAP-260X 0-63 3-62 40. - 620 µM 10 µM
200Y Y OAP-200Y 0-15 1-15 300 - 4500 µM 300 µM
- Total Accumulation (cnts) - AFSSP, AF300, APCAS, A200X, A260X, A200Y
- This measurement is the total number of particles detected by a PMS-1D
probe per unit time. This measurement has individual values for each
channel.
- Concentration (per channel) (N/cm3) - CFSSP, CF300, CPCAS
Concentration (per channel) (N/L) - C200X, C260X, C200Y
Concentration (all channels) (N/cm3) - CONCF, CONC3, CONCP
Concentration (all channels) (N/L) - CONCX, CONC6, CONCY
- These measurements are the particle concentrations with individual
values for each channel or a single, combined value using all channels.
For the scattering spectrometer probes (FSSP-100, FSSP-300 and PCAS),
the concentration value is modified by the probe activity
(FACT,
PACT).
(See description below.) The concentration is obtained from the total
number of particles detected and a calculated, probe-dependent sample
volume. Details of the concentration calculations can be found in
Bulletin No. 24.
- Mean Diameter (µM) - DBARF, DBAR3, DBARP, DBARX, DBAR6, DBARY
- The measurement is the mean diameter, the arithmetic average of all
particle diameters. It is calculated as follows:
Dbar = [Sum(i=1 to m) (ni di)]/Nt
where:
Sum = arithmetic summation
m = the number of channels
i = the ith channel
ni = the number of particles accumulated in channel i
di = the particle diameter for channel i, µM
Nt = the total number of sized particles (Sum of all ni)
- Dispersion (none) - DISPF, DISP3, DISPP, DISPX, DISP6, DISPY
- The dispersion is the ratio of the standard deviation of
particle diameters to the mean particle diameter.
- Liquid/Ice Water Content (g/M3) - PLWCF, PLWCX, PLWC6, PLWCY
- This variable is a derived calculation of the liquid water content
and is calculated as follows:
PLWC = rhow Pi/6 [Sum(i=1 to m) (ni die3)]
where:
Pi = 3.1415926536...
rhow = density of water
Sum = arithmetic summation
i = ith channel
m = number of channels
ni = number of particles accumulated in channel i
die = equivalent melted diameter of ith channel, µM
(The user must decide on method to be used to
determine these values. The equivalent melted
diameter is strongly dependent upon habit.)
- Reflectivity Factor (dbZ) - DBZF, DBZX, DBZ6, DBZY
- The probe reflectivity factor (optical array probes only) is defined
as the amount of reflectivity a measured distribution of particles
would have if detected by a radar. The reflectivity is dependent upon
the wavelength of the radar and the density of the particles. It is
calculated as follows:
dBZ = 10. log[Sum(i=1 to m) (ni die6)]
where:
dBZ = reflectivity (decibels)
log = common logarithm (base 10)
Sum = arithmetic summation
i = the ith channel
m = the number of channels
ni = the number of particles accumulated in channel i
die = equivalent melted diameter of ith channel, µM
(The user must decide on method to be used to
determine these values. The equivalent melted
diameter is strongly dependent upon habit.)
- FSSP-100 Range (none) - FRNG, FRANGE
- This is a variable which indicates the size range used for the
FSSP-100 probe.
Range Nominal Size Bin Width
0 2 µM - 47 µM diameter 3.0 µM
1 2 µM - 32 µM diameter 2.0 µM
2 1 µM - 15 µM diameter 1.0 µM
3 0.5 µM - 7.5 µM diameter 0.5 µM
- FSSP-100 Fast Resets (cnts) - FRST, FRESET
- This variable records the number of fast resets that occur during the
FSSP sampling. A fast reset occurs when a particle traverses the beam
outside the depth-of-field. When this occurs, the probe electronics
determine that the particle will not be accepted, and further delay
s avoided. This generates the fast reset. This variable is required
to account for the probe's total dead time. The fast reset time is a
measured circuit characteristic determined in the laboratory.
- FSSP-100 Total Strobes (cnts) - FSTB, FSTROB
- This variable records the number of particles detected within the
depth-of-field both inside and outside the "effective beam diameter."
The effective beam diameter is that portion of the beam defined by
velocity-averaging circuitry (within the FSSP). That velocity-averaging
circuitry is designed to reject particles passing through the outer
portions of the beam to provide a more accurate sizing within this
effective beam diameter. The velocity averaging circuitry keeps
track of particle transit times and rejects those below a certain,
probe-dependent threshold.
- FSSP-100 Beam Fraction (none) - FBMFR
- This is a derived variable that is the ratio of the number of
velocity-accepted particles (particles that pass through the effective
beam diameter) to the total number of particles detected in the
depth-of-field of the beam.
FBMFR = FSSP/FSTROB
where:
FSSP = the total number of particles velocity-accepted by
the FSSP
- FSSP-100 Calculated Activity Fraction (none) - FACT
- This is the calculated FSSP probe activity. It represents the probe
dead time, the time the probe electronics are busy processing particle
data. The probe activity is calculated from fast resets, total strobes,
and both slow and fast reset times (determined in the laboratory).
FACT = FSTROB T1 + FRESET T2
where:
T1 = slow reset time (µs)
T2 = fast reset time (µs)
- PCAS Raw Activity (none) - AACT/PACT
- This is a measurement of the PCAS probe dead time, the time that the
probe electronics are busy processing data and miss particles. The raw
activity obtained from the probe is corrected with a factor supplied by
the manufacturer (0.52).
Activity = [(raw activity) (sample rate)/1024] 0.52
(The value 1024 corresponds to 100% dead time.)
- PMS-2D Cloud Probe Particle Concentration (N/cm3) - CON2C1
- This measurement is the concentration of all particles sensed by the
PMS-2D Cloud Probe. It uses the probe's Shadow-Or count (SDWC1,SHDORC),
the total number of particles that has passed through its laser beam
during each sampling interval.
- PMS-2D Precip Probe Particle Concentration (N/cm3) - CON2P1
- This measurement is the concentration of all particles sensed by the
PMS-2D Precip Probe. It uses the probe's Shadow-Or count (SDWP1,SHDORP),
the total number of particles that has passed through its laser beam
during each sampling interval.
- PARTICLE MEASUREMENTS
RAF uses a modified TSI, Inc. Model 3760 condensation nucleus counter
to measure the concentration of particulates in the atmosphere larger than
about 0.01 µM diameter. Individual inlets have been designed for
each research aircraft to ensure isokinetic flow at research airspeeds.
The CN counter can be used as a stand-alone instrument or be placed
downstream of various instruments, such as a counterflow virtual impactor
or differential mobility analyzer. It is useful at altitudes up to about
11kM.
It operates by condensing n-butyl alcohol on the particles as they pass
through a cooling, condenser tube where supersaturation of a few hundred
percent occur. The particles grow large enough to be seen by a laser-diode
optical detector which outputs individual electrical pulses that are then
counted. The counter does not resolve particle concentration by size.
If large concentrations are encountered, two or more particles may
be present in the viewing volume at once and will produce only a single
pulse from the photodetector. This "coincidence" error, which increases
from about 0.6% at 1K/cm3 to 6% at 10K/cm3, will
create lower-than-true concentrations which are corrected for in RAF
post-processing. However, accuracy should be questioned at concentrations
greater than 20K/cm3.
- CN Counter Inlet Pressure (mbar) - PCN
- PCN is the absolute pressure inside the inlet tube as made by a Heise
Model 623 pressure sensor. The measurement is used to correct the sample
flow rates (FCN and XICN).
- CN Counter Inlet Temperature (C) - TEMP1, TEMP2, CNTEMP
- TEMP1, TEMP2 or CNTEMP is the output from a temperature sensor
mounted outside of the sampling tube immediately ahead of the counter.
The measurement, an approximation to the temperature of the air passing
through the tube, is used to correct the sample flow rates (FCN and XICN).
- Raw, Corrected CN Counter Sample Flow Rate (slpm, vlpm) - FCN, FCNC
- FCN is the raw sample flow rate in slpm (standard liters per minute)
measured with a Sierra 830 Mass Flow meter.
FCNC is the sample flow rate in vlpm (volumetric liters per minute)
corrected for pressure and temperature.
FCNC = FCN (1013.25/PCN) [T1K/(Tref +273.15)]
where:
T1K = TEMP1, K
Tref = reference temperature (21.11C), C
- Raw, Corrected CN Isokinetic Side Flow Rate (slpm, vlpm) - XICN, XICNC
- XICN is the raw isokinetic side flow rate in slpm measured with a
Sierra 830 Mass Flow meter.
XICNC is the isokinetic side flow rate in vlpm corrected for pressure
and temperature. For true isokinetic sampling, the flow rate at the inlet
entrance needs to equal the true airspeed. Also, the flow rate through
the CN counter should be maintained to at least 1.2 vlpm. A side flow
is added (and adjusted) so both of these conditions can be met.
XICNC = XICN (1013.25/PCN) [T1K/(Tref +273.15)]
where:
T1K = TEMP1, K
Tref = reference temperature (21.11C), C
- TSI CN Counter Output (cnts) - CNTS
- CNTS is the raw output count from the TSI, Inc. Condensation Nucleus
counter. Based on different project needs, the sample rate can be
adjusted between 1 and 50 sps. If necessary, the interface can use a
pre-scaler (to divide the counts by a selected multiple of 2) to keep
the counter from overflowing.
- Condensation Nucleus (CN) Concentration (N/cm3) - CONCN
- CONCN is the corrected concentration of condensation nuclei after
applying sample-volume and coincidence corrections.
CONCN = CNTS SR DIV/FCNC
where:
CNTS = raw counts
SR = sample rate (sps)
DIV = pre-scaler value (normally = 1)
FCNC = corrected sample flow rate (cm3/s)
Applying the coincidence correction:
CONCN = CONCN exp(Tvv CONCN FCNC)
where:
exp = exponentiation (natural antilogarithm, e = 2.71828...)
Tvv = time in view volume (0.25 µs) = 4.167x10-6
- AIR CHEMISTRY MEASUREMENTS
- Raw Carbon Monoxide Concentration (ppb) - CO
- CO is the raw, uncorrected output of the TECO model 48 CO
analyzer. This instrument is based on gas filter correlation
analysis of CO. The optics of the instrument have been modified
to increase the light through the absorption cell, and a zero
trap has been added to remove the CO from the sample air stream
to obtain an accurate zero. The instrument has a significant
temperature-dependent zero drift. This is compensated by making
a measurement for five minutes then making a zero measurement for
two minutes.
- Carbon Monoxide Analyzer Status (Vdc) - CMODE
Carbon Monoxide Baseline Zero Signal (Vdc) - COZRO
Raw Carbon Monoxide, Baseline Corrected (Vdc) - COCOR
- CMODE is the status line for the zero mode. When this value
is less than 0.2 volts, the instrument is in the "measure" mode.
When this value is greater than 8.0, volts the instrument is in
the "zero" mode. Data processing is done by selecting the signal
when the instrument is in the zero mode, and fitting these values
to a cubic spline. The value of this cubic spline becomes the
baseline zero signal (COZRO) for the instrument and is subtracted
from the entire data set to produce the variable COCOR.
- Corrected Carbon Monoxide Concentration (ppmv) - COCAL
- COCAL is the zero-corrected and calibrated signal after
the application of the appropriate calibration coefficients with
the units of ppmv. The quality of the zero fit and the data can
be judged by examining the offset at the zero points. If there
are relatively small changes in the baseline, the zero offset
will be only a few ppbv. If there have been rapid changes in the
baseline, the zero offset can be up to 50 ppbv. The magnitude of
the offset at the zero values gives a good measure of uncertainty
in the data set.
The detection limit is 10 ppbv, with an uncertainty of ± 15%.
The one sps data will have considerable noise, and it is recommended
that the 10-second average data be used.
- Raw TECO Ozone Output (ppb) - TEO3
- TEO3 is the uncorrected output of the TECO 49 UV ozone
analyzer. This instrument has been modified for separate
recording of the temperature and pressure inside the ozone
absorption cell.
- Internal TECO Ozone Sampling Pressure (mbar) - TEP, TEO3P
- TEP is the ozone cell pressure in mbar.
- Internal TECO Ozone Sampling Temperature (C) - TET
- TET is the ozone cell temperature in degrees Celsius.
- Corrected TECO Ozone Concentration (ppbv) - TEO3C
- TEO3C is the pressure- and temperature-corrected output from
the TECO 49 UV ozone analyzer. The instrument operates on a 10-second
data update time, where the data are actually collected in the 3
seconds preceding the update. It will sometimes give an artificially
high or low response when rapid changes in humidity are present.
This can occur when crossing the boundary layer or when going through
clouds. In operation on the ground prior to takeoff or immediately
after landing, a high concentration of hydrocarbons can cause positive
artifacts in the signal. The detection limit is 1 ppbv with an
uncertainty of ± 5%.
- NO Raw Counts (cnts) - XNO
NOy Raw Counts (cnts) - XNOY
- XNO and XNOY are the raw data counts from the NO and NOy
instruments respectively.
- NO Calibration Flow (slpm) - XNOCF
NOy Calibration Flow (slpm) - XNCLF
- XNCLF and XNOCF are calibration flows for the NO and NOy
instruments respectively.
- NO, NOy Measurement Status (none) - XNST
- XNST is the status for the NOy and NO instruments. The
instrument is in the measure mode for XNST of 0. For a XNST
reading of 5 the instruments are in the zero mode. XNST value of
10 is the calibration mode.
- NO Zero Air Flow (slpm) - XNOZA
NOy Zero Air Flow (slpm) - XNZAF
- XNOZA and XNZAF are zero air flows to back flush inlets,
typically at takeoff and landing, and also for calibration
on zero air. Even if the status, XNST, is 0, indicating the
instrument is in the measure mode and XNOZA and XNZAF are
approximately 1 slpm, the instrument is measuring zero air,
and not ambient air.
- NO Sample Flow (slpm) - XNOSF
NOy Sample Flow (slpm) - XNSAF
- XNOSF and XNSAF are the sample flows through the NO and NOy
instruments respectively. These values are typically about 1 slpm.
- NOy Reaction Chamber Pressure (mbar) - XNOYP
-
- Gold NOy Converter Temperature (C) - XNMBT
- XNMBT is the temperature of the gold NOy converter in degrees Celsius.
- Corrected NO Concentration (ppbv) - XNOCAL
Corrected NOy Concentration (ppbv) - XNYCAL
- XNOCAL and XNYCAL are the calibrated NO and NOy
concentrations, respectively, with units of ppbv. The NO and NOy
data are fit to a cubic spline for baseline subtraction, and then
the calibration coefficients are applied.
The quality of the data can be assessed by examining the
accuracy of the zero correction. This instrument has the
provision for the addition of water vapor to the sample stream to
eliminate the effect of ambient water on the final signal. The
water vapor addition is not sufficient to saturate the sample
stream, but enough to remove much of the interference.
The detection limit of the NO/NOy instruments is 50 ppmv for
a one-second averaging time. The uncertainty is ± 15%.
- Raw Chemiluminescent Ozone Signal (Vdc) - O3FS
- Raw output from the reverse chemiluminescence ozone
instrument, based on the reaction of nitric oxide with ozone.
- Chemiluminescent Ozone Sample Flow Rate (sccm) - O3FF
Chemiluminescent Ozone Nitric Oxide Flow Rate (sccm) - O3FN
Chemiluminescent Ozone Sample Pressure (mbar) - O3FP
Chemiluminescent Ozone Concentration (ppbv) - O3FC
- This is the corrected ozone concentration with units of
ppbv. This instrument is calibrated both on the ground and in
flight by comparison with the TECO 49 UV instrument. The final
data are corrected for the influence of water vapor on the
signal.
The detection limit is 0.1 ppbv with an uncertainty of 10%
for a one-second time response.
- EXPERIMENTAL VARIABLES
This bulletin normally does not include any experimental variables,
but exceptions occasionally are made for variables that have special
significance. That recently was the case for the GPS-Corrected position
and wind data when RAF decided that the measurements should no longer be
classified as experimental.
- OBSOLETE VARIABLES
RAF has retired its GENPRO processor, the software program
previously used to produce its final production output data sets.
Also, RAF has retired some of its older instrumentation.
Obsolete variable names that are associated only with GENPRO or a
retired instrument are shown below in italics. These are
included here to help those who are using older, archived RAF
data sets.
- Unaltered Tape Time (s) - TPTIME
- This variable is derived by converting the HOUR, MINUTE and
SECOND to elapsed seconds after midnight of the current day. If
time increments to the next day, its value is not reset to zero,
but 86400 seconds are added to produce ever-increasing values for
the data set.
- Processor Time (s) - PTIME
- This is an internal time variable created by the
GENPRO processor. It represents elapsed seconds after midnight.
It differs from TPTIME in that, after it has been set at the
beginning of the data set, it is incremented internally for each
second of data processed. If duplicate or missing raw data
records exist, it can differ from TPTIME. It is guaranteed to be
a monotonically-increasing series of values.
- INS - Data System Time Lag (s) - TMLAG
- This is the amount of time between the reference time of the
Litton LTN-5l Inertial Navigation System (INS) and the data
system`s clock in seconds. TMLAG will always be greater than
zero and less than 2.
- LORAN-C Latitude (deg) - CLAT
- Aircraft latitude output from the on-board LORAN-C receiver
at one sample per second (sps). Positive values are north.
- LORAN-C Longitude (deg) - CLON
- Aircraft longitude output from the on-board LORAN-C receiver
at 1 sps. Positive values are east.
- LORAN-C Circular Error of Probability (nmi) - CCEP
- The probable error in the LORAN-C position measurement
output from the on-board LORAN-C receiver.
- LORAN-C Ground Speed (M/s) - CGS
- The computed ground speed from the on-board LORAN-C
receiver.
- LORAN-C Status (none) - CSTAT
- The status word output from the LORAN-C receiver. A value
of 15 means that all is OK.
- LORAN-C Time (s) - CSEC
- The data system time when the LORAN-C record was generated.
This value is in whole seconds and needs to be combined with
CFSEC to get the actual time. The actual time then is used to
time-synchronize the LORAN-C data with the other measurements
made by the aircraft data system (ADS).
- LORAN-C Fractional Time (s) - CFSEC
- The data system fractional time showing when a LORAN-C
record was generated. This value needs to be combined with CSEC
to get the actual time.
- INS Latitude (deg) - ALAT
- Aircraft latitude output from the INS at one sample per
second (sps). Positive values are north. The resolution is 5
arc seconds (0.0014 deg).
- INS Longitude (deg) - ALON
- Aircraft longitude output from the INS at 1 sps. Positive
values are east. The resolution is 5 arc seconds (0.0014 deg).
- Raw INS Ground Speed X Component (M/s) - XVI
- This is an output from the INS (10 sps) of the ground speed
component along the x-axis of the INS platform. (During
alignment of the INS, the x-axis is parallel to the longitudinal
axis of the aircraft.) The resolution is 0.012 M/s.
- Raw INS Ground Speed Y Component (M/s) - YVI
- This is an output from the INS (10 sps) of the ground speed
component along the y-axis of the INS platform. (During
alignment of the INS, the y-axis is normal to the longitudinal
axis of the aircraft.) The resolution is 0.012 M/s.
- Raw INS True Heading (deg) - THI
- This is an output from the INS (5 sps) giving the true
heading of the aircraft. The resolution is 5 arc seconds (0.0014
deg).
- INS Wander Angle (deg) - ALPHA
- This is an output from the INS (5 sps) giving the
difference between the platform heading x-axis and true north.
ALPHA wanders slowly depending upon the east-west velocity of the
aircraft. The resolution is 5 arc seconds (0.0014 deg).
- INS Platform Heading (deg) - PHDG
- This is an output from an 8:1 platform heading axis
resolver. The resolution is 10 arc seconds (0.0028 deg).
- Raw Aircraft Vertical Velocity (M/s) - VZI
- This is an integrated output from an up/down binary counter
connected to the INS vertical accelerometer. Resolution is 0.012
M/s. Due to changes in local gravity, this is not an absolute
quantity but drifts considerably from zero.
- Aircraft True Heading (deg) - THF
- This a derived output of the horizontal projection of the
aircraft center line with reference to true north, calculated
from PHDG and ALPHA. Resolution is 10 arc seconds (0.0028 deg).
THF = PHDG + ALPHA
- Aircraft Ground Speed (M/s) - GSF
- This is a derived output from XVI and YVI giving the scalar
magnitude of the INS ground speed. (This variable shares its name
with the present IRS Ground Speed.)
GSF = (XVI2 + YVI2)0.5
- Aircraft Ground Speed East Component (M/s) - VEW
- This is a derived output using XVI, YVI, and ALPHA that
rotates the horizontal ground speed components to a geographic
reference frame. Positive values are toward the east. (This
variable shares its name with the present IRS Ground Speed East
Component.)
- Aircraft Ground Speed North Component (M/s) - VNS
- This is a derived output using XVI, YVI, and ALPHA that
rotates the horizontal ground speed components to a geographic
reference frame. Positive values are toward the north. (This
variable shares its name with the present IRS Ground Speed North
Component.)
- Wind Speed (M/s) - WSPD
Wind Direction (deg) - WDRCTN
- These variables are obtained in a straightforward manner
from both UI and VI.
WS = (UI2 + VI2)0.5
WD = atan4 [(-UI)/(-VI)] + 180.0
where:
UI = easterly component of the horizontal wind
measurement with respect to the earth. Positive
values are toward the east.
VI = northerly component of the horizontal wind
measurement with respect to the earth. Positive
values are toward the north.
atan4 = 4-quadrant arc-tangent converted to degrees
- Raw Attack (Fixed Vane) (g) - AFIXx
- This is an amplified output from a strain-gage, fixed-vane
sensor mounted in the horizontal plane of the aircraft at the end
of the gust boom. The "force" on the vane (calibrated in
"equivalent grams" at Jefferson County Airport gravity) varies as
a function of the aircraft attack angle and dynamic pressure.
Here x refers to left or right.
- Raw Sideslip (Fixed Vane) (g) - BFIXx
- This is an amplified output from a strain-gage, fixed-vane
sensor mounted in the vertical plane of the aircraft at the end
of the gust boom. The "force" on the vane (calibrated in
"equivalent grams" at Jefferson County Airport gravity) varies as
a function of the aircraft sideslip angle and dynamic pressure.
Here x refers to top or bottom.
- Attack Angle (Fixed Vane) (deg) - AKFXx
- This is a derived output of the attack angle computed from
AFIXx, and QCx (either boom or gust dynamic pressure). An
empirically-derived function, HSSATK, is used to determine the
attack angle based upon wind tunnel test data.
- Sideslip Angle (Fixed Vane) (deg) - SSFXx
- This is a derived output of the sideslip angle computed from
BFIXx, and QCx (either boom or gust dynamic pressure). An
empirically-derived function, HSSATK, is used to determine the
sideslip angle based upon wind tunnel test data.
- Dynamic Pressure (Boom) (mbar) - QCB, QCBC
Dynamic Pressure (Gust Probe) (mbar) - QCG, QCGC
- This is the output from a calibrated differential pressure
transducer. The measurement is the difference between a pitot
(total) pressure and a static pressure. The QCBC and QCGC values
are corrected for local flow-field distortion. The boom and gust
probe measurements referred to the same aircraft structure. The
different designations used for those measurements specified the
transducer used and its location. In the gust probe dynamic
pressure measurement (QCG), a Rosemount Model 1332 differential
pressure transducer was located closer to the sensor in the gust
probe itself, whereas in the boom measurement (QCB), a Rosemount
Model 1221 pressure transducer was typically located in the
aircraft nose.
- Total Temperature, Reverse Flow (C) - TTRF
- This is the output of the recovery temperature from a
calibrated, NCAR, reverse-flow temperature sensor. In the standard
output, the total temperature (and ambient temperature) variable name
also conveys the sensor type.
- Total Temperature (Fast Response) (C) - TTKP
- This is the output of recovery temperature from the NCAR
fast-response temperature probe originally designed by Karl Danninger.
(See discussion of
total temperature
in section VI above.)
- Ambient Temperature (C) - ATRF
- The ambient temperature computed using the NCAR reverse-flow
temperature sensor. (See discussion in
Section VI above.)
- Ambient Temperature (Fast Response) (C) - ATKP
- The ambient temperature computed using the fast-response temperature
probe. (See discussion of
ambient temperature
above.)
- Raw Cloud Technology (Johnson-Williams) Liquid Water Content (g/M3) - LWC
- This is the raw output of a Johnson-Williams liquid water
content sensor converted to grams per cubic meter. The
Johnson-Williams indicator measures the evaporative cooling
caused by the latent heat of vaporization of droplets contacting
the heated sensing element by sensing changes in its resistance
as it cools. Through calibration this resistance is converted to
a liquid water content. A "compensation" wire is also mounted in
the J-W sensor, parallel to the droplet stream, to compensate for
cooling effects due to convection. Typically the instrument is
set for a true airspeed of 200 knots. The instrument must be
zeroed in "cloud-free air."
The Johnson-Williams liquid water content sensor is designed
for the cloud droplet spectrum. There is some evidence to
indicate that droplets larger than 30 µM tend to be shed before
completely vaporizing on the sensor element. This tends to
underestimate the liquid water content.
- Corrected Cloud Technology (Johnson-Williams) Liquid Water Content (g/M3) - LWCC
- This is the corrected liquid water content obtained by using
the aircraft's true airspeed after removing the zero offset.
XLWCC = XLWC (true airspeed dial setting*)/TASref
where:
TASref = Reference Aircraft True Airspeed
*as mentioned above, 200 knots
- Water Vapor Pressure (mbar) - EDPC
- This is a derived intermediate variable used in the
calculation of several derived thermodynamic variables. The
vapor pressure over a plane water surface is obtained by the
method of Paul R. Lowe (1977), a derived, sixth-order, Chebyshev
polynomial fit to the Goff-Gratch Formulation (1946). The error is
much less than 1% over the range -50C to +50C. (Notes: 1. EDPC was
calculated using this method for most RAF research projects between
1993 and 1996. 2. This variable does not have the enhancement
factor applied. See
Appendix C.)
A. T < -50 C
EDPC = [4.4685 + T (0.27347 + T {6.83811x10-3
+ T [8.7094x10-5 + T (5.63513x10-7
+ T 1.47796x10-9)]})]
B. T >= -50C
EDPC = {6.107799961 + T [0.4436518521
+ T (0.01428945805 + T {2.650648471x10-4
+ T [3.031240396x10-6 + T (2.034080948x10-8
+ T 6.136820929x10-11)]})]}