PAM III Development Update
2/2/96 - Final
by Tom Horst
Atmospheric Technology Division
National Center for Atmospheric Research
1. Introduction
The Atmospheric Technology Division (ATD) at NCAR is now completing
development of the third generation of its Portable Automated Mesonet
(PAM) field observing facility.
PAM is a network of portable surface meteorological stations
designed to support observational field research projects
of the atmospheric science community.
The third generation, PAM III, has been designed not only to
meet the need for a traditional mesonet that provides measurements of
standard meteorological variables such as wind, temperature, humidity,
pressure, solar radiation, and precipitation
but also, in its enhanced Flux-PAM version, to measure fluxes of
momentum, sensible heat, water vapor, net radiation, and soil heat flux
(Militzer et al., 1995; Horst and Oncley, 1995).
User-provided sensors also can be interfaced to a PAM III station,
for example to measure concentrations or fluxes of trace chemical species.
As with PAM II, PAM III has been designed for maximum siting flexibility
and therefore relies principally on solar-charged-battery power
and real-time data transmission through the GOES satellite.
However, the PAM III data system has been designed with broad capabilities
so that stations can also be enhanced for such options as
on-site data recording or
direct, line-of-site data transmission to a central field base.
Three prototype Flux-PAM versions of PAM III have been built
and were deployed in Florida during the summer of 1995
to support a study of boundary layer roll circulations.
These stations will be deployed again in the summer of 1996
to support the Flatlands experiment in central Illinois.
This report briefly describes ongoing development of the PAM III
surface meteorological stations and plans for future expansion of the
network.
2. Ongoing PAM III Sensor Development
a. State Variables
The prototype PAM III stations measure wind at a height of 10 m,
using an R.M. Young 5103 propeller-vane that has been modified at NCAR
to incorporate an optical digital encoder for the measurement of
wind direction and an integral microprocessor to output calibrated data
as a digital serial message.
R.M. Young has now incorporated similar enhancements in a commercial
model of the 5103 anemometer and we are currently testing
a pre-production version,
both in the NCAR wind tunnel and at our Marshall field site.
This commercial anemometer is expected to provide performance similar
to the custom-built NCAR/R.M. Young anemometer but at a lower cost.
Temperature and humidity are measured at a height of 2 m
with a Vaisala 50Y Humitter that includes a platinum-resistance thermometer
and a solid-state capacitance sensor for relative humidity.
The NCAR hygrothermometer incorporates an integral microprocessor which
again sends calibrated data as a serial message.
The accuracy of the 50Y output is increased by calibrating the individual
temperature and humidity sensors at NCAR and by
applying this customized calibration to the 50Y output
by means of the hygrothermometer microprocessor.
The 50Y is enclosed in a mechanically-aspirated,
Gill-type, multi-plate radiation shield.
Field tests still in progress suggest that this shield suffers from
a radiation error of up to a few tenths of a degree C
under moderate insolation and light winds,
but that improved performance may be achieved
by enclosing the 50Y in an aspirated, dual-concentric-cylinder shield
as used by Flux-PAM for the measurement of water vapor fluxes
by the bandpass covariance technique.
Atmospheric pressure is measured with a Vaisala 220B barometer,
which has a factory-calibrated uncertainty in absolute pressure
on the order of 30 Pa (0.3 mb).
However, with periodic calibration at NCAR, it is likely that
PAM barometers could achieve an absolute accuracy of 10 Pa.
Moreover, recent field tests of these sensors suggest that,
with similar care, PAM barometers could potentially achieve
a relative accuracy of less than 5 Pa,
making it possible for PAM stations with a spacing of 100 km
to resolve the geostrophic wind within less than 0.5 m/s.
However at this level of accuracy, dynamic pressure errors can be important,
and thus we are currently investigating the field performance of
static pressure ports, including the PAM II Ser design and
the Nishiyama/Bedard quad disk pressure port.
In addition, the (relative) barometer elevation must be known to 0.5 m
or less, but this accuracy is readily achieved with current GPS technology.
b. Flux Sensors
Flux-PAM stations measure fluxes of momentum and virtual
temperature by eddy-correlation, using a 3-component sonic anemometer.
Two problems remain, to one degree or another,
with all existing sonic anemometers.
These are distortion of the measured flow field by the anemometer array
itself and reliable detection of the transmitted sound pulses by the
anemometer electronics over a wide range of environmental conditions.
An additional requirement for the Flux-PAM sonic anemometer is
low power consumption.
Although the ideal sonic anemometer has not yet been built,
several manufacturers are currently designing and producing new models
which we will be evaluating over the coming months,
both in the NCAR wind tunnel and in side-by-side field tests
using the NCAR
ASTER (Atmosphere-Surface Turbulent Exchange Research)
micrometeorological field facility (Businger et al., 1990).
We are currently evaluating the new Campbell Scientific sonic,
which has 0.5 cm diameter transducers and an 11.5 cm path length,
and in the immediate future we expect to evaluate a new version of
the Gill Instruments Solent anemometer that features, among other upgrades,
a new transducer array designed to reduce flow distortion.
Applied Technologies is also planning to have an entirely new design
of their electronics in the summer of 1996.
Finally, we have recently initiated a fundamental study of the
dependence of flow distortion on array geometry, with the goal of
developing a methodology for optimizing array design.
The common choice for measurement of water vapor fluxes by
eddy correlation is a fast-response optical-absorption hygrometer,
combined with a sonic anemometer to measure vertical velocity.
However, because of the cost, power consumption, maintenance requirements,
and calibration instability of optical-absorption hygrometers,
Flux-PAM stations are attempting to measure water vapor fluxes using
a solid-state capacitance relative humidity sensor and
a technique called bandpass-covariance which assumes cospectral similarity
between the water vapor and virtual heat fluxes (Horst and Oncley, 1994).
The water vapor flux is calculated from the directly-measured
virtual heat flux by means of a `virtual' Bowen ratio,
measured as the ratio of the virtual heat flux to the water vapor flux
within a bandpass frequency range where
the capacitance RH sensor has acceptable frequency response.
The water vapor flux can then be used to extract the sensible heat flux
from the measured virtual heat flux.
The initial implementation of the bandpass covariance technique
on the prototype Flux-PAM station has not been completely satisfactory.
There have been two reasons for this performance and
we are currently working on improvements for both.
The current bandpass hygrometer utilizes a Vaisala 50Y Humitter, housed
in an aspirated, dual-concentric-cylinder radiation shield.
Our recent laboratory investigation of the time response of the 50Y sensor
has shown that it has a first-order time constant that depends strongly
on temperature,
varying (non-linearly) from 0.5 sec at 35 degC to greater than 10 sec
at -15 degC, with a value of about 1 sec at 20 degC.
Although the response of the 50Y may be adequate at high temperatures,
the response at low temperatures is too slow to measure a useful
fraction of the turbulent water vapor flux.
As a consequence, we are currently evaluating the performance of
faster solid-state RH sensors manufactured by AIR and CORECI
as potential replacements for the 50Y in this application.
The second aspect requiring further development is the software
used to implement bandpass covariance.
Because of earlier uncertainty about the frequency response of the 50Y,
we use a dynamic algorithm to determine the upper limit of
the bandpass frequency range, and this algorithm appears to be
less than satisfactory.
Thus we plan to revisit this aspect of the software in the immediate future,
either improving the dynamic algorithm or simply using the laboratory
measurements of hygrometer response to specify the upper limit of the
bandpass frequency range.
4. Data Display and Analysis Software
PAM III data is archived on the PAM base-station computer in NetCDF files,
a format that is readable by several existing data display packages
including both Splus software previously developed for ASTER and
the ATD-developed Zebra program.
The ASTER Splus software was specifically developed for
in-field data quality control and post-project scientific analysis of
in-situ sensors at a single site and was easily adapted to PAM III data.
However, Zebra will also be used for display and analysis of PAM data
because of its capability for integrating and simultaneously displaying
spatially-distributed data from multiple measurement platforms including
satellites, weather radars, aircraft, radar wind profilers, rawindsondes,
and surface networks (Corbet et al., 1994).
The Zebra configuration used during the 1995 Flux-PAM deployment in Florida
proved to be less functional than the old PAM II ROBOT display software,
but it was useful for determining the additional capabilities
needed by Zebra in order to fully support
PAM field operations and post-project data analysis.
Work is currently proceeding to modify the Zebra configuration
(essentially the user interface) so that it will be more suitable
for the display of PAM data,
as well as to enhance Zebra's fundamental capabilities.
Near-term enhancements include output of graphical displays in
postscript format for hard-copy printing and batch mode processing
to allow automated plotting for in-field data quality control.
Longer term development plans include adding the capability to calculate
and display derived variables, which will significantly enhance
Zebra's value for in-field and post-project data analysis.
5. Flux-PAM Operation in SCMS
Three prototype Flux-PAM versions of PAM III were deployed in Florida
during the summer of 1995 to support a study of boundary layer roll
circulations in conjunction with the Small Cumulus Microphysics Study
(SCMS).
The principle role of the Flux-PAM network was to measure
the surface fluxes of momentum and virtual temperature
in order to quantify the stability of the boundary layer,
and the performance of the network was very satisfactory.
The siting flexibility of the PAM stations was essential to
success of the project because it was extremely difficult
to find sites that were representative of the variety
of surfaces present in the Florida land/waterscape, but at the same time
had fetches that were adequate for flux measurement.
A second highlight of this initial PAM III deployment was
direct data transmission from the remote stations to the PAM field base
via RF modems and local storage of data at the remote station,
both of which supplemented the standard transmission of data
through the GOES satellite and thus enhanced overall data recovery.
For SCMS these options provided redundant data transmission,
but in the future they could also support increased data bandwidth
which could be used either to record additional variables or to
increase the data rate, for example to archive the full turbulence time
series data for post-project analysis.
In addition, the RF modems also allowed two-way communication and thus
remote configuration and maintenance of the PAM III remote station software.
Perhaps the most disappointing aspect of this deployment was
damage to one of the stations during a lightning storm, and thus
we are currently enhancing the protection of the electronic components
against electrical surges.
Scientific analysis of SCMS Flux-PAM data has only recently begun.
6. Collaboration with GAME
The Global Energy and Water Cycle Experiment (GEWEX) has been initiated
as a major component of the World Climate Research Program with the goal
of understanding the energy fluxes and hydrological cycle and
their variability in the global climate system.
The GEWEX Asian Monsoon Experiment (GAME) has been proposed
to determine the role of the Asian monsoon
in global energy transport and in the global water cycle.
Since the surface energy budget is a fundamental forcing
of the climate system, GAME plans include the deployment,
beginning in 1997, of an Asian Automated Weather Station (AWS) Network
for the long--term monitoring of directly--measured energy fluxes.
A leading candidate for these surface meteorological stations is
the Flux-PAM station currently under development at NCAR.
In order to determine the suitability of PAM III stations
for the Asian AWS Network (AAN),
the AAN Working Group of the Japanese National Planning Committee for GAME
has requested that NCAR construct a prototype Flux-PAM station
for their evaluation.
Several enhancements are being made to the NCAR Flux-PAM design
to meet the needs of GAME.
These include development of a serial interface for the IMKO TRIME
Time-Domain Reflectometry system to enable accurate
measurement of soil moisture at multiple locations,
development of the capability to transmit data
through the Japanese GMS satellite, and
expansion of the capacity for local data storage on the station
through the use of removable PCMCIA flash memory cards.
Other sensors to be added to the prototype GAME station include
an infrared thermometer for the measurement of surface temperature,
a 4-component radiation system to independently measure upwelling
and downwelling, short and long wavelength radiation, and
a second hygrothermometer for the measurement of vertical temperature
and humidity gradients.
7. Construction Plans
The initial PAM III design phase was completed in 1994 and was the
basis for construction of the first three prototype stations.
We are currently in the second phase of development, as described in
this report, refining the initial design
based on field experience with the prototype stations and
on current laboratory and field tests of the various components.
Current plans are to complete this second development phase
in 1996, in order to permit the construction of multiple PAM III stations
in 1997 for use by the NSF scientific community.
However the modular design of the PAM III stations will make it easy
to upgrade individual sensors or data system components as desired
in the future.
In particular, development of PAM III will continue in order to
adapt the stations to the specific needs of the GAME program,
such as operation in the extreme environments encountered
in various locations on the Asian continent.
Construction of multiple stations for use by GAME is expected to extend
over a 3-year period beginning in late 1996.
References
Businger, J.A., W.F. Dabberdt, A.C. Delany, T.W. Horst, C.L. Martin,
S.P. Oncley and S.R. Semmer. 1990.
The NCAR Atmosphere-Surface Turbulent Exchange Research Facility.
Bulletin of the American Meteorological Society}, 1006--1011.
Corbet, J., C. Mueller, C. Burghart, K. Gould and G. Granger. 1994.
Zeb: software for integration, display and management of diverse data
sets.
Bulletin of the American Meteorological Society, 783--792.
Horst, T.W., and S.P. Oncley. 1995.
Flux-PAM measurement of scalar fluxes using cospectral similarity.
Proceedings of the 9th Symposium on Meteorological Measurements and
Instrumentation}, March 27--31, Charlotte, NC, 495--500.
Militzer, J.M., M.C. Michaelis, S.R. Semmer, K.S. Norris, T.W. Horst,
S.P. Oncley, A.C. Delany, and F.V. Brock. 1995.
Development of the prototype PAM III/Flux-PAM surface meteorological
station.
Proceedings of the 9th Symposium on Meteorological Measurements and
Instrumentation, March 27--31, Charlotte, NC, pp 490--494.
NOTE: Contact Tom Horst
if you would like hard copies of any of these
references.
