These data contain surface meteorology measurements of the Integrated Surface Flux System (ISFS) during the Shallow Cold Pool Study (SCP) at the Pawnee Grasslands, Colorado, during September, October and November of 2012.

For general information about the operations of the ISFS during SCP see https://www.eol.ucar.edu/field_projects/scp.

The ISFS five minute dataset contains first moments and some second moments of variables measured by the NCAR ISFS stations during SCP. 

The data are stored in NetCDF files. Information on the NetCDF file format and software is available at  http://www.unidata.ucar.edu/software/netcdf/.  Information specfic to ISFS NetCDF files is available at https://www.eol.ucar.edu/content/isfs-netcdf-files.

Datasets

Three datasets are available:

• geo_tilt_cor: http://data.eol.ucar.edu/codiac/dss/id=385.003 All wind vectors have been rotated to geographic coordinates (+U is wind to the east, +V is wind to the north), and 3D wind vectrors are corrected for tilt of the anemometer relative to a plane of mean flow (see #SonicTiltCorrection). Values for T, RH and P are derived at sonic anemometer locations where they were not measured, as described below. See #DataEdits and #Derived Variables. 

• geo_no_tilt_cor: http://data.eol.ucar.edu/codiac/dss/id=385.004 All winds are rotated to geographic coordinates, but 3D wind vectors have not been corrected for anemometer tilt. Values for T, RH and P were derived at sonic anemometer locations where they were not measured, as described below. See #DataEdits and #Derived Variables.

• instrument: http://data.eol.ucar.edu/codiac/dss/id=385.005 Winds from 3D sonic anemometers are in instrument, not geographic coordinates, with no correction for tilt. The data edits and derivations listed under #DataEdits and #Derived Variables have not been applied to this dataset.

NetCDF File Names

Each NetCDF file contains one day of data, from 00:00 to 24:00 UTC. The file names are of the following forms, depending on the dataset, where YYYYMMDD is the UTC date of year, month and day:

• isfs_qc_gtc_YYYYMMDD.nc ,  QC'd, geographic, tilt-corrected sonic winds
• isfs_qc_gntc_YYYYMMDD.nc, QC'd, geographic, non-tilt-corrected sonic winds
• isfs_nqc_inst_YYYYMMDD.nc, not QC'd, sonic winds in instrument coordinates

Measurements

The measurment sites (or stations) were given names A1-A19, C and M.  All "A" stations had a CSAT3 3D sonic anemometer and two NCAR aspirated hygrothermometers. As shown in the tables on the SCP project page, some "A" stations had an additional Handar 2-D sonic anemometer and/or a barometer. Station C was a 3 meter tower with 2 CSAT3 sonics, 4 hygrothermeters, a barometer and a 2D sonic. Station M, the main tower was a 20 meter tower with 13 levels of measurements.  Components of incoming and reflected surface radiation were sampled near the M tower. Soil temperature, moisture, heat flux and thermal properties were measured at two sites. A CSI EC150 3-D sonic with IR gas analyzer was added later in the project on Nov 12 at station A8.

Time Representation

The base_time variable contains one value, the time of the start of the file, as a number of POSIX (non-leap) seconds since 1970 Jan 1, 00:00 UTC.

Values for each time-varying measurement will be found in the NetCDF files, as a variable with a time dimension.  There are 288 5 minute periods per day, so the time dimension is 288.

The time variable contains the time to be associated with each sample, in units of seconds since base_time, or 00:00 UTC of the day.  Each time value is the middle of the averaging period, and will have  values of 150 (00:02:30 UTC), 450 (00:07:30), etc, up to 86250 (23:57:30 UTC).

Stations

A variable measured at more than one station, such as winds from the 3-D sonic at 1 meter, will have a station dimension of 21. Station indices 1-19 are for the A stations, 20 is the C tower and 21 is M, the main tower.  Variables that were measured at one or two sites generally do not have a station dimension. Instead the short_name will have a suffix indicating the station, such as ".C".

Dimensions

The NetCDF dimensions in each file are:

Short Name Attributes

Each measured variable will have a short_name NetCDF attribute. The field before the first period in the short_name is a generic variable name, such as T for temperature, Rsw for short wave radiation, or u for the U component of the wind. For second moments, such as variances and co-variances, the first field of the short_name will contain single quote marks after a variable to indicate it is an average of a deviation.  For example,  a leading short name of  w'tc', is short for: mean( (w-mean(w)) * (tc-mean(tc)) ), w is the vertical wind component, and tc is the temperaure from the speed of sound.

Higher Moments

For each of the  3-D sonic anemometers, the following second moments in u,v,w and tc are provided for the computation of eddy-correlation fluxes. Scalar values such as water vapor density and carbon dioxide were also measured at some 3-D anemometer locations. kh2o is a water vapor measurement from a Campbell krypton hygrometer at 5 and 10 meters on the "M" tower. h2o and co2 are water vapor and carbon-dioxide measurements from LICOR 7500 gas analyzers at 1 and 2 meters on the "M" tower, and from the EC150 at station "A8".

Heights

The height in meters above ground of the measurement, if appropriate, will be indicated in a second field after a period in the short_name, for example RH.0.5m, or u'tc'.20m.

Variable Names

The actual NetCDF variable names will have underscores, '_', in place of periods and single quotes.  Therefore a variable with a short_name attribute of w'co2'.3m.A8 will have a NetCDF variable name of w_co2__3m_A8.

Units and Long Names

Each variable will have NetCDF attributes containing the units of the measurement, and a long name giving more information on the measurement.

Counts Attributes

Variables from sensors used in eddy-covariance flux measurements will have a counts attribute indicating the number of samples that were included in each statistic.

Missing Data

The missing data value is 1x10^37.  A missing value indicates either that nothing was measured at the location indicated in the variable name and station index, or the sensor was not reporting at the given time, or it was detemined that the data value did not meet QC criteria during post-project analysis.

Non-Time Series Variables

The files also contain non-time series values for station latitude, longitude and altitude. The latitude and longitude were determined with a handheld GPS during the project. The altitudes were determined by survey with a theodolite during the project. The online field logbook has further information on these values.

Station Moves and Layouts

Stations 15 and 17 were moved on October 4th. The latitude, longitude and altitude variables therefore have a "layout" dimension of 2, to provide for the two locations of station 15 and 7. The sonic at station 15 was also raised from 1 to 2 meters above ground as part of the move, and the sonic at 17 lowered from 1 to 0.5 meters.

Wind Coordinates

The U and V components of the wind vectors from the 2D and 3D anemometershave been rotated to geographic coordinates, such that a positive U is the component of the wind blowing to the east, and a positive V is the component of the wind blowing to the north.

Sonic Tilt Correction

The 3D sonic anemometers were installed as level as was possible, but not perfectly "bubble" level.

The geo_tilt_cor dataset contains 3D wind vectors and second moments which have been rotated to a coordinate system where the mean W component is zero, as described in  /content/sonic-tilt-corrections.

Calibration Corrections

These corrections have been applied to all three datasets

• P from barometers: corrected for offsets seen in comparison with transfer standard.
• Pyrgeometers: post-project calibration factors applied.

Data Edits

The following corrections have been applied to the geo_tilt_corand geo_no_tilt_cor datasets, but not to the instrument dataset. Removing a data value means it was overwritten with the missing data value.

• co2, h2o from LI/COR 7500 on M tower: data removed during times of sensor maintenance as well as some instancess where excess moisture affected sensor operation.
• kh2o from krypton hygrometers on M tower: data removed during times of sensor maintenance, and low sensor voltages due to liquid water in the absorption path.
• co2,h2o from the Campbell Scientific EC150  IRGA gas analyzer at A8: data removed during times of sensor maintenance as well as some instances where excess moisture affected sensor operation.
• T,RH from NCAR hygrothermometers: data removed during times of sensor maintenance/cleaning as well as some instances prompted by erratic Ifan (aspiration fan current) values specific to each sensor
• Gsoil: re-applied Philip correction to heat flux plate based on heat conductivity as measured by the TP01
• Tsoil: corrected for data sampling problems of Tsoil at grass site between Oct 6  and Oct 12.
• Gsoil: re-applied Philip correction to heat flux plate based on heat conductivity as measured by the TP01
• u'h2o', v'h2o', w'h2o': Values computed from a CSAT3 with krypton hygrometer have been corrected for the absoption by oxygen, and the spatial decorrelation. Values from a CSAT3 and a Licor 7500 or CSI IRGA have been corrected for the spatial decorrelation.

Derived Variables

Barometric pressure was not measured at 1 meter AGL at sites 1,2,4,5,6,7,11,13,15,16,17,18 or 19. In the geo_tilt_cor and geo_no_tilt_cor data sets, those values have been derived using the formula:

P₂ = P₁ * e (-g * (z₂ - z₁) / ( Rair * T)

where P₁ and z₁ are the pressure and elevation of the reference barometer, Rair = 287 (m²/s²/K), g=9.7984 m/s^2,  T is the air temperature, and z₂ is the elevation of the desired pressure.

Low rate temperature and relative humidity measurements were not made at all the locations of the 3-D sonic anemometers. In order to compute air densidy, and flux corrections, these variables have been derived by a linear interpolation over height of the two nearest measurements. Note the wide interpolation and extrapolation of T and RH at 10 and 20 meters on the M tower.

• Long-wave radiation (Rlw):computed from the radiation measured by the thermopile (Rpile) and the case temperature (Tcase)
• Surface heat flux (Gsfc): heat flux at the surface was computed from value measured by the heat flux flux plate at 5 centimeters, and the heat storage above it, as computed from the soil temperatures and heat capacity.

Summary Table of Variables