ARM/GCIP NESOB-97 30 Minute Sensible, Latent, and Ground Heat Flux Composite 1.0 General Description This 30 minute Sensible, Latent and Ground Heat Flux Composite is one of two surface-layer flux data sets provided in the Atmospheric Radiation Measurement(ARM)/Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP) Near Surface Observation Data Set - 1997 (NESOB-97). This Sensible, Latent and Ground Heat Flux composite was formed from three data sources: the ARM Southern Great Plains (SGP) Clouds and Radiation Testbed (CART) Energy Balance/Bowen Ratio (EBBR) sites, the National Oceanic and Atmospheric Administration (NOAA)/Atmospheric Turbulence and Diffusion Division (ATDD) Little Washita Watershed site, and the ARM SGP Eddy Correlation (ECOR) sites. Data from 14 ARM/EBBR stations, 1 NOAA/ATDD station, and 8 ARM/ECOR stations were merged to form this composite. The University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS) did not do any quality control on the data set. This composite contains data for the ARM/GCIP NESOB-97 domain and time period (01 April 1997 through 31 March 1998). The ARM/GCIP NESOB-97 domain is approximately 100.5W to 94.5W longitude and 34N to 39N latitude. 2.0 Detailed Data Description Heat flux sensors consist of a differential temperature sensor which measures heat flow. Heat flux is a vector quantity of energy flowing through a 1 meter square surface in one second. Sensible heat flux is the transfer of sensible heat between the surface and the air, or vice versa. Latent heat flux is the transfer of latent heat (heat released or absorbed by water) between the surface and the air, or vice versa. Ground, or soil, heat flux is the transfer of sensible heat in the soil, either toward the surface or away from the surface. 2.0.1 Surface Energy Exchange Energy balance Bowen ratio or eddy correlation stations measure the rates of heat and moisture exchange between the surface of the Earth and the atmosphere at selected locations. The Energy Balance Bowen Ratio (EBBR) system is a ground-based system using in situ sensors to estimate the vertical fluxes of sensible and latent heat at the local surface. The flux values are representative of the grassy area within about 50 meters of the EBBR station. Flux estimates are made from observations of net radiation, soil heat flow, and the vertical gradients of temperature and relative humidity; these data are used in the Bowen ratio energy balance technique. The Eddy Correlation Flux Measurement System provides in situ half-hour averages of the surface vertical fluxes of momentum, sensible heat, and latent heat. The fluxes are obtained by the eddy-correlation technique, i.e. by correlating the vertical wind component with the horizontal wind component, the sonic temperature (which is approximately equal to the virtual temperature), and the water vapor density. In this composite data set, all energy flux densities have a negative sign when directed toward the surface and positive when directed away. For example, a representative value for latent heat flux could be 100-500 watts per meter squared during the daytime in the summer. 2.0.2 ARM EBBR Algorithms The components of the energy balance were determined with the Bowen Ratio Energy Balance (BREB) method. The BREB method is a combination of the transport and the energy balance equations. The Bowen ratio, B {a ratio of the gradient equations of sensible heat, H and latent heat, E} is given by: B = H / E (1) where: H = -rho*c(p)*K(h)*dT/dz E = -(rho*epsilon / P)*l(v)*K(v)*de/dz where symbols are defined as: e = Air vapor pressure epsilon = Ratio of the molecular weights of water vapor and dry air c(p) = Specific heat of air at constant pressure K(h) = Eddy diffusivity for heat K(v) = Eddy diffusivity for water vapor p = Atmospheric pressure rho = Air density T = Air temperature z = Height or depth l(v) = Latent heat of vaporization Substituting (1) in the energy balance equation (2) yields the BREB (3). Q is net radiation and G is soil heat flux density. Q + G + H + E = 0 (2) In this system surface-air interface is considered as a closed system. Any energy flux coming in is considered positive and going out is negative. E = -(Q + G) / (1 + B) (3) (FIFE, 1999) 2.0.3 ARM ECOR Algorithms The sonic anemometer makes observations of the orthogonal wind velocities by measuring the travel time of sound with and against the wind and of the temperature by measuring the speed of sound. The infrared hygrometer makes observations of the water vapor density by measuring the absorption of an infrared light beam. The hygrometer includes an A/D converter which samples the solid state temperature and barometric pressure devices. These are measured 10 times per second even though the sensors do not respond that rapidly to changes. Both instruments are serial data output devices which report 10 measurements per second. The instruments are kept in synchronization by the DCU computer. Data analysis is performed over 30 minute periods. Vertical fluxes of momentum, sensible heat, and latent heat are determined using the eddy-correlation technique (see section 2.0.2 above). Means, variances, and covariances of the input data, as well as of data from which linear trends or low frequency components have been removed, are computed. Three-dimensional coordinate rotations are applied to variances and covariances of data from which linear trends or low frequency components have been removed. The coordinate rotations result in zero mean vertical and transverse wind speeds. The mixing ratio, air density, specific heat of air at constant pressure, and latent heat of vaporization of water are computed from the mean water vapor density, air temperature, and barometric pressure. These quantities and the rotated covariances are used to compute the vertical fluxes of momentum, sensible heat, and latent heat. (ECOR, 2001) 2.0.4 ARM Automatic Exchange Mechanism Home Signals A unique aspect of the ARM system is the automatic exchange mechanism (AEM), which helps to reduce errors from instrument offset drift. The AEM extends from the north end of the frame. Aspirated radiation shields (which house the air temperature and relative humidity probes) are attached to the AEM. The openings of the aspirated radiation shields face north to reduce radiation error from direct sunlight. The EBBR system is capable of producing estimates of 30-minute average sensible and latent heat fluxes accurate to approximately +/- 10% of the estimated value or +/- 10 watts per meter squared, whichever is larger, at the 95% confidence level. Offset and calibration drift errors in the measurements of the temperature and relative humidity gradients are significantly reduced by the use of the AEM, which switches the positions of the upper and lower sensors every 15 minutes. There are a number of conditions under which the primary quantities may be incorrect. Generally, when the AEM is not functioning properly, the sensible and latent heat flux estimates are unreliable and should not be used. Examine the AEM home signals to ensure that only flux data produced when the AEM is functioning properly are used. 2.0.4.1 home_15: min = 35 mv, max = 70 mv The following is the explanation of the home_15 parameter. This is the AEM position indicator for the first 15 minutes of the 30 minute data period. The mv value is proportional to battery voltage and therefore can vary considerably in the max, min range. If it is less than 35, the battery voltage may be too low to produce correct data. Furthermore, the EBBR datalogger programming will think that the AEM is actually in the 15 to 30 minute position and will incorrectly calculate sensible and latent heat fluxes. The home_15 value can be larger than 70 and still indicate the proper AEM position, but this situation indicates that there is a problem with the AEM home signal voltage circuitry (a transistor short or a resistor failure can cause the home_15 and home_30 outputs to be electronically added). A home_15 value near zero indicates that the AEM has halted at a position between the top and bottom, resulting in incorrect sensible and latent heat flux values. Other conditions are also possible, although uncommon. 2.0.4.2 home_30: min = 15 mv, max = 34.999999 mv The following is the explanation of the home_30 parameter. This is the AEM position indicator for the last 15 minutes of the 30 minute data period. For battery low voltage conditions the home_30 value can be less than the min value and not mean that the 30 minute flux data are incorrect or that the AEM is not in the proper position.. However, the low battery condition itself may result in incorrect data. If the home_30 value is near zero, it probably indicates that the AEM has halted at a position between the top and bottom, resulting in incorrect sensible and latent heat flux values. Other conditions are also possible, although uncommon. (EBBR, 1999) 2.0.3 NOAA/ATDD Algorithm The Little Washita site uses the eddy covariance technique for measurements of sensible and latent energy fluxes. Using this technique, the average vertical turbulent eddy fluxes of sensible and latent heat (and other scalars) are determined as ____ w'X' = $(w-{w})(X-{X}) ________________ n where w is the vertical velocity component of the wind vector, and X is the scalar of interest (e.g. water vapor concentration). Here, the {bracketed} quantities denote an average or "mean" that is subtracted from the instantaneous values to obtain the fluctuating component. The $ represents the summation from i = 1 to n. Average vertical turbulent ____ fluxes (w'X') are computed in real time using a digital recursive filter (200 s time constant) for the determination of a running "mean" from which the instantaneous values are subtracted. An averaging period of 30 minutes (denoted by the overbar) is used and is considered large enough for statistical confidence in the covariance quantity but is short enough to resolve the structure of the diurnal cycle. (Meyers, 1999) 2.0.4 Correlating ARM data with ATDD data The data from the ARM EBBR sites recorded energy flux coming in as positive, and going out as negative. The ATDD site was just the opposite, with energy flux densities having a negative sign when directed toward the surface, and positive when directed away. To correlate the data correctly when UCAR/JOSS composited it, UCAR/JOSS multiplied all heat flux values in the ARM data by -1.0 in order to have the signage correspond with the data in the ATDD data set. 2.1 Detailed Format Description The GCIP/ARM NESOB-97 Sensible, Latent and Ground Heat Flux Composite contains 8 metadata parameters and 10 data parameters. The metadata parameters describe the date/time, network, station, and location at which the data were collected. The ten data parameters repeat once for each 30 minute period from UTC 0000 through UTC 2330. Data reported for a designated 30 minute time represents data collected during the previous 30 minute period. All times are reported in UTC, and flux data values are reported in watts per meter squared (W/m2). Each data value is followed by a Quality Control flag, but UCAR/JOSS does not Quality Control the data at the present time. The Quality Control flag is set to "U" for "Unchecked", unless the datum is missing, in which case the flag is set to "M". The EBBR sites report 3 data values (home_15, home_30, and Bowen Ratio) which can serve as Quality Control indicators. When the instruments are operating reliably the home_15 parameter is 35-70 mv, and the home_30 parameter is 15-39.99999 mv. If either is outside their range then the sensible and latent heat flux estimates are unreliable, and should not be used. See the section on Automatic Exchange Mechanism (AEM) home signals (2.0.2.1) for further information. When Bowen Ratio values are near -1 a spike results in the flux data that is not real. This typically occurs near stability transition times (morning and evening), particularly the evening one. The table below details each parameter in the composite data set. Parameters Units ---------------------- ----------------------------------- Date of Observation UTC Time of Observation UTC Network Identifier Abbreviation of platform name Station Identifier Network dependent Latitude Decimal degrees, South is negative Longitude Decimal degrees, West is negative Station Occurrence Unitless Station Elevation Meters above sea level Latent Heat Flux W/m2 QC flag U or M Sensible Heat Flux W/m2 QC flag U or M Soil Heat Flux 1 W/m2 QC flag U or M Soil Heat Flux 2* W/m2 QC flag U or M Soil Heat Flux 3* W/m2 QC flag U or M Soil Heat Flux 4* W/m2 QC flag U or M Soil Heat Flux 5* W/m2 QC flag U or M Bowen Ratio* Unitless QC flag U or M home_15 value* mv QC flag U or M home_30 value* mv QC flag U or M * these parameters are only reported from the EBBR sites 2.2 Data Remarks The Little Washita site records only 1 soil heat flux value. The EBBR sites record soil heat flux values from 5 different sensors. The ARM soil sensors are located in a half-circle approximately 2 meters in diameter under the net radiometer, which extends to the south about 1 meter from the EBBR frame. The soil conditions at the EBBR sites are varied from very sandy soil to very clay-laden soil. However, all of the sensors for one particular EBBR site are in the same soil type. Information on the soil characteristics at each of the ARM Soil Water and Temperature System (SWATS) sites (which are located nearby the ARM EBBR sites) is available as part of the ARM/GCIP NESOB-97. (These include the "Organic Carbon and Matter", "Soil Texture", "Parameters for Soil Water Retention Models", "Bulk Density", Particle Size", and "Soil Water Retention" data sets.) The EBBR sites also record Bowen Ratio, home_15 and home_30 values, as well, whereas the Little Washita site does not. Since this is a composite data set, only the first 3 fields of data from the Little Washita site will have values, while the rest of the parameters on a line will always be missing. Missing values are -999.99999. 3.0 Quality Control Processing This data set was not Quality Controlled by the Joint Office of Science Support (UCAR/JOSS). 4.0 References FIFE Bowen Ratio Surface Flux: Smith Data Set Guide Document, http://www-eosdis.ornl.gov/FIFE/Datasets/Surface_Flux/Bowen_Ratio_Smith.html; September 8, 1999 Energy Balance Bowen Ratio (EBBR), http://www.arm.gov/docs/instruments/static/ebbr.html; September 6, 1999 Eddy Correlation (ECOR), http://www.arm.gov/docs/instruments/static/ecor.html; February 16, 2001 Meyers, Tilden, 1999, GCIP/EOP NOAA/ATDD Little Washita Watershed Long Term Flux Site, information supplied with data