GEWEX Longwave Daily-Average Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.1_LW_DAILY data set. The data set contains daily average global fields of six longwave (LW) surface and Top of Atmosphere (TOA) radiative parameters derived with the Longwave algorithm of the NASA World Climate Research Programme/Global Energy and Water-Cycle Experiment (WCRP/GEWEX) Surface Radiation Budget (SRB) Project. If users have questions, please contact the Langley Atmospheric Sciences Data Center (ASDC) Science, Users and Data Services Office at: Atmospheric Sciences Data Center Science, Users and Data Services Office Mail Stop 157D 2 South Wright Street NASA Langley Research Center Hampton, Virginia 23681-2199 U.S.A. E-mail: larc@eos.nasa.gov Phone: (757)864-8656 FAX: (757)864-8807 URL: http://eosweb.larc.nasa.gov This readme includes the following sections: 1.0 Introduction 2.0 Data Set Description 2.1 Data Quality 2.1.2. Indian Ocean Gap Artifact 2.2 Input Information 2.3 Grid Description 2.4 Points of Contact 3.0 Format and Packaging 4.0 Science Parameters Information 5.0 Sample Read Software Description 6.0 Implementing the Sample Read Software 7.0 Sample Output 8.0 Additional Derivable Parameters 2.0 Data Set Description There are a total of six parameters in these files as follows: 1. TOA Upward Clear-Sky Flux/Clear-sky Outgoing Longwave Radiation (OLR) (clr_toa_up) 2. Surface Clear-sky Upward Longwave Flux (clr_sfc_up) 3. Surface Clear-sky Downward Longwave Flux (clr_sfc_down) 4. TOA Upward Longwave Flux/OLR (toa_up) 5. Surface Upward Longwave Flux (sfc_up) 6. Surface Downward Longwave Flux (sfc_down) These parameters are derived originally on a 3-hourly temporal resolution. The 3-hourly values are averaged into the daily values given in these files. The current version of the data sets is identified as Release 2.1. The GEWEX LW algorithm uses the Fu et al. (1997, JAS, Vol. 54, 2799-2812) Thermal Infrared radiative transfer code with cloud and surface parameters requiring cloud, atmospheric profile information, and surface properties. The sources for these inputs are briefly described below. A detailed description of the algorithm is currently being prepared for publication (Stackhouse et al., 2005). Please contact the Dr. Paul W. Stackhouse Jr. at the address below for further details. 2.1 Data Quality An assessment of the quality of these daily average fluxes was accomplished by comparisons with corresponding ground-measured fluxes over a period of four years (1992-1995) from a number of sites of the Baseline Surface Radiation Network (BSRN). From the aggregate data set for all sites and years, mean bias was determined to be about 1.7 W/m**2 (0.5%, model fluxes higher), and the root mean square difference is 23.2 W/m**2 (7.4%). Uncertainties associated with operational BSRN measurements during this period are believed to be about +/- 3-5 W/m**2 (1-1.5%, Ellsworth Dutton, NOAA, BSRN Manager). Thus, the mean bias for the present results is within the uncertainty for BSRN measurements. Errors for individual daily values are subject to bias and random errors due to local meteorological conditions. 2.1.2. Indian Ocean Gap Artifact There is a visible and common artifact in much of the data set period, due to a lack of coverage from geostationary satellites over an area centered on 70 degrees east longitude. This situation, commonly called the Indian Ocean gap, occurs for all of the July 1983 - June 1998 time period, except for April 1988 - March 1989, when data from the INSAT satellite is available to cover the gap. In July of 1998, Meteosat-5 was moved over the gap area, eliminating the gap. When the Indian Ocean gap occurs, the gap area is covered by polar orbiting satellites, which can result in only one or two daytime overpasses per day. Geosynchronous temporal sampling during the daytime is 3-5 times per daytime depending upon the latitude (between 55 degrees North and South) and the time or year. In addition, the limbs of the geostationary satellites which bound the gap may suffer from spuriously high cloud amounts, due to large view angles. This results in an abrupt drop-off of cloud fraction in the gap as compared to the gap boundary. Downward longwave radiation is lower in the gap, creating an appearance of a flux discontinuity. For daily averaged fluxes a discontinuity of magnitude less than 20 W/m**2 for TOA fluxes and less than 5 W/m**2 for surface fluxes may appear in the Indian Ocean gap region. 2.2 Input Information Inputs to the algorithm were obtained from the following sources: Cloud parameters were derived from the International Satellite Cloud Climatology Project (ISCCP; Rossow and Schiffer, 1999,BAMS, 80, 2261-2287) DX data product. The cloud pixels were separated into categories of high, middle and low where middle and low clouds could be composed of ice or water. Cloud fractions and cloud optical depths were determined within these categories. Cloud particle sizes were assumed and cloud physical thicknesses were also assigned based upon information from literature. Random overlap is used between the high, middle and low layers to better approximate undercast conditions. Temperature and moisture profiles were obtained from a 4-D data assimilation product provided by the Data Assimilation Office at NASA GSFC and were produced with the Goddard Earth Observing System model version 1 (GEOS-1). Surface emissivities were taken from a map developed at NASA LaRC (Wilber et al. 1999, NASA/TP-1999-209362, 35 pp.). 2.3 Grid Description The fluxes are generated on a nested grid, which contains 44016 cells. The grid has a resolution of 1 degree latitude globally, and longitudinal resolution ranging from 1 degree in the tropics and subtropics to 120 degrees at the poles. The first cell is Latitude 89-90 degrees South, Longitude 0-120 degrees East. The cells start at the Greenwich meridian and proceed east around the globe, then shift one degree to the north. The number of cells per latitude band starting at the South Pole are: 3, 45, 45, 45, 45, 45, 45, 45, 45, 45, 90, 90, 90, 90, 90, 90, 90, 90, 90, 90, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 360, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 180, 90, 90, 90, 90, 90, 90, 90, 90, 90, 90, 45, 45, 45, 45, 45, 45, 45, 45, 45, 3 The read software described below contains a subroutine to regrid the fluxes to 1 degree latitude by 1 degree longitude equal-angle grid using replication. 2.4 Points of Contact Scientific contact: Dr. Paul W. Stackhouse Jr. Mail Stop 420 21 Langley Boulevard NASA Langley Research Center Hampton, VA 23681-2199 U.S.A. E-mail: p.w.stackhouse@larc.nasa.gov Production Contact: Atmospheric Sciences Data Center Science, Users and Data Services Office Mail Stop 157D 2 South Wright Street NASA Langley Research Center Hampton, VA 23681-2199 U.S.A. 3.0 Format and Packaging Each data file contains an entire month of daily global fields of the parameters described in Section 4.0 on an approximately 1 deg x 1 deg equal-area grid described in Section 2.3. The files are contain binary data and are named according to the following convention: srb_rel2.1_longwave_daily_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2.1 Release number for these data (Release 2.1) longwave Name of the algorithm, GEWEX Longwave daily Time resolution of the data file yyyy 4-digit year mm 2-digit month binary file format 4.0 Science Parameters Information The files contain global fields of daily values of the six radiative parameters on the nested grid. Each file has 6 records, containing one global field for every time period in each record. The parameters are: Name: Top-of-Atmosphere Clear-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Surface Clear-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 800 Fill Values: -999.0 Scale Factor: None Name: Surface Clear-sky Downward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Top-of-Atmosphere All-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Surface All-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 800 Fill Values: -999.0 Scale Factor: None Name: Surface All-sky Downward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None 5.0 Sample Read Software Description Sample read software written in Fortran-90, read_longwave_daily.f90, was developed for reading these data. The software constitutes the name of the input data file, accesses and reads it, using the information provided in the namelist file (longwave_daily.nml). The input files are read as direct-access binary on the nested (44016 box) grid. The software reads one or more of the 6 parameter fields, regrids them to an equal-angle 1 deg x 1 deg grid, and writes the output as ascii or binary format. The choice of file format (ascii or binary) and of the location of the output files is also made through the namelist file. A sample namelist file that would be used to read the July 1992 data file and write all parameters to an ascii format output file is presented below: &time_vars yr=1992 mon=7 ascii=.true. binary=.false. path_in='**** input file path here****' path_out='**** output file path here****' little_endian=.false. clr_toa_up=.true. clr_sfc_up=.true. clr_sfc_down=.true. toa_up=.true. sfc_up=.true. sfc_down=.true. / There is a choice to convert the input fields from big endian to little endian byte order with the logical variable "little_endian" in the namelist. This applies to operating systems where byte order is stored opposite that of the Sun and SGI machines used to create the data set, such as Linux. If possible, a better choice for doing the conversion in these cases would be to use a compiler option. If using a compiler option, do not set little_endian to true. Both, input and output fields have the same orientation: they start at the Greenwich meridian-south pole and go east and north from there. A limitation of this code is that it provides a complete global field of the specified parameters in the above orientation. The user should be easily able to extract values for any box or lat-lon region from these fields. 6.0 Implementing the Sample Read Software The sample read software can be compiled with any Fortran 90 or 95 compiler. To compile: % f90 -o run_longwave_daily read_longwave_daily.f90 The providers used a NAG F95 compiler but any F90/F95 compiler should work. Edit the namelist file to select month and year to be processed, choose the parameters to be read and the format of the output file. Run the software: % run_longwave_daily 7.0 Sample Output The six tables of numbers below show the values of the six parameters contained in these files for latitude bands 45-51 (starting at the south pole) and longitude boxes 100-104 (starting at the Greenwich meridian) for day 14 of the month. Values for only a small lat-lon box for a single time are printed to the screen. When the is code run, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.1_longwave_daily Read Software * * * * Version: 1.0 * * * * Date: November 23, 2004 * * * Contact: Atmospheric Sciences Data Center * * Science, Users and Data Services Office * * Mail Stop 157D * * 2 South Wright Street * * NASA Langley Research Center * * Hampton, Virginia 23681-2199 * * U.S.A. * * * * E-mail: larc@eos.nasa.gov * * Phone: (757)864-8656 * * FAX: (757)864-8807 * * * ***************************************************************** srb_rel2.1_longwave_daily_199207.binary input file is opened Variable clr_toa_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 243.644 243.158 243.158 243.394 243.394 lat band # 46 245.504 245.434 245.744 245.890 246.569 lat band # 47 247.717 248.330 248.297 248.944 249.685 lat band # 48 250.380 251.333 252.100 252.474 253.389 lat band # 49 253.510 254.314 255.114 255.357 256.335 lat band # 50 256.460 257.270 257.579 258.476 258.945 lat band # 51 258.891 259.657 260.130 260.828 261.432 file clr_toa_up_daily_199207.ascii has been written Variable clr_sfc_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 351.679 352.534 352.534 353.177 353.177 lat band # 46 357.002 357.262 357.279 357.302 357.416 lat band # 47 360.769 360.898 360.658 360.419 360.330 lat band # 48 363.206 363.253 362.778 362.302 362.103 lat band # 49 365.139 365.170 364.665 364.162 363.948 lat band # 50 366.556 366.658 366.329 365.998 365.859 lat band # 51 368.223 368.320 368.240 368.160 368.311 file clr_sfc_up_daily_199207.ascii has been written Variable clr_sfc_down_Day = 14 lon # = 100 101 102 103 104 lat band # 45 262.052 265.728 265.728 267.517 267.517 lat band # 46 267.970 269.345 270.075 271.118 271.053 lat band # 47 271.706 272.467 273.004 273.570 273.489 lat band # 48 273.461 274.046 274.527 274.897 275.101 lat band # 49 275.654 276.108 276.488 276.934 277.283 lat band # 50 278.132 279.152 279.517 279.760 279.867 lat band # 51 281.369 281.723 281.878 282.042 281.718 file clr_sfc_down_daily_199207.ascii has been written Variable toa_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 179.844 177.534 177.534 169.204 169.204 lat band # 46 182.418 181.701 166.647 176.766 175.903 lat band # 47 188.113 188.201 178.418 182.741 179.376 lat band # 48 179.881 175.355 175.128 174.759 180.393 lat band # 49 187.041 175.039 184.680 204.810 199.692 lat band # 50 201.547 204.122 219.252 203.607 214.264 lat band # 51 220.074 226.595 224.231 226.633 231.402 file toa_up_daily_199207.ascii has been written Variable sfc_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 352.351 353.121 353.121 353.850 353.850 lat band # 46 357.524 357.787 357.880 358.005 358.064 lat band # 47 361.429 361.687 361.318 361.105 360.961 lat band # 48 363.889 363.930 363.350 362.908 362.824 lat band # 49 365.838 365.878 365.465 364.962 364.704 lat band # 50 367.321 367.450 367.058 366.844 366.482 lat band # 51 369.124 369.037 368.882 368.923 369.054 file sfc_up_daily_199207.ascii has been written Variable sfc_down_Day = 14 lon # = 100 101 102 103 104 lat band # 45 306.616 304.903 304.903 312.591 312.591 lat band # 46 302.813 304.418 310.173 318.150 314.419 lat band # 47 315.766 325.182 317.039 319.515 315.699 lat band # 48 318.946 319.235 312.739 315.380 323.432 lat band # 49 322.338 323.500 330.136 330.593 328.013 lat band # 50 329.435 332.331 328.491 336.624 321.749 lat band # 51 341.868 329.917 325.062 333.438 331.725 file sfc_down_daily_199207.ascii has been written 8.0 Additional Derivable Parameters The net LW flux at the top-of-atmosphere (TOA) is simply the TOA upward LW flux. The net LW flux at the surface can be defined as: Net LW Flux = Downward LW Flux - Upward LW Flux and is, therefore, generally a negative number. Net fluxes can be computed for the clear-sky and all-sky conditions. The estimates of clear-sky and all-sky fluxes also allow the estimation of the contribution by clouds to the all-sky fluxes. This is commonly referred to as the cloud radiative forcing (CRF) and is computed according to: CRF = Flux (all-sky) - Flux (clear-sky) Thus, the cloud radiative forcing on the downward longwave flux is generally positive because clouds act to increase the emission to the surface. In this way, the effect of the cloud emission on the fluxes can be estimated for each flux component. Lastly, providing TOA and surface fluxes allows one to derive the net radiative flux of the atmosphere. This is given by the relation Net Atmos. Flux = Net TOA Flux - Net Surface Flux For the LW, this flux is negative meaning that the atmosphere is cooling over the LW wavelengths.