GEWEX Longwave Monthly-Average Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.1_LW_MONTHLY data set. The data set contains monthly 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 monthly averages 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 Dr. Paul W. Stackhouse Jr. at the address below for further details. 2.1 Data Quality An assessment of the quality of these monthly 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 13 W/m**2 (4.0%). 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 monthly 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. All algorithms compute monthly averages from the Daily averaged fluxes. Thus, any discontinuity in the daily averaged fluxes will be averaged over the course of an entire month and are observed to persist. For Daily averaged fluxes any discontinuity in instantaneous fluxes will be exacerbated by the temporal gaps of coverage in the Indian Ocean gap region. LW and LWQC daily averages are less effected by the temporal gap because the 2 night time observations of the region are also used in determining the daily average. For monthly 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 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 monthly averaged 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_monthly_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 monthly Time resolution of the data set yyyy 4-digit year for these data mm 2-digit month for these data binary file format 4.0 Science Parameters Information The files contain global fields of monthly averages of the six parameters on the nested grid. Each file has 6 records, containing one global field in each record. Name: Top-of-Atmosphere Clear-sky Upward LW Flux (Clear-sky OLR) 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 (OLR) 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_monthly.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_monthly.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_monthly read_longwave_monthly.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_monthly 7.0 Sample Output The six tables of numbers below show the values of the 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). Values for only a small lat-lon box are printed to the screen. When the is code run, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.1_longwave_monthly 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_monthly_199207.binary input file is opened Variable clr_toa_up_ lon # = 100 101 102 103 104 lat band # 45 246.215 246.419 246.419 246.679 246.679 lat band # 46 248.129 248.141 248.170 248.254 248.250 lat band # 47 249.693 249.823 249.726 249.768 249.752 lat band # 48 251.275 251.263 251.190 251.198 251.138 lat band # 49 252.941 252.949 252.834 252.761 252.757 lat band # 50 254.710 254.801 254.690 254.752 254.674 lat band # 51 256.850 256.957 256.965 257.089 257.157 file clr_toa_up_monthly_199207.ascii has been written Variable clr_sfc_up_ lon # = 100 101 102 103 104 lat band # 45 351.629 352.493 352.493 353.249 353.249 lat band # 46 357.047 357.306 357.359 357.411 357.563 lat band # 47 360.854 360.986 360.782 360.580 360.516 lat band # 48 363.288 363.341 362.927 362.511 362.326 lat band # 49 365.220 365.274 364.845 364.416 364.207 lat band # 50 366.650 366.780 366.536 366.293 366.158 lat band # 51 368.321 368.492 368.505 368.518 368.665 file clr_sfc_up_monthly_199207.ascii has been written Variable clr_sfc_down_ lon # = 100 101 102 103 104 lat band # 45 253.944 254.675 254.675 255.203 255.203 lat band # 46 259.244 259.590 259.691 259.784 259.836 lat band # 47 263.116 263.364 263.326 263.403 263.298 lat band # 48 266.307 266.455 266.387 266.260 266.118 lat band # 49 268.604 268.721 268.620 268.503 268.373 lat band # 50 270.115 270.198 270.125 270.077 270.014 lat band # 51 271.514 271.612 271.666 271.702 271.860 file clr_sfc_down_monthly_199207.ascii has been written Variable toa_up_ lon # = 100 101 102 103 104 lat band # 45 201.879 202.803 202.803 204.099 204.099 lat band # 46 206.542 204.922 206.027 207.287 207.791 lat band # 47 211.670 211.973 211.285 211.919 211.933 lat band # 48 214.089 214.524 214.782 216.358 216.254 lat band # 49 220.994 217.807 218.996 221.987 221.353 lat band # 50 226.417 224.738 224.838 225.041 225.642 lat band # 51 231.719 231.257 229.598 228.692 227.215 file toa_up_monthly_199207.ascii has been written Variable sfc_up_ lon # = 100 101 102 103 104 lat band # 45 352.421 353.297 353.297 354.059 354.059 lat band # 46 357.865 358.090 358.187 358.190 358.358 lat band # 47 361.593 361.785 361.590 361.375 361.306 lat band # 48 364.085 364.164 363.710 363.292 363.085 lat band # 49 366.023 366.094 365.669 365.174 364.963 lat band # 50 367.408 367.556 367.278 367.020 366.862 lat band # 51 369.012 369.217 369.280 369.294 369.425 file sfc_up_monthly_199207.ascii has been written Variable sfc_down_ lon # = 100 101 102 103 104 lat band # 45 305.516 307.027 307.027 308.057 308.057 lat band # 46 312.873 310.925 313.978 310.835 311.876 lat band # 47 311.817 315.981 316.574 315.736 315.243 lat band # 48 318.914 320.784 318.163 317.878 316.294 lat band # 49 321.666 322.944 323.115 318.627 318.448 lat band # 50 320.284 321.544 319.201 318.198 316.602 lat band # 51 317.290 319.594 322.973 323.065 322.175 file sfc_down_monthly_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.