GEWEX Longwave Monthly Averaged 3-Hourly (Diurnal) Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.1_LW_3HRLY_MONTHLY data set. The data set contains monthly averaged 3-hour 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 Description of Sample Read Software 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 (i.e., a global instantaneous gridded field every 3 hours). The 3-hourly values are used to compute monthly averages separately for each of the 8 UT hours (00, 03, 06, 09, 12, 15, 18, and 21 UT). The current version of the datasets 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 3-hourly 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 1.8 W/m**2 (0.5%, model fluxes higher), and the root mean square difference is 18.8 W/m**2 (5.8%). 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, mean bias for the present results is within the uncertainty for BSRN measurements. Errors for individual 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 3-hourly fluxes a discontinuity may appear in the Indian Ocean depending upon the prevalent meteorological conditions. Significant areas within this region may also be missing depending upon the hour due to the lack of geosynchronous coverage. For 3-hourly/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 files contains monthly average/3-hourly 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 contain binary data and are named according to the following convention: srb_rel2.1_longwave_3hrlymonthly_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 3hrlymonthly 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 monthly averaged/3-hourly 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 Description of Sample Read Software Sample read software written in Fortran-90, read_longwave_3hrlymonthly.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_3hrlymonthly.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_3hrlymonthly read_longwave_3hrlymonthly.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_3hrlymonthly 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) at hour 06. 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_3hrlymonthly 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_3hrlymonthly_199207.binary Variable clr_toa_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 247.228 247.284 247.284 247.139 247.139 lat band # 46 248.894 248.733 248.548 248.449 248.220 lat band # 47 250.155 250.112 249.737 249.593 249.512 lat band # 48 251.538 251.218 250.940 250.656 250.651 lat band # 49 253.083 252.746 252.332 252.121 252.144 lat band # 50 254.811 254.681 254.253 254.125 254.076 lat band # 51 256.859 256.700 256.523 256.452 256.538 file clr_toa_up_3hrlymonthly_199207.ascii has been written Variable clr_sfc_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 351.607 352.491 352.491 353.272 353.272 lat band # 46 357.035 357.299 357.364 357.429 357.586 lat band # 47 360.851 360.987 360.793 360.592 360.541 lat band # 48 363.292 363.351 362.943 362.533 362.354 lat band # 49 365.237 365.293 364.871 364.448 364.240 lat band # 50 366.677 366.815 366.578 366.339 366.203 lat band # 51 368.360 368.538 368.554 368.570 368.713 file clr_sfc_up_3hrlymonthly_199207.ascii has been written Variable clr_sfc_down_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 252.673 254.412 254.412 256.230 256.230 lat band # 46 258.346 258.954 259.705 260.456 260.851 lat band # 47 262.587 263.041 263.565 263.808 264.326 lat band # 48 266.117 266.529 266.847 267.088 267.276 lat band # 49 268.911 269.194 269.491 269.762 269.743 lat band # 50 270.822 271.288 271.602 271.873 271.815 lat band # 51 272.713 273.166 273.419 273.679 273.680 file clr_sfc_down_3hrlymonthly_199207.ascii has been written Variable toa_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 204.690 204.354 204.354 204.839 204.839 lat band # 46 207.101 205.484 205.710 207.962 207.413 lat band # 47 214.120 212.073 211.834 214.347 216.660 lat band # 48 215.171 218.486 214.830 219.194 217.841 lat band # 49 220.055 217.214 223.170 221.972 217.739 lat band # 50 225.522 226.843 227.369 224.320 223.561 lat band # 51 233.121 235.129 232.036 229.153 227.069 file toa_up_3hrlymonthly_199207.ascii has been written Variable sfc_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 352.300 353.247 353.247 354.042 354.042 lat band # 46 357.815 358.070 358.176 358.139 358.357 lat band # 47 361.549 361.766 361.559 361.406 361.251 lat band # 48 364.104 364.132 363.761 363.286 363.094 lat band # 49 366.054 366.116 365.649 365.150 364.995 lat band # 50 367.472 367.624 367.245 367.095 366.901 lat band # 51 369.105 369.242 369.320 369.365 369.504 file sfc_up_3hrlymonthly_199207.ascii has been written Variable sfc_down_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 297.617 303.510 303.510 306.440 306.440 lat band # 46 309.197 309.253 312.786 307.051 311.368 lat band # 47 308.510 314.135 313.909 317.248 311.041 lat band # 48 319.536 318.064 320.924 316.824 316.233 lat band # 49 322.931 323.667 320.870 316.177 319.758 lat band # 50 323.469 324.798 315.775 321.914 318.158 lat band # 51 322.165 319.862 324.161 326.383 326.036 file sfc_down_3hrlymonthly_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.