GEWEX Quality-Check Longwave Daily-Average Data set README File 1.0 Introduction This README file provides information on the SRB_REL2_QCLW_DAILY data set. The data set contains daily average global fields of three longwave (LW) surface radiative parameters derived with the Quality-Check LW (QCLW) algorithm of the NASA World Climate Research Programme /Global Energy and Water-Cycle Experiment (WCRP/GEWEX) Surface Radiation Budget (SRB) Project. If users have any 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 three parameters in these files as follows: 1. Surface Downward Longwave Flux (DLF), 2. Surface Net Longwave Flux (NLF), and 3. Surface Longwave Cloud Radiative Forcing (LWCRF). These parameters were derived originally on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours). 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. Detailed description of the algorithm used in deriving these parameters can be found in: Gupta et al. (1992) - J. Appl. Meteor., 31, 1361-1367. Gupta (1989) - J. Climate, 2, 305-320. Wilber et al. (1999) - NASA/TP-1999-209362, 35 pp. (available on the web from http://techreports.larc.nasa.gov/ltrs/ltrs.html) 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 5 W/m**2 (model fluxes higher), and the random error to be about +/- 24 W/m**2. 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). These errors should be considered very reasonable and the users also need to keep in mind that ground-based measurements are not totally error-free. 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 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. 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; see reference above). 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 contain binary data and are named according to the following convention: srb_rel2_qclw_daily_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2 Release number for these data (Release 2) qclw Name of the algorithm, Quality-Check Longwave daily 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 daily averages of the following three parameters for the whole month on the nested grid. Each file has up to 93 records; 3 records for each day of the month; one for each parameter in the order they are listed below. The first 3 records are for day 1, the next 3 for day 2, and so on. Name: Surface Downward LW Flux (DLF) Units: Watts per square meter Type: Real Range: 50 to 650 Fill Values: -999.0 Scale Factor: None Name: Surface Net LW Flux (NLF) Units: Watts per square meter Type: Real Range: -180 to 20 Fill Values: -999.0 Scale Factor: None Name: Surface LW Cloud Radiative Forcing (LWCRF) Units: Watts per square meter Type: Real Range: 0 to 120 Fill Values: -999.0 Scale Factor: None 5.0 Sample Read Software Description Sample read software written in Fortran-90, read_qclw_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 (qclw_daily.nml). The input files are direct-access binary on the nested (44016 box) grid. The software reads one or more of the 3 parameter fields, regrids them to an equal-angle 1 deg x 1 deg grid, and writes them 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. DLF=.true. NLF=.true. LWCRF=.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 software 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_qclw_daily read_qclw_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_qclw_daily 7.0 Sample Output When the is code run, the following information appears on the screen: The three tables of numbers below show the values of the three 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 day are printed to the screen. ***************************************************************** * * * * * Data Set srb_rel2_qclw_daily Read Software * * * * Version: 1.0 * * * * Date: February 12, 2003 * * * * 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_qclw_daily_199207.binary input file is opened Variable DLF_Day = 14 lon # = 100 101 102 103 104 lat band # 45 304.119 308.085 308.085 309.389 309.389 lat band # 46 303.002 307.588 311.418 317.425 314.625 lat band # 47 319.193 324.882 317.712 319.754 317.884 lat band # 48 322.131 320.166 311.339 318.915 323.732 lat band # 49 325.181 324.146 326.144 333.914 327.895 lat band # 50 333.124 334.845 335.343 336.074 321.248 lat band # 51 341.623 331.667 329.154 334.289 334.872 file DLF_daily_199207.ascii has been written Variable NLF_Day = 14 lon # = 100 101 102 103 104 lat band # 45 -49.794 -46.666 -46.666 -45.996 -45.996 lat band # 46 -56.220 -51.915 -48.123 -42.176 -45.064 lat band # 47 -43.950 -38.431 -45.278 -42.998 -44.761 lat band # 48 -43.493 -45.474 -53.724 -45.724 -40.744 lat band # 49 -42.400 -43.448 -40.945 -32.723 -38.459 lat band # 50 -35.930 -34.307 -33.471 -32.405 -46.948 lat band # 51 -29.151 -39.107 -41.512 -36.340 -35.921 file NLF_daily_199207.ascii has been written Variable LWCRF_Day = 14 lon # = 100 101 102 103 104 lat band # 45 38.489 38.167 38.167 37.008 37.008 lat band # 46 30.760 33.709 36.562 41.679 38.929 lat band # 47 43.391 47.994 40.266 41.691 40.148 lat band # 48 44.092 41.534 32.431 39.723 45.011 lat band # 49 45.041 43.705 45.616 53.282 47.703 lat band # 50 51.257 52.276 52.924 53.812 39.913 lat band # 51 57.722 47.819 45.767 51.370 53.227 file LWCRF_daily_199207.ascii has been written 8.0 Additional Derivable Parameters It is important to keep in mind that NLF is computed as NLF = DLF - Upward LW Flux (ULF) and is, therefore, generally a negative number. Also, the three parameters provided in these files can be used to compute two additional surface LW parameters, if needed. ULF can be computed as ULF = DLF - NLF Clear-sky DLF (CSDLF) can be computed as CSDLF = DLF - LWCRF To compute these additional parameters, both quantities on the right hand side of the equations have to be available.