GEWEX Quality-Check Longwave 3-Hourly README File 1.0 Introduction This README file provides information on the SRB_REL2_QCLW_3HRLY 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 on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours), namely, at UT hours 00, 03, 06, 09, 12 15, 18, and 21 for every day of the month. 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 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 5 W/m**2 (model fluxes higher), and the random error to be about +/- 30 W/m**2. 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. Uncertainties associated with BSRN measurements during this period are believed to be about +/- 5 W/m**2 (Ellsworth Dutton, NOAA, BSRN Manager). Thus, mean bias for the present results is within the uncertainty for BSRN measurements. Of course, errors for individual 3-hourly values are subject to the above-stated random error. 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 of magnitude less than 5 W/m**2 may appear in the Indian Ocean depending upon the meteorological conditions. 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 These files contain an entire month of 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 are contain binary data and are named according to the following convention: srb_rel2_qclw_3hrly_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 3hrly 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 3-hourly values of the following three parameters for the whole month on the nested grid. Each file has up to 744 records; 3 records every 3 hours during the month; one for each parameter in the order they are listed below. The first 3 records are for hour 00 of day 1, the next 3 for hour 03, and so on. Name: Surface Downward LW Flux (DLF) Units: Watts per square meter Type: Real Range: 50 to 750 Fill Values: -999.0 Scale Factor: None Name: Surface Net LW Flux (NLF) Units: Watts per square meter Type: Real Range: -250 to 50 Fill Values: -999.0 Scale Factor: None Name: Surface LW Cloud Radiative Forcing (LWCRF) Units: Watts per square meter Type: Real Range: 0 to 150 Fill Values: -999.0 Scale Factor: None 5.0 Sample Read Software Description Sample read software written in Fortran-90, read_qclw_3hrly.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_3hrly.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 code is that it provides one file for each parameter containing complete global fields for all 3-hourly times of the month. From these files the user should be easily able to extract a field for any time of the month and also, the values for any box or lat-lon region. 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_3hrly read_qclw_3hrly.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_3hrly 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 hour 06 of day 14 of the month. Values for only a small lat-lon box for a single time are printed to the screen. ***************************************************************** * * * * * Data Set srb_rel2_qclw_3hrly 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_3hrly_199207.binary input file is opened Variable DLF_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 303.331 299.047 299.047 314.324 314.324 lat band # 46 305.206 305.956 319.659 316.255 326.113 lat band # 47 356.793 357.113 336.166 322.402 313.980 lat band # 48 360.080 334.433 312.081 311.319 311.566 lat band # 49 328.049 315.672 319.227 323.023 323.019 lat band # 50 325.415 325.762 330.205 325.762 335.039 lat band # 51 355.805 340.674 336.381 341.162 358.859 file DLF_3hrly_199207.ascii has been written Variable NLF_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 -50.584 -55.632 -55.632 -41.121 -41.121 lat band # 46 -54.050 -53.546 -39.975 -43.350 -33.700 lat band # 47 -6.716 -6.516 -27.013 -40.393 -48.647 lat band # 48 -5.913 -31.357 -53.008 -53.270 -52.817 lat band # 49 -39.578 -51.864 -47.820 -43.537 -43.314 lat band # 50 -43.592 -43.332 -38.590 -42.652 -33.316 lat band # 51 -15.131 -30.217 -34.387 -29.568 -12.192 file NLF_3hrly_199207.ascii has been written Variable LWCRF_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 40.354 31.995 31.995 42.838 42.838 lat band # 46 35.829 36.333 49.826 46.219 54.577 lat band # 47 83.489 84.729 65.021 52.551 43.051 lat band # 48 83.390 58.993 38.446 39.552 39.448 lat band # 49 49.582 38.165 43.207 48.522 48.578 lat band # 50 46.836 47.195 51.918 47.754 57.166 lat band # 51 76.757 61.266 56.722 61.254 79.183 file LWCRF_3hrly_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.