GEWEX Shortwave 3-hourly README file 1.0 Introduction This README file provides information on the SRB_REL2_SW_3HRLY data set. The data set contains daily average global fields of fifteen shortwave (SW) surface radiative parameters derived with the Shortwave 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 The data is generated using the Pinker/Laszlo shortwave algorithm (R.T. Pinker and I. Laszlo, 1992: Modeling Surface Solar Irradiance for Satellite Applications on a Global Scale, J. Appl. Met., 31, 194-211). These parameters were derived originally on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours), at UT hours 00, 03, 06, 09, 12, 15, 18, and 21 for every day of the month. The current version of the data is identified as Release 2. There are a total of 15 parameters in these files as follows: 1. TOA Downward Flux 2. All Sky TOA Upward Flux 3. All Sky Surface Downward Flux 4. All Sky Surface Downward Diffuse Flux 5. All Sky Surface Upward Flux 6. Clear Sky TOA Upward Flux 7. Clear Sky Surface Downward Flux 8. Clear Sky Surface Upward Flux 9. All Sky Global Photosynthetically Active Radiative Flux (PAR) 10. All Sky Diffuse PAR 11. Aerosol Optical Depth* 12. Cloud Optical Depth* 13. Cloud Fraction 14. Cosine Solar Zenith Angle from Satellite 15. Cosine Solar Zenith Angle from Astronomy (center of 3 hour period) The last two are very similar; they differ only slightly because the satellite retrieval time is not always centered on the 3-hourly ISCCP time stamp (0, 3, 6, 9, 12, 15, 18 and 21 UT). *Note that in the Pinker-Laszlo algorithm, aerosol and cloud optical depths are used as tuning parameters. That is, any difference between ISCCP clear-sky composite radiance and instantaneous radiance, is ascribed to aerosol in the clear fraction of the gridbox, and to cloud optical depth in the cloudy fraction. The resulting aerosol field in particular is not representative of a realistic aerosol field. The cloud optical depth returned by the algorithm agrees fairly well with the ISCCP-derived optical depth, except over ice. 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 -0.2 W/m**2 (0.5%, model fluxes higher), and the root mean square difference is 81.7 W/m**2 (9.3%). Uncertainties associated with operational BSRN measurements during this period are believed to be about +/- 8-20 W/m**2 (Ellsworth Dutton, NOAA, BSRN Manager). Thus, mean bias for the present results is within the uncertainty for BSRN measurements. Errors for individual 3-hourly 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 shortwave radiation is therefore higher in the gap, creating an appearance of a flux discontinuity. The discontinuity approaches 60 W/m**2 raising the uncertainty of the fluxes in this region. For 3-hourly fluxes a discontinuity may appear in the Indian Ocean depending upon the meteorological conditions. Significant areas within this region may also be missing depending upon the hour. 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). Column ozone for the July 1983 to November 1994 period were taken from the Total Ozone Mapping Spectrometer (TOMS) data from flights aboard Nimbus-7 and Meteor-3. Column ozone for December 1994 to October 1995 were taken from TIROS Operational Vertical Sounder (TOVS) data. 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.gocv 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 file contains 3-hourly average global fields of the 15 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_shortwave_3hrly_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2 Release number for these data (Release 2) shortwave Name of the algorithm, GEWEX Shortwave 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 averages of the following fifteen parameters on the nested grid. Name: TOA Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 1400 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 1100 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 1200 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky Surface Downward Diffuse SW Flux Units: Watts per square meter Type: Real Range: 0 to 900 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 700 Fill Values: -1000.0 Scale Factor: 10 Name: Clear Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 700 Fill Values: -1000.0 Scale Factor: 10 Name: Clear Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 1200 Fill Values: -1000.0 Scale Factor: 10 Name: Clear Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 700 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky Global Photosynthetically Active Radiation Flux Units: Watts per square meter Type: Real Range: 0 to 550 Fill Values: -1000.0 Scale Factor: 10 Name: All Sky Diffuse Photosynthetically Active Radiation Flux Units: Watts per square meter Type: Real Range: 0 to 450 Fill Values: -1000.0 Scale Factor: 10 Name: Aerosol Optical Depth Units: Dimensionless Type: Real Range: 0 to 1.5 Fill Values: -1000.0 Scale Factor: 1000 Name: Cloud Optical Depth Units: Dimensionless Type: Real Range: 0 to 500 Fill Values: -1000.0 Scale Factor: 10 Name: Cloud Fraction Units: Dimensionless Type: Real Range: 0 to 1 Fill Values: -1000.0 Scale Factor: 1000 Name: Cosine Solar Zenith Angle From Satellite Units: Dimensionless Type: Real Range: 0 to 1 Fill Values: -1000.0 Scale Factor: 1000 Name: Cosine Solar Zenith Angle From Astronomy (center of 3 hour period) Units: Dimensionless Type: Real Range: 0 to 1 Fill Values: -1000.0 Scale Factor: 1000 5.0 Description of Sample Read Software Sample read software written in Fortran-90, read_shortwave_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 (shortwave_3hrly.nml). The input files are direct-access binary on the nested (44016 box) grid. The software reads one or more of the 15 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 January 1992 data file and write all parameters to an ascii format output file is presented below: &time_vars yr=1992 mon=1 ascii=.true. binary=.false. path_in='**** input file path here ****' path_out='**** output file path here ****' little_endian=.false. toa_down=.true. toa_up=.true. sfc_down=.true. sfc_diff=.true. sfc_up=.true. clr_toa_up=.true. clr_sfc_down=.true. clr_sfc_up=.true. par=.true. diff_par=.true. aer_opt_dep=.true. cld_opt_dep=.true. cld_frac=.true. cos_sza=.true. ave_cos_sza=.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 the Fortran 95 compiler. Problems may occur when this program is compiled with the Fortran 90 compiler. To compile: % f95 -o run_shortwave_3hrly read_shortwave_3hrly.f90 The providers used a NAG F95 compiler. 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_shortwave_3hrly 7.0 Sample Output The fifteen 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). Values for only a small lat-lon box are printed to the screen. When the code runs, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2_shortwave_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_shortwave_3hrly_199201.binary input file is opened Variable toa_down_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 1253.900 1249.600 1249.600 1244.200 1244.200 lat band # 46 1262.200 1260.000 1257.500 1254.800 1251.800 lat band # 47 1271.100 1268.800 1266.300 1263.500 1260.500 lat band # 48 1279.500 1277.300 1274.700 1271.900 1268.800 lat band # 49 1287.600 1285.300 1282.800 1279.900 1276.700 lat band # 50 1295.300 1293.000 1290.400 1287.500 1284.300 lat band # 51 1302.700 1300.300 1297.600 1294.700 1291.400 file toa_down_3hrly_199201.ascii has been written Variable toa_up_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 249.500 238.700 238.700 251.300 251.300 lat band # 46 250.000 379.800 317.800 206.500 151.300 lat band # 47 328.100 257.400 221.100 227.300 221.900 lat band # 48 410.200 289.600 284.300 302.300 391.300 lat band # 49 287.200 271.400 273.500 314.500 302.400 lat band # 50 525.800 441.400 405.200 362.000 335.000 lat band # 51 505.000 526.700 531.100 570.600 488.000 file toa_up_3hrly_199201.ascii has been written Variable sfc_down_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 850.900 860.000 860.000 839.500 839.500 lat band # 46 862.700 695.200 770.500 910.100 984.400 lat band # 47 769.700 857.400 903.900 890.200 893.000 lat band # 48 665.300 817.600 821.600 795.100 677.200 lat band # 49 818.800 837.000 831.000 776.000 787.500 lat band # 50 510.300 615.400 659.000 711.400 742.100 lat band # 51 533.900 505.000 497.200 445.900 545.300 file sfc_down_3hrly_199201.ascii has been written Variable sfc_diff_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 788.800 679.100 679.100 659.300 659.300 lat band # 46 798.600 693.100 742.100 776.400 493.900 lat band # 47 745.400 796.200 806.000 814.100 803.300 lat band # 48 663.400 773.100 774.500 758.100 675.200 lat band # 49 773.100 782.900 778.700 746.600 752.000 lat band # 50 510.300 614.000 657.000 708.600 724.900 lat band # 51 533.700 505.000 497.200 445.900 544.900 file sfc_diff_3hrly_199201.ascii has been written Variable sfc_up_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 50.600 49.000 49.000 47.800 47.800 lat band # 46 50.700 41.700 45.400 53.300 57.200 lat band # 47 45.600 50.100 55.400 53.200 53.400 lat band # 48 39.800 48.600 49.200 47.400 40.600 lat band # 49 49.400 50.100 49.000 46.100 46.700 lat band # 50 30.600 36.900 39.500 42.600 44.300 lat band # 51 32.000 30.300 29.800 26.800 32.700 file sfc_up_3hrly_199201.ascii has been written Variable clr_toa_up_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 114.500 114.600 114.600 99.200 99.200 lat band # 46 108.300 108.700 99.400 113.000 109.300 lat band # 47 104.000 103.800 130.400 118.800 119.100 lat band # 48 90.200 113.100 119.000 113.400 113.100 lat band # 49 124.700 119.100 107.600 107.400 107.300 lat band # 50 113.300 113.100 107.500 107.800 107.800 lat band # 51 113.900 108.100 107.800 102.000 101.700 file clr_toa_up_3hrly_199201.ascii has been written Variable clr_sfc_down_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 1031.700 1018.300 1018.300 1037.700 1037.700 lat band # 46 1044.300 1041.400 1037.800 1035.700 1047.300 lat band # 47 1050.400 1048.400 1046.700 1042.400 1038.000 lat band # 48 1049.900 1050.000 1047.100 1043.400 1039.200 lat band # 49 1048.700 1046.400 1043.300 1040.700 1037.000 lat band # 50 1043.800 1042.400 1039.500 1036.300 1032.800 lat band # 51 1039.900 1037.900 1035.900 1033.200 1030.400 file clr_sfc_down_3hrly_199201.ascii has been written Variable clr_sfc_up_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 55.200 49.500 49.500 48.300 48.300 lat band # 46 48.400 48.500 37.900 53.700 59.100 lat band # 47 43.200 43.100 73.400 60.400 60.500 lat band # 48 27.600 54.100 60.700 54.200 54.100 lat band # 49 67.600 61.100 48.000 47.900 47.900 lat band # 50 55.200 55.000 48.500 48.600 48.500 lat band # 51 55.800 49.100 49.000 42.400 42.300 file clr_sfc_up_3hrly_199201.ascii has been written Variable par_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 389.500 390.600 390.600 383.800 383.800 lat band # 46 393.400 334.400 361.200 409.100 431.300 lat band # 47 362.300 392.800 408.800 404.400 405.200 lat band # 48 326.100 381.800 383.000 373.700 330.800 lat band # 49 386.300 392.300 390.100 370.400 374.400 lat band # 50 268.700 312.000 329.600 350.700 361.500 lat band # 51 281.200 268.400 264.700 241.400 285.300 file par_3hrly_199201.ascii has been written Variable diff_par_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 357.000 311.300 311.300 303.600 303.600 lat band # 46 360.000 333.000 346.200 343.200 216.000 lat band # 47 349.400 360.900 359.200 364.700 359.200 lat band # 48 324.900 358.400 358.300 354.100 329.500 lat band # 49 362.100 363.800 362.400 354.700 355.600 lat band # 50 268.700 311.100 328.300 348.900 352.100 lat band # 51 281.100 268.400 264.700 241.400 285.000 file diff_par_3hrly_199201.ascii has been written Variable aer_opt_dep_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.128 0.197 0.197 0.000 0.000 lat band # 46 0.128 0.128 0.129 0.128 0.000 lat band # 47 0.128 0.128 0.127 0.128 0.128 lat band # 48 0.129 0.128 0.128 0.128 0.128 lat band # 49 0.127 0.128 0.128 0.128 0.128 lat band # 50 0.128 0.128 0.128 0.128 0.128 lat band # 51 0.128 0.128 0.128 0.128 0.128 file aer_opt_dep_3hrly_199201.ascii has been written Variable cld_opt_dep_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 2.500 2.800 2.800 3.100 3.100 lat band # 46 2.500 5.300 3.900 1.700 1.000 lat band # 47 4.100 2.700 1.900 2.000 1.900 lat band # 48 6.400 3.400 3.300 3.600 5.800 lat band # 49 3.400 3.000 3.100 4.000 3.700 lat band # 50 10.700 7.700 6.500 5.000 4.400 lat band # 51 9.700 10.800 11.100 13.400 9.300 file cld_opt_dep_3hrly_199201.ascii has been written Variable cld_frac_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 1.000 0.833 0.833 0.857 0.857 lat band # 46 1.000 1.000 1.000 1.000 0.667 lat band # 47 1.000 1.000 1.000 1.000 1.000 lat band # 48 1.000 1.000 1.000 1.000 1.000 lat band # 49 1.000 1.000 1.000 1.000 1.000 lat band # 50 1.000 1.000 1.000 1.000 1.000 lat band # 51 1.000 1.000 1.000 1.000 1.000 file cld_frac_3hrly_199201.ascii has been written Variable cos_sza_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.894 0.890 0.890 0.890 0.890 lat band # 46 0.905 0.900 0.900 0.900 0.893 lat band # 47 0.910 0.910 0.905 0.905 0.900 lat band # 48 0.918 0.917 0.913 0.910 0.910 lat band # 49 0.923 0.920 0.920 0.920 0.918 lat band # 50 0.930 0.930 0.928 0.923 0.920 lat band # 51 0.930 0.930 0.930 0.930 0.930 file cos_sza_3hrly_199201.ascii has been written Variable ave_cos_sza_Hour = 3 Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.892 0.889 0.889 0.885 0.885 lat band # 46 0.898 0.896 0.894 0.893 0.890 lat band # 47 0.904 0.903 0.901 0.899 0.897 lat band # 48 0.910 0.909 0.907 0.905 0.902 lat band # 49 0.916 0.914 0.912 0.910 0.908 lat band # 50 0.921 0.920 0.918 0.916 0.913 lat band # 51 0.927 0.925 0.923 0.921 0.919 file ave_cos_sza_3hrly_199201.ascii has been written 8.0 Additional Derivable Parameters Additional parameters can be computed if needed, e.g.: Cloud Radiative Forcing = All Sky Surface Downward Flux - Clear Sky Surface Downward Flux Surface Albedo = All Sky Surface Upward Flux / All Sky Surface Downward Flux