SRB_REL2.8_SHORTWAVE_3HRLY - GEWEX Shortwave 3-hourly README file 1.0 Introduction This README file provides information on the SRB_REL2.8_SHORTWAVE_3HRLY data set. The data set contains 3-hourly global fields of eleven 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 Science Data Center User 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 2.2 Input Data 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 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 2.8. Version History: Release 1.0: 8 year dataset (July 1983-June 1991) on 2.5 degree equal angle grid using ISCCP C1 data and algorithm of Darnell et al. (1992) Release 1.1: 4 year dataset (March 1985-December 1988), with Pinker/Laszlo now the primary algorithm. Release 2.0: 12 year dataset (July 1983-October 1995), on nested grid (described in Section 2.3), using ISCCP DX pixel data. Release 2.5: Atmospheric transmissivity/reflectivity lookup tables extended to cosine solar zenith angles as low as 0.01. Revamping of the methodology used to fill data gaps. These changes allowed data to be computed for locations with low sun angles the entire month (polar twilight areas). Release 2.6: Improvement of the TOA insolation calculation. Previously each January 1 the Earth began in the same orbital point. Leap years were handled by making day 366 a duplicate of day 1. The new scheme was a Julian day based approach from the Astronomical Almanac. Release 2.7: The effective solar constant was increased to 1367 W/m2 from 1359 W/m2, for consistency with other products. The Pinker/Laszlo algorithm computes radiation in the range from 0.2-4.0 microns. That does not cover the full range of solar output, which extends past 4 microns. The extra energy was placed in the 0.7-4.0 micron band. A bug was fixed which had incorrectly handled the treatment of 3-hourly time periods with low sun angles. This has had the effect of increasing the extent of the solar terminator. Lookup tables for atmospheres at altitude were constructed and added. Surface fluxes at non sea level elevations are now increased. Release 2.81: Further improvement was made to the treatment of 3-hourly periods which bracket sunrise or sunset. There are minor improvements in the treatment of filling gaps in the input data. Several new output fields were added for diagnostic use by the SRB group. There are a total of 11 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 Upward Flux 5. Clear Sky TOA Upward Flux 6. Clear Sky Surface Downward Flux 7. Clear Sky Surface Upward Flux 8. All Sky Global Photosynthetically Active Radiative Flux (PAR) 9. Cloud Fraction 10. Cosine Solar Zenith Angle from Satellite 11. 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). 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 thirteen years (1992-2005) 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 -12.9 W/m**2 (-4.0%, model fluxes higher), and the root mean square difference is 90.4 W/m**2. Uncertainties associated with operational BSRN measurements during this period are believed to be about +/- 8-20 W/m**2 (Ellsworth Dutton, NOAA, BSRN Manager). 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 Data 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 the 4-D data assimilation Goddard EOS Data Assimilation System, level-4 (GEOS-4) obtained from the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center (GSFC) (Bloom et al., 2005. Documentation and Validation of the Goddard Earth Observing System (GEOS) Data Assimilation System - Version 4 . Technical Report Series on Global Modeling and Data Assimilation 104606 , 26 http://gmao.gsfc.nasa.gov/pubs/docs/Bloom168.pdf) Column ozone values for the entire duration of this dataset (July 1983 to December 2004) were obtained primarily from the Total Ozone Mapping Spectrometer (TOMS) archive. For the early period (July 1983-November 1994), TOMS data came from NIMBUS-7 and Meteor-3 satellites. There was an interruption of about 20 months (December 1994-July 1996) after which TOMS data from EP-TOMS became available in August 1996 and continued until December 2004. All gaps in TOMS data, including those over the polar night areas every year, were filled with column ozone values 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: Paul.W.Stackhouse@nasa.gov Production Contact: Atmospheric Science Data Center User 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 11 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_rel2.8_shortwave_3hrly_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2.8 Release number for these data (Release 2.8) 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 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: 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 11 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_up=.true. clr_toa_up=.true. clr_sfc_down=.true. clr_sfc_up=.true. par=.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 any Fortran 90 or 95 compiler. To compile: % f90 -o run_shortwave_3hrly read_shortwave_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_shortwave_3hrly 7.0 Sample Output The eleven 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 code runs, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.8_shortwave_3hrly Read Software * * * * Version: 1.0 * * * * Contact: Atmospheric Science Data Center * * User 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_rel2.8_shortwave_3hrly_199207.binary input file is opened Variable toa_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 485.600 481.300 481.300 475.900 475.900 lat band # 46 505.300 503.100 500.600 497.900 494.900 lat band # 47 525.800 523.500 521.000 518.300 515.200 lat band # 48 546.100 543.900 541.300 538.500 535.400 lat band # 49 566.300 564.000 561.400 558.500 555.400 lat band # 50 586.300 584.000 581.300 578.400 575.200 lat band # 51 606.200 603.800 601.100 598.100 594.900 file toa_down_3hrly_199207.ascii has been written Variable toa_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 242.500 252.300 252.300 235.500 235.500 lat band # 46 237.400 239.600 238.400 237.100 229.600 lat band # 47 228.200 186.400 235.600 234.600 234.400 lat band # 48 213.500 190.900 216.700 227.500 234.500 lat band # 49 229.700 247.100 222.400 217.100 240.800 lat band # 50 276.800 264.500 220.900 248.900 201.600 lat band # 51 217.600 200.500 178.900 202.700 168.000 file toa_up_3hrly_199207.ascii has been written Variable sfc_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 148.800 134.100 134.100 149.800 149.800 lat band # 46 171.400 168.300 168.900 168.900 175.300 lat band # 47 201.200 245.500 191.200 190.300 188.300 lat band # 48 235.800 258.500 229.100 215.400 205.700 lat band # 49 235.700 215.300 240.300 244.200 216.000 lat band # 50 200.800 213.100 259.400 227.100 276.800 lat band # 51 283.900 301.400 324.000 296.100 333.600 file sfc_down_3hrly_199207.ascii has been written Variable sfc_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 8.900 8.000 8.000 9.000 9.000 lat band # 46 10.300 10.100 10.100 10.100 10.500 lat band # 47 12.100 14.700 11.500 11.400 11.300 lat band # 48 14.200 15.500 13.800 13.000 12.400 lat band # 49 14.100 12.900 14.500 14.700 13.000 lat band # 50 12.000 12.800 15.600 13.600 16.800 lat band # 51 17.200 18.100 19.400 17.800 21.100 file sfc_up_3hrly_199207.ascii has been written Variable clr_toa_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 73.300 78.000 78.000 76.500 76.500 lat band # 46 74.100 74.000 77.800 81.500 77.200 lat band # 47 72.900 73.100 69.600 99.800 94.300 lat band # 48 77.900 77.300 105.800 94.400 94.300 lat band # 49 73.800 106.100 100.300 94.500 94.800 lat band # 50 100.900 100.800 89.600 90.500 100.600 lat band # 51 101.700 85.400 80.000 80.000 95.200 file clr_toa_up_3hrly_199207.ascii has been written Variable clr_sfc_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 331.900 327.700 327.700 326.400 326.400 lat band # 46 347.500 347.200 347.200 346.700 344.400 lat band # 47 367.600 366.800 364.700 365.600 362.900 lat band # 48 385.400 384.300 384.800 382.200 380.200 lat band # 49 402.500 403.400 401.400 399.600 396.900 lat band # 50 422.000 420.100 418.200 415.600 415.100 lat band # 51 438.800 436.800 435.200 433.700 433.400 file clr_sfc_down_3hrly_199207.ascii has been written Variable clr_sfc_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 22.200 27.200 27.200 26.400 26.400 lat band # 46 22.200 22.000 26.500 31.000 26.200 lat band # 47 20.100 20.200 16.000 50.800 44.600 lat band # 48 24.400 24.100 56.700 43.900 43.800 lat band # 49 19.200 56.300 49.700 43.400 43.600 lat band # 50 49.700 49.600 37.100 37.600 49.500 lat band # 51 49.900 31.300 25.300 25.200 43.000 file clr_sfc_up_3hrly_199207.ascii has been written Variable par_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 74.000 67.200 67.200 74.100 74.100 lat band # 46 84.800 83.100 83.100 83.000 85.600 lat band # 47 98.100 116.600 93.300 92.800 91.800 lat band # 48 113.300 122.800 110.200 104.300 99.900 lat band # 49 114.100 105.300 115.800 117.200 105.100 lat band # 50 99.800 105.100 124.600 110.600 131.200 lat band # 51 136.000 143.000 151.300 140.100 154.300 file par_3hrly_199207.ascii has been written Variable cld_frac_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 1.000 1.000 1.000 1.000 1.000 lat band # 46 1.000 1.000 1.000 1.000 1.000 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_199207.ascii has been written Variable cos_sza_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.384 0.378 0.378 0.381 0.381 lat band # 46 0.396 0.395 0.395 0.395 0.393 lat band # 47 0.415 0.412 0.408 0.407 0.405 lat band # 48 0.425 0.427 0.423 0.424 0.422 lat band # 49 0.443 0.440 0.440 0.440 0.436 lat band # 50 0.458 0.456 0.458 0.450 0.450 lat band # 51 0.470 0.470 0.468 0.466 0.467 file cos_sza_3hrly_199207.ascii has been written Variable ave_cos_sza_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.367 0.364 0.364 0.360 0.360 lat band # 46 0.382 0.380 0.378 0.376 0.374 lat band # 47 0.397 0.396 0.394 0.392 0.389 lat band # 48 0.413 0.411 0.409 0.407 0.405 lat band # 49 0.428 0.426 0.424 0.422 0.420 lat band # 50 0.443 0.441 0.439 0.437 0.435 lat band # 51 0.458 0.456 0.454 0.452 0.450 file ave_cos_sza_3hrly_199207.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