SRB_REL2.8_SHORTWAVE_MONTHLY - GEWEX Shortwave Monthly README file 1.0 Introduction This README file provides information on the SRB_REL2.8_SHORTWAVE_MONTHLY data set. The data set contains monthly average 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. The 3-hourly values were first averaged into local solar time daily values (see README_SRB_REL2.8_GSW_DAILY.txt). The Daily averages were then averaged into monthly averages. The current version of the data sets 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 monthly average fluxes was accomplished by comparisons with corresponding ground-measured fluxes over a period of fourteen 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 -8.2 W/m**2 (-4.3%, model fluxes lower), and the root mean square difference is 23.8 W/m**2. Uncertainties associated with the BSRN measurements during this time period are believed to be about +/- 5 - 15 W/m**2 (Ellsworth Dutton, NOAA, BSRN Manager) depending on environmental conditions. This includes a possible thermal offset which would result in a systematic bias of up to 3% (personal communication, Rolf Philipona, World Radiation Center) depending on atmospheric humidity and cloudiness conditions. These flux estimates are within these uncertainties. 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 20 W/m**2 raising the uncertainty of the fluxes in this region. All algorithms compute monthly averages from the Daily averaged fluxes. Thus, any discontinuity in the daily averaged fluxes will be averaged over the course of an entire month and are observed to persist. For Daily averaged fluxes any discontinuity in instantaneous fluxes will be exacerbated by the temporal gaps of coverage in the India Ocean gap region. In this region, the shortwave local time daily average will be based upon fluxes from at most 2 daytime observations of the region while areas with geosynchronous coverage are sampled 3-5 times during the daylight hours. 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 an entire month of monthly average 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_rel2.8_shortwave_monthly_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 monthly 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 monthly averages of the following eleven parameters on the nested grid. Name: TOA Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 550 Fill Values: -1000.0 Scale Factor: None Name: All Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 400 Fill Values: -1000.0 Scale Factor: None Name: All Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 425 Fill Values: -1000.0 Scale Factor: None Name: All Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 350 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 375 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 425 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 350 Fill Values: -1000.0 Scale Factor: None Name: All Sky Global Photosynthetically Active Radiation Flux Units: Watts per square meter Type: Real Range: 0 to 200 Fill Values: -1000.0 Scale Factor: None Name: Cloud Fraction Units: Dimensionless Type: Real Range: 0 to 1 Fill Values: -1000.0 Scale Factor: None Name: Cosine Solar Zenith Angle From Satellite Units: Dimensionless Type: Real Range: 0 to 1 Fill Values: -1000.0 Scale Factor: None 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: None 5.0 Description of Sample Read Software Sample read software written in Fortran-90, read_shortwave_monthly.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_monthly.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 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. 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_monthly read_shortwave_monthly.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_monthly 6.1 Read Software Incompatibilities With some Fortran compilers the RECL keyword in the OPEN statement assumes record lengths are specified in 4 byte increments. If that is the case, then the following statement in the read program: open (10,file=infile,status='old',form='unformatted',access='direct',recl=4*nreg) should be modified to: open (10,file=infile,status='old',form='unformatted',access='direct',recl=nreg) The same should be done with the output binary file: open (15,file=outfile, form='unformatted', access='direct', & recl=nlon*nlat*4, status='replace') to: open (15,file=outfile, form='unformatted', access='direct', & recl=nlon*nlat, status='replace') 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 is code run, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.8_shortwave_monthly 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_monthly_199207.binary input file is opened Variable toa_down_ lon # = 100 101 102 103 104 lat band # 45 127.700 127.700 127.700 127.700 127.700 lat band # 46 134.400 134.400 134.400 134.400 134.400 lat band # 47 141.000 141.000 141.000 141.000 141.000 lat band # 48 147.700 147.700 147.700 147.700 147.700 lat band # 49 154.400 154.400 154.400 154.400 154.400 lat band # 50 161.100 161.100 161.100 161.100 161.100 lat band # 51 167.800 167.800 167.800 167.800 167.800 file toa_down_monthly_199207.ascii has been written Variable toa_up_ lon # = 100 101 102 103 104 lat band # 45 51.100 52.800 52.800 51.000 51.000 lat band # 46 54.100 55.600 55.500 54.300 53.800 lat band # 47 54.100 54.700 54.700 59.800 58.400 lat band # 48 55.700 56.100 62.500 59.700 59.300 lat band # 49 56.700 64.900 62.600 59.200 60.100 lat band # 50 61.100 61.500 59.600 58.900 58.400 lat band # 51 60.200 57.900 58.600 60.500 61.400 file toa_up_monthly_199207.ascii has been written Variable sfc_down_ lon # = 100 101 102 103 104 lat band # 45 53.800 51.900 51.900 53.600 53.600 lat band # 46 56.100 54.600 54.600 55.800 56.000 lat band # 47 61.600 60.900 60.900 56.100 57.500 lat band # 48 65.800 65.400 59.100 62.200 62.500 lat band # 49 70.300 62.100 65.000 68.400 67.600 lat band # 50 72.100 71.600 73.700 74.500 75.700 lat band # 51 79.200 81.800 80.200 78.200 77.800 file sfc_down_monthly_199207.ascii has been written Variable sfc_up_ lon # = 100 101 102 103 104 lat band # 45 3.400 3.400 3.400 3.400 3.400 lat band # 46 3.400 3.600 3.500 3.500 3.400 lat band # 47 3.800 3.800 3.700 4.200 4.500 lat band # 48 4.200 4.100 4.500 4.700 4.700 lat band # 49 4.200 4.400 5.100 5.100 5.300 lat band # 50 5.300 5.400 5.500 5.600 6.300 lat band # 51 6.100 6.100 5.200 5.400 6.200 file sfc_up_monthly_199207.ascii has been written Variable clr_toa_up_ lon # = 100 101 102 103 104 lat band # 45 24.600 25.600 25.600 24.500 24.500 lat band # 46 24.500 25.100 25.000 25.500 25.800 lat band # 47 25.800 25.400 25.100 32.100 32.600 lat band # 48 26.900 25.800 34.600 33.000 32.700 lat band # 49 25.900 35.400 35.700 32.700 33.000 lat band # 50 34.700 35.700 34.300 32.800 34.100 lat band # 51 35.300 33.600 31.300 32.500 35.500 file clr_toa_up_monthly_199207.ascii has been written Variable clr_sfc_down_ lon # = 100 101 102 103 104 lat band # 45 81.500 80.900 80.900 81.300 81.300 lat band # 46 86.600 87.000 86.800 86.200 86.500 lat band # 47 91.400 91.300 91.400 90.500 89.100 lat band # 48 96.700 97.400 96.100 95.900 94.900 lat band # 49 101.900 102.100 99.700 100.400 100.600 lat band # 50 105.200 103.600 103.300 105.700 106.300 lat band # 51 110.600 110.500 110.800 109.700 109.500 file clr_sfc_down_monthly_199207.ascii has been written Variable clr_sfc_up_ lon # = 100 101 102 103 104 lat band # 45 6.500 7.300 7.300 6.400 6.400 lat band # 46 6.100 7.200 7.000 6.900 7.500 lat band # 47 6.800 6.300 6.100 13.000 12.500 lat band # 48 7.700 7.000 15.500 13.600 12.500 lat band # 49 5.900 16.400 14.900 12.200 12.700 lat band # 50 13.400 13.300 11.500 11.800 13.700 lat band # 51 13.600 11.700 9.400 9.900 13.100 file clr_sfc_up_monthly_199207.ascii has been written Variable par_ lon # = 100 101 102 103 104 lat band # 45 24.300 23.600 23.600 24.400 24.400 lat band # 46 25.600 25.000 25.000 25.500 25.600 lat band # 47 28.100 27.900 27.900 25.800 26.300 lat band # 48 30.200 30.000 27.300 28.600 28.700 lat band # 49 32.300 28.900 30.000 31.300 31.000 lat band # 50 33.200 32.900 33.700 34.000 34.500 lat band # 51 36.200 37.200 36.700 35.800 35.700 file par_monthly_199207.ascii has been written Variable cld_frac_ lon # = 100 101 102 103 104 lat band # 45 0.866 0.875 0.875 0.882 0.882 lat band # 46 0.887 0.891 0.897 0.872 0.862 lat band # 47 0.859 0.869 0.876 0.860 0.833 lat band # 48 0.862 0.868 0.866 0.849 0.833 lat band # 49 0.850 0.892 0.847 0.824 0.802 lat band # 50 0.808 0.797 0.761 0.763 0.745 lat band # 51 0.761 0.744 0.796 0.768 0.759 file cld_frac_monthly_199207.ascii has been written Variable cos_sza_ lon # = 100 101 102 103 104 lat band # 45 0.198 0.195 0.195 0.197 0.197 lat band # 46 0.210 0.205 0.207 0.208 0.208 lat band # 47 0.220 0.220 0.218 0.217 0.218 lat band # 48 0.230 0.232 0.229 0.230 0.229 lat band # 49 0.243 0.240 0.241 0.242 0.240 lat band # 50 0.255 0.252 0.253 0.251 0.250 lat band # 51 0.264 0.265 0.264 0.262 0.262 file cos_sza_monthly_199207.ascii has been written Variable ave_cos_sza_ lon # = 100 101 102 103 104 lat band # 45 0.193 0.193 0.193 0.193 0.193 lat band # 46 0.203 0.203 0.203 0.203 0.203 lat band # 47 0.213 0.213 0.213 0.213 0.213 lat band # 48 0.223 0.223 0.223 0.223 0.223 lat band # 49 0.233 0.233 0.233 0.233 0.233 lat band # 50 0.243 0.243 0.243 0.243 0.243 lat band # 51 0.253 0.253 0.253 0.253 0.253 file ave_cos_sza_monthly_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