SRB_REL2.8_SHORTWAVE_DAILY - GEWEX Shortwave Daily README file 1.0 Introduction This README file provides information on the SRB_REL2.8_SHORTWAVE_DAILY data set. The data set contains daily 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 are averaged into daily values using a normalization correction to account for the discretization of the solar cycle. 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). All fluxes are in Watts per square meter. It is important to note that the daily averages are representative of the local day, not the period from 0Z to 2359Z. 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 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 -8.3 W/m**2 (-3.8% model fluxes lower), and the root mean square difference is 38.5 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 (personal communication, Rolf Philipona, World Radiation Center) depending on atmospheric humidity and cloudiness conditions. These flux estimates are within these uncertainties. An assessment of the surface PAR fluxes with SURFRAD sites yields a mean bias of -2.3 W/m**2 and a root mean square difference of 14.5 W/m**2. 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. For Daily averaged fluxes, any discontinuity in instantaneous fluxes will be exacerbated by the temporal gaps of coverage in the Indian 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 daily 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 are contain binary data and are named according to the following convention: srb_rel2.8_shortwave_daily_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 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 eleven parameters on the nested grid. Name: TOA Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 575 Fill Values: -1000.0 Scale Factor: None Name: All Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 450 Fill Values: -1000.0 Scale Factor: None Name: All Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 475 Fill Values: -1000.0 Scale Factor: None Name: All Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 400 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky TOA Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 425 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky Surface Downward SW Flux Units: Watts per square meter Type: Real Range: 0 to 475 Fill Values: -1000.0 Scale Factor: None Name: Clear Sky Surface Upward SW Flux Units: Watts per square meter Type: Real Range: 0 to 400 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 Sample Read Software Description Sample read software written in Fortran-90, read_shortwave_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 (shortwave_daily.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=.true. 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_daily read_shortwave_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_shortwave_daily 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 code runs, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.8_shortwave_daily 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_daily_199207.binary input file is opened Variable toa_down_Day = 14 lon # = 100 101 102 103 104 lat band # 45 123.367 123.367 123.367 123.367 123.367 lat band # 46 130.031 130.031 130.031 130.031 130.031 lat band # 47 136.711 136.711 136.711 136.711 136.711 lat band # 48 143.403 143.403 143.403 143.403 143.403 lat band # 49 150.100 150.100 150.100 150.100 150.100 lat band # 50 156.800 156.800 156.800 156.800 156.800 lat band # 51 163.497 163.497 163.497 163.497 163.497 file toa_down_daily_199207.ascii has been written Variable toa_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 66.039 64.235 64.235 59.718 59.718 lat band # 46 63.979 64.390 63.547 61.352 58.797 lat band # 47 63.063 57.175 63.289 62.346 63.293 lat band # 48 61.557 58.800 61.255 64.646 65.118 lat band # 49 63.923 72.031 65.269 61.831 64.327 lat band # 50 70.222 67.868 62.782 65.983 56.365 lat band # 51 65.541 55.629 50.743 58.369 52.176 file toa_up_daily_199207.ascii has been written Variable sfc_down_Day = 14 lon # = 100 101 102 103 104 lat band # 45 32.610 34.232 34.232 39.392 39.392 lat band # 46 40.395 40.135 41.327 43.864 46.577 lat band # 47 47.598 54.226 47.754 48.899 48.289 lat band # 48 55.263 58.369 57.046 52.563 51.906 lat band # 49 58.532 50.015 57.353 61.142 58.140 lat band # 50 57.513 60.265 65.929 62.370 72.631 lat band # 51 68.563 79.186 84.601 76.255 82.996 file sfc_down_daily_199207.ascii has been written Variable sfc_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 1.957 2.054 2.054 2.364 2.364 lat band # 46 2.424 2.408 2.480 2.632 3.051 lat band # 47 2.856 3.252 2.865 2.935 3.810 lat band # 48 3.317 3.503 5.686 3.837 3.119 lat band # 49 3.513 3.004 3.454 3.689 3.717 lat band # 50 3.488 3.679 3.975 3.810 4.718 lat band # 51 4.163 5.335 5.569 4.753 5.743 file sfc_up_daily_199207.ascii has been written Variable clr_toa_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 23.085 23.980 23.980 23.313 23.313 lat band # 46 23.715 23.735 24.249 24.737 23.163 lat band # 47 23.706 23.379 23.444 28.240 30.233 lat band # 48 25.455 24.787 33.630 30.773 30.672 lat band # 49 24.076 34.963 33.122 30.636 33.671 lat band # 50 33.216 33.257 32.538 30.389 34.139 lat band # 51 33.532 33.551 30.647 32.782 37.120 file clr_toa_up_daily_199207.ascii has been written Variable clr_sfc_down_Day = 14 lon # = 100 101 102 103 104 lat band # 45 76.988 76.876 76.876 77.592 77.592 lat band # 46 81.954 82.080 82.582 83.056 83.039 lat band # 47 87.762 87.878 88.019 88.565 88.979 lat band # 48 93.173 93.435 94.316 94.108 94.142 lat band # 49 99.556 99.602 99.308 99.270 93.695 lat band # 50 104.760 105.196 105.600 104.242 101.115 lat band # 51 111.095 107.595 107.980 102.521 100.869 file clr_sfc_down_daily_199207.ascii has been written Variable clr_sfc_up_Day = 14 lon # = 100 101 102 103 104 lat band # 45 6.121 7.121 7.121 6.584 6.584 lat band # 46 6.331 6.241 6.854 7.468 5.624 lat band # 47 5.836 5.319 5.333 10.743 13.065 lat band # 48 7.140 6.479 16.528 13.141 12.956 lat band # 49 5.798 17.686 15.314 12.447 11.023 lat band # 50 15.018 15.347 14.707 11.498 12.653 lat band # 51 15.796 12.533 9.541 7.607 10.922 file clr_sfc_up_daily_199207.ascii has been written Variable par_Day = 14 lon # = 100 101 102 103 104 lat band # 45 15.879 16.619 16.619 18.887 18.887 lat band # 46 19.531 19.369 19.865 20.939 22.009 lat band # 47 22.767 25.537 22.757 23.232 22.855 lat band # 48 26.160 27.479 26.701 24.895 24.690 lat band # 49 27.796 24.109 27.233 28.815 27.285 lat band # 50 27.529 28.680 31.107 29.432 33.436 lat band # 51 32.433 36.412 38.524 35.163 37.534 file par_daily_199207.ascii has been written Variable cld_frac_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 0.917 lat band # 47 1.000 1.000 1.000 1.000 0.875 lat band # 48 1.000 1.000 0.812 0.917 1.000 lat band # 49 1.000 1.000 1.000 1.000 0.887 lat band # 50 1.000 1.000 1.000 0.938 0.667 lat band # 51 0.917 0.708 0.700 0.700 0.692 file cld_frac_daily_199207.ascii has been written Variable cos_sza_Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.190 0.189 0.189 0.193 0.193 lat band # 46 0.200 0.199 0.200 0.202 0.201 lat band # 47 0.212 0.211 0.210 0.210 0.211 lat band # 48 0.220 0.222 0.221 0.222 0.222 lat band # 49 0.238 0.231 0.232 0.233 0.232 lat band # 50 0.248 0.248 0.251 0.241 0.242 lat band # 51 0.258 0.258 0.258 0.252 0.252 file cos_sza_daily_199207.ascii has been written Variable ave_cos_sza_Day = 14 lon # = 100 101 102 103 104 lat band # 45 0.186 0.186 0.186 0.186 0.186 lat band # 46 0.197 0.197 0.197 0.197 0.197 lat band # 47 0.207 0.207 0.207 0.207 0.207 lat band # 48 0.217 0.217 0.217 0.217 0.217 lat band # 49 0.227 0.227 0.227 0.227 0.227 lat band # 50 0.237 0.237 0.237 0.237 0.237 lat band # 51 0.247 0.247 0.247 0.247 0.247 file ave_cos_sza_daily_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