SRB_REL3.0_SHORTWAVE_MONTHLY (netCDF) - GEWEX Shortwave Monthly README file 1.0 Introduction This README file provides information on the SRB_REL3.0_SHORTWAVE_MONTHLY_NC 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 Science Data Center (ASDC) User and Data Services Office at: Atmospheric Science Data Center User and Data Services Office Mail Stop 157D NASA Langley Research Center Hampton, Virginia 23681-2199 U.S.A. E-mail: support-asdc@earthdata.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 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_REL3.0_SW_DAILY.txt). The Daily averages were then averaged into monthly averages. The current version of the data sets is identified as Release 3.0. 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 1998), with Pinker/Laszlo now the primary algorithm. Release 2.0: 12 year dataset (July 1983-October 1985), 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 include sunrise and 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. Release 3.0: Previous versions showed occasional instances of clear sky surface downward fluxes less than cloudy sky values at the same scene. This related to issues in the ISCCP DX radiance inputs to the algorithm. Now any 3-hourly period at any grid cell which shows an average TOA cloudy radiance less than the TOA clear sky radiance is considered a cell with a zero cloud fraction. The clear sky radiance is recalculated to include the cloudy sky values. Changes in the initial guess aerosol are made. The value of the surface albedo depends on the ISCCP DX clear sky composite radiance and the initial guess aerosol optical depth. Previously the initial guess depended only on surface type. Now the initial guess is from a monthly climatology of aerosols from the MATCH chemical transport model. In addition, the final aerosol optical depth calculated by the algorithm is capped at 0.05 over snow and ice. 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. Solar Zenith Angle from Satellite 11. Average 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 -4.1 W/m**2 (-2.3%, surface data higher), and the root mean square difference is 18.0 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 first 20.5 years 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. Starting in January 2005, ozone data was taken from the Stratosphere Monitoring Ozone Blended Analysis. 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 fluxes contined within these netCDF files have been regridded to a 1 degree latitude by 1 degree longitude equal-angle grid using a replication method. 2.4 Points of Contact Scientific contact: Dr. Paul W. Stackhouse Jr. Mail Stop 420 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 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 are in netCDF format and are named according to the following convention: srb_rel3.0_shortwave_monthly_(method)_yyyymm.nc, where srb Project name, Surface Radiation Budget rel3.0 Release number for these data (Release 3.0) shortwave Name of the algorithm, GEWEX Shortwave monthly Time resolution of the data set (method) Averaging method, either utc or local yyyy 4-digit year for these data mm 2-digit month for these data nc file format 4.0 Science Parameters Information The files contain global fields of monthly averages of the following eleven parameters on the 1x1 grid. Name: TOA Downward SW Flux Units: Watts per square meter Range: 0 to 550 Fill Values: -1000.0 Name: All Sky TOA Upward SW Flux Units: Watts per square meter Range: 0 to 400 Fill Values: -1000.0 Name: All Sky Surface Downward SW Flux Units: Watts per square meter Range: 0 to 425 Fill Values: -1000.0 Name: All Sky Surface Upward SW Flux Units: Watts per square meter Range: 0 to 350 Fill Values: -1000.0 Name: Clear Sky TOA Upward SW Flux Units: Watts per square meter Range: 0 to 375 Fill Values: -1000.0 Name: Clear Sky Surface Downward SW Flux Units: Watts per square meter Range: 0 to 425 Fill Values: -1000.0 Name: Clear Sky Surface Upward SW Flux Units: Watts per square meter Range: 0 to 350 Fill Values: -1000.0 Name: All Sky Global Photosynthetically Active Radiation Flux Units: Watts per square meter Range: 0 to 200 Fill Values: -1000.0 Name: Cloud Fraction Units: Dimensionless Range: 0 to 1 Fill Values: -1000.0 Name: Solar Zenith Angle From Satellite Units: Degrees Range: 0 to 90 Fill Values: -1000.0 Name: Average Solar Zenith Angle From Astronomy (center of 3 hour period) Units: Degrees Range: 0 to 90 Fill Values: -1000.0 5.0 Sample Read Software Description Certain graphics packages, such as GrADS (http://grads.iges.org/grads/head.html) and Panoply (http://www.giss.nasa.gov/tools/panoply/) allow easy rendering of the data sets. However, if the data user would like to read these data through a Fortran-90 code, one is provided with the data. The data user will need to have netCDF libraries installed on their computer in order to compile the read software. This sample read software is read_srb_rel3_shortwave_monthly_nc.f90. The software reads the 11 parameters into data arrays, and adjusting for the scaled values if necessary. The month, year, and averaging method can be changed inside the read code. The data start at the Greenwich meridian-south pole and go east and north from there. 6.0 Implementing the Sample Read Software The sample read software can be compiled with any Fortran 90 or 95 compiler. To compile: % xlf90 -o run_shortwave_monthly -q64 -qextname -I/usr/local/include -L/usr/local/lib -lnetcdf read_srb_rel3_shortwave_monthly_nc.f90 /usr/local/lib/libnetcdf.a The providers used an IBM XLF compiler but any F90/F95 compiler with access to the netCDF libraries should work. Note that the location of the netCDF libraries referenced in the compile statement could be different and the compiler flags are unique to the IBM XLF compiler. Run the software: % run_shortwave_monthly 7.0 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