SRB_REL3.0_QCSW_DAILY (netCDF) - GEWEX Quality-Check Shortwave Daily README File 1.0 Introduction This README file provides information on the SRB_REL3.0_QCSW_DAILY_NC data set. The data set contains daily average global fields of six shortwave (SW) surface radiative parameters derived with the Quality-Check SW (QCSW) algorithm of the NASA World Climate Research Programme /Global Energy and Water-Cycle Experiment (WCRP/GEWEX) Surface Radiation Budget (SRB) Project. If users have any 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 There are a total of six parameters in these files as follows: 1. Top-of-atmosphere (TOA) Shortwave Downward Flux (insolation) 2. Surface Pristine-sky Shortwave Downward Flux 3. Surface Clear-sky Shortwave Downward Flux 4. Surface All-sky Shortwave Downward Flux 5. Surface All-sky Shortwave Net Flux 6. Surface All-sky Shortwave Albedo These parameters were derived originally on a daily temporal resolution and archived at the same resolution. The current version of the data sets is identified as Release 3.0. It covers a period of 24.5 years from July 1983 to December 2007. Detailed description of the algorithm used in deriving these parameters can be found in: Gupta et al. (2001) - NASA/TP-2001-211272, Dec. 2001, 31 pp. (available on the web at http://techreports.larc.nasa.gov/ltrs/ltrs.html) Darnell et al. (1992) - J. Geophys. Res., 97, 15741-15760. Darnell et al. (1988) - J. Climate, 1, 820-835. 2.0.1. Improvements over the algorithm described in the above references. The algorithm used for producing the current dataset differs from the one described in the above references in the following important regards: (a) Broadband aerosol optical properties used for the current data set were derived using aerosol optical depths from the Model of Atmospheric Transport and Chemistry (MATCH; Rasch, Collins,and Eaton,2001: JGR-Atmospheres, 106, 7337-7355) assimilation products and single scattering albedo and asymmetry parameter from the Optical Properties of Aerosols and Clouds (OPAC; Hess, Koepke, and Schult, 1998: BAMS, 79, 831-844) database. Aerosol properties used in earlier versions of this data set were based on the information provided in Deepak and Gerber (1983; Report of the experts' meeting on aerosols and their climatic effects. WCP-55, 107 pp.) (b) Monthly climatological top-of-atmosphere clear-sky albedos used for the current data set were derived from 70 months (March 2000 - December 2005) of CERES data from the Terra satellite. Corresponding data used in earlier versions of this data set were based on 5 years (1985-1989) of Earth Radiation Budget Experiment (ERBE) measurements. (c) The 4-class (ocean/coast/land/desert) surface type map used for the current data set was derived from the 1/6 deg. surface type map from the International Geosphere-Biosphere Program (IGBP). Corresponding map used in earlier versions of this data set was developed for ERBE processing from older information. (d) TOA insolation computation method used in earlier versions of this data set was based on approximate relations presented in Iqbal (1983) in the interest of simplicity and speed. For the current release, TOA insoaltion is computed using the astronomical almanac algorithm of Michalsky (1988). 2.1 Data Quality For information on validation of these products, the user is referred to the Data Quality Summary available at: http://gewex-srb.larc.nasa.gov/ 2.1.1 Calibration shifts The SRB algorithms rely heavily on radiances and cloud properties from ISCCP. Great care is taken by the ISCCP team to produce a well-calibrated, homogeneous dataset from dozens of satellites over the course of several decades. However, there are a few known issues and discontinuities in the ISCCP products which are likely to be reflected in SRB products. ISCCP uses a reference afternoon polar orbiter as a calibration standard for the other satellites (geostationary and polar orbiting) in the constellation. The afternoon orbiters are subject to orbital drift and are replaced every few years. The dates of transition from one reference satellite to the next are the dates where small discontinuities in the SRB products are possible. The transition dates are as follows: February 1, 1985: NOAA-7 replaced by NOAA-9 November 1, 1988: NOAA-9 replaced by NOAA-11 September 30, 1994: NOAA-11 goes out of service February 1, 1995: NOAA-14 goes into service For the October 1994-January 1995 timeframe there was no reference orbiter available, so an interpolation between NOAA-11 and NOAA-14 is used. SRB results show noticeable anomalies in this period, some of which are likely artifacts of the calibration situation. October 1, 2001: NOAA-14 replaced by NOAA-16 January 1, 2006: NOAA-16 replaced by NOAA-18 From NOAA-16 onward, the visible calibration is bi-linear, which has led to some changes from the previous linear calibrations. The 2001 transition to NOAA-16 is accompanied by fairly strong radiance increases over ice, which has led to polar values of SRB surface albedo and cloud optical thickness which are probably anomalously high, and surface downward fluxes which may be too low. The NOAA-18 calibration appears to be raising overall reported visible radiances, especially reducing the frequency of very low radiance scenes, such as clear skies over ocean near the day-night terminator. The effect on SRB products from 2006 onward has been mainly to cause a jump in surface albedo over much of the planet. Surface downward fluxes are less affected. Care should be taken when computing long term time series from SRB data, with special notice taken of known transition dates, including those noted in Section 2.1.2 below. 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. 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 most of the 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. Column ozone data continued to be available beyond December 2004 from OMI instrument aboard Aura satellite but TOVS data, which is essential for filling the gaps in OMI data, developed some unexplained gaps of its own and became unusable. Beginning in January 2005, GEWEX/SRB started using a daily analysis ozone product from NOAA Climate Predictions Center (CPC), known as the Stratospheric Monitoring-group Ozone Blended Analysis (SMOBA). Surface albedos are derived with a parameterization using monthly climatological clear-sky TOA albedos which are based on 70 months of CERES Terra observations. 2.3 Grid Description The grid on which these fluxes are originally computed is a quasi-equal-area grid consisting of 44016 cells. The cell size is 1 deg. in latitude throughout and 1 deg. in longitude between 45N and 45S. Poleward of these latitudes, the cell size is progressively increased in longitude to accommodate a sufficient number of 30 km ISCCP pixels in each cell. This grid is also called the "nested grid." 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 data 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 in netCDF format and are named according to the following convention: srb_rel3.0_qcsw_daily_yyyymm.nc, where srb Project name, Surface Radiation Budget rel3.0 Release number for these data (Release 3.0) qcsw Name of the algorithm, Quality-Check Shortwave daily Time resolution of the data set 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 daily averages of the following six parameters for the whole month on the 1x1 grid. Name: TOA Shortwave Downward Flux or TOA Insolation Units: Watts per square meter Range: 0 to 700. Fill Values: -999.0 Name: Surface Pristine-sky Shortwave Downward Flux or Pristine-Sky Surface Insolation Units: Watts per square meter Range: 0 to 600. Fill Values: -999.0 Name: Surface Clear-sky Shortwave Downward Flux or Clear-Sky Surface Insolation Units: Watts per square meter Range: 0 to 600. Fill Values: -999.0 Name: Surface All-sky Shortwave Downward Flux or All-Sky Surface Insolation Units: Watts per square meter Range: 0 to 500. Fill Values: -999.0 Name: Surface All-sky Shortwave Net Flux Units: Watts per square meter Range: 0 to 500. Fill Values: -999.0 Name: Surface All-sky Shortwave Albedo Units: Dimensionless Range: 0 to 1 Fill Values: -999.0 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_qcsw_daily_nc.f90. The software reads the 6 fluxes into data arrays, and adjusting for the scaled values if necessary. The month and year 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_qcsw_daily -q64 -qextname -I/usr/local/include -L/usr/local/lib -lnetcdf read_srb_rel3_qcsw_daily_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_qcsw_daily 8.0 Additional Derivable Parameters The parameters available from these files can be used to derive additional surface SW radiative parameters. SW cloud radiative forcing (SWCRF) at the surface can be derived from all-sky and clear-sky downward fluxes as SWCRF = FALL - FCLR Upward SW flux (FUP) can be derived from all-sky downward and surface net fluxes as FUP = FALL - FABS Aerosol Radiative Forcing (ARF) at the surface under clear-sky condition can be derived from pristine-sky and clear-sky fluxes as ARF = FPRS - FCLR To compute these additional parameters, both quantities on the right hand side of the equations have to be available.