SRB_REL3.0_QCLW_3HRLYMONTHLY (netCDF)- GEWEX Quality-Check Longwave Monthly Averaged 3-Hourly README File 1.0 Introduction This README file provides information on the SRB_REL3.0_QCLW_3HRLYMONTHLY_NC data set. The data set contains monthly average/3-hourly (also called diurnally-resolved monthly average) global fields of three longwave (LW) surface radiative parameters derived with the Quality-Check LW (QCLW) 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 There are a total of three parameters in these files as follows: 1. Surface Downward Longwave Flux (DLF), 2. Surface Net Longwave Flux (NLF), and 3. Surface Clear Sky Downward Longwave Flux (CSDLF). These parameters were derived originally on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours), namely, at UT hours 00, 03, 06, 09, 12, 15, 18, and 21 for every day of the month. The 3-hourly values were used to compute monthly averages separately for each of the 8 GMT hours. 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. (1992) - J. Appl. Meteor., 31, 1361-1367. Gupta (1989) - J. Climate, 2, 305-320. Wilber et al. (1999) - NASA/TP-1999-209362, 35 pp. (available on the web from http://techreports.larc.nasa.gov/ltrs/ltrs.html) A refinement recently implemented in this algorithm has been submitted for publication to the Journal of Applied Meteorology and Climatology (2010). 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 longwave radiation is lower in the gap, creating an appearance of a flux discontinuity. For 3-hourly/monthly averaged fluxes a discontinuity of magnitude less than 5 W/m**2 for surface fluxes may appear in the Indian Ocean gap region. 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) Surface emissivities were taken from a map developed at NASA LaRC (Wilber et al. 1999; see reference above). 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." A detailed description of the grid is also presented in the Data Quality Summary. The read software described below contains a subroutine to regrid the fluxes to a 1 degree latitude by 1 degree longitude equal-angle grid using replication. 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/3-hourly global fields (resolved at 8 times) 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_qclw_3hrlymonthly_yyyymm.nc, where srb Project name, Surface Radiation Budget rel3.0 Release number for these data (Release 3.0) qclw Name of the algorithm, Quality-Check Longwave 3hrlymonthly Indicates 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 diurnally-resolved monthly averages of the three parameters on the 1x1 grid. Name: Surface Downward LW Flux (DLF) Units: Watts per square meter Range: 50 to 650 Fill Values: -9.99e+08 Name: Surface Net LW Flux (NLF) Units: Watts per square meter Range: -180 to 20 Fill Values: -9.99e+08 Scale Factor: None Name: Surface Clear Sky Downward Longwave Flux (CSDLF) Units: Watts per square meter Range: 50 to 700 Fill Values: -9.99e+08 Scale Factor: None 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_qclw_3hrlymonthly_nc.f90. The software reads the 3 parameters into data arrays, 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_qclw_3hrlymonthly -q64 -qextname -I/usr/local/include -L/usr/local/lib -lnetcdf read_srb_rel3_qclw_3hrlymonthly_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_qclw_3hrlymonthly 7.0 Additional Derivable Parameters It is important to keep in mind that NLF is computed as NLF = DLF - Upward LW Flux (ULF) and is, therefore, generally a negative number. Also, the three parameters provided in these files can be used to compute two additional surface LW parameters, if needed. ULF (Upward Longwave Flux) can be computed as ULF = DLF - NLF Longwave Cloud Radiative Forcing (LWCRF) can be computed as LWCRF = DLF - CSDLF To compute these additional parameters, both quantities on the right hand side of the equations have to be available.