SRB_REL3.0_QCLW_3HRLYMONTHLY - 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 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 Sample Output 8.0 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 Longwave Cloud Radiative Forcing (LWCRF). 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 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 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 contain binary data and are named according to the following convention: srb_rel3.0_qclw_3hrlymonthly_yyyymm.binary, 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 binary file format 4.0 Science Parameters Information The files contain global fields of diurnally-resolved monthly averages of the three parameters on the nested grid. Each file has 24 records, 3 records for each of the 8 times containing global fields the 3 parameters in the order listed below. Name: Surface Downward LW Flux (DLF) Units: Watts per square meter Type: Real Range: 50 to 650 Fill Values: -999.0 Scale Factor: None Name: Surface Net LW Flux (NLF) Units: Watts per square meter Type: Real Range: -180 to 20 Fill Values: -999.0 Scale Factor: None Name: Surface LW Cloud Radiative Forcing (LWCRF) Units: Watts per square meter Type: Real Range: 0 to 150 Fill Values: -999.0 Scale Factor: None 5.0 Sample Read Software Description Sample read software written in Fortran-90, read_srb_rel3_qclw_3hrlymonthly.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 (srb_rel3_qclw_3hrlymonthly.nml). The input files are direct-access binary on the nested (44016 box) grid. The software reads one or more of the 3 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. DLF=.true. NLF=.true. LWCRF=.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 IBM P6 machine 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 for all 8 times of the day. The user should be easily able to extract values for any box or lat-lon region from these fields for any time of the day. 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_qclw_3hrlymonthly read_srb_rel3_qclw_3hrlymonthly.f90 The providers used a GFORTRAN F90 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_qclw_3hrlymonthly 7.0 Sample Output When the is code run, the following information appears on the screen: The three 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) at hour 06. Values for only a small lat-lon box are printed to the screen. ***************************************************************** * * * * * Data Set srb_rel3.0_qclw_3hrlymonthly Read Software * * * * Version: 1.0 * * * * Contact: 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 * * * ***************************************************************** /srb_rel3.0_qclw_3hrlymonthly_199207.binary input file is opened Variable DLF_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 309.433 313.619 313.619 316.510 316.510 lat band # 46 317.567 319.554 322.397 314.377 321.827 lat band # 47 320.361 323.345 323.194 324.797 318.878 lat band # 48 329.355 327.333 327.119 325.402 322.628 lat band # 49 328.964 331.714 327.897 324.752 323.851 lat band # 50 331.264 331.732 324.283 328.789 325.067 lat band # 51 330.095 326.314 331.839 332.689 333.330 file DLF_3hrlymonthly_199207.ascii has been written Variable NLF_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 -45.612 -42.221 -42.221 -39.842 -39.842 lat band # 46 -41.901 -40.074 -37.482 -45.549 -37.968 lat band # 47 -43.007 -39.864 -39.901 -38.059 -43.402 lat band # 48 -36.573 -38.214 -38.042 -39.182 -41.209 lat band # 49 -38.417 -35.389 -38.700 -41.145 -41.362 lat band # 50 -37.827 -37.334 -44.382 -39.376 -42.641 lat band # 51 -41.149 -45.248 -39.776 -38.754 -38.146 file NLF_3hrlymonthly_199207.ascii has been written Variable LWCRF_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 50.957 54.387 54.387 56.224 56.224 lat band # 46 54.189 56.288 59.226 51.257 58.315 lat band # 47 53.383 56.820 57.024 58.860 53.409 lat band # 48 60.239 58.793 59.094 57.877 55.595 lat band # 49 58.648 61.843 58.395 55.732 55.154 lat band # 50 59.835 60.442 53.320 58.139 54.383 lat band # 51 57.683 53.699 59.166 59.981 60.370 file LWCRF_3hrlymonthly_199207.ascii has been written 8.0 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 can be computed as ULF = DLF - NLF Clear-sky DLF (CSDLF) can be computed as CSDLF = DLF - LWCRF To compute these additional parameters, both quantities on the right hand side of the equations have to be available.