SRB_REL2.1_LW_3HRLY - GEWEX Longwave 3-Hourly Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.1_LW_3HRLY data set. The data set contains 3-hourly global fields of six longwave (LW) surface and Top of Atmosphere (TOA) radiative parameters, in addition to a day/night flag, derived with the Longwave 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 Sciences Data Center Science, Users 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 2.1.2. Indian Ocean Gap Artifact 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 seven parameters in these files as follows: 1. Day/Night flag (daynite; 1=Day, 0=Night) 2. TOA Upward Clear-Sky Flux/Clear-sky Outgoing Longwave Radiation (OLR) (clr_toa_up) 3. Surface Clear-sky Upward Longwave Flux (clr_sfc_up) 4. Surface Clear-sky Downward Longwave Flux (clr_sfc_down) 5. TOA Upward Longwave Flux/OLR (toa_up) 6. Surface Upward Longwave Flux (sfc_up) 7. Surface Downward Longwave Flux (sfc_down) These parameters were derived originally on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours), at UT hours 00, 03, 06, 09, 12, 15, 18, and 21 for every day of the month. The current version of the data set is Release 2.1. The GEWEX LW algorithm uses the Fu et al. (1997, JAS, Vol. 54, 2799-2812) Thermal Infrared radiative transfer code with cloud and surface parameters requiring cloud, atmospheric profile information, and surface properties. The sources for these inputs are briefly described below. A detailed description of the algorithm is currently being prepared for publication (Stackhouse et al., 2005). Please contact Dr. Paul W. Stackhouse Jr. at the address below for further details. 2.1 Data Quality An assessment of the quality of these 3-hourly fluxes was accomplished by comparisons with corresponding ground-measured fluxes over a period of four years (1992-1995) 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 about 1.7 W/m**2 (0.5%, model fluxes higher), and the root mean square difference is 30.3 W/m**2 (9.3%). Uncertainties associated with operational BSRN measurements during this period are believed to be about +/- 5 W/m**2 (1.5%, Ellsworth Dutton, NOAA, BSRN Manager). Thus, mean bias for the present results is within the uncertainty of BSRN measurements. Errors for individual 3-hourly values are subject to bias and random errors due to local meteorological conditions. 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 averaged fluxes, a discontinuity of magnitude less than 20 W/m**2 for TOA and 5 W/m**2 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. The cloud pixels were separated into categories of high, middle and low where middle and low clouds could be composed of ice or water. Cloud fractions and cloud optical depths were determined within these categories. Cloud particle sizes were assumed and cloud physical thicknesses were also assigned based upon information from literature. Random overlap is used between the high, middle and low layers to better approximate undercast conditions. Temperature and moisture profiles were obtained from a 4-D data assimilation product provided by the Data Assimilation Office at NASA GSFC and were produced with the Goddard Earth Observing System model version 1 (GEOS-1). Surface emissivities were taken from a map developed at NASA LaRC (Wilber et al. 1999, NASA/TP-1999-209362, 35 pp.). 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: p.w.stackhouse@larc.nasa.gov Production Contact: Atmospheric Sciences Data Center Science, Users 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 data file contains an entire month of 3-hourly 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 contain binary data and are named according to the following convention: srb_rel2.1_longwave_3hrly_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2.1 Release number for these data (Release 2.1) longwave Name of the algorithm, GEWEX Longwave 3hrly Time resolution of the data file yyyy 4-digit year mm 2-digit month binary file format 4.0 Science Parameters Information The files contain global fields of 3-hourly values of the day/night flag and the six radiative parameters on the nested grid. Each file has 7 records, containing one global field for every time period in each record. The parameters are: Name: Day/Night flag (Day=1, Night=0) Units: none Type: real Range: 0.0 or 1.0 Fill Values: n/a Scale Factor: None Name: Top-of-Atmosphere Clear-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Surface Clear-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 800 Fill Values: -999.0 Scale Factor: None Name: Surface Clear-sky Downward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Top-of-Atmosphere All-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None Name: Surface All-sky Upward LW Flux Units: Watts per square meter Type: Real Range: 50 to 800 Fill Values: -999.0 Scale Factor: None Name: Surface All-sky Downward LW Flux Units: Watts per square meter Type: Real Range: 50 to 600 Fill Values: -999.0 Scale Factor: None 5.0 Sample Read Software Description Sample read software written in Fortran-90, read_longwave_3hrly.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 (longwave_3hrly.nml). The input files are read as direct-access binary on the nested (44016 box) grid. The software reads one or more of the 7 parameter fields, regrids them to an equal-angle 1 deg x 1 deg grid, and writes the 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. daynite=.true. clr_toa_up=.true. clr_sfc_up=.true. clr_sfc_down=.true. toa_up=.true. sfc_up=.true. sfc_down=.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 code 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_longwave_3hrly read_longwave_3hrly.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_longwave_3hrly 7.0 Sample Output The seven tables of numbers below show the values of the seven 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) for hour 06 of day 14 of the month. Values for only a small lat-lon box for a single time are printed to the screen. When the software is run, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.1_longwave_3hrly Read Software * * * * Version: 1.0 * * * * Date: November 23, 2004 * * * * Contact: Atmospheric Sciences Data Center * * Science, Users 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_rel2.1_longwave_3hrly_199207.binary input file is opened Variable daynite_ Hour = 06 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 1.000 lat band # 47 1.000 1.000 1.000 1.000 1.000 lat band # 48 1.000 1.000 1.000 1.000 1.000 lat band # 49 1.000 1.000 1.000 1.000 1.000 lat band # 50 1.000 1.000 1.000 1.000 1.000 lat band # 51 1.000 1.000 1.000 1.000 1.000 file daynite_3hrly_199207.ascii has been written Variable clr_toa_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 244.323 242.894 242.894 243.500 243.500 lat band # 46 245.262 244.871 244.906 245.419 245.365 lat band # 47 245.523 245.918 246.575 247.134 248.530 lat band # 48 247.440 249.265 249.699 249.832 250.640 lat band # 49 251.632 251.362 252.915 253.153 253.937 lat band # 50 256.030 256.190 255.842 256.706 256.813 lat band # 51 258.839 259.396 259.461 259.904 259.852 file clr_toa_up_3hrly_199207.ascii has been written Variable clr_sfc_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 351.621 352.472 352.472 353.179 353.179 lat band # 46 356.920 357.151 357.160 357.170 357.336 lat band # 47 360.715 360.788 360.521 360.255 360.220 lat band # 48 363.148 363.157 362.655 362.153 361.984 lat band # 49 365.073 365.088 364.565 364.042 363.840 lat band # 50 366.484 366.573 366.246 365.919 365.788 lat band # 51 368.146 368.253 368.183 368.113 368.272 file clr_sfc_up_3hrly_199207.ascii has been written Variable clr_sfc_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 258.353 261.559 261.559 266.652 266.652 lat band # 46 262.654 262.325 262.544 262.803 265.667 lat band # 47 267.666 265.115 264.309 263.370 266.114 lat band # 48 269.236 267.713 266.529 265.411 267.218 lat band # 49 270.928 270.375 269.595 268.866 269.960 lat band # 50 272.744 273.136 273.399 273.648 274.266 lat band # 51 275.555 276.240 276.819 277.393 277.655 file clr_sfc_down_3hrly_199207.ascii has been written Variable toa_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 159.142 144.000 144.000 166.970 166.970 lat band # 46 158.157 153.388 179.292 182.073 195.004 lat band # 47 229.082 230.951 213.297 190.921 171.740 lat band # 48 231.413 203.552 166.961 169.199 166.795 lat band # 49 191.303 164.263 179.668 188.523 191.124 lat band # 50 180.019 183.485 205.830 184.009 210.318 lat band # 51 241.281 221.477 217.645 219.853 240.196 file toa_up_3hrly_199207.ascii has been written Variable sfc_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 352.289 352.971 352.971 353.808 353.808 lat band # 46 357.538 357.699 358.152 357.870 358.162 lat band # 47 361.902 361.938 361.653 361.173 360.784 lat band # 48 364.334 364.191 363.292 362.643 362.468 lat band # 49 366.059 365.594 365.227 364.710 364.539 lat band # 50 367.199 367.238 366.880 366.590 366.457 lat band # 51 369.260 369.323 369.088 369.203 369.323 file sfc_up_3hrly_199207.ascii has been written Variable sfc_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 302.286 294.401 294.401 308.610 308.610 lat band # 46 303.422 298.334 328.196 309.007 320.554 lat band # 47 346.425 341.097 339.093 323.854 303.151 lat band # 48 347.909 336.123 308.544 297.393 298.950 lat band # 49 336.430 303.756 313.407 313.032 316.194 lat band # 50 320.324 317.307 315.558 318.315 318.640 lat band # 51 349.745 347.692 337.158 350.335 347.743 file sfc_down_3hrly_199207.ascii has been written 8.0 Additional Derivable Parameters The net LW flux at the top-of-atmosphere (TOA) is simply the TOA upward LW flux. The net LW flux at the surface can be defined as: Net LW Flux = Downward LW Flux - Upward LW Flux and is, therefore, generally a negative number. Net fluxes can be computed for the clear-sky and all-sky conditions. The estimates of clear-sky and all-sky fluxes also allow the estimation of the contribution by clouds to the all-sky fluxes. This is commonly referred to as the cloud radiative forcing (CRF) and is computed according to: CRF = Flux (all-sky) - Flux (clear-sky) Thus, the cloud radiative forcing on the downward longwave flux is generally positive because clouds act to increase the emission to the surface. In this way, the effect of the cloud emission on the fluxes can be estimated for each flux component. Lastly, providing TOA and surface fluxes allows one to derive the net radiative flux of the atmosphere. This is given by the relation Net Atmos. Flux = Net TOA Flux - Net Surface Flux For the LW, this flux is negative meaning that the atmosphere is cooling over the LW wavelengths.