SRB_REL2.5_LW_3HRLY - GEWEX Longwave 3-Hourly Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.5_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 2.2 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 2.5. 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 the Dr. Paul W. Stackhouse Jr. at the address below for further details. 2.0.1. Differences with Release-2.0 Data Set The only important difference between the current data set and the corresponding Release-2.0 data set is the use of GEOS-4 meteorological inputs for the current data set in place of GEOS-1 for Release-2.0. 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 thirteen years (1992-2004) 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 1.19 W/m**2 , and the root mean square difference is 33.6 W/m**2 . 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 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 entire 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. 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. Paul.W.Stackhouse@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 are contain binary data and are named according to the following convention: srb_rel2.5_longwave_3hrly_yyyymm.binary, where srb Project name, Surface Radiation Budget rel2.5 Release number for these data (Release 2.5) 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 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_rel2.5_longwave_3hrly Read Software * * * * Version: 1.0 * * * * 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_rel2.5_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 # 45 242.856 244.619 244.619 247.754 247.754 lat band # 46 247.106 249.278 251.876 253.253 254.182 lat band # 47 253.563 255.856 256.730 257.418 257.686 lat band # 48 259.188 259.338 259.535 259.737 260.119 lat band # 49 261.231 261.298 261.525 262.092 262.810 lat band # 50 263.356 263.369 263.954 265.210 266.240 lat band # 51 265.624 266.550 267.494 269.264 270.334 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 # 45 353.591 354.206 354.206 354.514 354.514 lat band # 46 358.036 358.060 358.207 358.249 357.848 lat band # 47 361.945 361.670 361.526 361.237 360.529 lat band # 48 364.432 364.049 363.691 363.119 362.267 lat band # 49 365.944 365.689 365.246 364.565 363.832 lat band # 50 367.686 367.729 367.437 366.885 366.446 lat band # 51 369.887 370.329 370.368 370.181 370.168 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 # 45 274.135 278.198 278.198 277.779 277.779 lat band # 46 281.425 280.714 279.482 278.380 277.511 lat band # 47 282.275 280.776 279.588 278.741 278.699 lat band # 48 282.625 281.591 281.558 280.608 279.568 lat band # 49 284.686 284.073 283.209 282.135 280.290 lat band # 50 286.163 284.256 283.288 282.065 280.825 lat band # 51 285.407 283.899 282.484 281.437 280.717 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 # 45 159.631 144.115 144.115 167.441 167.441 lat band # 46 158.697 153.820 181.608 184.007 198.734 lat band # 47 236.643 239.956 221.256 195.384 173.024 lat band # 48 241.882 209.639 168.822 171.227 168.408 lat band # 49 196.086 166.390 180.047 191.463 194.156 lat band # 50 181.532 185.426 209.517 186.236 214.705 lat band # 51 246.786 225.181 225.858 224.958 248.419 file /toa_up_3hrly_199207.ascii has been written Variable sfc_up_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 354.160 354.631 354.631 355.102 355.102 lat band # 46 358.548 358.513 359.081 358.861 358.611 lat band # 47 362.961 362.630 362.529 362.037 361.011 lat band # 48 365.446 364.990 364.233 363.529 362.686 lat band # 49 366.875 366.129 365.830 365.144 364.459 lat band # 50 368.321 368.332 368.017 367.507 367.083 lat band # 51 370.894 371.334 371.318 371.260 371.232 file /sfc_up_3hrly_199207.ascii has been written Variable sfc_down_ Hour = 06 Day = 14 lon # = 100 101 102 103 104 lat band # 45 312.585 306.829 306.829 317.497 317.497 lat band # 46 316.138 311.229 338.558 319.595 328.899 lat band # 47 350.982 345.360 347.281 332.641 310.992 lat band # 48 351.070 345.011 318.050 308.011 307.553 lat band # 49 347.499 313.592 322.640 321.138 322.359 lat band # 50 329.087 324.913 322.314 323.842 323.330 lat band # 51 353.283 351.470 346.136 353.753 351.597 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.