SRB_REL2.5_LW_3HRLY_MONTHLY - GEWEX Longwave Monthly Averaged 3-Hourly (Diurnal) Data Set README File 1.0 Introduction This README file provides information on the SRB_REL2.5_LW_3HRLY_MONTHLY data set. The data set contains monthly averaged 3-hour global fields of six longwave (LW) surface and Top of Atmosphere (TOA) radiative parameters 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 six parameters in these files as follows: 1. TOA Upward Clear-Sky Flux/Clear-sky Outgoing Longwave Radiation (OLR) (clr_toa_up) 2. Surface Clear-sky Upward Longwave Flux (clr_sfc_up) 3. Surface Clear-sky Downward Longwave Flux (clr_sfc_down) 4. TOA Upward Longwave Flux/OLR (toa_up) 5. Surface Upward Longwave Flux (sfc_up) 6. Surface Downward Longwave Flux (sfc_down) These parameters are derived originally on a 3-hourly temporal resolution (i.e., a global instantaneous gridded field every 3 hours). The 3-hourly values are used to compute monthly averages separately for each of the 8 UT hours (00, 03, 06, 09, 12, 15, 18, and 21 UT). The current version of the datasets is identified as 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 monthly average 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 1.41 W/m**2 (0.1%, model fluxes higher), and the root mean square difference is 16.4 W/m**2 (5.5%). Uncertainties associated with operational BSRN measurements during this period are believed to be about +/- 3-5 W/m**2 (1-1.5%, Ellsworth Dutton, NOAA, BSRN Manager). Thus, mean bias for the present results is within the uncertainty for BSRN measurements. Errors for individual 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 fluxes a discontinuity may appear in the Indian Ocean depending upon the prevalent meteorological conditions. Significant areas within this region may also be missing depending upon the hour due to the lack of geosynchronous coverage. For 3-hourly/monthly averaged fluxes a discontinuity of magnitude less than 20 W/m**2 for TOA fluxes and less than 5 W/m**2 for surface fluxes may appear in the Indian Ocean gap region. 2.2 Input Information 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 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: 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 files contains monthly average/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_3hrlymonthly_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 3hrlymonthly 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 monthly averaged/3-hourly values of the six radiative parameters on the nested grid. Each file has 6 records, containing one global field for every time period in each record. The parameters are: 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 Description of Sample Read Software Sample read software written in Fortran-90, read_longwave_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 (longwave_3hrlymonthly.nml). The input files are read as direct-access binary on the nested (44016 box) grid. The software reads one or more of the 6 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. 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_3hrlymonthly read_longwave_3hrlymonthly.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_3hrlymonthly 7.0 Sample Output The six 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. When the is code run, the following information appears on the screen: ***************************************************************** * * * * * Data Set srb_rel2.5_longwave_3hrlymonthly 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_3hrlymonthly_199207.binary Variable clr_toa_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 251.302 251.788 251.788 251.905 251.905 lat band # 46 253.676 254.035 254.416 254.547 254.506 lat band # 47 256.010 256.384 256.642 256.781 256.715 lat band # 48 258.108 258.265 258.540 258.617 258.574 lat band # 49 259.864 259.989 260.010 260.072 260.171 lat band # 50 261.734 261.813 261.914 262.078 262.025 lat band # 51 264.161 264.355 264.487 264.747 264.668 file /clr_toa_up_3hrlymonthly_199207.ascii has been written Variable clr_sfc_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 352.662 353.419 353.419 353.918 353.918 lat band # 46 357.021 357.159 357.382 357.504 357.315 lat band # 47 360.889 360.706 360.594 360.344 359.830 lat band # 48 363.368 363.013 362.636 362.082 361.365 lat band # 49 364.830 364.535 364.070 363.410 362.741 lat band # 50 366.513 366.485 366.161 365.628 365.214 lat band # 51 368.656 369.005 369.009 368.834 368.865 file /clr_sfc_up_3hrlymonthly_199207.ascii has been written Variable clr_sfc_down_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 258.042 258.874 258.874 260.043 260.043 lat band # 46 263.289 263.277 263.370 263.514 263.805 lat band # 47 267.126 267.026 266.940 266.856 266.602 lat band # 48 269.874 269.755 269.622 269.350 268.750 lat band # 49 271.615 271.534 271.312 271.068 270.612 lat band # 50 273.018 273.021 272.977 272.773 272.754 lat band # 51 274.213 274.417 274.602 274.642 274.897 file /clr_sfc_down_3hrlymonthly_199207.ascii has been written Variable toa_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 207.939 207.428 207.428 208.292 208.292 lat band # 46 210.736 209.687 209.917 212.011 211.978 lat band # 47 219.061 216.929 217.430 219.570 221.904 lat band # 48 220.403 224.761 220.339 225.644 223.279 lat band # 49 225.634 222.378 229.312 227.827 223.319 lat band # 50 230.467 232.270 233.693 230.246 229.136 lat band # 51 238.861 241.519 238.351 235.327 233.195 file /toa_up_3hrlymonthly_199207.ascii has been written Variable sfc_up_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 353.338 354.177 354.177 354.707 354.707 lat band # 46 357.790 357.919 358.189 358.218 358.092 lat band # 47 361.569 361.487 361.377 361.148 360.546 lat band # 48 364.169 363.791 363.458 362.822 362.110 lat band # 49 365.649 365.372 364.845 364.137 363.510 lat band # 50 367.319 367.295 366.836 366.392 365.935 lat band # 51 369.404 369.711 369.798 369.643 369.665 file /sfc_up_3hrlymonthly_199207.ascii has been written Variable sfc_down_ Hour = 06 lon # = 100 101 102 103 104 lat band # 45 302.121 308.126 308.126 311.486 311.486 lat band # 46 313.699 312.871 316.140 310.312 314.655 lat band # 47 312.000 318.401 318.431 319.685 313.706 lat band # 48 322.697 321.061 323.881 318.206 317.912 lat band # 49 325.817 326.826 322.487 319.106 321.407 lat band # 50 326.433 326.592 317.613 323.310 320.465 lat band # 51 323.849 321.275 326.806 328.222 327.771 file /sfc_down_3hrlymonthly_199207.ascii has been written 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.