Satellite Products from SSM/I: Pentad and monthly estimates from SSM/I of rainfall, cloud liquid water, and integrated water vapor have been produced for the period from June 1992 through October 1998. There are a total of 15 files on the tape. File 1 -> README and doc files File 2 -> Pentad files for 1992 File 3 -> Pentad files for 1993 File 4 -> Pentad files for 1994 File 5 -> Pentad files for 1995 File 6 -> Pentad files for 1996 File 7 -> Pentad files for 1997 File 8 -> Pentad files for 1998 File 9 -> Monthly files for 1992 File 10 -> Monthly files for 1993 File 11 -> Monthly files for 1994 File 12 -> Monthly files for 1995 File 13 -> Monthly files for 1996 File 14 -> Monthly files for 1997 File 15 -> Monthly files for 1998 Produced by Wesley Berg at the NOAA/CIRES Climate Diagnostics Center ------------------------------------------------------------------------------- Pentad (5-day) composite files: Five day averages were produced from all of the available satellites (including F10, F11, F13, and F14). The data has been gridded into a 0.5 by 0.5 degree rectangular lat/lon grid from 60S to 60N and 0 to 360E (720 columns by 240 rows). To minimize storage space, the data are stored as unformatted direct access files of 2-byte (short) integer values which have been multiplied by 100. To get the actual estimates, therefore, it is necessary to divide the values by 100 when reading the data. The files were written on a Sun computer so it may be necessary to perform byte swapping for PC's or certain other computer systems. The lat/lon coordinates specified below are for the center of the bins. References for the retrieval algorithms are included below. Both gridded data files *.dat and browse images *.gif are included and are separated into subdirectories by year. The filename convention is given below. rn2 -> rainfall rate in mm/hour (Ferriday algorithm). To convert to mm/day multiply by 24 and divide by 100 (the data scale factor). wvp -> column integrated water vapor in g/cm^2 (Schluessel algorithm) clw -> column integrated cloud liquid water in g/m^2 (Weng algorithm) DATA TYPE: 2-byte integer SCALE: multiply by 0.01 to get data values (no offset) LONGITUDE: 720 values from 0.25 E to 359.75 E incrementing by 0.5 deg LATITUDE: 240 values from 59.75 N to 59.75 S incrementing by 0.5 deg MISSING VALUE: -1 MASK VALUE: -5 (indicates land for the wvp field valid over ocean only) Monthly composite files: Corresponding monthly averages were also produced for the same period. The same algorithms were used and the rainfall rates are also in mm/hr*100. -------------------------------------------------------------------------- Pentad filenames (shown for year 98, pentad 06 -> Jan 26-30, 1998): pentad9806_rn2.dat -> SSM/I rainfall data file pentad9806_clw.dat -> SSM/I cloud liquid water data file pentad9806_wvp.dat -> SSM/I integrated water vapor data file pentad9806_rn2.gif -> SSM/I rainfall image pentad9806_clw.gif -> SSM/I cloud liquid water image pentad9806_wvp.gif -> SSM/I integrated water vapor images Monthly filenames (shown for year 98, month 04 -> April 1998): month9804_rn2.dat -> SSM/I rainfall data file month9804_clw.dat -> SSM/I cloud liquid water data file month9804_wvp.dat -> SSM/I integrated water vapor data file month9804_rn2.gif -> SSM/I rainfall image month9804_clw.gif -> SSM/I cloud liquid water image month9804_wvp.gif -> SSM/I integrated water vapor images *Note: The files pentads.doc and lpentads.doc contain listings of the dates and julian days for each pentad and each month for both normal and leap years. -------------------------------------------------------------------------- Algorithm References: Ferriday global rainfall algorithm: This algorithm is based on a fairly simple combination of the SSM/I channels utilizing all four SSM/I frequencies as well as dual polarization information. Different algorithms are used over ocean and land to provide global coverage. The algorithm was developed by James Ferriday and is discussed in detail in the following paper. Ferriday, J. G. and S. K. Avery, 1994: Passive microwave remote sensing of rainfall with SSM/I: Algorithm development and implementation, J. Appl. Meteor., Vol 33, pp. 1587-1596. * Note: Different algorithms are utilized over land and ocean with the ocean algorithm using both liquid water emission and ice scattering information while the land algorithm uses a simple ice scattering retrieval technique. The emission technique over the ocean is a more direct measure of cloud liquid water or rainfall. Since the land algorithm only detects scattering by ice particles, not only is low level warm rain is not detected, but the relationship between the ice scattering signal and precipitation is difficult to quantify. Looking at the daily images one will note that large systems which contain a significant ice layer will produce a relatively consistant rainfall estimate over land/ocean boundaries, however smaller systems often show significant discontinuities over the land/ocean boundary. Another problem over land is discriminating between cloud ice and snow cover. This is manifest as a persistant rainfall signal over both the Chilean Andes and the Rocky Mountains during northern hemisphere winter. Over oceans a sea ice detection algorithm has been used as well to eliminate a similar problem, although this should only be a problem at higher latitudes. In addition, there are problems over coastal boundaries due to geolocation errors resulting in the use of the ocean algorithm for pixels with partial land coverage. Because of these limitations the user is cautioned when using the land cover rain estimates. They can certainly be useful in a qualitative sense, but quantitative accuracy is suspect. For additional information refer to the Ferriday and Avery 1994 paper or contact Wesley Berg at the address given below. Weng cloud liquid water algorithm: This is an updated version of the operational cloud liquid water path retrieval algorithm. Estimates are provided only over ocean regions because a radiometrically cold background such as the ocean is required to detect the emission signal of cloud liquid water. Weng, F. and N. C. Grody, 1994: Retrieval of cloud liquid water using the special sensor microwave imager, J. Geophys. Res., Vol 99, pp. 25,535. Weng, F., N. C. Grody, R. R. Ferraro, A. Basist, and D. Forsyth, 1996: Cloud liquid water climatology from the special sensor microwave imager, submitted to J. Climate. Schluessel total column integrated water vapor: Estimates of the total column integrated water vapor amount are estimated using information from the 22.235 GHz SSM/I channel, which is centered on a weak water vapor line. Integrated water vapor is the simplest and most accurate product to retrieve from the SSM/I and is therefore very useful for quantitative comparisons with model results or other data sets. Estimates are not available over land due to significant variability in surface emissivity obscurring the water vapor signal. Schluessel, P., and W. J. Emery, 1990: Atmospheric water vapour over oceans from SSM/I measurements, Int. J. of Remote Sensing, Vol 11, pp. 753. Schulz, J., P. Schluessel, and H. Grassl, 1993: Water vapour in the atmospheric boundary layer over oceans from SSM/I measurements, Int. J. Rem. Sens., Vol 14, pp. 2773. Bauer, P. and P. Schluessel, 1993: Rainfall, total water, ice water, and water vapor over sea from polarized microwave simulations and special sensor microwave/imager data, J. Geophys. Res., Vol 98, pp. 20,737. -------------------------------------------------------------------------- Disclaimer: As noted above the quantitative accuracy of these estimates is suspect over land regions, especially over areas with snow or ice. If you have any problems or questions please contact Wesley Berg at the following address. -------------------------------------------------------------------------- Contact information: Dr. Wesley Berg (303) 492-4724 Cooperative Institute for Research (303) 492-2468 (fax) in Environmental Sciences (CIRES) email: wkb@cdc.noaa.gov Campus Box 449 University of Colorado Boulder, CO 80309-0449 WWW: http://www.cdc.noaa.gov/~wkb/