GIST National Radar Precipitation Rate Composite


     1.0  Introduction

          The United States Radar Precipitation Rate Composite dataset produced
     by NASA/Marshall Space Flight Center (MSFC) is one of various image data
     sets provided for the GEWEX Continental-scale International Project (GCIP)
     Integrated systems Test (GIST).  This dataset contains 15-minute composited
     radar derived precipitation rates for the conterminous United States for
     the complete GIST time period (01 April 1994 through 31 August 1994).  The
     GIST domain is approximately 91W to 107W longitude and 31N to 40N latitude.
     These composites were produced by NASA/MSFC using radar reflectivity
     composite data from WSI's National United States NOWRAD product. 


     2.0  Detailed Data Description

          The composite radar data (radar reflectivity, dBZ) is created by WSI
     Inc. using the currently operational network of 10 cm and 5 cm National
     Weather Service radars (WSR-57S, WSR-74C, WSR-88D) in the Continental
     United States. The composite radar data (WSI NOWRAD product) image size is
     1837 rows X 3661 columns and has a pixel resolution of 2 km X 2 km.  The
     real-time manually quality controlled radar data image is a snapshot
     received via satellite link at NASA/MSFC every 15 minutes from WSI.  The
     radar data comprises 16 levels reflectivity, every 5 dBZ, beginning at
     level 1 (0-5 dBZ). Additionally, composite retains information about radar
     sites in the image flagging them as present of absent based on whether a
     site was operational/non-operational during the making of a particular
     composite.  This information is superimposed on the reflectivity levels at 
     each pixel.  In case of signal-washout due to severe weather in any region 
     of the composite, a severe weather warning box is included in the composite
     that encloses such regions of missing echoes.

          Software developed at NASA/MSFC converts each image to rainfall rates
     using the empirical Z-R relation proposed by Woodley et al. (1975).  The
     same Z-R relation is the default equation used by the National Weather
     Service WSR-88D radars to generate a rainfall product (FMH-11, 1992).
     Furthermore, the radar site flags in the composite are used to generate
     site identification (identifying a particular radar site as a WSR-57 or
     WSR-88D). This information along with the operational status of the radar
     sites are recorded in a metafile for each instantaneous composite.  The
     high resolution (2 km X 2km) rainfall product is then resampled to a
     coarser resolution of 8 km X 8 km.  In addition to the instantaneous PR
     products, there is also a daily PR file, that contains the 24 hour
     accumulation from 0000Z to 2359Z.  The daily PR is created by averaging
     the instantatneous high resolution (2 km X 2 km) PR images and then
     resampling the final PR image to a coarser resolution of 8 km X 8 km.  
     Finally, the rainfall rates, metafile and a readme_file  (data
     descriptor) are written to a file using the  Hierarchial Data Format (HDF).

          The MSFC radar precipitation rate imagery were collected every 15-min
     for the period 01 April through 31 August 1994.  MSFC translated these
     imagery to Graphical Interchange Format (GIF) and provided them to UCAR/
     Office of Field Project Support (OFPS).  From this dataset, a
     representative daily image was selected at 1200 UTC (or closest available
     time) to coincide with the National Meteorological Center Eta Model Weather
     Map imagery (1200 UTC).  These GIF images can be viewed using any standard
     GIF display software package.  A color bar legend indicating precipitation
     rate has been included on each image.

     2.1  Detailed Format Description

          General information on GIF files, structure and format are provided
     in the references (Section 4.0).

     2.3  Data Remarks

          All daily images are for 1200 UTC or closest available time.
     Imagery was unavailable for the following dates: 21 May 1994 and 
     22 May 1994.

     3.0  Quality Control Processing

          No additional quality control (QC) or re-navigation was performed
     on the imagery by the UCAR/OFPS.

     4.0  References

     Kay: Graphics File Formats, 2/E.  ISBN: 0-07-034025-0, McGraw Hill.

     Woodley, W.L., A.R. Olsen, A. Herndon and V. Wiggert, 1975: Comparison of
          gage and radar methods of convective rain measurement .  J. Appl.
          Meteor. 14, 909-928.

     Federal Meteorological Handbook No. 11, 1992: Doppler Radar Meteorological
          Observations, Office of the Fed. Coord. for Meteorological Services
          and Supporting research, Rockville, MD.