1. Introduction

This README file contains information on the structure of the U.S. composite 2 km by 2 km daily total rainfall data sets derived from National Weather Service Radars (NWS) data, instructions for accessing the HDF library, and pertinent scientific references. Users of these data products must make reference, in all publications, to NOWrad(TM), a registered trademark of WSI Corporation.

The Global Hydrology Resource Center (GHRC) generates 2 km by 2 km daily total rainfall products for the Continental United States (CONUS) based upon composites of radar reflectivity data received from WSI. The radar reflectivity data provided by WSI is created from the NWS 5 cm and 10 cm radars operational in the CONUS. (NOTE: The last WSR-57 was decomissioned in November 1997, and since then, WSI has used the WSR-88D exclusively for the radar products). The WSI radar reflectivity data is in image form with a size of 1887 rows by 3661 columns and has a resolution of 2 km by 2 km. This image has been quality controlled by WSI in real-time before it is received at the GHRC.

2. Derivation of U.S. Composite Daily Total Rainfall from NWS Radars

The GHRC receives instantaneous snapshots of the reflectivity data from WSI every 15 minutes. The GHRC converts this reflectivity data into a composite daily rainfall total using the Z-R relation proposed by Woodley, et al. (1975). The daily rainfall totals contain the 24 hour accumulation from 00:00Z to 23:59Z.

The data producer stores the rainfall totals in HDF format as raster images. However, the files do not contain HDF images within them. The sole purpose of using the HDF raster image format is to conserve space. The HDF raster 8 normal image group does not contain a map overlay and the user must derive the geolocation using the navigation information provided in the HDF description buffer.

Images of the daily rainfall totals can be obtained in Graphical Interchange Format (GIF). These images have a resolution of 8 km by 8 km. The GIF files do have a map overlay of the United States. If you are interested in obtaining information or ordering the GIF files, please contact the GHRC User Services Office at (256) 961-7932 or by email at ghrc@eos.nasa.gov.

3. Daily Rainfall Average HDF File Structure

The purpose of the HDF files is to contain pixel only data, a file descriptor and the meta-data. The meta-data contains information regarding the particular radars operating at each site and the operational status of those radars.

Each daily rainfall total file has the following contents:

The daily rainfall totals comprises 13 levels with the following assignment to rainfall accumulations.

4. Latitude/Longitude Values

The navigation for the daily rainfall totals is provided in the HDF description buffer of each file. The following is the navigation information from an HDF description buffer.

Image Navigation (radians)
Projection: Cylindrical Equidistant
Center Longitude: -1.658063e+00
Top Latitude: 9.250243e-01
Difference Longitude: -6.108653e-01
Radians/Line: 3.135320e-04
Radians/Element: 3.337150e-04
Navigation Transformation information:
lat = north_lat - row * lin_res
lon = ctr_lon + lon_diff + column * ele_res
row = (north_lat - lat) / lin_res
column = (lon - ctr_lon - lon_diff) / ele_res

5. Accessing the HDF Library and Tools

See http://hdf.ncsa.uiuc.edu.

6. References

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.

Data Services Catalog, WSI Corporation, Billerica, MA, April, 1995. Federal Meteorological Handbook No. 11, 1992: Doppler Radar Meteorological Observations, Office of the Fed. Coord. for Meteorological Services and Supporting research, Rockville, MD.