1. Introduction

This README file contains information on the structure of the U.S. composite 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 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 which are currently operational in the CONUS (i.e. WSR-57S, WSR-74C, WSR-88D). The WSI radar reflectivity data is in image form with a size of 1837 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 MSFC.

The distribution of the 15 minute data is restricted to educational or non-commercial research purposes and can only be distributed after 48 hours of data acquisition. Researcher receiving the 15 minute instantaneous rainfall rate data are prohibited from redistributing this data for any purpose or user.

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

The GHRC receives images from WSI every 15 minutes and these images are instantaneous snapshots from the combined NWS radar data. The GHRC converts these images into composite rainfall rates for the CONUS using the empirical Z-R relation proposed by Woodley, et al. (1975). In order to reduce the size of the files, the resolution of the WSI images is degraded to 8 km by 8 km. In order to save additional file space, the data producer stores the rainfall rates in HDF format as raster images. However, the files do not contain HDF images within them. The HDF raster 8 image does not contain a map overlay and the user must derive the geolocation using the navigation information provided in the HDF description buffer.

Two rainfall rate products are generated at GHRC from the WSI images. The first product is the instantaneous rainfall rates over the CONUS and the second is the daily rainfall totals over the CONUS. The daily rainfall totals contain the 24 hour accumulation from 00:00Z to 23:59Z.

3. Rainfall Rate File Structure

NOTE: A change has been incorporated into the HDF products in the levels that comprise the rainfall rate raster images. All data has been shifted down a level to start at level zero (0) rather than at level one. This change affects all products made after January 1, 1995 and will be incorporated through reprocessing of the 1994 Rainfall HDF product archive. Please contact GHRC User Services if further information is desired at (256) 961-7932.

For both the instantaneous and daily rainfall products, a file is created using the HDF. The purpose of the HDF files are 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.

3.1 Instantaneous Rainfall Rate HDF File Structure

Each instantaneous rainfall rate file has the following contents:

The RIS8 rainfall rate image comprises 16 levels with the following assignment to rain rates:

If for any reason, the annotations describing the non-operational radar sites as well as the co-located WSR-57 and WSR-88D radar sites are absent in the HDF file, then the next item after the file descriptor item will say "Meta-file absent in this Image".

3.2 Daily Rainfall HDF File Structure

Each daily rainfall file has the following contents:

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

3.3 Latitude/Longitude Values

The navigation for the rainfall rate products is as follows:

The precipitation array has a horizontal (East-West) dimension of 915 and a vertical (North-South) dimension of 459. The latitude bounds are from 20N to 53N in the North-South direction and from 130W to 60W in the East-West direction. The latitude and longitude can be computed for the center of each pixel in the following manner:

N latitude = 53. - ((row# - 1/2) x (0.07189542484))
W longitude = 130. - ((col# - 1/2) x (0.07650273224))

For example:

The (row#,col#) of the upper left hand (northwestern corner) pixel is (1,1). Therefore, the latitude and longitude of the center of pixel (1,1) is:

N latitude = 53. - ((1 - 0.5) x (0.07189542484)) = 52.96405228759
W longitude = 130. - ((1 - 0.5) x (0.07650273224)) = 129.9617486339

The (row#,col#) of the lower right hand (southeastern corner) pixel is (459,915). Therefore, the latitude and longitude of the center of pixel (459,915) is:

N latitude = 53. - ((459 - 0.5) x (0.07189542484)) = 20.03594771242
W longitude = 130. - ((915 - 0.5) x (0.07650273224)) = 60.03825136612

4. Accessing the HDF Library and Tools

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

5. 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.

You can contact user and data services by phone or email using the number or address below:

Global Hydrology Resource Center
320 Sparkman Drive
Huntsville, AL 35805

Phone: 256-961-7932
FAX: 256-961-7723
E-mail: ghrc@eos.nasa.gov