1. Introduction

This README file contains information on the structure of the U.S. composite 2 km 15 minute instantaneous reflectivity data set 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 composite 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 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 reflectivity data is restricted to educational or non-commercial research purposes and can only be distributed after 48 hours of data acquisition. Researchers and educators receiving the 15 minute instantaneous reflectivity data are prohibited from redistributing this data for any purpose or use.

2. U.S. Composite Reflectivity Data from NWS Radar Data

The GHRC receives national composite images from WSI every 15 minutes and these images are instantaneous snapshots from the combined NWS radar data. In order to save file space, MSFC reformats and stores the reflectivity data in HDF format as run length encoded raster 8 images. These HDF reflectivity files have a resolution of 2 km by 2 km. The sole purpose of using the HDF raster image format is as a space-saving technique. 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. Reduced resolution browse images (8 km by 8 km) associated with the reflectivity products can be obtained in Graphical Interchange Format (GIF). 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.

The reflectivity products were generated at the GHRC on a Silicon Graphics ONYX class computer with version 3.3, release 3 of the HDF library. These files are transferred to tape using the UNIX tar utility.

3. File Structure

The 15 minute 2 km reflectivity products are created using the HDF file format. The purpose of the HDF files are to contain pixel only data, a file descriptor and the metadata. The metadata contains information regarding the particular radars operating at each site and the operational status of those radars.

3.1 HDF File Structure

Each 15 minute reflectivity data file has the following information. This information can be generated by an hdfls command.

3.2 HDF Description Buffer

The HDF description buffer is a summary provided by WSI on the radars available for the composite. The buffer also contains a statistical breakdown of how many pixels are associated with each of the 16 reference levels. The following is a sample of a HDF description buffer from a 15 minute reflectivity data file. Please note that the status for the NOWrad radar sites is only a partial summary in order to conserve space in this file.

Image Header: =NA   12:45 NOWRADHD 
Image Dimensions: Height(rows): 1837 Width(cols): 3661
Image Label: NOWrad Master Sector12:45 15-Oct-95
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

Image Statistics:
Total # of pixels in image: 6725257
Level   # Pixels    Reflectivity(dbZ):
   0     6597155      0 <= <  5
   1       13362      5 <= < 10
   2       18492     10 <= < 15
   3       19370     15 <= < 20
   4       23703     20 <= < 25
   5       30974     25 <= < 30
   6       16200     30 <= < 35
   7        4809     35 <= < 40
   8        1025     40 <= < 45
   9         147     45 <= < 50
  10          20     50 <= < 55
  11           0     55 <= < 60
  12           0     60 <= < 65
  13           0     65 <= < 70
  14           0     70 <= < 75
  15           0     75 <= 

Radar location and indeterminate data box intensity levels:
16-31 Present site indicator over modulus(level, 16)
32-47 Absent site indicator over modulus(level, 16)
48-63 Indeterminate data box border over modulus(level, 16)
64    Indeterminate data box inside over level 0

Pixels covered by Warning boxes: 0

Radar Sites Status:

             **************************************************
             *             NOWRAD NETWORK STATUS              *
             **************************************************

                     Sun Oct 15 12:45:00 1995 (GMT)

status for ALL NOWrad radar sites

*****************************************************************************
| ABERDEEN, SD       | abr | nexrad  |   not included   |     no echoes  =     |
| ALBANY, NY         | enx | nexrad  |     included     |  echoes in area=     |
| ALBUQUERQUE, NM    | abx | nexrad  |   not included   |     no echoes  =     |
| ALLIANCE, NE       | aia | network |     offline      |  status unknown=     |
| ALPENA, MI         | apn |  local  |     included     |  possible echoes =   |
| AMARILLO, TX       | ama | nexrad  |   not included   |     no echoes  =     |
| APALACHICOLA, FL   | aqq | network |   no data rcvd   |  status unknown=     |
| ATLANTA, GA        | ffc | nexrad  |   not included   |     no echoes  =     |
| AUSTIN/SAN ANT., TX| ewx | nexrad  |   not included   |     no echoes  =     |
| BILLINGS, MT       | bil |  local  |   not included   |     no echoes  =     |
| BINGHAMTON, NY     | bgm | nexrad  |     included     |  echoes in area=     |
| BIRMINGHAM, AL     | bmx | nexrad  |     included     |  echoes in area=     |
| BISMARCK, ND       | bis | nexrad  |   not included   |     no echoes  =     |
| BOISE, ID          | cbx | nexrad  |   not included   |     no echoes  =     |
| TAUNTON(BOSTON), MA| box | nexrad  |     included     |  echoes in area=     |
| BROWNSVILLE, TX    | bro | nexrad  |   not included   |     no echoes  =     |
| BUFFALO, NY        | buf | network |   no data rcvd   |  echoes in area=     |
| BURLINGTON, VT     | btv |  local  |     included     |  echoes in area=     |

Tools such as hdf, available from NCSA, may be used to read this descriptive information.

3.2 Latitude/Longitude Values

The navigation for the RIS8 reflectivity image is provided in the HDF description buffer for each 15 minute reflectivity file. The following is the navigation information from a 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

4. Accessing the HDF Library and Tools

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

5. References

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: