AMS Weather and Forecasting : Vieux, B. E. and P. B. Bedient

Estimation of Rainfall for Flood Prediction from WSR-88D Reflectivity: A Case Study, 17-18 October 1994

Baxter E. Vieux, Ph.D., P.E. Member AMS
University of Oklahoma, School of Civil Engineering and Environmental Science
202 West Boyd Street, Norman, OK 73019, USA
Tel. (405)325-3600 Fax (405)325-4217 E-mail bvieux@ou.edu Homepage EM-GIS Lab

Philip B. Bedient, Ph.D.
Rice University, Department of Environmental Science and Engineering
MS 317, 6100 Main Street, Houston, TX 77005-1892, USA

An extreme storm event occurred in south Texas over a period of 4 days, 15-19 October 1994. Selected reflectivity derived from the KHGX WSR-88D radar for an intense period is presented below as a series of images. The WSR-88D reflectivity shown is solely from the lowest tilt at 0.5 degrees. Using a Geographic Information System (GIS), maps of reflectivity greater than 20 dBZ are superimposed over terrestrial features showing county boundaries, radar location, and rain gauges. Each image is named according to the volume scan number 100 through 116. The corresponding collection times of Archive Level II reflectivity data in GMT were taken from tape N2961 for each volume scan. For example the reflectivity contained in the file ARCHIVE2.100 10/18/94 08:07:19.8 is shown below as 100.gif.

Selected Reflectivity Images Derived from the KHGX WSR-88D Radar

Volume scans from 08:07:19.8 GMT to 09:42:10.1 GMT on October 18, 1994

Click a thumbnail to see the image
100 at 08:07:19.8 GMT	101 at 08:13:16.4 GMT	102 at 08:19:11.9 GMT
103 at 08:25:07.4 GMT	104 at 08:31:02.6 GMT	105 at 08:36:58.7 GMT
106 at 08:42:54.5 GMT	107 at 08:48:49.9 GMT	108 at 08:54:46.0 GMT
109 at 09:00:41.6 GMT	110 at 09:06:37.3 GMT	111 at 09:12:33.6 GMT
112 at 09:18:29.5 GMT	113 at 09:24:24.6 GMT	114 at 09:30:19.5 GMT
115 at 09:36:15.0 GMT	116 at 09:42:10.1 GMT	Animation

Superimpositon of reflectivity over the hydrography

Click above image for the larger image.

Rainfall estimates for flood forecasting were obtained by processing the reflectivity using the tropical Z-R relationship and then superimposing each volume scan over the Clear Creek watershed. These rainfall estimates were then used in the hydrologic model HEC-1 to produce stream flow rates. Reflectivity and derivative rainfall maps were resampled from polar coordinates in the Archive Level II data into a 1 km resolution rectilinear coordinate system (UTM). The superposition of this reflectivity over the hydrography is shown left together with county boundaries, rain gauges used in this analysis, and the KHGX radar location.

October 17, 1994

October 18, 1994

Click above image for the larger image. Particularly intense periods are shown in the hyetographs on the left. The rainfall intensity derived by averaging the rainfall rates in each volume scan is shown as bars, whereas, accumulated rainfall is shown in red.

GIS as used in this analysis permits the detailed analysis and prediction of flooding over a river basin by combining radar estimates of rainfall and the watershed area in a common georeferenced coordinate system for the purposes of radar to raingauge calibration and flood prediction.

Special thanks to Jean E. Vieux for final preparation of figures and GIS manipulation using ArcView, ESRI, Redlands, CA, and to Yumiko Yokoyama for preparation of the HTML document for the CDROM and Webpage.