NEXRAD Rainfall Validation, Flash-Flood Forecasting and Modeling

Precipitation, characterized by high spatial and temporal variation, is one of critical inputs for hydrological modeling. It is also an important factor influencing agriculture, water resources and ecosystems. Accurate measurements of precipitation are very important for all rainfall-related applications. Traditionally, rain gauges physically measure rainfall accumulation at a point (~ 324 cm²) and generally provide good quality data for a small area. These point measurements of precipitation have been used in all kinds of hydrological models (Jayakrishnan et al., 2004). Problems with rainfall measurements by gauge were documented in several studies (e.g., Legates and DeLiberty, 1993; Finnerty et al., 1997).

Weather radar measurements of precipitation, which provide precipitation data with much higher spatial resolution compared to rain gauges, have served meteorology for over 40 years and hydrology for around 20 years (Krajewski and Smith, 2002). The installation of the Next Generation Weather Radar (NEXRAD) system across the United States in the early to mid 1990s by National Weather Service (NWS), which includes more than 160 WSR-88D (Weather Surveillance Radar-1988 Doppler) radars, has revolutionized the NWS forecast and warning programs through improved detection of severe wind, rainfall, hail, and tornadoes (Fulton, 2002). NEXRAD has two series of products: Levels 2 and 3, and Stages II, III, and IV (Fulton, 2002; Jayakrishnan et al., 2004; Xie et al., 2005, 2006). The Multisensor Precipitation Estimator (MPE), developed by the NWS Office of Hydrology in March 2000, is a product that merges rainfall measurements from rain gauges, and rainfall estimates from NEXRAD and Geostationary Operational Environmental Satellite (GOES) products. The NWS West Gulf River Forecast Center (WGRFC) switched from Stage III to MPE as the preferred precipitation estimation program in October 2003, and ended Stage III in December, 2004. Thus, since January 1, 2005, only MPE has been produced and distributed by the WGRFC (Greg Story, personal communication, April 2005).

In spite of better spatial representation of rainfall variability by radar compared with rain gauge networks, there are limitations of radar estimates due to data contamination and uncertainty issues (Smith et al., 1996; Legates, 2000; Xie et al., 2006; Wang et al. 2008). In particular, radar rainfall overestimation is caused by the presence of hail, large raindrops, or melting; underestimation occurs due to small raindrops, dry ice, attenuation, truncation error, and beam blockage. Minimizing these errors has been one of the major tasks in radar meteorology for decades (Schmid & Wuest, 2005).

Objectives

The goals of the LRSG group for the rainfall and modeling study, supported by NASA, NOAA, and USGS, are to:

1. Validate the Stage III or MPE products and document their possible errors
2. Study the spatial-temporal variation of rainfall in one radar cell using high dense rain gauge network
3. Use NEXRAD rainfall and MODIS bio-physical states to predict runoff for ungauge basins, and use NEXRAD rainfall as a forcing for hydrological and ecological modeling for the Texas coastal region
4. Perform flash-flooding forecasting for the Central Texas region, the most flash-flood prone region in North America