Assessing Gauge Adjusted Radar Rainfall Estimation for Use in Local Flash Flood Prediction

The Urban Drainage and Flood Control District (UDFCD) operates a Flash Flood Prediction (FFP) Program, to forecast rainfall events up to 24 hours in advance within the Denver Metropolitan Area (Figure 1). Program meteorologists from the private sector communicate directly with local governments to prepare for and assess flash flood threats. Spatially comprehensive rainfall estimates that are both timely and accurate are critical in this mission. As part of the FFP program, UDFCD maintains an ALERT gauging network, for real-time monitoring of rainfall. Weather Decision Technologies (WDT) Inc. provides UDFCD with rainfall estimates in real-time that are automatically derived from NEXRAD Level II mosaicked radar data. The WDT rainfall estimates are derived on a CONUS scale, 'clipped' and then adjusted using the ALERT rain gauge data (Figure 2). The WDT rainfall data are provided in Geographic Information System (GIS) format on a high-resolution 1 km grid. Traditionally, meteorologists for UDFCD have used, coarser resolution, NWS single site radar data in conjunction with their point ALERT data to estimate basin rainfall and communicate the potential flash flood threat with local agencies. During the 2007 program, WDT integrated the ALERT rain gauge data into their high resolution gridded radar rainfall mosaics, updated every 15 minutes. WDT provided both the radar-only Quantitative Precipitation Estimates (QPE) using multiple radars and the gauge-adjusted QPE products (QPE_GC) at 1 km resolution in real-time. Significant rainfall events were archived over the 2007 season. Observations of 24-hour rainfall from the Community Collaborative Rain Hail and Snow (CoCoRaHS) database provided a dense network of independent rain gauge data to perform statistical assessment of gauge adjusted QPE products on a daily basis. The National Weather Service 24-hour precipitation analysis is used as a baseline comparison to measure the performance of the WDT QPE against. This paper will discuss the gauge-adjusted radar QPE techniques, the integration of QPE products in local flash flood prediction processes and verification of the rainfall estimates for cases from the 2007 season.