Validation of Operational NESDIS Algorithms against NexRAD Stage IV and Hourly Rain Gauges

Evaluating the quality of any satellite-based rainfall product is useful and required for improving its algorithm. The objective of this study is to develop a statistical approach for evaluating the satellite-based NESDIS rainfall products from Hydro-Estimator (HE) algorithm, GOES Multi-Spectral Rainfall Algorithm (GMSRA), and IR/microwave Blended Algorithm (Blended). Capability of each NESDIS rainfall product is examined with respect to seasonal variability, climate conditions, and storm types over different topographic regions.

Ground-based radar and gauge rainfall observations, at high resolution (hourly), are used for validating NESDIS product. In this study, high resolution NESDIS product (hourly 4 Km x 4 Km) is evaluated regionally in details over several small size study sites with high hourly (daily when hourly is not available) rain-gauge density, for instance: specific 1°x 1° degrees. The size of every study site varies from 0.5°x 0.5° to 2°x 2° degrees, depending on the density and distribution of available hourly rain-gauge stations over the study site and storm size. Study time period is from April 2003 to present, when archived NESDIS rainfall products are available.

Three NESDIS rainfall products have been compared with NEXRAD Stage-IV rainfall, rain gauge observations, and GOES infrared (IR) cloud images for four storms during the winter and summer seasons over a 2°x 2° validation study area located in Hernando County, Florida (28°N-30°N and 81°W-83°W). The first test (I) was conducted for a six-hour storm event that took place on 02/24/2004 from 14:00 to 20:00 UTC, the second test (II) was for a six-hour storm that took place on 03/16/2004, from 00:00 to 06:00 UTC, the third test (III) was for a six-hour storm that took place on 08/22/2003, from 18:00 to 24:00 UTC, and the forth test (IV) was for a six-hour storm that took place on 09/03/2003, from 18:00 to 24:00 UTC. The primary results from tests I and II demonstrated that HE estimates rainfall with lower intensity and also over different locations compared to NEXRAD rainfall during the winter but, from tests III and IV, a remarkable improvement has been noticed in the performance of the algorithm which was also confirmed when compared to the rain gauge observations and the GOES infrared cloud images. Those preliminary results confirmed the fact that Hydro-Estimator (HE) has shown a substantially improved ability to estimate precipitation compared to GMSRA and Blend algorithms.

An hourly, six hourly and daily time series are also plotted with the rainfall of January,February & March 2004 and July,August and September 2004 to gauge the seasonal performance of the algorithms. Its observed that generally the algorithms performed better during the summers in comparison to the winters.