Effects of drop size distribution on NEXRAD rain rate estimation

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Sunday, 17 January 2010
Exhibit Hall B2 (GWCC)
Nazario D. Ramirez-Beltran, Univ. of Puerto Rico, Mayaguez, Puerto Rico, PR; and E. J. Suarez-Vazquez and S. Cruz-Pol

Handout (328.2 kB)

The purpose of this research effort is to develop and analyze the droplet size distribution (DSD) for different types of rainfall events, and to measure how the DSD affects the rain-rate radar estimation. The cloud DSD plays a paramount role in the parameterization of cloud microphysics in climate models and plays a crucial role in determining the cloud radiative properties, which is usually obtained by radar and satellite retrieval algorithms. The optical disdrometer is used to collect in-situ samples of droplet size, which will be used to develop the parameters of the DSD. A preliminary analysis was conducted throughout of the modified Gamma distribution which is used to represent the cloud DSD. The analysis of the DSD is conducted through the derivation of the distribution moments and their physical interpretation. The analysis includes the discussion of the mean, variance, effective radius, and effective variance of the DSD. It was noted that the effective radius is the ratio of expected volume of water falling in an expected area and the average radius if the arithmetic mean of the droplets and the effective radius is larger than the average of radius. Rain gauges were located in the same place of the disdrometer with the purpose of measuring the accumulated rainfall during 5 minutes. NEXRAD rain rate will be aggregated to compare with the accumulated rainfall and, therefore, the DSD effects on radar rain rate will be estimated. It can be noted the small DSD is associated with small error in rain rate estimation, whereas larger DSD is usually associated with larger error in rain rate estimation. Thus, it is expected to develop an algorithm to improve the rain rate radar measurements based on the knowledge of the DSD.