Diagnosing the Intercept Parameter for Exponential Raindrop Size Distribution Based on Video Disdrometer Observations: Model Development

The exponential distribution N(D) = N-0 exp(-Lambda D) with a fixed intercept parameter N-0 is most commonly used to represent raindrop size distribution (DSD) in rainfall estimation and in single-moment bulk microphysics parameterization schemes. Disdrometer observations show that the intercept parameter is far from constant and systematically depends on the rain type and intensity. In this study, a diagnostic relation of N-0 as a function of rainwater content W is derived based on two-dimensional video disdrometer (2DVD) measurements. The data reveal a clear correlation between N-0 and W in which N-0 increases as W increases. To minimize the effects of sampling error, a relation between two middle moments is used to derive the N-0-W relation. This diagnostic relation has the potential to improve rainfall estimation and bulk microphysics parameterizations. A parameterization scheme for warm rain processes based on the diagnostic N-0 DSD model is formulated and presented. The diagnostic N-0-based parameterization scheme yields less evaporation and accretion for stratiform rain than that using fixed N-0.