A new method to estimate the characteristic raindrop size from space-borne Doppler radars: validation with the Monte Carlo model

This paper describes a physically-based algorithm for estimating the mass-weighted raindrop diameter ( D m ) and the corresponding Doppler velocities ( V D ) which are the radar reflectivity-weighted terminal velocities of the raindrops. The algorithm exploits the attenuation-corrected radar reflectivity ( Z e ) and the specific attenuation ( k ) observed by Ku- and W-band space-borne Doppler radars and estimate D m assuming a specified drop size distribution DSD. The estimated D m were examined using a Monte Carlo model which evolves DSDs by coalescence and breakup in a convective rain as well as measured data with a disdrometer. A detailed error analysis is also presented in the retrieved D m arising from the measurement errors. While discrepancies between the assumed DSDs (DSD LUT ) and actual DSDs introduce errors in the estimated values, the errors were generally small for small D m values. The uncertainties related to the variability of DSD were reduced by selecting an appropriate DSD LUT using the dependence of Z – R relationship ( R : rain rate) on D m . Since k is the DSD-dependent value in the W-band, the influence of the DSD can be partially mitigated but is limited. Utilizing parameters representing the shape of DSD by k , Z e and V D offer the potential to reduce the DSD-related errors furthermore.