The impact of raindrop collisional processes on the polarimetric radar variables

The impact of the collisional warm-rain microphysical processes on the polarimetric radar variables is quantified using a coupled microphysics-electromagnetic scattering model. A one-dimensional explicit bin-microphysical rain shaft model that resolves explicitly the evolution of the drop size distribution (DSD) under the influence of collisional coalescence and breakup, drop settling, and aerodynamic breakup is coupled with electromagnetic scattering calculations that simulate vertical profiles of the polarimetric radar variables Z_H, Z_DR, and K_DP. The polarimetric radar fingerprint of each individual microphysical process is quantified as a function of the shape of the initial DSD and for different values of nominal rainfall rate. In addition, a sensitivity analysis is performed using the most common microphysical parameterizations found in the literature.

The resulting simulated vertical profiles of the polarimetric variables are compared with radar and disdrometer observations. These comparisons suggest that the parameterizations of drop breakup most frequently used are overly aggressive for the largest rainfall rates, resulting in very “tropical” DSDs heavily skewed towards smaller drops. Implications for storm-scale modeling are discussed.