Modeling rainfall drop size distribution moments using an S‐band polarimetric radar in complex terrain

Abstract

Extreme rainfall is fundamentally driven by microphysical processes aloft, which can be strongly influenced by complex terrain. The 2022 Prediction of Rainfall Extremes Campaign in the Pacific (PRECIP 2022) provided a unique opportunity to study these relationships through the deployment of the NSF NCAR S‐band dual‐polarization radar (S‐Pol) in Taiwan, a region characterized by steep topography and frequent extreme precipitation. This study examines a Mei‐Yu front case characterized by widespread, long‐duration rainfall with embedded deep convective cells occurring both upstream and over Taiwan's topography. Our objective is to link microphysical processes to the varying terrain in context of rainfall during this high‐impact event. Microphysical characteristics are inferred from vertical gradients of dual‐polarization radar variables and modeled drop size distribution (DSD) moments, supported by ground‐based disdrometer observations that resolve both the drizzle and precipitation modes. A normalized double‐moment generalized gamma DSD model is applied to S‐Pol to obtain DSD moments that are linked to dominant microphysical processes aloft including radar‐inferred graupel presence. Results show that collision coalescence is the most frequent warm‐rain process in convection, contributing to large drop production and increased rainfall rates. Additionally, graupel presence is associated with precipitation enhancement, with the magnitude of graupel‐related amplification increasing with terrain height. This study highlights the benefit of modeling both the drizzle and precipitation modes of the DSD using dual‐polarization radars to connect microphysical processes aloft to complex terrain.