Raindrop sorting induced by vertical drafts in convective clouds

Abstract

Evidence of raindrop sorting induced by a convective updraft is presented. Using a vertically pointing 94-GHz Doppler radar (⊣ = 3.2 mm) and capitalizing on the resonant nature of the backscattering cross-section as a function of the raindrop size (Mie scattering), the vertical air motions to an accuracy of 0.1 ms−1, and the shape of the raindrop size distribution are retrieved from the Doppler spectra. The interaction of vertical drafts and raindrops is documented for the first time by high resolution radar data. The updraft structure clearly causes horizontal and vertical sorting of the raindrops. In the updraft core, small raindrops (D ≤ 1.7 mm) that have terminal velocities less than the updraft velocities (6-7 ms−1) and a clear absence of drops ≥ 3 mm are observed. Towards the updraft periphery, a gradual increase in the raindrop sizes is documented where large raindrops (D ≥ 3 mm) are observed. The observations demonstrate the importance of updrafts in distributing the raindrops in space.