Dual-frequency profiler study of hydrometeor fall speeds in tropical deep convection

Abstract. This study investigates hydrometeor fall speeds using a dual-frequency profiling radar operating during the 2005–2006 monsoon season near Darwin, Australia. Our focus is on tropical deep convection where the observations provide a new perspective on hydrometeor fall speeds within and near intense drafts having mixed-phase media. The techniques we employ avoid undue assumptions on the air motion or media distributions, offering a convenient path to estimate bulk radar reflectivity(Z)-weighted hydrometeor fall speed Vt. While these mixed-phase media estimates are not specific to size or density, they may be replicated by models and are practical for radar-based retrievals that necessitate Vt assumptions.

Tests performed under rain and snow conditions show comparable performance to disdrometer and other references. The standard deviation of residuals for rain and snow relationships are ≅ 1 ms^-1 and ≅ 0.4 ms^-1, respectively. In convective core regions aloft, Darwin observations align with existing graupel Vt-Z treatments, however mixed-phase media typically falls faster (> 1–2 ms^-1) for Z < 35 dBZ than prior relationships. Breakdowns suggest that Active and Break monsoon conditions favor a similar Vt-Z behavior in strong cores. However, Break conditions – those more favorable to intense daytime tropical convection – potentially indicate the presence of additional lofted liquid or melting media mixed in volumes at convective core peripheries Z < 35 dBZ. Break events also show higher variability in Vt-Z pairs, with select samples having Vt faster than rain for a given Z that argues for partially-melted graupel coupled with size-sorting.