Variability of Graupel and Snow Observed in Tropical Oceanic Convection by Aircraft during TRMM KWAJEX

Abstract

Empirical characterization of graupel and snow in precipitating tropical convective clouds is important for refining satellite precipitation retrieval algorithms and cloud-resolving and radiative transfer models. Microphysics data for this analysis were collected by the University of North Dakota (UND) Citation and the National Aeronautics and Space Agency (NASA) DC-8 aircraft during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) in the western tropical Pacific Ocean. An ice particle identification algorithm was applied to two-dimensional optical array probe data for the purpose of identifying ice particle ensembles dominated by graupel or snow particles. These ensembles were accumulated along 1-km flight segments at temperatures below 0°C. A third category, mixed graupel/snow, has characteristics between those of the predominately graupel and snow ensembles and can be used either in combination with the other two categories or separately. Snow particle ensembles compose 80% of UND Citation and 98% of NASA DC-8 ensemble data. For the UND Citation, graupel ensembles compose ?5% of the total with mixed graupel/snow ensembles composing ?15%. There were no graupel ensembles in the NASA DC-8 data, which were collected primarily at temperatures <?35°C. Particles too small to classify (<150-?m maximum dimension) compose 56% of UND Citation and 64% of NASA DC-8 particle images. Nearly all these ?tiny? particles occur coincident with particles >?150 ?m. Combining data from both aircraft, snow and mixed graupel/snow ensembles were evident over the full range of subfreezing temperatures (from 0° to ?65°C) sampled by the aircraft. In contrast, graupel ensembles were present primarily at temperatures >?10°C. Accurate graupel identification was further supported by all graupel ensembles observed either coincident with or within a 10-km horizontal distance of radar-identified convective precipitation structures.