Ice crystal sizes in high ice water content clouds. Part 2: Statistics of mass diameter percentiles in tropical convection observed during the HAIC/HIWC project.
Résumé
High ice water content (IWC) regions in Mesoscale Convective Systems (MCS) are a potential threat to commercial aviation as they are suspected to cause in-service engine power-loss events and air data probe malfunctions. To investigate this, the High Altitude Ice Crystals (HAIC) / High Ice Water Content (HIWC) projects set up a first field campaign in Darwin (Australia) in 2014. The airborne instrumentation was selected to provide the most accurate measurements of both the bulk total water content (TWC), using a specially developed isokinetic evaporator, and the individual ice crystals properties using particle imaging probes.This study focuses on determining the size ranges of ice crystals responsible for the mass in high IWC regions, defined here as cloud regions with IWC greater than 1.5gm-3. It is shown that for high IWC areas in most of the encountered MCS systems, median mass diameters (MMDs) of ice crystals range from 250 to 500μm and decrease with increasing TWC and decreasing temperature. At the same time, the mass contribution of the smallest crystals (below 100μm) remains generally low (below 15%).In contrast, data from two flight missions in a long-lasting quasi-stationary tropical storm reveal that high IWC values can also be associated with MMDs in the range 400-800 μm and peak values of up to 2mm. Ice crystal images suggest a major growth contribution by vapor deposition (columns, capped columns) even for such larger MMDs values.
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