Observations and Simulations of High Ice Water Content Clouds: What Causes Large Concentrations of Small Ice Crystals

Abstract:

High Ice Water Content (HIWC) clouds have been frequently observed over oceanic convective systems in regions with reflectivity less than 20 dBZ, suggesting the clouds are composed mainly of smaller ice crystals. Such clouds have been recognized as an aviation hazard since the 1990s. To investigate causes of HIWC clouds, two phases of the High Altitude Ice Crystals (HAIC)-HIWC international field program were conducted out of Darwin Australia 2014 and French Guiana in 2015. In this presentation, in-situ cloud probe and airborne radar data acquired by instruments on board the French Falcon 20 and Canadian National Research Council (NRC) Convair-580 are used to examine how the ice water content, total number concentration and median mass diameter vary with vertical air velocity and environmental conditions, such as the age of the convective system, distance from the peak of convection, and location of the convective system (e.g., coastal, oceanic or continental, Australia or French Guiana). Further, the representation of the measured size distributions as unimodal or bimodal gamma functions is explored using a unique methodology that determines volumes of equally realizable solutions of the gamma fit parameters in the phase space of said parameters. Comparison of the observations against a Weather Research and Forecasting (WRF) simulation of a convective system observed on 26 May 2015 off the coast of French Guiana is made. Although a simulation with the Morrison microphysical scheme generally reproduces the vertical thermodynamic and dynamic environment, the intensity and spatial extent of the reflectivity above the melting layer is overestimated. The simulations also miss the peak of the observed ice size distributions for 0.1 < D < 1 mm. Implications for the processes leading to the formation of small ice crystals in HIWC regions is discussed.