High ice water content in tropical mesoscale convective systems (a conceptual model)

Abstract. The phenomenon of high ice water content (HIWC) occurs in mesoscale convective systems (MCS) when a large number of small ice particles with typical sizes of a few hundred micrometers, concentrations of the order of 10²–10³ L^-1 and IWC exceeding 1 g m^-3 are found at high altitudes. HIWC regions in MCSs may extend vertically up to 10 km above the melting layer and horizontally up to hundreds of kilometers, filling large volumes of the convective systems. HIWC has great geophysical significance due to its effect on precipitation formation, the hydrological cycle, and the radiative properties of MCSs. It is also recognized as a hazard for commercial aviation operations since it can result in engine power loss and in the malfunctioning of aircraft data probes. This study summarizes observational and numerical simulation efforts leading to the development of a conceptual model for the production of HIWC in tropical MCSs based on the data collected during the HAIC-HIWC campaign in French Guiana in 2015. It is hypothesized that secondary ice production (SIP) in the vicinity of the melting layer plays a key role in the formation and sustainability of HIWC. In-situ observations suggest that the major SIP mechanism in the vicinity of the melting layer is related to the fragmentation of freezing drops (FFD). Both in-situ data and numerical simulations suggest that the recirculation of drops through the melting layer could lead to the amplification of SIP. However, laboratory measurements remain insufficient to support the accurate model representation of FFD conclusively. The proposed conceptual model and simulation results motivate further efforts to extend reproducible laboratory measurements.