Influence of secondary ice formation on tropical deep convective clouds simulated by the Unified Model

Abstract

Secondary ice production (SIP) plays an important role in tropical deep convection. This study implements multiple SIP mechanisms, including droplet fragmentation and ice-ice collisional breakup, into the CASIM microphysics scheme of the UK Met Office Unified Model, and evaluates their impacts through a real-case simulation of a Hector thunderstorm. SIP enhances ice number concentration in upper cloud layers, with values up to 3 orders of magnitude higher than the no-SIP case, particularly above -10 °C. Ice water content (IWC) increases by a factor of 3-5 in the anvil region, contributing to more extensive upper-level cloud coverage. These microphysical changes reduce outgoing longwave radiation (OLR) by ∼ 3.2 Wm-2 (1.3 %) and increase outgoing shortwave radiation (OSR) by ∼ 4.5 Wm-2 (1.8 %) over a 6 h analysis period and a 110 km × 110 km domain. SIP modifies precipitation spatially, yielding a more localized, compact rainfall pattern near the convective core, while reducing domain-averaged precipitation by ∼ 8 %. Peak rainfall rates remain only slightly affected, consistent with the minor changes (< 1 ms-1) in maximum updraft velocity. Among the tested mechanisms, ice-ice collisional breakup shows negligible impact on simulated ice concentration, consistent with limited graupel-involved collision energetics under warm profiles. Ensemble experiments confirm that these effects are robust and exceed the influence of meteorological variability. These results highlight the importance of representing SIP processes in cloud-resolving models of tropical convection and accounting for their environmental dependence.