Secondary ice production in tropical deep convection: Insights from high-resolution simulations with the Unified Model

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 W m−2 (1.3 %) and increase outgoing shortwave radiation (OSR) by ∼ 4.5 W m−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 (