Influence of secondary ice formation on tropical deep convective clouds simulated by the Unified Model
Year: 2025
Type: article
Abstract: . Secondary ice production (SIP) plays an important role in tropical deep convection, yet its representation in models remains uncertain. This study incorporates multiple SIP mechanisms, including droplet fragmentation (Mode 1 and Mode 2) 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-hour analysis period and a 110 km × 110 km domain. SIP modifies precipitation structure, enhancing local rainfall near the convective core while reducing domain-averaged precipitation by ~8 %. Peak rainfall rates remain only slightly affected, consistent with the minor changes (<1 m s−1) in maximum updraft velocity. Among the tested mechanisms, ice–ice collisional breakup shows negligible impact under warm, graupel-sparse tropical conditions. 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.
Language: English
Subfield: Atmospheric Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $1,725
OpenAlex ID: W4417485583