The role of parametrized shallow convection in tropical cloud systems

Abstract

In current storm-resolving models, the parameterization of shallow cumulus convection is based on the mass-flux framework, originally tailored for coarse mesh sizes O(10-50km). Recent finer grids present a unique opportunity to study the coupling between clouds, convection, and the large-scale circulations. This finer resolution also prompts a critical inquiry into the role of shallow convection parameterization (SCP). Within the context of EUREC4A-MIP, we use HARMONIE-AROME with a grid spacing of 2.5 km to test the mesoscale cloud sensitivity to SCP. While cloud patterns are discernible at this resolution, individual shallow cumuli may not be fully resolved. Our investigation reveals that mesoscale properties of tropical shallow cumulus fields and associated circulations exhibit a pronounced dependence on sub-grid parametrization, with differences in cloud cover up to 20%. We simulate the period from January 1st to February 28th 2020, and compare three configurations of HARMONIE-AROME: 1) the control with active SCP, 2) UVmix-off without momentum mixing by shallow convection, 3) SCP-off without any mixing by shallow convection. Instead of an incremental effect, our results show that UVmix-off and SCP-off can produce opposite responses in the cloud field. UVmix-off produces large anvils, less precipitation, and a cooler lower-troposphere. In contrast, SCP-off produces many smaller clouds which precipitate more in a warmer lower-troposphere due to a more unstable environment, and the buildup of CAPE and turbulent kinetic energy. Importantly, our results underscore that the removal of SCP (and to a lesser extent, the removal of UV mixing) strengthens mesoscale circulations and augments their coverage through increased wind variance, predominantly at scales larger than 25 km. Stronger resolved vertical motion in SCP-off produces stronger circulations, whereas the altered wind mixing in UVmix-off primarily affects the coverage of circulations. The ability to look at parameterized tendencies provides further insight into where the convection is strengthening or weakening the winds. This nuanced exploration contributes valuable insights into the intricate dynamics of mesoscale cloud systems under varying shallow convective parameterizations.