Cloud-Resolving Modeling of a Forming Cirrus Cloud in the Tropical Transition Layer over the Southwestern Indian Ocean

Abstract:

A yet forming cirrus was reported on 11^th January 2019 in the tropical transition layer (TTL) over the southwestern Indian Ocean, with the use of sondes launched from Reunion Island. Although TTL cirrus are very frequent there, gaps in our understanding of the mechanisms controlling them remain. We analyze the processes leading to the formation of the observed TTL cirrus.

Two nested cloud-resolving simulations accounting for the aerosol-microphysics interaction have been performed with the mesoscale model Meso-NH. Both share the vertical grid, with 100 m resolution in the TTL. A simulation at 2.5 km is run over a large domain (2000 x 2000 km²) covering the southwestern Indian Ocean. The other, at 500 m, covers a region (500 x 300 km²) where oceanic convective clouds were observed during the balloon flight. This simulation is used to investigate the role of convective gravity waves on the cirrus formation and properties.

The simulated clouds, assessed against satellite observations, are realistic. The spatial distribution of the TTL cirrus is controlled by eastward moving cold temperature anomalies, related with convection over Madagascar, and the southerly advection of moisture. Furthermore, heterogeneous nucleation drives the cirrus at the regional scale. Homogeneous freezing becomes important at smaller scales, most likely triggered by rapid cooling induced by gravity waves. We determine the contribution of homogeneous freezing to ice formation in the TTL at the scale of an isolated convective cloud using the 500-m simulation. For this simulation, the vertical wind speeds in the TTL are better resolved (up to 3 m/s) than for the 2.5-km simulation (1 m/s).