The Influence of Microphysics and Convection over Land on TTL Cirrus in the DYAMOND Simulations

Abstract:

Pervasive, optically thin cirrus clouds in the tropical tropopause layer (TTL), the 14-18 km transition layer between the troposphere and stratosphere, strongly influence stratospheric water vapor and have a significant local radiative warming effect. TTL cirrus are closely tied to the moisture and ice injected by deep convection, which influences their formation. Previous modeling studies of TTL cirrus using global climate models were limited by their coarse resolution and the biases introduced by parameterized convection, particularly over land where convection penetrates the TTL more often. To address these issues, we use five global storm-resolving models (GSRMs) with sub-5km horizontal grid spacing and explicit convection from the DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) intercomparison: FV3, ICON, SAM, GEOS, and NICAM. We also compare these models to ERA5 reanalysis data, CERES satellite observations, DARDAR radar-lidar retrievals, and TRMM precipitation measurements. To focus on convection over land, this study analyzes a 10°x10° latitude-longitude box over the Sahel in tropical western Africa.

There are large differences in the model microphysics, especially in the composition of the TTL. Convection, including the diurnal cycle of precipitation, is generally well-represented, although some biases remain in individual models. Despite its relative infrequency in the models, deep convection is primarily responsible for mass injection into the TTL with most mass coming from ice. Large variations in vertical velocity dominate the differences in mass transport between models. These results stress the importance of accurately representing deep convection to study TTL cirrus. GSRMs are a promising tool for this purpose, but further refinements in model microphysics and dynamics are needed to more accurately simulate the formation of TTL cirrus and ultimately advance our knowledge of their influence on the climate.