Shaping the Tropical Anvil Cloudiness: the relative roles of net convective detrainment and vapor deposition in controlling the tropical high cloud fraction in an extratropically-warmed climate

Abstract

Improving the estimates of global climate sensitivity relies on understanding the mechanisms that control the fractional coverage of tropical anvil clouds. Even small changes in the tropical anvil cloud coverage have been shown to significantly impact the radiative budget of the Earth. Most general circulation models and cloud resolving models depict a decrease in the tropical anvil cloud cover with surface warming. According to the "stability-iris" hypothesis, this reduction is thermodynamically controlled by the changes in the upper-tropospheric static stability, which in turn is governed by the peak of the radiatively-driven clear-sky convergence. However, the influence of the changes in the atmospheric dynamics independent of the local SST changes remains relatively less explored due to the difficulty in segregating the dynamical influence from the local thermodynamic influence on the tropical anvil cloud cover. Using idealized general circulation model simulations from the Met Office Unified Model, our study aims to understand the dynamical impact on the fractional cloudiness of tropical high clouds with global warming. To achieve this, we propose a novel method to separate the dynamical effects from the local thermodynamical effects by warming the extratropics and keeping the tropical sea surface temperatures unchanged. We thereby focus on the mechanisms underpinning the changes in the tropical high clouds resulting from changes in the atmospheric dynamics induced by extratropical warming. We find that the depositional growth of ice cloud condensates has relatively greater significance than the net convective detrainment of condensates in controlling the reduction of the fractional cloudiness over a considerable altitude range of the upper troposphere in the deep tropics.