The interaction between cloud, radiation and convection self-aggregation

Abstract

Convective self-aggregation in the radiative-convective equilibrium (RCE) is a phenomenon of significant interest because of its potential relevance to the organization of tropical storms. RCE is defined as the balanced situation that the radiative cooling in a column of the atmosphere is ideally compensated by convective heating. Self-aggregation spontaneously organizes into one or several isolated clusters although under homogeneous boundary conditions and forcing.Previous studies found that self-aggregation can regulate tropical climate due to dependence on temperature. It is also associated with tropical cyclone and Madden-Julian Oscillation(MJO). The development of self-aggregation involves radiation, surface flux and advective term. The entire development can be quantified by computing the variance of frozen moist static energy (MSE) and its budget. Previous analysis suggested that longwave radiation is necessary for non-rotating aggregation to occur, while shortwave radiation may contribute to self-aggregation but not necessary. It’s still uncertain that the advection is a necessary condition for triggering self-aggregation or contributes only after the non-adiabatic process Self-aggregation is sensitive to cloud microphysics, which interacts with radiation. Some of the previous theories on RCE instability neglected or simplified cloud-radiative interaction in the theoretical models and suggested that water vapor-radiation feedback is responsible for destabilizing a homogeneous RCE state. Here, we compare two microphysics schemes (Thompson and Morrison) and experiment different size thresholds (DCS) for the ice-to-snow autoconversion in one of them (Morrison) to control the strength of the cloud-radiative feedback. We find the RCE simulation using the Thompson scheme can organize a stable self-aggregation, while the one using the Morrison scheme couldn’t. Their difference in the effect size of cloud ice cause radiation changes. When the DCS value in the Morrison scheme increases, self-aggregation is more likely to occur and develops at a faster rate. The key to the formation of stable self-aggregation depends on the opposite influence of radiation and advection on MSE anomaly. High DCS value results in more cloud ice in the upper atmosphere, lower outgoing longwave radiation (OLR) for the same deep convection below, therefore enhancing differential heating across the domain and favoring the growth of self-aggregation. Through the numerical experiments with varying cloud microphysics, we confirm that predictions from previous simplified theories are valid, in that dry patches can always form in an initially homogeneous domain, even when cloud ice is substantially reduced. However, sustained growth of those dry patches is not guaranteed. Developing deep convective circulation can engulf the dry patches, and water vapor feedback is not sufficient for overcoming this barrier. Only when the early-stage cloud-radiative forcing is of enough strength can self-aggregation fully develop. Therefore, cloud-radiative feedback is essential for the eventual realization of convective self-aggregation.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Xiaoming SHI (Supervisor)