[A-AS03] Large-scale moisture and organized cloud systems

[AAS03-04] The role of imposed large-scale forcing on convective self-aggregation

*Ching-Shu Hung¹, Hiroaki Miura¹ (1.The University of Tokyo)

Keywords:self-aggregation, radiative-convective equilibrium, convective organization

Convective self-aggregation (CSA) is a unique atmospheric phenomenon widely observed in idealized numerical simulations under radiative convective equilibrium (RCE). The initially scattered convection can spontaneously organize into spatially coherent clusters through internal interactions among convection, radiation, moisture, and surface flux. While RCE is known to be a valid approximation for the global atmosphere, the reginal atmosphere is characterized by regimes of vertical motion, which are often shown in close relation with the distribution of cloud activities. To understand the role of large-scale forcing on the convective structures and the development of CSA, a time-invariant upward motion is implemented into the RCE framework in a convection-permitting model (i.e., SCALE-RM). Under large-scale upward motion, cloud activities become active to balance the moistening and adiabatic cooling from large-scale forcing, while the extensive high clouds and abundant water vapor weaken the cooling by radiation. Cloud object analysis further reveals that individual cloud systems become larger and stronger under large-scale upward motion. However, the clear moisture contrast accompanied with CSA does not develop in the forced runs, as the moistening by large-scale upward motion continuously outweighs the drying by radiation-driven subsidence. Sensitivity experiments further find that the regime boundaries between the scattered and aggregated states are very sensitive to the existence of large-scale vertical motion. Even a weak upward motion could change the behavior of CSA significantly.