Role of Spatial Distribution of Rain in Formation of Open Cellular Circulation

Abstract:

Precipitation alone is not sufficient to transform closed cellular circulation capped by stratocumulus cloud to open cellular circulation with cumulus clouds; observations often show high rainrates existing within the closed cell state. It has been suggested that the spatial distribution of precipitation plays an important role in the transition from closed-to-open convection but this hypothesis has to date not been rigorously tested. In this study, a series of idealized 3-dimensional simulations are conducted to evaluate the dependency of areal coverage of rain on the transformation, and to explore the role of interactions between multiple rainy areas in the formation of the open cell state. This is done by inserting a low aerosol concentration patch (or multiple patches separated by specified distance) into a non-precipitating closed cell state. We show that when rain is restricted to a small area, even significant rain does not result in a transition. The rain event is quickly filled in by adjacent non-precipitating closed cells and the rain ceases without significant trace. With increasing areal coverage of the rain, the transition becomes possible provided the rainrate is sufficiently large. When multiple rain regions interact with each other, the transition to the open cell state is reinforced over a wider area, provided the distance between the rain regions is small enough. When the distance between the rain areas is large, rainy areas are initially filled in by neighboring closed cells, but the transition eventually occurs, albeit slowly. These results suggest a connection to the remote control of open cell formation hypothesized in the recent past.