Influence of spatial distribution of rain on transformation of open cellular circulation

Although, rain in closed cellular stratocumulus has been shown to be a triggering mechanism for open cellular circulation, aircraft observations often show high rainrates existing within the closed cell state. A few published studies have focused on the spatial distribution of precipitation. In these numerical modeling studies rain is initiated homogeneously within the domain, which favors the formation of open cellular systems. In this study, idealized 3-dimensional simulations with various spatial distributions of rain are conducted to examine conditions favoring the closed-to-open transition. To categorize these transitions we have developed methods of quantifying spatial variability between closed and open cellular systems with spatial statistics. We show that when rain is restricted to small areas, 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 event disappears without significant trace. With increasing areal coverage of the rain, transition becomes possible provided the rainrate is sufficiently large. When the rain is distributed over the entire domain the closed cell state readily transitions to the open cell state. Thus both the areal coverage and magnitude of precipitation are important factors controlling the transition.