A statistical model for isolated convective precipitation events

TL;DR

This paper introduces a cell tracking algorithm for convective precipitation, revealing that solitary tracks follow simple linear relations, while merged tracks exhibit complex behaviors, and highlights the role of boundary layer cooling in precipitation dynamics.

Contribution

We develop a novel cell tracking method that analyzes merging and fragmentation in convective clouds, proposing a simplified model for solitary rain events and examining the effects of boundary layer cooling.

Findings

Solitary tracks show linear relations with auxiliary fields.

Merged tracks are more complex and often involve repeated merging.

Precipitation intensity is weakly dependent on initial CAPE, influenced more by boundary layer cooling.

Abstract

To study the diurnal evolution of the convective cloud field, we develop a precipitation cell tracking algorithm which records the merging and fragmentation of convective cells during their life cycles, and apply it on large eddy simulation (LES) data. Conditioning on the area covered by each cell, our algorithm is capable of analyzing an arbitrary number of auxiliary fields, such as the anomalies of temperature and moisture, convective available potential energy (CAPE) and convective inhibition (CIN). For tracks that do not merge or split (termed "solitary"), many of these quantities show generic, often nearly linear relations that hardly depend on the forcing conditions of the simulations, such as surface temperature. This finding allows us to propose a highly idealized model of rain events, where the surface precipitation area is circular and a cell's precipitation intensity falls off linearly with the distance from the respective cell center. The drop-off gradient is nearly independent of track duration and cell size, which allows for a generic description of such solitary tracks, with the only remaining parameter the peak intensity. In contrast to the simple and robust behavior of solitary tracks, tracks that result from merging of two or more cells show a much more complicated behavior. The most intense, long lasting and largest tracks indeed stem from multi-mergers - tracks involved in repeated merging. Another interesting finding is that the precipitation intensity of tracks does not strongly depend on the absolute amount of local initial CAPE, which is only partially consumed by most rain events. Rather, our results speak to boundary layer cooling, induced by rain re-evaporation, as the cause for CAPE reduction, CIN increase and shutdown of precipitation cells.