Influence of Cloud-Radiative Processes on Predecessor Rain Events

A series of idealized numerical simulations is conducted to examine the effects of cloud-radiative processes on the formation of predecessor rain events (PREs). PREs are coherent mesoscale rainstorms that occur well in advance of recurving tropical cyclones (TCs) and have a high potential to cause flooding and adverse societal impacts. Simulations are performed using an aquaplanet version of the Weather Research and Forecasting v.3.6 model with a straight jet. This study finds that cloud-radiative forcing (CRF), the interaction of hydrometeors with longwave (LW) and shortwave (SW) radiation, produces a more robust PRE storm structure with stronger convective activity and, ultimately, more precipitation. It is hypothesized that LW cooling associated with low-level clouds outside of the PRE region induces a horizontal pressure gradient which enhances convergence near the surface and drives more vigorous ascent. In contrast, in-cloud warming in the middle troposphere generally serves to increase the static stability within the PRE structure. Therefore, the primary radiation driver of PRE formation occurs outside of the PRE itself and is based on how the model responds radiatively to low clouds. Because the recipe for PRE development requires many individual ingredients to come together in just the right way, accurately predicting heavy rainfall events associated with tropical cyclones remains a daunting forecast challenge.