Understanding the contrasting environments for localised and widespread heavy rainfall in the tropics

Localised and widespread heavy rainfall cause different impacts and are increasingly recognised to be associated with different atmospheric regimes. Here we develop a simple model of a convective ensemble based on a spectrum of entraining plumes. The model relates both rainfall intensity and mean rainfall over the ensemble to the large-scale thermodynamic environment. According to this spectral plume model, widespread heavy rainfall occurs when the free-troposphere is relatively moist, whereas localised heavy rainfall occurs under a drier and more unstable troposphere. These results are confirmed using 20 years of weather radar observations and reanalysis data near Darwin, NT, Australia. Further, using the connection between the convective environment and the entrainment distribution within the plume spectrum model, a bulk estimate of convective entrainment is diagnosed from the observational data. This estimate suggests that entrainment decreases with increasing boundary layer depth, but is largely independent of the size or organisation of convective rain cells. The analysis framework developed here is a useful tool for both diagnosing processes and evaluating high-resolution models.