The Response of Tropical Rainfall to Idealized Small-Scale Thermal and Mechanical Forcing

Predicting the spatiotemporal distribution of rainfall remains a key challenge in Tropical Meteorology, partly due to an incomplete understanding of the effects of different environmental factors on atmospheric convection. In this work, we use numerical simulations of tropical ocean domains to study how rainfall responds to imposed localized thermal and mechanical forcings to the atmosphere. We use the Normalized Gross Moist Stability-NGMS-to quantify the net precipitation response associated with a given net atmospheric heating. We find that NGMS values differ considerably for different forcings, but show that the relationship between precipitation and column relative humidity collapses along a universal curve across all of them. We also show that the contributions from mean vertical advection of moist and dry static energy only approximate the NGMS well at scales larger than a couple hundred kilometers, indicating that general horizontal mixing processes are not negligible at smaller scales. Predicting where rain tends to occur in tropical areas is challenging. In this work, we simulate a small area of atmosphere over a tropical ocean to study how rainfall changes when we alter the surface temperature, the atmospheric heating rate at different heights, and the pressure gradients that drive the winds near the surface. We find that such alterations lead to self-consistent but different relationships between the amount of rainfall produced and the net heating of the atmosphere. We show that the spatial extent of the alteration affects how well this relationship can be inferred from horizontally averaged atmospheric properties. In contrast, we find that the relationship between rainfall and the average relative humidity in the atmosphere remains the same across all types of environmental alterations. Convective rainfall triggered by different thermodynamic and mechanical forcings can exhibit markedly different Gross Moist Stabilities Mechanically induced, low-level convergence is an effective mechanism of net precipitation enhancement All forcing types collapse onto the same curve relating column relative humidity and rainfall