On the relationship of coastal tropical rainfall and the large-scale atmosphere

TL;DR

This study investigates how tropical coastal rainfall differs from inland and open-ocean rainfall in relation to large-scale atmospheric conditions, highlighting the need for improved model parameterizations near coasts.

Contribution

It introduces an objective method to identify coastally-influenced rainfall and analyzes its distinct relationship with large-scale atmospheric states compared to other regions.

Findings

Coastal rainfall occurs in more stable, drier atmospheres.

Dependence of rainfall on large-scale conditions is weaker near coasts.

Model representations should account for unique coastal convection characteristics.

Abstract

Rainfall in coastal areas of the tropics is often shaped by the presence of circulations directly associated with the topography, such as land-sea and/or mountain-valley breezes. In many regions the coastally-affected rainfall consitutes more than half of the overall rainfall received. Weather and climate models with parametrized convection produce large errors in rainfall in tropical coastal regions, most commonly underestimating rainfall over land and overestimating it over the ocean. Building on an algorithm to objectively identify rainfall that is associated with land-sea interaction we investigate whether the relationship between rainfall in coastal regions and the large-scale atmosphere differs from that over the open ocean or over inland areas. We combine 3-hourly satellite estimates of rainfall with estimates of the large-scale atmospheric state from reanalyses. We find that when grouped by rainfall intensity, medium-intensity coastal rainfall in the tropics occurs in more stable conditions and drier atmospheres and is associated with less large-scale convergence than its open-ocean and inland counterparts. Overall, the dependence of the amount of rainfall on the large-scale state is significantly weaker when the rainfall is coastally influenced. Our findings indicate that the representation of convection in weather and climate models must cater for the special character of the large-to-small scale relationship near coasts rather than rely on a common set of rules across the globe.