How important is tropospheric humidity for coastal rainfall in the tropics?

Abstract

Recent research has community have shown that tropical convection and rainfall is sensitive to mid-tropospheric humidity. Therefore it has been suggested to improve the representation of moist convection by making cumulus parameterizations more sensitive to mid-tropospheric moisture. Climate models show considerable rainfall biases in coastal tropical areas, where approx. 33 % of the overall rainfall received is associated with coastal land-sea interaction. 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 atmospheric humidity differs from that over the open ocean or over inland areas. We combine 3-hourly satellite estimates of rainfall with humidity estimates from reanalyses and investigate if coastal rainfall reveals the well known relationship between area-averaged precipitation and column integrated moisture. We find that rainfall that is associated with coastal land-sea effects occurs under much drier mid-tropospheric conditions than that over the ocean and doesn't exhibit a pronounced critical value of humidity. In addition, the dependence of the amount of rainfall on mid-tropospheric moisture 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 near coasts rather than rely on a common set of rules across the globe.