The role of coastline-induced precipitation in the tropics

The representation of tropical rainfall in weather and climate models remains a challenge. Areas that are particularly poorly represented are the tropical coastal regions. Over the Maritime Continent some climate models produce too much rain over the ocean areas and too little over land, others show the opposite problem. One factor that contributes to difficulties in the representation of precipitation over tropical coastal regions is the diurnal-cycle of precipitation, which is often associated with land-sea-breeze and orographic circulations. Influenced by a variety of conditions the land-sea-breeze in the presence of orography is a particularly challenging phenomenon.

This study develops and applies an algorithm to objectively identify land-sea-breeze-initiated precipitation. The algorithm relies on the use of three-hourly precipitation estimates from satellites. Pattern-recognition techniques are applied to the data to determine the occurrence and intensity of coastline-affected rainfall features. The effects of changing parameters in the algorithm are investigated. The algorithm is validated and evaluated with high-resolution radar data.

Having identified precipitation affected by coastlines, its climatological features and diurnal behaviur are studied. We find that a significant percentage of rainfall in the Maritime Continent results from coastline effects. We also find that major modes of climate variability, such as the Madden-Julia-Oscillation and the El Niño/Southern Oscillation have a strong effect on the existence of coastal precipitation features. We find that suppressed conditions of the MJO favor coastline-induced precipitation over the Maritime Continent to the extent that differences in rainfall between active and suppressed MJO conditions over the islands are small. This has potential implications for the dynamics of the MJO as it traverses the Maritime Continent Region.

In support of potentially improving parametrizations of coastal convection in GCMs, we also study the large-scale conditions that are associated with coastline-induced rainfall. In particular, we identify the conditions that give rise to or suppress the development of sea breezes and precipitating convection associated with them.