Coastal Convection in the Tropics

Weather and climate models produce large rainfall errors in tropical coastal regions where the rain is often shaped by land-sea interaction like sea-breeze circulation systems. However, it has been difficult to quantify the contribution of these meso-scale interactions to the overall local rainfall. A technique to objectively identify precipitation associated with land-sea interaction is developed and the results suggest that about 30% of the global rainfall near coasts is associated with coastal processes. Investigating the influence of the Madden-Julian-Oscillation and the El Niño Southern Oscillation on coastal rainfall suggests that during large-scale suppressed conditions coastal effects tend to enhance the rainfall in coastal areas. This result raises the question whether the relationship between coastal rain and the large-scale atmosphere differs from that over the ocean or inland areas. By combining the dataset of coastal rainfall with observations of the atmosphere it is shown that medium-intensity coastal rainfall occurs in more stable and drier atmospheres than its open-ocean or inland counterparts. Land-sea interactions that act on scales not resolved by a climate model are hypothesized to be crucial for this behaviour. This raises the question of how to parametrize convection that is influenced by this sub-grid scale land-sea interaction. This question is investigated by developing a simple cloud model that includes land-sea effects and testing its ability to capture the main characteristics of coastal convection. The model consists of a trigger function that determines the presence and strength of land-sea interactions and a stochastic model that determines the area coverage of deep convection. When coastal effects are present well known spatio temporal organization of clouds near coasts can be captured by the model. The ideas encapsulated in the model have potential to help to improve the simulation of coastal tropical convection in weather and climate models.