Effect of coastline-driven organization on rain climatology of a GCM with two-plume organized convection scheme

Description

The mesoscale organization of precipitation convection is highly relevant to next-generation global numerical weather prediction models. An organized convection scheme is introduced and tested in the newest Community Atmosphere Model (CAM5.1) with a new unified shallow-deep convection scheme, a two-plume version of the University of Washington (UW scheme). The single variable org represents the roles of sub-grid evaporation and downdraft, sub-grid geography and breezes, shear rolls, deformation filaments, and other unknown stochastic components in the initiation and development of convection. The study is to investigate the effect of breeze-initiated organization on the precipitation climatology.

Sea and land breezes (SLB) play a significant role in the initiation and organization of convection and do make an important contribution to the climatology of precipitation. It is needed to better represent the convective organization over complex land-sea terrains. In this two-plume organized convection scheme, the role of coastline is represented by a constant or semidiurnal variable, called “coast2org”, as one of the org sources. Experiments with and without coastline show some interesting but surprising results. Based on the effect of coastline-driven organization on the future precipitation, there are two different regions that we called normal regions and mystery regions. In normal regions like the African coastline, Indian coastline, eastern South American, and northeastern U.S. coastline, as might be expected, a positive feedback exists: the coastline makes the organization stronger, which favors the occurrence of deep convection and has more precipitation; then the larger precipitation rate generates the convective organization by the evaporation of downdraft (cold pool), and so on. However, there still exist some mystery regions like Indonesian maritime continents and the Gulf of Mexico Coast, etc., where the total precipitation is reduced with coastlines, although their convective organization becomes stronger. A stability study shows that when taking the coast-driven organization into account, the mean states in these mystery regions become significantly more stable and CAPE has a dominant importance to control the convection; the net stabilization effect defeats the evaporation positive effect. Further studies are needed to explore the mechanism for puzzling results.