Convective and Stratiform Components of the Precipitation-Water Vapor Relationship

Abstract:

The empirical relationship between tropical oceanic precipitation in a grid and the moisture content in the column atop the grid is well established. There exists a critical value of column moisture below which the mean precipitation is negligible, and above which it rises rapidly or “picks-up”. We re-examine this relationship with a closer look at its convective and stratiform aspects, using data from the DYNAMO field campaign, Tropical Rainfall Measuring Mission (TRMM) and Modern-Era Retrospective Analysis for Research and Application (MERRA). On daily and hourly time scales, and across all tropical ocean basins, we find that the pick-up is pronounced for stratiform rainfall, while convective rainfall, in contrast only displays a weak pick-up above column moisture. The non-linearity of the precipitation-column moisture curves and the differences between convective and stratiform curves relax at the monthly timescale. We conclude that the environmental moisture content is a stronger constraint on stratiform than convective rain. We also speculate that mesoscale dynamics are responsible for producing the strong non-linearity of the stratiform precipitation curve. These findings suggest that to accurate capture sub-grid scale convection in Global Climate Models (GCMs), we must make strides towards parameterizing mesoscale convective systems (MCSs).