Tropical precipitation in relation to the large-scale circulation

Abstract

This study uses Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) observations from 1998--2000 to characterize the convective and stratiform components of the tropical precipitation field. National Centers for Environmental Prediction (NCEP) reanalysis fields of relative humidity, temperature, and zonal wind are jointly analyzed with the TRMM PR observations to highlight the large-scale environmental characteristics of stratiform rain (i.e., rain associated with deeper cloud systems and in which precipitation processes occur above the 0°C level). This study also models the steady-state atmospheric response to the four-dimensional latent heating field implied by the TRMM PR tropics-wide convective and stratiform precipitation climatology.The TRMM PR provides an illuminating view of the ensemble of precipitating clouds across the tropics; shallow convective rain elements dominate the outer fringes of the tropical rain area but give way to deeper, more organized convective systems and associated stratiform areas toward heavy rain regions. The PR shows geographical and temporal variations in the proportion of rain that is stratiform, most notably a trans-Pacific gradient with low proportions of stratiform rain over the maritime continent and high proportions over the eastern-central Pacific. This gradient becomes even more pronounced during El Nino. Variations in stratiform rain production may be attributed to the environment's ability to sustain moderate convection and differences in zonal wind shear and momentum transport. Horizontal variations in the vertical heating profile implied by the PR stratiform rain fraction pattern lead to variations in the height and vertical extent of circulation anomalies not present when the model is forced with latent heating derived from a geographically-uniform stratiform rain fraction. Accurate representation of convective and stratiform precipitation (as detected by the TRMM PR) is essential in modeling a realistic large-scale circulation response to precipitating cloud systems in the tropics.