Characteristics of precipitation event life cycles in the tropical western Pacific

Propagating convection contributes substantially to tropical rainfall. There is a two-way interaction between propagating convection and the environment: the large scale circulation controls the characteristics of propagating convection, while at the same time, any changes in the spectrum of the propagating precipitation events imply an upscale feedback to the large scale disturbances. These important interactions warrant continued investigation of the spectrum of tropical convective systems, particularly as current climate models advance and parameterizations continually mature. Our work focuses on the characteristics of convection event life cycles, factors that control event propagation, and aspects of feedback to the environment in the tropical Pacific. The data used include CMORPH precipitation, NCEP reanalysis for environmental wind shear, GHRSST, QuickSCAT high-resolution winds for mesoscale cold pool detection and convergence boundaries, AIRS for environmental thermodynamics, and TRMM for precipitating cloud characteristics.

All precipitation events are observed within a 4-year (2006.4-2010.4) period over the tropical western Pacific. Events are categorized according to their longevity (up to 3 days). In general longer events are associated with heavier rainfall. Events start from relatively warm SST and optimal shear, and terminate at cooler SSTs and decreased shear. The extent to which wind shear and small-scale convergence boundaries and/or cold pool dynamics influence the event propagation direction is investigated. SST/moisture along the path, representing the potential energy, plays a role in the amplification or dissipation of the precipitation events. TRMM Precipitation Radar data shows a clear evolution of the vertical structure of convection as events propagate in space and time. The rainfall event lifecycle starts from shallow raining cumulus cloud clusters, evolves to unorganized deep cloud clusters within 6 hours, then increases in organization and dissipates to stratiform by the end of the lifecycle.

Precipitation system diabatic heating (Q1) and moistening (Q2) terms are calculated from the TRMM Spectral Latent Heating product. The analysis shows that the maximum level for heat release is near 5km, and is stronger for longer events. Two dominant peaks in moistening occur: one within the PBL and the other near the lifted condensation level. The feedbacks of different convective cloud clusters to the large-scale environment are different; shallow convection helps to transport heat and moisture to the mid-troposphere, while deep convection consumes the mid-troposphere energy.

Accurate analysis of the global heat and momentum budgets critically depends on an adequate understanding of propagating convective systems. The results of this study highlight the importance of properly modeling precipitation events in order to achieve realistic global climate model simulations.