Precipitation event life cycle in the tropical western Pacific

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Yanping Li (Department of Physics and Oceanic Sciences, School of Physics, Peking University, Beijing, China)
The systematic propagation of the organized convection contributes most to the total amount of rainfall and but is also more vulnerable to the changes of the large scale circulation. On the contrary, the shift of the spectrum of the precipitation events will have an upscale feedback to large scale disturbances. So it is very important to address the observed spectrum of the tropical convection in current climate models. Our work has studied the characteristics of the precipitation event life cycle in the tropical Pacific and what controls event propagation, especially the long-lived events. 
 All the precipitation events are within a 4-year (2006.4-2010.4) period over tropical western Pacific. The propagating/non-propagating precipitation events can be categorized to four major categories. Category one, long-lived westward propagating events, with individual systems persist for 1 to 4 days and propagate 500-2000 km from their origins. This category frequently produces heavy rainfall amounts. Category two, suppress conditions, which are periods of little or no precipitation over highest SST; weak winds, likely because of the large scale descent with the dry phase of Kelvin waves. Category three, quasi-stationary, diurnal, short-lived events, which has a well-defined diurnal cycle, nearly no phase propagation; very common in the northern hemisphere winter, mostly within the warmer SST sub-regions. Category four, less organized, eastward propagating type frequent near EQ in wet season, persist 1-3 days, travel 500-1500 km from origin, likely to be associated with EIG or Kelvin waves. 
 For the long-lived westward propagating events, mainly category one, the detailed analysis shows that it is usually associated with environmental zonal wind, weak westerly at surface and moderate easterly aloft, with critical level presented around 650hPa. Their preferred excitation locations imply a leeward “shore” excitation, usually over westward-directed SST gradient zones or the eastern side of the mesoscale warm patches. The SST gradient generated surface wind convergences with the environmental westerly. The leeward location has its thermodynamical advantage, the excited moist convection has an opportunity to become heavy event. After its onset, the steering easterly wind carry it to travel along the whole warm patch, which helps the convection system to keep gaining potential energy, thus providing the potential for heavier rainfall early in the event lifecycle.
 The results of this study suggest that the systemic consequence of the precipitation events is critical to the performance of the current climate models. The global heat and momentum budgets are critically dependent on adequate representation of the phenomenon.
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