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Kimberly K. Comstock, Christopher S. Bretherton, and Sandra E. Yuter


Observations from the East Pacific Investigation of Climate (EPIC) 2001 field campaign are well suited for exploring the relationships among the diurnal cycle, mesoscale (10–100 km) structure, and precipitation in the stratocumulus region in the southeast Pacific. Meteorological time series and observations from a scanning C-band radar, vertically pointing cloud radar, and ceilometer, as well as satellite data, are used to show that drizzle is associated with increased variability in cloud and boundary layer properties compared to nondrizzling periods. The stratocumulus-topped boundary layer is typically well mixed at night, transitioning to less well mixed in the afternoon, with drizzle most frequently occurring in the early morning. Coherent patches of drizzle, or “cells,” can have large areas with radar reflectivities of greater than 5 dBZ of up to about 100 km2. Individual cells have long lifetimes, up to 2 h, and appear to be replenished by moisture in the boundary layer.

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Robert A. Houze Jr., Shuyi S. Chen, David E. Kingsmill, Yolande Serra, and Sandra E. Yuter


Deep convection over the western tropical Pacific warm pool is analyzed in terms of its relation to the atmospheric Kelvin–Rossby wave, which dominates the large-scale flow during the austral summer. The study uses Doppler radar data collected by aircraft and ship radars during different time periods in the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment to characterize the mesoscale circulations of organized convective cloud systems occurring throughout the season. The study focuses on convection in two contrasting phases of the wave: the “westerly onset region” just west of the point within the wave where low-level easterlies change to westerlies, and the “strong westerly region” (or “westerly wind burst”) lying between the large-scale counterrotating gyres of the Kelvin–Rossby wave.

In the westerly onset region the zonal wind component had midlevel easterlies overlying low-level westerlies. In the strong westerly region a deep layer of westerlies extended from the surface up to the upper troposphere, with a maximum of westerly component at about the 850-mb level. The different vertical shear of the zonal wind in these two regions of the wave led to different momentum transport by the mesoscale circulations that develop into very large “super convective systems” (cloud tops colder than −65°C over regions of ∼300 km or more in lateral dimension). The super convective systems developed strong midlevel inflow jets. The direction of the jet was determined by the environmental shear, which in turn was determined by the dynamics of the large-scale wave. In the westerly onset region, the large-scale shear determined that the jet had an easterly component. In the strong westerly region, the jet had a westerly component. In both cases, the inflow intensified within the cloud system as the convective cells of the super convective system filled a broad region with a deep stratiform ice cloud, from which ice particles fell. Evidently, as the particles sublimated and melted, they cooled the air at midlevels in the cloud system. The cooling evidently modified the mesoscale pressure field in the system so as to accelerate the flow of ambient air into the system and to encourage the inflow to subside. In this way, the mesoscale inflow to super convective systems transported easterly momentum downward in the westerly onset region and westerly momentum downward in the strong westerly region, so that the mesoscale momentum feedback of the mesoscale inflow jets were negative in the westerly onset region and positive in the strong westerly region (accelerating the westerly wind burst). These momentum transports by the broad mesoscale midlevel inflow of super convective systems affected broad horizontal regions and were sometimes different in sign from the momentum transports of individual convective-scale cells in the same system.

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