12.5
Long-lived drizzle cells in inversion-topped boundary layers
Sandra E. Yuter, North Carolina State University, Raleigh, NC; and A. M. Hall, C. Burleyson, D. Leon, and W. A. Brewer
Drizzle within inversion-topped marine stratocumulus regions such as the Southeast Pacific is an example of precipitation produced by a liquid-phase cloud. Radar observations have shown that drizzle-cells are a common feature of marine stratocumulus. These cells, which tend to become more pronounced and larger as the depth of the boundary layer increases, are characterized by low or mid-level convergence into a higher-reflectivity core paired with divergence aloft. Fallstreaks originating near the top of the cloud layer are tilted inward towards the center of the cells by the shear between the inflow and outflow. The drizzle cells often resemble diminutive cumulus congestus with anvils. Heavier drizzle (> 1 mm/day) in boundary layers 1 km or deeper can yield shallow, evaporatively-cooled outflows at the surface. The role of these outflows in maintaining the drizzle cell, or in triggering new convection elsewhere is unclear. A substantial number of drizzle cells in the Southeast Pacific have a lifetime > 1 hour -- more than twice the time required to rainout the observed liquid water path. The implication of this observation is that drizzle cells are able to recycle a portion of the water substance from the precipitating portions of the cell back into the cell updraft. Evidence based on high vertical resolution cross-sections of drizzle cells obtained by the University of Wyoming Cloud Radar on the NCAR C-130, and the C-band radar and Doppler lidar aboard the NOAA Ship Ronald H. Brown indicates that part of this recycling occurs as virga falling into the converging flow below the cell. This study examines the dynamical and microphysical mechanisms contributing to long drizzle cell lifetimes and how they relate to the convective and mesoscale structures of clouds and drizzle in the Southeast Pacific stratocumulus region.
Session 12, Results from recent testbed investigations and field research programs II
Wednesday, 3 August 2011, 4:00 PM-6:00 PM, Marquis Salon 456
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