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Colloquium

Department of Atmospheric Science

Thurs., Dec. 1, 3:10 pm, EN6085

Dynamics and Organization of Cloud-top Convection in Winter Cyclones

Dr. Jason Keeler

Department of Earth and Atmospheric Sciences

University of Nebraska -- Lincoln

Abstract

Cloud-top precipitation generating cells (GCs) were first observed in the early 1950s. In the more than 6 decades since their discovery, subsequent research has established their typical dimension, kinematics, and microphysical properties. Recent field observations from the Profiling of Winter Storms (PLOWS) campaign suggest that GCs are ubiquitous in the warm-frontal and comma-head regions of midlatitude winter cyclones. Given this ubiquity and the presence of fall streaks emanating from the GCs that persist either to the surface or until merging into precipitation bands, GCs are likely a critical component of the precipitation process in these cyclones.
This study establishes an understanding of the dynamics and thermodynamics that control GC development and maintenance using very high-resolution idealized Weather Research and Forecasting (WRF) simulations: first under stability and shear conditions representative of the 14-15 February 2010 cyclone, then under modified radiative forcing, and idealized shear and stability profiles. This talk will discuss the development, maintenance, and structure of GCs (or lack thereof) under a wide range of upper-tropospheric conditions in the context of PLOWS observations. Under initially unstable conditions at cloud-top, GCs develop regardless of radiative forcing, but only persist clearly with radiative forcing. Cloud-top destabilization due to longwave cooling leads to development of GCs even under initially neutral and stable conditions, providing a physical explanation for the observed ubiquity of GCs atop winter cyclones. GCs do not develop in initially stable simulations with no radiative forcing. Decreased range in vertical velocity spectra under daytime radiative forcing is consistent with offset of the destabilizing influence of longwave cooling by shortwave heating. The primary role of shear is to modulate the organization of GCs, with closed convective cells favored with weak or no shear and linear streets parallel to the shear vector with shear greater than 2 m s-1 km-1.

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