32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Thursday, 7 August 2003: 11:30 AM
Drizzle-induced mesoscale variability of boundary layer clouds in a regional forecast model
David B. Mechem, CIMMS/Univ. of Oklahoma, Norman, OK; and Y. L. Kogan
Poster PDF (565.6 kB)
The U. S. Navy Coupled Ocean/Atmosphere Mesoscale Prediction System has been applied to a case study of coastal California summer season stratocumulus, with the results analyzed in the context of understanding derived from large eddy simulation. The mesoscale model produces a pronounced diurnal cycle and decent agreement with satellite-derived measurements of liquid water path. When the effects of drizzle are included, an organized mesoscale pattern in the form of cloud bands emerges, along with the transition from a well mixed cloud topped boundary layer to a potentially unstable regime accompanied by ensembles of shallow cumuli. The transition and the emergence of mesoscale variability in this simulation appear to be the regional model analogue to the drizzle-induced cloud breakup in large eddy simulation studies. The mesoscale model's subgrid parameterization dictates the dynamics of the well mixed stratocumulus mode, while the convective regime exhibits the resolved vertical velocities characteristic of an ensemble of updrafts. A weak drizzle-induced decoupling of the cloud and subcloud layers causes moisture to pool at the surface, which then leads to the resolved convective updrafts. Some similarities to the propagation of deep convection are also noted.

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