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Implementation of a new Microphysical Parameterization for Marine Stratocumulus Clouds in Regional Forecast Models |
Coastal Meteorology Research Program/University of Oklahoma,
Cooperative Institute for Mesoscale Meteorological Studies/University of Oklahoma,
Coastal Meteorology Research Program/University of Oklahoma
A 4-moment bulk microphysical parameterization has been incorporated into the US Navy Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) which should enable the model to better simulate broad regions of stratocumulus clouds with highly variable aerosol concentrations. Cloud and drizzle water number concentrations are predicted in addition to the mixing ratios for both species. Results of idealized experiments for both "clean" and "polluted" environments are compared to those from the CIMMS LES model which have been extensively verified against data obtained during the FIRE-II ASTEX field campaign.
In these experiments COAMPS produces reasonable drizzle rates and a realistic decrease of in-cloud liquid water with time. Trends in boundary layer stability brought about by drizzle evaporation are apparent in vertical profiles of virtual liquid water potential temperature and total liquid water, and boundary layer decoupling is evident in the profile of vertical velocity variance. The two models correspond reasonably well in the evolution of cloud droplet concentration, both characterized by a significant decrease with time.
As a test of the drizzle scheme at mesoscale resolutions, COAMPS was run over an area of the eastern Pacific containing the California coastal region. The model produces a broad region of drizzling stratocumulus which is in agreement with satellite observations. Stratocumulus cloud top height slopes downward toward the coast and is strongly correlated with SST, which varies slowly in time. Significant spatial structure is visible in the instantaneous surface drizzle rates, and local drizzle rate maxima, associated with peaks in liquid water path and surface moisture convergence, range up to 1.75 mm/day. Over the course of the 24 hour simulation, much of the boundary layer CCN is scavenged, indicating a need for accurate formulation in the model of CCN sources.
