Extended Abstract (68K)4.6
Surface fluxes and stratocumulus clouds in DECS: A modeling study
PAPER WITHDRAWN
Shouping Wang, NRL, Monterey, CA; and Q. Wang
It is well known that stratocumulus clouds in California central coast area undergo strong diurnal variation. During late evening and night, stratocumulus clouds develop over the land area with relatively weaker onshore flow. The clouds dissipate after the sunrise,a process usually called "morning burnoff". This phenomenon was frequently observed in Development and Evolution of Coastal Stratocumulus (DECS) project. To understand the evolution, we conducted several COAMPS simulations over the central coast area for a period of the diurnal variation observed during DECS. Three nested grids are used, they are 5km, 15km and 45 km to cover 570´510km2, 1530´1350 km2, 2970´2700 km2 centered at (237.2°E, 36.8°N). The model simulation started at 6 July, 1999 and was run for one week. The COAMPS simulation show that the dissipation of the clouds during the daytime is significantly influenced by the land surface heat flux and is associated with deepening boundary layer. During the daytime, the land surface is heated up by solar radiation, and thus the heat flux is significantly increased. The enhanced heat flux considerably increases buoyancy production of turbulent kinetic energy, which promotes stronger cloud-top entrainment. Consequently, the clouds dissipate with deepening of the boundary layer. This interpretation is also consistent with some overland observations which show a rapid rise of the boundary layer during a clearing cloud episode. These results suggest that the land surface skin temperature and other elements defining the surface heat flux are controlling parameters of the evolution of the stratocumulus clouds. To test the sensitivity of the proposed process to the details of the surface flux parameterization, we repeated the similar simulations with modified skin temperature and surface flux parameterization. The cloud clearing extent sensitively depends on the magnitude of the surface sensible heat flux. Another process contributing to the cloud dissipation is direct cloud solar warming which tends to evaporate cloud droplets. However, the decoupling process resulting from the solar warming contributes little to the dissipation.
Session 4, Atmospheric Processes
Friday, 9 November 2001, 8:30 AM-10:45 AM
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