Dynamics of Diurnal Variation of Stratus Clouds in Central Coast of California

It is well-known that stratus clouds on the central coast of California undergo diurnal variation, which is most pronounced during summer. In the evening, clouds or fog move inland with onshore flow and reach maximum coverage just before the sun rise. In the morning, clouds begin to retreat and dissipate. Visible satellite images show that the di-urnal variation is particularly significant in the Salinas Valley of the central coast, which is about 40 km wide, 150 km long, and bordered with the mountain ranges about 2 km high. Stratus clouds penetrate into the valley with various cloud thickness and cloud-top heights. Many processes are involved in regulating the diurnal variation and the cloud properties. Sea breeze may provide moisture to the cloud formation over the land; the di-urnal change of land surface temperature is believed to play a role in the cloud “morning burn-off”; cloud solar absorption is considered the key driving force for the cloud diurnal variation over oceans; the local valley circulation has an important impact on the cloud depth; and the synoptic condition associated with the subtropical high pressure and the strong lower troposphere stability provides the fundamental forcing for the long lasting stratus coverage on the central coast.

In order to provide insight into the diurnal variation, we perform a high resolution simu-lation over the central coast region using NRL COAMPS. The focus of our simulation and analyses is on the cloud diurnal variation in the Salinas Valley. The simulation has 5 nested grids with the finest horizontal resolution 333 m. Satellite data, surface observa-tions and RASS profiles are used to validate the simulation. We further evaluate the moisture and heat budgets in the simulation to determine impacts on the clouds from various processes. To sort out the impact of each individual process, we perform single column and LES model simulations forced by the mesoscale forcing diagnosed from the COAMPS simulation. It is found that sea breeze and diurnal variation of land surface temperature are the most important processes in regulating the cloud diurnal variation; while the valley generated local circulation also affects the variation.