Dynamics of Diurnal Variation of Stratus Clouds in Monterey Bay Area

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 sun rise. In the morn-ing, clouds begin to retreat and dissipate. Visible satellite images show that the diurnal variation is particularly significant over the Monterey Bay, 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 proc-esses are involved in regulating the diurnal variation and the cloud properties. Sea breeze may provide moisture to the cloud formation over the land; the diurnal 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 open 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 tro-posphere 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 high resolution simula-tions over the central coast region using NRL COAMPS. The focus of our simulation and analyses is on the cloud diurnal variation in Monterey Bay. The simulation has 4 nested grids with the finest horizontal resolution 1 km. Satellite data, surface observations and RASS profiles are used to validate these simulations. It is found that one main driving force of the diurnal variation in Monterey Bay is the so-called “Santa Cruz Eddy” that is formed due to Santa Cruz Mountains and sea breeze circulation. In the morning, the land surface heating over the mountain produces direct circulation that enhances the down-ward motion, pushing down the boundary layer height below the condensational level and leading to the cloudiness decrease. In the evening, sea breeze significantly strength-ens “Santa Cruz Eddy” to create an intense convergence zone and upward motion in the bay area. This strong upward motion leads to the cloud formation and increased cloudi-ness in the late evening. Consequently, this stratocumulus cloud diurnal variation is fun-damentally driven by the costal dynamics, in the contrast to that over open oceans, which is fully forced by cloud solar absorption. We further evaluate the impact of clouds on the mesoscale circulation including the sea breeze and Santa Cruz Eddy.