Liquid Water Budgets of Numerically Simulated Non-Precipating Boundary Layer Clouds

In the past decades, attention have been focused on stratocumulus clouds over open ocean due to their potential impact on the earth climate. Several field experiments in the open ocean have significantly contributed our understanding of these clouds. However, many questions related to coastal stratocumulus clouds remain unanswered, such as the breakup of the clouds and the interaction between the clouds and land-sea circulation etc. This study attempts to understand the evolution of stratocumulus clouds in sea breeze in California coast. Particularly, we will focus on how the stratocumulus clouds break up over land during the day.

Cloud evolution in a downstream PBL air column following the lower branch of the sea breeze circulation essentially reflects the influences of underlying surfaces and other time-dependent forcing like solar radiation and large-scale wind fields. Thus, it is appropriate to use a Lagrangian approach with an eddy-resolving cloud model to study the roles of the land surfaces.

The Lagrangian simulation starts about 50 km off the coast with initial solid marine stratocumulus clouds. Then, integration is carried out to follow onshore flow air mass. The simulated PBL column evolves under the influences of changes from ocean to land surfaces. The simulated stratocumulus clouds starts to break up 30 km after the landing. The following breakup process operates in the simulations. After the landing of the PBL column, surface heat flux significantly increases and moisture flux decreases, resulting in a warmer and dryer PBL column. The strong surface heat flux promotes a considerable increase in turbulent kinetic energy throughout the entire PBL. Then, the entrainment at the cloud considerably intensifies, leading to a massive influx of dry air into the cloud layer. Consequently, the breakup of the stratocumulus clouds results. In this process, the cloud-top entrainment is a direct and leading mechanism to break up the clouds. The surface heat flux provides the trigger for the massive entrainment.

We also performed mesoscale model simulations in the same region and obtain a qualitatively similar results. However, the mesoscale simulations provide significantly more details of the breakup process. It appears that both the Lagrangian and mesoscale simulations can provide much insight into the cloud evolutions.