Stratocumulus clouds are a major influence on the weather of the California coastal regions especially in the summer months. The clouds develop in the late afternoon and evening hours, and clear the next morning. The morning clearing is usually referred to as the cloud “burnoff”. Although many studies have focused on stratocumulus burn off, few studies have focused on the mechanisms of the morning clearing of the cloud over the coastal land surfaces. This study represents the initial effort to understand the physical processes involved in the morning breakup process using both observations and a mixed layer model.

The observations used in this study were made during the Development and Evolution of Coastal Stratocumulus (DECS) Experiment in the summer of 1999. Rawinsonde and continuous wind profiler/Radio Acoustic Sounding System (RASS) measurements were used to determine the boundary layer top and the evolution of the boundary layer mean thermodynamic properties during the cloud breakup period. The meteorological observations also included a laser ceilometer, which was used to detect the variation of cloud base height, and a meteorological station with ground-based instruments. In addition, we used a 1-dimensional mixed layer model modified for coastal land surfaces to help understand the observed phenomena. The model results revealed the role of surface heating and thus surface heat flux in enhancing entrainment at the cloud top and eventually led to cloud breakup.