Those familiar with the weather off the California coast can attest to the variability of cloudiness. Sometimes a deck of strato-cumulus clouds may persist for hundreds of miles for hours or days. At other times such a layer of clouds may form and break up within a relatively short period of time. Cloud-top entrainment instability is the mechanism whereby a stratocumulus deck breaks up into scattered cumulus clouds. It is generally accepted that the onset of the process depends on the temperature and mixing ratio discontinuity across the inversion. That is, whenever the equivalent potential temperature difference is smaller than a critical value, the inversion is unstable and entrainment can occur freely. However, recent evidence suggests that despite this instability the cloud deck may remain solid. This requires that other mechanisms sustain the entrainment instability; some examples include turbulence within the cloud layer, coupling between the cloud and subcloud layers, and shear induced entrainment. Under this grant, Dr. David Rogers will study these physical mechanisms using a mesoscale model of the planetary boundary layer over the ocean. The results should increase our understanding of the evolution of marine stratocumulus.
