Tuesday, 16 January 2007
Sensitivities of stratocumulus organization to precipitation
Exhibit Hall C (Henry B. Gonzalez Convention Center)
In Arakawa's 1975 WMO report, stratocumulus are identified as one of the Canonical Cloud forms whose representation is essential to accurate simulations of the general circulation. In the intervening decades, work by himself, his students and collaborators has greatly advanced our understanding of this critical cloud regime. In this paper, we use large-eddy simulations on large domains (25.5 x 25.5 x 1.5 km^3) to explore the effect of precipitation on stratocumulus structure and dynamics. In particular, we investigate the hypothesis that drizzle can induce a transition in cloud planform from closed cellular circulations, characterized by well mixed boundary layers with significant cloud cover, to open cellular circulations, characterized by decoupled boundary layers and less cloud cover. The simulations are shown to support this hypothesis and are consistent with available measurements. Additional agreement between the observations and our simulations is that stratocumulus can have long-lasting and locally intense drizzle, which is also associated with the increased horizontal variability in the temperature and moisture fields, but reduced horizontal variance of vertical velocity, as well as with pools of elevated equivalent potential temperature.