| Description |
The stratocumulus topped boundary layers that tend to form off the west coast of major continents have been an important topic in cloud physics research due to their large impact on global climate, while uncertainty exists about their formation and behavior. This uncertainty is largely due to the substantial range of scales that govern the characteristics and evolution of these cloud decks. Specifically, small scale processes near cloud top have remained unexplored in the field due to limits in technology that inhibit the resolution necessary for these small scale processes to be investigated and studied. The entrainment interface layer (EIL), a region near cloud top where free tropospheric air is mixed with cloudy boundary layer air, is especially suspected to have an effect on these clouds. Three important processes occur in the EIL: mixing between free tropospheric air and cloudy mixed layer air, radiative heating and cooling, and heating and/or cooling due to phase changes of water, all of which may have an important effect on cloud properties and behavior. The Physics of Stratocumulus Top (POST) field campaign took place in July and August of 2008 off the coast of Monterey, California with the mission of gathering data from the stratocumulus topped boundary layers that regularly form there each summer. The POST mission was unique in that it employed the use of a small aircraft, which allowed for colocation of instrumentation during research flights. This colocated instrumentation, coupled with vertical flight paths specifically designed to sample the region near cloud top of these cloud capped boundary layers, produced a data set especially equipped to probe into the small scale processes that are so pivotal to the behavior of these clouds. Here, high rate data from POST are combined with a conserved variables analysis to define the location and extent of the EIL for five flights during the summer of 2008. Two daytime flights and three evening flights were selected for analysis. Overall, the existence of an EIL is detected and defined for all five of the flights analyzed. In addition, for each case, the EIL is found to be noticeably thicker than what was found in past studies, with a typical extent of several tens of meters. Further use of conserved variables to approximate virtual potential temperature changes within the EIL reveals the effects of radiation and phase changes to be of the same order as each other, on average, for most of the flights. In addition, radiative cooling occurs most frequently during evening flights while solar warming occurs most frequently for daytime flights. Finally, evaporative cooling occurs much more frequently than condensational warming within the EIL for all flights, regardless of whether the flight was during the daytime or during the evening. |
