A Large-Eddy Simulation of Fog in Stable Boundary Layer

Although stratocumulus clouds have been a focus of many field experiments and modeling study projects, fog has attracted significantly less attention, possibly due to its seemingly similarity to the stratocumulus clouds or due to the complexity of its various interactive physical processes. Fog in a stable boundary layer presents a particular challenge to the modeling community because of the small scale nature of the turbulence and the complexity in the microphysics process. During the intensive observation period of Coupled Boundary Layers /Air-Sea Transfer in Low Winds field experiment in the east coastal region in Summer of 2003, fog has been frequently observed. Specifically, fog was observed in the stable marine boundary layer on August 22, 2004 . Our objective is to use COAMPS-LES to simulate this case of fog and understand the interaction among various physical processes at different scales. These processes include mesoscale horizontal advection, boundary layer turbulence mixing, cloud microphysics and the radiation. We pay particular attention to the moistening mechanism and the process that maintains the stable marine boundary layer. We hypothesize that horizontal moist advection is necessary to balance the negative surface moisture flux due to the saturated stable layer. Furthermore, the stable boundary layer is maintained by horizontal warm advection which balances part of radiative cooling at the top of fog. The COAMPS-LES is configured to cover 1 km x 1 km x 0.5 km domain with 6.4 meters in all three directions. The initial and large-scale conditions are derived from CBLAST-Low observations and COAMPS real time forecast. The preliminary results show the importance of low sea surface temperature in the fog formation. In addition, without significant warm air advection, the simulated fog layer is rapidly transformed into a well-mixed stratus cloud layer which was not observed. Currently, we are performing various sensitivity tests to evaluate the individual role of different physical mechanisms in the fog evolution.