Microphysical Sensitivity of Marine Boundary Layer Clouds Associated with Coastally Trapped Disturbances: A Case Study of the 22-25 June 2006 Case

Forecasting the life cycle (initiation, maintenance, and dissipation) and spatial and vertical coverage of marine low clouds and fogs in coastal environments remains an outstanding challenge due to the inherently complex relationship between the ocean-land-atmosphere system. This is especially important in the context of climate models due to the profound radiative impact of these clouds. This study utilizes results from Weather Research and Forecasting (WRF) model simulations of a coastally trapped disturbance (CTD) case (22-25 June 2006). The dynamics of the CTD system are accurately represented and are relatively insensitive to the choice of physics parameterizations; however, a suite of sensitivity runs suggests that the treatment of model physics strongly influences the evolution of the associated cloud field. Specifically, characteristics of the stratiform clouds and fogs are highly sensitive to the choice of planetary boundary layer (PBL), microphysics, and radiation parameterizations. For example, differences between vertical mixing and entrainment in the Yonsei University (non-local) and Mellor-Yamada-Janjic (local) PBL schemes strongly affect cloud properties. Also, modifying cloud droplet sedimentation is found to play a role in both cloud and radiation properties. The contributions to drizzle formation from autoconversion and accretion are also explored. Moreover, because previous observational results from satellite retrievals indicate that CTD cloud decks have, on average, higher cloud droplet number concentrations than their non-CTD counterparts, the utility of treating aerosol prognostically in WRF is explored.