Investigating the Sensitivity of Marine Fog to Physical and Microphysical Processes Using Large-Eddy Simulation

Abstract

Over the past few years large-eddy simulation (LES) has demonstrated success in modelling continental radiation fog, and several recent studies have used LES to investigate the sensitivity of fog formation to physical processes such as turbulent mixing and surface heat and moisture exchange, as well as to the parametrization of microphysical processes such as cloud droplet activation. Here we extend these sensitivity studies to marine fog. There are several important differences in the formation of marine and continental fog, however moisture availability is no longer a decisive factor, and surface temperature changes over a much longer time scale. Here LES is used to examine the sensitivity of simulated marine-fog formation and maintenance to the cloud-droplet number concentration, turbulent mixing, and air–sea temperature difference. The strength of the fog (in terms of liquid water content) is found to be highly sensitive to all three factors. Varying only the cloud-droplet number concentration, even within a range of physically realistic values for marine regions, can mean the difference between fog halving or doubling in liquid water content. The sensitivities demonstrated herein indicate the great need and challenge for constraining these parameters in numerical weather prediction. Similarities and differences to the findings for continental radiation fog are examined, and important considerations for future improvements in marine-fog forecasting are discussed.