Surface deposition of marine fog and its treatment in the Weather Research and Forecasting (WRF) model

Abstract

Abstract. There have been many studies of marine fog, some using Weather Research and Forecasting (WRF) and other models. Several model studies report overpredictions of near-surface liquid water content (Qc), leading to visibility estimates that are too low. This study has found the same. One possible cause of this overestimation could be the treatment of a surface deposition rate of fog droplets at the underlying water surface. Most models, including the Advanced Research Weather Research and Forecasting (WRF-ARW) Model, available from the National Center for Atmospheric Research (NCAR), take account of gravitational settling of cloud droplets throughout the domain and at the surface. However, there should be an additional deposition as turbulence causes fog droplets to collide and coalesce with the water surface. A water surface, or any wet surface, can then be an effective sink for fog water droplets. This process can be parameterized as an additional deposition velocity with a model that could be based on a roughness length for water droplets, z0c, that may be significantly larger than the roughness length for water vapour, z0q. This can be implemented in WRF either as a variant of the Katata scheme for deposition to vegetation or via direct modifications in boundary-layer modules.

Constant flux layers with gravitational settling: links to aerosols, fog and deposition velocities

“…The initial idea behind this analysis was that, in marine fog, cloud droplets can both fall towards the underlying surface through gravitational settling and be diffused towards the surface by turbulence, and on contact they can coalesce with an underlying water surface. Taylor et al (2021) apply these ideas to fog modelling with the WRF (Weather Research and Forecasting) model. During reviews of that work and an earlier version of the current paper, it became clear that some reviewers were reluctant to accept that turbulence could cause fog droplets to collide and coalesce with an underlying surface and even more reluctant to see this as a constant-fluxlayer situation.…”

“…This could be the case with long-range transport of aerosol in air blowing out over a rural area, a lake or the ocean. Another example could be fog droplets formed at the top of a fog layer and deposited at the underlying surface (Taylor et al, 2021).…”

Marine Stratus—A Boundary-Layer Model

“…In collaboration with the Fatima [1] project, my group at York University has been studying and forecasting fog occurrence over the Canadian, coastal N. Atlantic, and over the Yellow Sea near Korea. Our early studies [2,3] with the forecast model, WRF, introduced the idea that the ocean water surface should be a sink for cloud droplets as well as a source for water vapor. In parallel with our WRF modeling, we have developed a separate boundary-layer cloud model.…”