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.
Highlights
This study was initiated when it was found that predicting fog in areas offshore from Atlantic Canada using the National Center for Atmospheric Research and University Corporation for Atmospheric Research (NCAR/UCAR) Weather Research and Forecasting model (WRF-ARW) was generally satisfactory in terms of fog occurrence but gave high values of cloud water mixing ratio, leading to visibilities that were too low compared to observations
Gravitational settling is included within the Thompson microphysics module and, within the MYNN boundary layer module, Eq (4) is used to compute deposition velocities associated with turbulent diffusion with Vd = u∗k/ ln((z1 + z0c)/z0c), where z1 is the first Qc model level above the surface
Original WRF runs with just gravitational settling show seriously limited visibility (< 100 m) on some occasions when METAR visibility was closer to 1 km, while, with added turbulent Qc deposition and a range of z0c values, the optical range was a better match to the observations. These are sample cases and a more extensive comparison is planned. It has been known for many years that fog water can be deposited on vegetation, and this has been incorporated into some boundary layer fog models
Summary
This study was initiated when it was found that predicting fog in areas offshore from Atlantic Canada using the National Center for Atmospheric Research and University Corporation for Atmospheric Research (NCAR/UCAR) Weather Research and Forecasting model (WRF-ARW) was generally satisfactory in terms of fog occurrence but gave high values of cloud water mixing ratio, leading to visibilities that were too low compared to observations. Fog could be caused by mixing two slightly subsaturated air parcels and causing saturation due to curvature of the saturated mixing ratio versus temperature line, most fog formation is initialized by cooling the lower parts of a column of moist, but unsaturated, air. This can arise because of longwave radiative heat loss from the underlying surface (radiation fog), vertical displacement of the air column as it travels over sloping terrain or horizontal advection over a cooler surface. Our focus is on the advection fog situation over ocean waters, a frequent occurrence over areas such as the Grand Banks and offshore areas of eastern Canada as the wind blows moist air from over the Gulf Stream towards the Labrador Current (Taylor 1917; Isaac et al, 2020)
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