Surface deposition of marine fog and its treatment in the WRF model 1 2

Surface deposition of marine fog and its treatment in the WRF model 1 2 Peter A. Taylor1, Zheqi Chen1, Li Cheng1, Soudeh Afsharian1, Wensong Weng1, George A. 3 Isaac1,2, Terry W. Bullock3, Yongsheng Chen1 4 5 1 Centre for Research in Earth and Space Science, Lassonde School of Engineering, York University, Toronto, 6 Ontario, M3J 1P3, Canada 7 2 Weather Impacts Consulting Incorporated, 20 Pine Ridge Trail, Barrie, Ontario, L4M 4Y8, Canada 8 3 Met-Ocean & Digital Environment Solutions, 133 Crosbie Road, St. John’s, NL, A1B 4A5, Canada 9 10 Correspondence to: Peter Taylor (pat@yorku.ca) 11 12

location (Burkhard et al, 2002) report significant differences in downward flux at different levels (flux at 22m can be 193 45% less than at 35m), perhaps illustrating the difficulty of making representative measurements close to the canopy 194 top. Evaporation of fog droplets is also cited as a possible cause of these differences. It is perhaps also worth adding 195 that fog water collectors (e.g. Schemenauer and Cereceda, 1991) can enhance the amount of fog water that is 196 removed at ground level and provide an important source of clean water for some isolated communities. a removal 197 efficiency of 20% is estimated for a 2-layer, 12m x 4m polypropylene mesh.

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These and other papers confirm the strong size dependence of deposition velocity and acknowledge wind speed 213 dependence but are often concerned with long term estimates of the deposition of chemical species to the ocean or 214 lake rather than short term events. One way in which wind speed plays a role is via wave breaking and "broken" 215 water surfaces, a concept used in a model proposed by Williams (1982). This proposes that dry deposition of aerosol 216 particles is considerable different between smooth and broken patches of the water surface with a much higher 217 resistance over the smooth areas.

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To briefly summarize we believe that there are observations to support the idea that the underlying land or water 220 surface can be an effective sink for fog droplets, and other, similar sized, aerosol. The deposition velocity will have 221 a dependence on droplet size, especially over water, but there is a lack of reliable data, even over land, to calibrate can be made.

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and provides an excellent summary of the key components needed to model fog formation and its life cycle, 232 including radiation, turbulent diffusion and gravitational settling. They note that " liquid water (as well as water 233 vapour) is also lost to the ground by turbulent diffusion and gravitational settling of droplets." and their lower 234 boundary conditions include w = 0 for z = 0 and t > 0, where w is their liquid water mixing ratio. Brown and Roach 235 assert that "Kh , Kq , Kw , exchange coefficients for heat, water vapour and liquid water respectively" are assumed 236 equal in their model. In adiabatic conditions they state K= kzu* but avoid discussion of roughness length.

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Extrapolating their w vs log z profiles to w = 0 would indicate a z0c value, for liquid water, of slightly less than 10 ˗2

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It is interesting to note that the removal of Qc at the lower boundary has minimal impact on the predicted 468 temperature and water vapour, Qv profiles (Fig. 3). It could however be important when fog starts to evaporate if the 469 air temperature rises. Note that in generating these results we have not included radiation (short wave or long wave) 470 effects in order to focus on the impacts of turbulent deposition at the water surface. Radiation can play a significant 471 role once fog has formed, and in particular long wave radiational cooling at the fog top (Yang and Gao, 2020) can