Bidirectional Turbulent Fluxes of Fog at a Subtropical Montane Cloud Forest Covering a Wide Size Range of Droplets

Abstract

AbstractSize-resolved turbulent fluxes of fog droplets are investigated above a subtropical montane cloud forest in Taiwan. By integrating an aerosol spectrometer into an eddy-covariance set-up, we measure droplet number fluxes and liquid water fluxes in a size range of aerosol particles and droplets with diameters ranging from 0.25 $${\upmu }\!\mathrm{m}$$ μ m to 17.3 $${\upmu }\!\mathrm{m}$$ μ m . We find two flux-direction changes within this size range: a downward flux occurs for accumulation-mode aerosols of diameters between 0.25 $${\upmu }\!\mathrm{m}$$ μ m and 0.83 $${\upmu }\!\mathrm{m}$$ μ m , an upward flux occurs for hydrated aerosols with diameters between 1.1 $${\upmu }\!\mathrm{m}$$ μ m and 2.4 $${\upmu }\!\mathrm{m}$$ μ m , and a downward flux occurs again for activated fog droplets between diameters of 3 $${\upmu }\!\mathrm{m}$$ μ m and 17.3 $${\upmu }\!\mathrm{m}$$ μ m . The droplet size distributions can be modelled by a trimodal log-normal distribution, and the modes correlate with the different flux directions. The formation of the three modes and the establishment of the respective flux directions can be explained by combining the Köhler theory on the basis of measured ion concentrations in fog with the turbulent transport of droplets. Finally, from the combined analysis of droplet fluxes and size distributions, we infer relevant processes of droplet development and dissolving during various phases of the life cycles of the fog events.