Abstract
Size-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} to 17.3 {upmu }!mathrm{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} and 0.83 {upmu }!mathrm{m}, an upward flux occurs for hydrated aerosols with diameters between 1.1 {upmu }!mathrm{m} and 2.4 {upmu }!mathrm{m}, and a downward flux occurs again for activated fog droplets between diameters of 3 {upmu }!mathrm{m} and 17.3 {upmu }!mathrm{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.
Highlights
Fog is of great importance, both locally and globally: it is an essential source of water for plants (Ju et al 2012), animals (Parker and Lawrence 2001), and entire ecosystems (Dawson 1998) and has been utilized for human water supply (Klemm et al 2012)
The fog-droplet size distribution was fitted by a model which was linked to the droplet number fluxes (DNFs) (Sect. 3.7)
Bidirectional turbulent fluxes of fog droplets were measured between 27 July 2019 and 19 August 2019 at the top of a 40-m high tower in a subtropical montane cloud forest in Xitou, Taiwan
Summary
Fog is of great importance, both locally and globally: it is an essential source of water for plants (Ju et al 2012), animals (Parker and Lawrence 2001), and entire ecosystems (Dawson 1998) and has been utilized for human water supply (Klemm et al 2012). Fog affects the net radiation on Earth by reflecting shortwave radiation (Klemm et al 2006). In most parts of the world, the occurrence of fog has decreased over the past years to decades, and the causes for this trend are not fully understood (Hanesiak and Wang 2005; Vautard et al 2009; Klemm and Lin 2016). Physical and chemical conditions (temperature, humidity, and the number, size, and chemical composition of condensation nuclei) of potentially foggy airmasses are important drivers of the occurrence of fog (Gultepe et al 2007). Understanding the microphysics of fog in detail is of great importance in atmospheric-boundary-layer research
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