Up and down: Bidirectional fluxes of fog droplets at two subtropical mountain forest sites

Abstract

Bidirectional fog droplet fluxes were observed at two mountainous subtropical forests in East Asia. To investigate the various boundary conditions and that cause these bidirectional droplet fluxes, we set up identical eddy covariance systems including a droplet spectrometer (FM100 Fog monitor) at two study sites in the Ailaoshan (SW China) and Xitou (central Taiwan) areas for several weeks in winter 2015/2016 and spring 2017, respectively. Normally, fog droplets are expected to have a downward flux toward the surface for all sizes due to impaction at the vegetation’s surface. However, we also observed temporally positive fluxes, for fog droplets up to a diameter of 10 μm. Two different cases of bidirectional fog droplet fluxes were identified in our experiments: At Xitou, the positive droplet number fluxes for smaller droplets occurred together with a positive sensible heat flux in combination with large critical diameters according to the Köhler theory. At Ailaoshan, similar fluxes occurred yet together with negative sensible heat fluxes in combination with small critical diameters. We analyze the occurrence of these bidirectional droplet number fluxes in conjunction with the boundary layer conditions and the activation state of the droplets with diameters up to 10 μm. Plausible reasons for the identical flux directions at the two sites under very different microphysical fog conditions are presented.