Vertical Flux Divergence During Fog Deposition

According to field measurements carried out during the “Cloudand Aerosol Chemistry Experiment (CACHE)” project, Kowalski and Vong [1999] observed a persistent and significant divergence of the liquid water fluxes during fog events. This result encouraged us to carry out a field experiment in order to address the question whether their findings can be confirmed or not. During a measuring campaign in the Fichtelgebirge mountains in Germany, two almost identical eddy covariance measurement set-ups (Burkard et al. 2001) were running simultaneously for measuring the liquid water flux directly. They were mounted on a meteorological tower at different heights. First results show that in contrast to Kowalski and Vong [1999] a flux convergence (corresponding to a sink of liquid water between the two measuring heights) was observed during fog. Calculations have shown that condensation due to updrafts by the mean vertical wind makes a contribution of 9% to the convergence of the turbulent liquid water fluxes between the two levels. The difference in the size of the flux footprints and in the distance between the points of the maximum surface influence (both calculated with the flux source–area model by Schmid 1994), may partly explain the flux convergence. However, in consideration of the analyses carried out so far, we suggest that the main reason for a flux convergence can be attributed to re-evaporation processes.