The Parameterization of the Gravitational Water Flux in Fog Models

Abstract

The parameterization of the mean terminal fallspeed of the fog droplets ? has been analyzed in terms of the ratio between the liquid water content and the droplet concentration, W/N, as linked to the gravitational water flux G via G = ?W. Computations have been carried out on the basis of 239 experimental droplet size distributions selected from the literature and grouped according to the fog type. The results show that the (?, W/N) correlation is poor; the values of the correlation coefficients are 0.46, 0.44, 0.48 and 0.56 for valley, advection and radiation fog, and for all the fog types together, respectively. On the other hand, G turns out to be virtually determined by the contribution of the fog droplets (G2) only, while the contribution to G of the haze droplets (G1) is negligible. This result leads us to consider the parameterization of G2 only through a new (?, W/N) relationship, where ?, W and N are due only to droplets grown on activated condensation nuclei; this new relationship is shown, as a whole, to be slightly better than the first one and, in particular, to be reliable only for radiation fogs (the correlation coefficients are 0.60, 0.64, 0.92 and 0.71 for the above reported fog groups).