The Relation Between Humidity and Liquid Water Content in Fog: An Experimental Approach

Gonser S, Klemm O, Griessbaum F, Chang S-C, Chu H-S, Hsia Y-J

Abstract

Microphysical measurements of orographic fog were performed above a montane cloud forest in north-eastern Taiwan (Chilan mountain site). The measured parameters include droplet size distribution (DSD), absolute humidity (AH),relative humidity (RH), air temperature, wind speed and direction, visibility, and solar short wave radiation. The scope of this work was to study the short term variations of DSD, temperature, and RH, with a temporal resolution of 3 Hz. Theresults show that orographic fog is virtually randomly composed of various air volumes that are intrinsically rather homogeneous, but exhibit clear differences between each other with respect to their size, RH, LWC and DSD. Three general types of air volumes have been identified via the recorded DSD. A statistical analysis of the characteristics of these volumes yielded large variabilities in persistence, RH, and LWC. Further, the data revealed an inverse relation between RH and LWC. In principle, this finding can be explained by the condensational growth theory for droplets containing soluble or insoluble material. Droplets with greater diameters can exist at lower ambient RH than smaller ones. However, condensational growth alone is not capable to explain the large observed differences in DSD and RH because the respective growth speeds are too slow to explain the observed phenomena. Other mechanisms play key roles as well. Possible processes leading to the large observed differences in RH and DSD include turbulence induced collision and coalescence, and heterogeneous mixing. More analyses including fog droplet chemistry and dynamic microphysical modeling are required to further study these processes. To our knowledge, this is the firstexperimental field observation of the anti-correlation between RH and LWC in fog.