Drop size distribution and LWC in Po valley fog

In this paper results are presented of ground-based fog microphysical measurements obtained during the CHEMDROP experiment in the Po Valley (Northern Italy) in November 1994. Altogether about 85 hours of drop microphysical data are analyzed. At the beginning of the experiment a comparison of some of the different microphysical instruments, operated during the experiment, was performed. It has revealed some differences between the Liquid Water Content LWC measured by Particle Volume Monitors (PVMs), and by several Forward Scattering Spectrometer Probes (FSSPs). Possible explanations for the discrepancies are discussed. The FSSP derived drop size distributions (number and mass) were parameterized in terms of log-normal distributions. The statistical analysis of the fittings has shown that the overwhelming majority of the drop mass size distributions was characterized by a bimodal shape. The most frequent values of the mode parameters (median diameter, geometric standard deviation) are given in Table 3 of this paper. An investigation of the temporal evolution of the drop size distribution revealed two typical phases of fog formation. In the first step both modes of the drop mass size distribution increase more or less uniform, whereas in the second phase the large drop mode drastically rises. Furthermore, the second phase is characterized by quasi-periodic oscillations in nearly all mode parameters of the drop size distribution with a period between ten and 15 minutes. In the last part of the paper the frequent occurrence of drizzle within the fog was studied by comparing the measurements with respective model calculations.