Examination of Mechanisms Underlying the Variations of Microphysical Properties in Different Fog Phases from the Perspective of Entrainment

As one of the main processes affecting cloud and fog, the entrainment-mixing process has an important impact on the cloud/fog life cycle, precipitation formation, radiative transfer, aerosol indirect effect evaluation, and so on. This study discussed the entrainment-mixing mechanisms in a radiation fog from the microphysical and dynamical perspectives, which not only improved the theoretical understanding of entrainment-mixing mechanisms but also revealed the development and dissipation of radiation fog from a new perspective. Using the comprehensive field observational data in Nanjing during the 2006 and 2007 winter, entrainment-mixing mechanisms in nine fog cases were analyzed. First, a radiation fog event during December 10–11, 2007, was studied to understand the microphysical relationships and entrainment-mixing mechanisms during different phases in detail. Results showed that the extreme inhomogeneous entrainment-mixing was found in the mature phase, in which the volume-mean radius slightly changed as the number concentration and liquid water content decreased. The homogeneous entrainment-mixing was found in the rapid dissipation phase, in which all microphysical properties decreased simultaneously with positive correlations. Except for microphysical properties, the scale number was calculated as a dynamical measure for entrainment-mixing mechanisms. In the mature (rapid dissipation) phase, the scale number was small (large), indicating that the extreme inhomogeneous (homogeneous) entrainment-mixing was most likely to occur. The microphysical relationships of the other eight fog events were then examined, which indicated that the volume-mean radius had positive correlations with the liquid water content in general, i.e., homogeneous entrainment-mixing dominated. The research results were helpful to the development of parameterization schemes of entrainment-mixing mechanisms and provided a reference for the simulation and prediction of radiation fog.