A Radiation Fog Model with a Detailed Treatment of the Interaction between Radiative Transfer and Fog Microphysics

Abstract

A one-dimensional radiation fog model is presented which includes a detailed description of the interaction between atmospheric radiative transfer and the microphysical structure of the fog. Aerosol particles and activated cloud droplets are treated using a two-dimensional joint size distribution whereby the activation process of aerosols is explicitly modeled. For this purpose a new positive definite semi-Lagrangian advection scheme is developed that produces only small numerical diffusion and is numerically very efficient. For the radiative calculations, time dependent attenuation parameters are determined from the actual particle size distributions. The diffusional growth of the particles is calculated by considering radiative effects in the droplet growth equation. Numerical results elucidate that the strong interaction between the radiatively induced droplet growth and their gravitational settling is responsible for a distinct reduction of the liquid water content which also shows quasi-periodic oscillations with periods of 15?20 min. Application of the model to a measured fog episode demonstrates its ability to predict reasonably well the main features of a fog event. This is also true for the oscillations of the liquid water content which are calculated with timescales similar to the observations.