Abstract:

During  calm and clear-sky nights,  radiation is the principal mode of heat transfer in the atmospheric boundary layer, and it governs the vertical temperature distribution near the ground. Under these conditions temperature measurements in the field, close to the ground, indicate development of an anomalous temperature profile with the minimum temperature occurring a few decimeters above the ground. In recent studies it is demonstrated that such preferential cooling cannot occur in a homogeneous atmosphere. In a set of experiments, we show that the presence of aerosols and their number density determines the intensity of the near surface radiative-cooling and the final value of the minimum temperature observed. Aerosols, through radiation in the window-band, provides an alternate path for the surface air layer to lose heat to the cold upper atmosphere. This cooling produces few-decimeter thick, cold-air layer between warmer ground and upper air layers. Thickness of the cold layer is determined by the length scale associated with the non-linear vertical distribution of aerosols and the diffusion process. We also present filed experiments, to show that the fog formation is closely related to this anomalous temperature profile. Thus we show that the accounting for the aerosols and their radiative process is important in modeling micro-meteorological processes, including the initiation of radiation fog.