Field and laboratory experiments on aerosol-induced cooling in the nocturnal boundary layer

Abstract

Heat transfer processes in the nocturnal boundary layer (NBL) influence the surface energy budget and play an important role in many micrometeorological processes, including the formation of inversion layers, radiation-fog and in the control of air-quality near the ground. Under calm and clear-sky conditions, radiation plays an important role in determining the characteristics of the NBL. In this article, we report observations, close to ground, of hypercooling that has a radiative origin, and which leads to anomalous vertical temperature profiles with elevated minima. In addition, a laboratory experimental set-up is developed that is capable of capturing the thermal structure of the NBL, close to ground, under various conditions. Results from the laboratory experiments indicate that the high cooling rates near the ground, observed in the field experiments, arise from a near-surface heterogeneity in the (aerosol-laden) NBL; a feature ignored in radiation models used for atmospheric simulations. Many of these models nevertheless predict preferential near-ground cooling in apparent agreement with our field observations. However, the cooling is spurious, and arises from the use of an incorrect frequency-averaged transmittance in the radiation model. Based on our observations, a non-dimensional number is proposed that characterizes the evolution in the lowest metres of the NBL; in particular, the effect of radiation on the NBL thermal structure. Our results should help in parametrizing NBL transport process, and highlight the need to account for both the effects of aerosols close to the ground and a varying ground emissivity, via appropriate boundary conditions in general circulation and climate models.