Evolution of nocturnal temperature inversions: a numerical study

Abstract

A series of numerical simulations using a one-dimensional energy balance model suggest that both the depth and the intensity of the nocturnal temperature inversion depend on surface emissivity eg and a ground cooling rate parameter β (which in the model is a surrogate for the inverse square root of the soil thermal diffusivity), especially under calm conditions. It is found that, after a transient that may last a few hours after nominal sunset, both depth and intensity follow the classical parabolic growth law, but only under calm conditions. If the ground cools faster the transient for the inversion depth is longer and the inversion deeper. If the surface is radiatively darker, the transient is again longer but the inversion depth is lower. The temperature at the top of the inversion is not strongly influenced by g or β, but, depending on whether the reference is taken at the surface or at screen height, the intensity of the inversion decreases (or increases) with a drop in g; it also increases with increase in ground cooling rate but with either choice of reference temperature. With wind, the inversion may be deeper during the transient than under calm conditions, but eventually becomes both shallower and weaker, and may disappear altogether at high winds. The effect of wind is found to be negligible when the friction velocity is less than 0.2 ms⁻¹. Comparison with observations shows general qualitative agreement, but also suggests that the highly variable results reported in the literature on inversion parameters may be due to site-dependent surface characteristics whose effects, till now ignored, need explicit attention in future field observations and models.