Influence of nighttime radiation fog on the development of the daily boundary layer: An large eddy simulation study

Abstract

<p>Apart from hazards associated with deep fog, its presence significantly alters the properties of the nocturnal boundary layer (NBL). <br>The NBL is typically characterized by a stable stratification resulting in weak or sometimes intermittent turbulence. <br>In contrast, the NBL during deep fog is often convective, as for the longwave radiation optical thick fog layer, the net radiative loss takes place at the fog top, destabilizing the atmosphere from above.<br>Therefore, processes as modified longwave cooling, shortwave absorption, turbulent mixing, reduction of the total water content through droplet settling or modified dewfall, is able to induce differences between the stable NBL (SNBL) and foggy NBL. <br>Albeit after sunrise the SNBL is quickly transformed into a convective boundary layer (CBL), properties of the NBL are transferred into the day and affect the CBL. <br>Even though fundamental and applied research have significantly improved fog forecasts and contributed to a broader and deeper understanding at the process-level in the last decades, common numerical weather prediction (NWP) models still miss a significant amount of fog events.<br>A number of complex small-scale processes (such as turbulent mixing, land-atmosphere interactions, aerosol and cloud microphysics and radiation) interacting on different scales have to be correctly resolved or parameterized.<br>Likewise, the prerequisite formation conditions must be presented precisely as they are highly sensitive to slight changes in temperature, humidity or soil moisture, entailing that even small biases in the forcing data could lead to an incorrect representation of subtle supersaturations and might result in failing to predict fog.</p><p>Thus, we will present in this talk results of idealized large eddy simulations pairs (with and without the possibility to form fog) covering the diurnal cycle based on a typical fog event observed in Cabauw considering radiative conditions between February and April. <br>As we performed several parameter studies we will demonstrate, that the CBL in cases without fog is warmer and obtain higher inversion heights than in simulations with fog during night.<br>Further, we show that this temperature deviations are mainly driven by an stronger integrated  longwave cooling during night in the foggy cases.<br>Moreover, we identified the liquid water path as a crucial parameter determining the strength of the fog impact on CBL development. </p>