Impact of black carbon on daytime valley and slope winds in idealised simulations

Abstract. Thermally-driven valley circulation plays a key role in transporting air pollutants and heat in mountainous regions, yet the influence of absorbing aerosols on these local winds remains poorly understood. This study investigates how black carbon (BC) affects the daytime valley and slope winds using high-resolution idealised WRF-Chem simulations. The study consist of two simulations: one including realistic BC concentrations that interact with meteorological fields through absorption of incoming solar radiation, and a reference case without BC. When comparing the two simulations, absorption by BC leads to warming in the upper boundary layer and cooling in lower levels during daytime, enhancing boundary-layer stability and reducing surface heating. As a result, the up-slope winds that develop near the slope surface are weaker and flow in a shallower layer in the BC simulation. Although BC also weakens the pressure-gradient force between the plain and the valley that drives the up-valley winds, the up-valley winds in the afternoon become stronger than in the reference simulation. Momentum budget analysis for the valley volume shows that weaker up-slope winds reduce the export of along-valley momentum associated with up-valley winds out of the valley atmosphere, allowing stronger up-valley winds to form despite the weaker forcing. Overall, the results show that absorbing aerosols can modify the thermal structure in the valley and the exchange of heat and momentum between the valley atmosphere and surroundings, revealing a pathway through which aerosols can influence the valley and slope wind characteristics.