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

Thermally-driven valley circulation governs heat, momentum, and pollutant transport in mountains and is affected by the valley topography, large-scale weather, surface properties, and thermal forcing. Aerosols alter the heat distribution in the atmosphere through absorption and scattering of the incoming solar radiation, influencing the boundary layer (BL) development. From studies considering urban BL over flat terrain, it is known that depending on the radiative properties and vertical distribution of the aerosol population, aerosols can either enhance or suppress the buoyancy and mixing in BL, and cause simultaneous cooling and warming at different altitudes within BL. The impact of aerosols on the thermally-driven valley circulation remains poorly understood, a shortcoming addressed by this study.

This study examines how the absorption of incoming solar radiation by black carbon (BC) affects the daytime valley and slope winds in high-resolution idealised simulations using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). The simulations have an idealised valley topography that has a sinusoidal shape in the cross-valley direction and is 100 km long, 20 km wide, and 2 km deep. The study consists of two simulations: one including realistic BC concentrations interacting with the meteorological fields through absorption of shortwave radiation, and a reference simulation without BC. Heat and momentum budgets for the valley volumes are computed to understand the mechanisms behind the differences in the winds between the two simulations.

BC absorption acts to warm the upper BL and cool the lower levels during daytime, enhancing stability and reducing surface heating. Consequently, up-slope winds are weaker and confined to a shallower layer in the BC simulation. In the afternoon the up-valley winds are stronger in the BC simulation, although BC weakens the daytime temperature difference between the valley atmosphere and the BL above the plain. Based on the classic valley wind theory, the stronger temperature difference, hence a stronger pressure-gradient force, should lead to stronger up-valley winds. The average up-valley wind speed in the afternoon is 2.6 m s-1 in the BC simulation and 2.3 m s-1 in the simulation without BC. However, in the evening when the up-valley winds peak in magnitude, the maximum wind speed is stronger in the simulation without BC with a 0.5 m s-1 margin.

Momentum budget analysis shows that in the simulation without BC the pressure-gradient force is indeed stronger than in the BC simulation, which is in line with the stronger temperature difference. The advection term shows that the vertical export of along-valley momentum out from the valley by the cross-valley circulation, which is seen in the simulation without the BC, is suppressed or even absent in the BC simulation. This occurs likely due to the weaker up-slope winds which allow the stronger up-valley winds to develop in the afternoon despite the weaker pressure-gradient forcing. These results show that realistic BC concentrations can affect the thermally-driven valley circulation and fluxes of heat and momentum, revealing a pathway through which absorbing aerosols can modify the daytime slope and valley wind characteristics.