Impact of aerosol heat radiation absorption on the dynamics of an atmospheric boundary layer in equilibrium

The objective of this work is to investigate the influence of the shortwave radiation (SW) absorption by aerosols on the dynamics and heat budget of the atmospheric boundary layer (ABL). This study is relevant for areas characterized by large concentrations of light-absorbing aerosol, which are known to warm up the ABL 2-10 K/day depending on the amount and vertical distribution of the aerosols. We investigate if aerosols, by warming the ABL and the entrainment zone, contribute to a destabilization of the thermal inversion layer, either by reducing the capping inversion or by stabilizing the lower part of the entrainment zone. The impact of the reduced amount of radiation reaching the surface due to aerosols SW absorption is taken into account.

As such numerical experiments are carried out by using a large-eddy simulation (LES) model (DALES code) to study the ABL vertical structure and its time evolution. Mixed-layer theory (0-order bulk model - MXL) is also employed to support DALES results. A radiative transfer code (TUV) is used to calculate the impact of the aerosols absorption on the SW radiation field. The numerical simulations are made for a reference case of a dry, non-polluted free convective ABL in equilibrium, i.e. the boundary layer height is in steady-state. In addition, three other simulations are designed: average (AC), high and extreme aerosol concentrations. They differ from the reference case only by the increasing aerosol concentration (homogeneously distributed) within the ABL. An extra simulation (called TOP) is performed with the AC concentration placed only at the ABL top to investigate if the vertical distribution of aerosols plays a role in changing the entrainment zone dynamics. The simulations are integrated towards a new steady state.

The DALES results indicate a shallower ABL for all simulations, compared to the reference case. The ABL-depth decreasing is even more pronounced for the TOP case. Different processes explain this decrease. At the surface, the reduction of SW, due to the aerosols absorption, leads to decrease the surface sensible heat flux (SH), and in consequence it yields to a shallower ABL. In turn, the aerosols absorption also weakens the capping inversion in all the experiments because of the aerosol heat absorption. The absorption also drives an increased stabilization of the upper region of the ABL. Stabilization and the reduced SH together suppress the weakening of the inversion layer, reducing the ABL height. Turbulence characteristics are further analyzed: first, the entrainment velocity increases when the aerosols are located at the top of ABL. The entrainment flux depth changes when aerosols are added, however its magnitude remains the same as for the control case, around 16%. Second, the stabilization below the ABL top leads to a reduction of the temperature variance.