Radiative effect of black carbon aerosol on a squall line case in North China

- Aerosols accumulate in upper free troposphere due to transport by deep convection.
- Black carbon aerosol reduces low-level temperature, surface fluxes, vapor amount.
- Black carbon aerosol does not significantly affect the evolution of the squall line.

The radiative effect of black carbon aerosol (BC) on a squall line case in north China is studied with the Weather Research and Forecasting model. Before the initiation of the squall line, the surface-emitted BC is mixed only in the boundary layer (BL). BC is then transported from the BL into the free troposphere by the updrafts in the squall line system. Once distributed in the atmosphere, BC absorbs solar radiation and heats the surrounding air. The maximum increase of temperature is 0.05 K for the moderately polluted case bc2 and 0.37 K for the heavily polluted case bc20. In case bc2, where the BC concentration is not very high, the solar flux reaching the surface, the sensible heat flux, and the latent heat flux are not significantly affected by BC. In case bc20, the solar flux reaching the surface, the sensible heat flux, and the latent heat flux are reduced by up to 80, 30, and 21 W m− 2, respectively. The reduced surface evaporation leads to a reduced vapor amount at the early stage. After some time, the heating effect causes a large-scale convergence and brings slightly more vapor into the domain. The effect of BC on the cold pool strength and low-level wind shear is small and hence does not significantly affect the triggering of new convections. In addition, our results show that the effect of BC is negligible on the strength and rain rate of the squall line case.