The Direct Effect of Aerosol on Planetary Boundary Layer (PBL)

As a example of studying aerosol's direct effect on Planetary Boundary Layer (PBL) for clear sky, the simulations of dust events including an agricultural burning episode at Shouxian site in China is carried out by using the improved chemistry version of Weather Research and Forecasting (WRF) model (WRF-Chem) with the highest resolution at 2.5 km from May 20 to 31 in 2008. Model simulation is evaluated based on the observation at Shouxian. The direct effect of aerosol on PBL is inferred by comparing two simulations with and without aerosol direct effect. The results show that the PBL height is decreased during the period with high aerosol loading in daytime. This is because the boundary layer atmosphere is stabilized by reducing surface radiation that cools the surface and leads to reduced sensible and latent heat fluxes, weakening the convection. With the contribution of absorbing aerosols, PBL height may be even lower since that the boundary layer may be more stable due to the heating of the lower atmosphere. This heating layer above PBL may further lead to the corresponding lower relative humidity and even upward motion instead of subsidence, which means more larger convective available potential energy. Below PBL, the weak convection is more favorable for intensifying the low level jet (LLJ) if present.