Evaluating the impacts of updated aerodynamic roughness length in the WRF/Chem model over Pearl River Delta

The aerodynamic roughness length ( Z 0 ) plays an important role in the land–air exchange. At present, the atmospheric numerical models cannot accurately describe the Z 0 in different underlying surfaces, which affects the simulation performance and the description of the momentum and heat exchange and atmospheric environment. In this study, the regional air quality model WRF/Chem with updated Z 0 of the various underlying surface categories is used to study the impacts of Z 0 on the boundary layer meteorological conditions and atmospheric environment over the Pearl River Delta (PRD) region. Two numerical experiments are conducted, including default Z 0 (Base experiment) and updated Z 0 (Case experiment). By comparing the simulations with the observations from 9 meteorological stations and 30 national air quality monitoring stations, the updated Z 0 improves the model simulation performance of 10 m wind speed in PRD. The impacts of updated Z 0 on 2 m temperature and relative humidity are relatively small, while the influences of updated Z 0 on surface skin temperature (TSK), ground heat flux (GRDFLX), sensible heat flux (HFX), friction velocity ( U *), 10 m wind speed (WS) and planetary boundary layer height (PBLH) are evident. The impacts of updated Z 0 in the urban are greater than that in the evergreen broadleaf forest. In the urban, the updated Z 0 reduces TSK, lowers GRDFLX, increases HFX during the daytime, and increases U *, causing the reduction of WS, while the vertical velocity also increases, which is conducive to the elevation of PBLH. With the updated Z 0 , the surface carbon monoxide (CO) concentrations generally increase, especially in the central urban areas, due to the impact of wind speed, turbulent motion and planetary boundary layer height. The updated Z 0 has a little impact on CO concentration profile during the daytime, which is affected by horizontal wind speed and turbulent motion; while at night, the updated Z 0 produces an increase of CO concentration below 300 m, which is affected by the horizontal wind speed.