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

2019 
The aerodynamic roughness length (Z0) plays an important role in the land–air exchange. At present, the atmospheric numerical models cannot accurately describe the Z0 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 Z0 of the various underlying surface categories is used to study the impacts of Z0 on the boundary layer meteorological conditions and atmospheric environment over the Pearl River Delta (PRD) region. Two numerical experiments are conducted, including default Z0 (Base experiment) and updated Z0 (Case experiment). By comparing the simulations with the observations from 9 meteorological stations and 30 national air quality monitoring stations, the updated Z0 improves the model simulation performance of 10 m wind speed in PRD. The impacts of updated Z0 on 2 m temperature and relative humidity are relatively small, while the influences of updated Z0 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 Z0 in the urban are greater than that in the evergreen broadleaf forest. In the urban, the updated Z0 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 Z0, 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 Z0 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 Z0 produces an increase of CO concentration below 300 m, which is affected by the horizontal wind speed.
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