Investigate the Boundary Layer Features in Beijing During the SURF-2015 Field Campaign

Understanding and correctly modeling the planetary boundary layer (PBL) structures are critical for weather and air-quality forecast in complex urban environments. We studied the PBL evolution by combining 6-day Doppler lidar wind measurements and observations from the 325-m Institute of Atmospheric Physics (IAP) meteorological tower in central Beijing during the summer 2015 Study of Urban Rainfall Impacts and Fog/Haze (SURF) field campaign. We developed a composite method for estimating the diurnal variations of the PBL depth using the Doppler lidar. A threshold method using vertical velocity variance (σ_w² > 0.1 m² s^-2) is applied to the convective boundary layer (CBL). Results show that this method determines PBL depths better than the conventional maximum wind-shear method, but has limitations for the nocturnal boundary layer (NBL). We instead determined the NBL top as the height at which the vertical velocity variance decreases to 10 % of its near-surface maximum minus a background variance. The PBL depths determined by combining these methods have average values ranging from ~270 m to ~1500 m for the six days. Release of stored heat and anthropogenic heat generation contribute to sustaining turbulence until late evening, preventing the NBL falling into very stable regime. A number of high-resolution simulations, employing the WRF (Weather Research and Forecasting)-Urban model with various PBL and urban canopy parameterizations were used to identify the deficiencies in WRF-Urban regarding its ability to capture the observed CBL and NBL characteristics.