Atmospheric boundary layer characteristics over the Pearl River Delta, China during summer 2006: measurement and model results
1Department of Atmospheric Sciences, Sun yat-sen University, Guangzhou, 510275, China
2Climate Center of Guangdong Province, Guangzhou, 510080, China
3Macao Meteorological and Geophysical Bureau, Macao, China
Abstract. Atmospheric conditions are often connected with the occurrence of high pollution episodes especially in urban areas. As part of the PRIDE-PRD2006 intensive campaign, atmospheric boundary layer (ABL) measurements were carried out at Qingyuan, Panyu and Xinken in the Pearl River Delta (PRD) from 1 July to 30 July of 2006. It was found that in summer, the surface winds in PRD are more controlled by the south, and there usually is vertical wind shear at the height of 800 m or so, therefore, PRD is often influenced by the tropical cyclone/typhoon. The subsidence and precipitation from a tropical cyclone will affect the air quality of PRD. Under the subsidence, the wind speed in ABL and the height of ABL will decrease and result in high level concentrations. When the background wind speed is small or calm, the wind profile at Panyu and Xinken change dramatically with height, which is perhaps caused by the local circulations, such as the sea land breeze. For more understanding about the ABL of PRD, the simulations by the WRF mesoscale model were used to analyse the ABL characteristics in PRD. From three kinds of weather condition simulations (subsidence days, rainy days and sunny days) by WRF model, it was found that the simulated temperature, wind fields in these three cases were moderately consistent with the measurements. The results show that the diurnal variation of ABL in subsidence days and sunny days are obvious, but the diurnal variation of ABL on rainy days is not obvious. The ABL is obviously affected by the local circulation and the features of ABL are different in various stations. A simulation focus on high pollution episode during the subsidence days from 12–15 July 2006, occurred under high pressure conditions, accompanied by a tropical cyclone "Bilis". Comparing the simulated vertical wind fields and temperature structure with the ABL measurements at Xinken, Panyu and Qingyuan station, it was found that, the modelled and measured atmospheric fields reveal that there are two different kinds of ABL characteristics in PRD: when the surface winds in PRD were light or almost calm, the local circulation was dominated, such as the sea-land breeze at Xinken station and the mountain-valley circulation at Qingyuan station. When the surface winds were strong, the stations were under the same background weather system and the wind directions were almost the same. Furthermore, the modelled results also suggest that the high Air Pollutant index (API) episode was caused predominately by subsidence.