Regionalization of the Boundary-Layer Height and its Dominant Factors in Summer over China

The underlying surface is complex in China, and there are numerous factors that drive the boundary layer. At present, the boundary-layer height and factors influencing it throughout China lack a comprehensive analysis. This paper uses the K-prototype algorithm to cluster the boundary-layer height and its twelve possible factors and studies the regional features of the boundary-layer height and its dominant factors in summer over China based on high-resolution radiosonde measurements from the China radiosonde network during 2012–2016. The results show that the boundary-layer height of 100 stations at 0800 BJT (Beijing time), 1400 BJT, and 2000 BJT can be divided into four regions: northeastern China, southeastern China, northwestern China, and southwestern China. Accordingly, this paper analyzes the dominant factors of the boundary-layer height in different regions at different times and studies the possible mechanism by which these factors affect boundary-layer development under different thermodynamic conditions. The results suggest that the development of a stable boundary layer is mainly affected by the turbulent dynamics of wind speed at 0800 BJT and 2000 BJT and is closely related to intermittent turbulence at 1400 BJT. Furthermore, the development of the neutral and convective boundary layers is mainly driven by wind speed in the morning, while at noon it is mainly driven by turbulent thermal caused by higher surface temperature and larger net surface radiation flux. Moreover, the cloud amount and specific humidity in the northeastern China, latent heat flux in the southeastern China, sensible heat flux and specific humidity in the northwestern China, and sensible heat flux and soil moisture in the southwestern China can affect the development of the neutral and convective boundary layers through direct or indirect effects on thermal turbulence. At night, because of the time difference, wind speed tends to be the main factor influencing the neutral and convective boundary-layer heights in eastern China, while thermal factors may still be the main factors in western China. In addition, the leaf area index in northern China regulates the specific humidity through vegetation transpiration, and the specific humidity can affect the boundary-layer height by regulating the surface energy distribution. Finally, the surface pressure can affect the development of the nocturnal boundary layer in northeastern China by influencing the ascending and descending airflow movements.