On the Summertime Planetary Boundary Layer with Different Thermodynamic Stability in China: A Radiosonde Perspective

Strongly influenced by thermodynamic stability, the planetary boundary layer (PBL) is key to the exchange of heat, momentum, and moisture between the ground surface and free troposphere. The PBL with different thermodynamic stability across the whole of China, however, is not yet well understood. In this study, the occurrence frequency and spatial distribution of the convective boundary layer (CBL), neutral boundary layer (NBL), and stable boundary layer (SBL) were systematically investigated, based on intensive summertime soundings launched at 1400 Beijing time (BJT) throughout China's radiosonde network (CRN) for the period 2012 to 2016. Overall, the occurrences of CBL, NBL, and SBL account for 70%, 26%, and 4%, respectively, suggesting that CBL dominates in summer throughout China. In terms of the spatial pattern of PBL height, a prominent north-south gradient can be found with higher PBL height in northwest China. In addition, the PBL heights of CBL and NBL were found to be positively (negatively) associated with near-surface air temperature (humidity), whereas no apparent relationship was found for SBL. Furthermore, clouds tend to reduce the occurrence frequency, irrespective of PBL type. Roughly 70% of SBL cases occur under overcast conditions, much higher than those for NBL and CBL, indicating that clouds govern to some extent the occurrence of SBL. In contrast, except for the discernible changes in PBL height under overcast conditions relative to those under clear-sky conditions, the changes in PBL height under partly cloudy conditions are no more than 170 m for both NBL and CBL types.