Variation of Atmospheric Boundary Layer Height Over the Northern, Central, and Southern Parts of the Tibetan Plateau During Three Monsoon Seasons

The atmospheric boundary layer (ABL) over the Tibetan Plateau (TP) is important to the study of the interactions between the underlying surface and the atmosphere. Radiosondes were released during three intensive ABL experimental periods (monsoon-onset, monsoon, and post-monsoon) in 2014 and 2019. Using these observational data, this study explored the features of the ABL height (ABLH) in the northern, central, and southern parts of the TP during the three periods and their possible association with the westerlies. Measurements showed that the ABLH over the northern part of the TP in the monsoon season was the highest (average 3 km above ground level [AGL]) among the three intensive observation periods. Meanwhile, the southern part of the TP had the highest ABLH (almost 3.5 km AGL) in post-monsoon. Using ERA5, we discovered that the ABLH over the TP was not only affected by the surface sensible heat flux but also was influenced by the seasonal north-south movement of the westerlies. The westerlies correspond to an upper-level high potential vorticity (PV) center that is associated with the weak atmospheric stability below the westerlies and is conducive to the development of the ABL. When the westerlies are located above the southern TP in winter, they provide conditions that favor ABL increase. Westerlies are above the north TP in summer, and this provides favorable conditions for ABL increase in the northern TP. The role of westerlies in the ABLH explains why the northern and southern parts of the TP have different seasonal variation patterns.