Atmospheric-Boundary-Layer-Height Variation over Mountainous and Urban Sites in Beijing as Derived from Radar Wind-Profiler Measurements

The evolution of the atmospheric boundary layer (ABL) varies greatly with terrain, so that the spatial and temporal variabilities of the ABL height remain poorly understood over complex terrain. Using radar wind-profiler measurements obtained from rural mountainous (Yanqing) and adjoining urban-plain (Haidian) landscapes of Beijing, China in 2019, ABL heights are calculated based on a normalized signal-to-noise-ratio threshold. The seasonally contrasting features of ABL height variation and growth rate over the two sites are revealed for clear-sky conditions. Interestingly, the ABL in spring remains suppressed during the morning and evolves rapidly in the afternoon over Haidian; however, a usual diurnal ABL evolution is observed over Yanqing. During the winter, more rapid evolution of the ABL is observed over Haidian, although on average the daytime ABL height remains less than 800 m above ground level. The growth rate of ABL height is found to undergo a more pronounced seasonal variation over Haidian while being relatively less variable over Yanqing. As expected, the lowest (highest) growth rate of 90 m h(-1) (188 m h(-1)) occurs in winter (summer) over Haidian. The analysis of the seasonal variations in wind profiles reveals deeper insights into the development of the local plain-to-mountain flow circulation over the region and possible implications on the contrasting seasonal ABL variations, particularly during the spring and summer. Additionally, the slower ABL evolution over Haidian in autumn and winter could be associated with an aerosol-induced stable ABL as well as stronger urban heat accumulation. The findings have implications for the better understanding of air pollution meteorology in regions with mountainous terrain.