Spatio-temporal Characteristics of Boundary Layer Height Derived from Soundings

Using K-means cluster method, the whole country is divided into four regions (Qinghai-Tibet region, northwest region, central region and eastern region) by boundary layer height (BLH) derived from potential temperature gradient method based on L-band radar sounding secondly data of 119 stations from January 2010 to December 2018. Characteristics of BLH and frequency of different boundary layer state are investigated, including convective boundary layer (CBL), neutral boundary layer (NBL) and stable boundary layer (SBL), through their interannual, annual and diurnal variations respectively. Results show that there is no significant difference in the annual average BLH and the frequencies of different boundary layer states in four regions at 0800 BT and 2000 BT from 2010 to 2018. At 0800 BT, the annual average BLH is around 200-600 m and mainly in SBL. At 2000 BT, the annual average BLH in Qinghai-Tibet region is the highest (about 1500 m), followed by northwest region and central region (about 1000 m and 500 m), and that of eastern region is the lowest (about 400 m). Qinghai-Tibet region and northwest region are mainly with CBL and NBL, while central region and eastern region are mainly with NBL. Besides, the annual variation of BLH in four regions is similar at 0800 BT, but it's significantly different at 2000 BT. At 0800 BT, the difference of one-year monthly average BLH in four regions are not obvious, and there is no clear difference among these regions. But at 2000 BT, the monthly average BLH in each region reaches maximum in spring and summer, and minimum in autumn and winter. As for corresponding annual variation of different boundary layer state frequencies, SBL's frequency first increases then decreases while the frequencies of CBL and NBL first decrease then increase overall at 0800 BT and 2000 BT. And their turning point is in May to July. In general, the variation range of monthly average BLH and boundary layer state frequencies gradually descend from Qinghai-Tibet region, northwest region, central region to eastern region. The diurnal variations of BLH are different in four regions. In particular, the diurnal variations of Qinghai-Tibet region, northwestern region and central region show distinct seasonal difference. In Qinghai-Tibet region, the amplitude of diurnal variation can reach 2000-2300 m in spring and summer, but relatively weaker in autumn and winter. The diurnal variation in eastern region is similar in all seasons, and amplitudes are around 600 m.