Observation and analysis of nocturnal low-level jet characteristics over Beijing in summer
北京夏季夜间低空急流特征观测分析

By using the data of tethered balloon sounding observed in summer for three years at three sites in Beijing, the properties of nocturnal low-level jet (NLLJ) were analyzed. NLLJs are present in 30% of nocturnal soundings, and the mean height of NLLJs is around 200 m. More than 90% of NLLJs have their wind speed maxima below 320 m. The wind direction of NLLJ is predominately from W, SW and SE during nighttime. Speeds, directions and heights of NLLJ are varied from different sites, in which jet heights at urban site is mostly bigger than that in suburban site. Case study shows a good relationship between evolution of NLLJ and diurnal variation of mountain and valley breezes in nocturnal stable boundary layer. Further analyses revealed that baroclinicity over sloping terrain and mountain-valley breezes are mainly potential causes of nocturnal NLLJ over Beijing during summer.