北京地区边界层低空急流的观测研究

Observed Characteristics of Boundary Layer Low-Level Jet over Beijing Area

关键词:  边界层低空急流  季节特征  悬空逆温  日变化特征  城郊差异

Key words:  boundary layer low-level jet  seasonal characteristics  elevated inversion layer  diurnal variation  differences between urban and suburban areas

摘要:

利用 2016—2017 年北京常规探空资料与 2016 年 8 月 28 日—9 月 2 日多点同步加密探空资料，探讨了北京地区边界层低空急流与气象要素的相互关系，以及城市下垫面对急流的影响，并简要分析了急流的形成机制。结果显示：(1) 秋冬季急流表现为发生频率高、急流轴高，强度强，急流风向以偏北风为主；春夏季急流发生频率较低，急流轴低，强度弱，急流风向以偏南风为主。(2) 急流与大气边界层垂直结构有密切关系。急流发生时多伴有悬空逆温的出现。悬空逆温高度和急流上方最小风速高度、风向转变高度有较好一致性关系。(3)边界层低空急流的生消与位温的日变化过程基本一致，惯性振荡是北京地区夜间晴好天气条件下急流形成的主要机制。(4) 边界层低空急流受下垫面热力差异影响在城、郊有不同的表现特征。城市急流具有显著的突发性和不连续性，维持时间短，发生频率低。郊区急流则发生频率高，维持时间长。典型急流过程中，城市与郊区相比，急流轴高150 m，急流风速小1.6 m·s-1。

Abstract:

Based on two-year routine sounding data and intensive radiosonde data collected at multiple stations from August 28 to September 2, 2016, over the Beijing area, this study investigated the relationship between boundary layer low-level jet (BLLJ) and meteorological elements, analyzed the differences in BLLJ characteristics between urban and suburban areas, and briefly explored its formation mechanism. The results show that: (1) The BLLJ in autumn and winter was more frequent, higher, and stronger, and the main jet direction was southerly. In spring and summer, it became less frequent, lower in height, and weaker, and the jet was primarily northerly. (2) The jet was closely related to the vertical structure of the atmospheric boundary layer. In most cases, an elevated inversion layer was observed above the jet. The height of inversion exhibited good consistency with the minimum wind speed above jet height and the wind direction change height. (3) The diurnal variation of potential temperature was similar to the cycle of the BLLJ from its formation to dissipation. The main driver of the nocturnal BLLJ in Beijing was inertial oscillations under clear weather conditions. (4) The difference in BLLJ characteristics between urban and suburban areas was significant. Urban jets were usually abrupt and not coherent, short-lived, and rarely observed. By contrast, suburban jets were frequent and long-lived. In a typical urban jet case, the jet core height was 150 m higher, and the jet intensity was 1.6 m s-1 smaller than those of a suburban jet at the same time.