北部湾一次持续性混合型海雾过程分析

Analysis of a Persistent Mixed-Type Sea Fog Process over Beibu Gulf

摘要:

利用北部湾沿岸海岛站观测资料和ERA5数据，对混合型（高压后部型转西南倒槽型）海雾出现概率和一次典型过程进行了分析。结果表明：①2015—2020年混合型海雾过程出现概率最高，累计出现15次（58天），占比34.9%（39.5%）。②2016年3月15—23日海雾过程具有持续时间长、范围广的特点。前期北部湾受入海高压后部偏东气流影响，后期高原槽发展东移，北部湾逐渐转为受西南倒槽边缘偏南气流控制。③热力条件上，低空急流的强暖湿输送，一方面使北部湾上空比湿增大，饱和层从近地面延伸至800 hPa，另一方面使逆温层建立，逆温层与其下方高湿区配合形成稳定状态。动力条件方面，垂直方向维持一定强度的风切变，有利于边界层的湍流混合，并与斜压大气形成有利于低层稳定层结维持的配置。此外800 hPa以下为弱上升运动，利于水汽在低层抬升凝结，促进海雾发展及维持。④海雾发生期间，前期（后期）气海温差大多在0～1.5 ℃（1.5～2.5 ℃）之间。

Abstract:

Based on the observation data from the coastal island stations in Beibu Gulf and the fifth generation ECMWF reanalysis (ERA5) data, the probability of occurrence of mixed-type (that is, high-pressure rear type to southwest inverted trough type) sea fog processes in Beibu Gulf is analyzed. Furthermore, the typical mixed-type sea fog process from 15 March to 23 March 2016 is selected for in-depth analysis. The results show the following: (1) Compared with other types of sea fog processes, the probability of occurrence of the mixed-type sea fog processes was the highest during 2015-2020, with a cumulative number of 15 times (58 days), accounting for 34.9% (39.5%). (2) The typical mixed-type sea fog process from 15 March to 23 March 2016 was characterised by long duration and wide range. In the early stage, the Beibu Gulf was influenced by the easterly flow behind the high pressure entering the sea. In the later period, the plateau trough developed and moved eastward, which guided the warm and humid south-western airflow to expand inland, and the easterly airflow gradually turned to the southerly airflow at the edge of the south-west inverted trough in Beibu Gulf. (3) In terms of thermal conditions, the strong warm and humid transport of the low-level jet stream increased the specific humidity over the Beibu Gulf, and then the water vapour saturated layer extended from the near-surface layer to 800 hPa on the one hand. It also contributed to the establishment of the temperature inversion layer on the other hand. And the inversion layer cooperated with the high humidity region below to reach a stable state. As for the dynamical conditions, the wind shear in the vertical direction maintained a certain intensity, which was favourable to the turbulent mixing in the boundary layer. The cooperation of the wind shear with the baroclinic atmosphere was the configuration that favoured the maintenance of a stable layer in the low-layer. In addition, the weak upward motion below 800 hPa was conducive to water vapour uplift and condensation in the low-layer, which promoted the development and maintenance of the sea fog. (4) During the occurrence of the typical mixed-type sea fog, the air temperature was mostly higher than the sea surface temperature in each sea area. And the difference between the air temperature and sea surface temperature in the early stage (late stage) was between 0 and 1.5 ℃ (1.5 and 2.5 ℃).