湛江地区近海岸雾产生的天气条件及宏微观特征分析

利用2011年2～3月广东省湛江市东海岛雷达站观测得到的12次雾过程中雾滴谱、能见度、边界层风温场资料以及常规气象资料,对沿岸海雾发生时的天气系统进行分类,分析了雾发生时的气象条件、边界层特点、微物理特征等,并对典型个例进行讨论。总结发现:海雾易出现在低压前部、高压后部和冷锋前部等天气形势下；雾过程中,地面以偏东风为主,高空以偏东、偏南风为主；雾滴数浓度(N)的平均值变化范围为170～372 cm-3,液态水含量(LWC)为0.018～0.170 g m-3,平均半径为1.71～3.28 μm。选取了一个典型个例来研究典型天气形势下海雾宏微观特征,该过程发展初期以核化凝结为主,在水汽不充足时,大量的雾滴凝结核争食水汽,使得N增加而平均半径几乎不变。根据自动转化阈值T可以看出,成熟时期碰并效率增加,大滴端数密度增加,滴谱拓宽。结合边界层气象要素演变分析发现,在夜间急流显著时,不利于雾滴生长,LWC较低；而高空风速较小,逆虚温较强可能是消散阶段平均半径出现跃增的原因。雾滴谱呈双峰分布,且瞬时谱符合Gamma分布；液态水含量与消光系数、有效半径呈正相关关系,相关系数分别为0.95和0.97。

Study on the Synoptic System and Macro-Micro Characteristicsof Sea Fog along the Zhanjiang Coastal Area

Based on data of droplet spectra, visibility, temperature field, wind field, and meteorological elements for twelve cases from a comprehensive fog experiment carried out at the radar station on Donghai Island of Guangdong from February to March 2011, we classified the weather synoptic system for fog occurrence and analyzed the meteorological elements, boudary layer structures, and microphysical properties. Furthermore, a typical case was studied. It is summarized that when Zhanjiang is located at forepart of depression, cold front and rearward of high pressure, the frequency of fog occurrence was high. During the fog events, the wind direction was mainly easterly near the surface, and southerly and easterly in the upper air. The range of fog droplets number concentration (N), liquid water content (LWC), and average radius was 170-372 cm-3, 0.018-0.170 g m-3, and 1.71-3.28 μm, respectively. The initial stage of the development of Case 11 was dominated by droplet activation and condensational growth. N increased with a slight variation of radius when the water vapor was not sufficient for the growth of fog condensation nuclei. In the mature stage, according to the auto conversion threshold functions, the collision-coalescence efficiency and the number density of large droplets increased, and the drop-size distribution expanded. Combined with the meteorological elements in the boundary layer, we demonstrated that the wind jets were significant at night, inhibiting the growth of the fog droplets, and the LWC was low. The reasons for the sharp increase in average radius might be the strong inversion of the virtual temperature and the low wind speed. Statistical analysis showed that the droplets spectrum had a bimodal distribution and could be described by the Gamma distribution.The relationships between LWC and the extinction coefficient and the effective radius were positive with a correlation coefficient 0.95 and 0.97, respectively.