济南冬季雾的微物理结构及其对能见度的影响

The Microphysical Characteristics of Winter Fog in Jinan and Its Effect on Visibility

2016年12月19日至2017年1月9日,受静稳天气影响,济南接连出现了10次大雾等级以上天气过程,期间最低能见度不足50 m,持续的大雾天气严重影响了工农业生产和人民生活。本文利用10次冬季雾期间雾滴谱仪、自动站等观测资料,分析了济南不同强度冬季雾的微物理结构特征,分析了其中的微物理过程及强度,探讨了微物理结构、微物理过程对能见度(V)的影响。结果表明:(1)济南冬季雾强度不同,其谱分布具有明显的差异,在雾变浓的过程中,谱型由"单峰"结构逐渐向"多峰"结构发展。(2)数浓度对能见度具有较好的指示意义,液态含水量、离散度等对能见度指示意义不稳定。(3)环境温度与核化、凝结和碰并增长(或蒸发)等微物理过程密切相关;核化、凝结增长是济南冬季雾发展过程中最主要的微物理过程,在整个雾过程中起主导作用。(4)碰并过程主要发生在发展和成熟阶段,在生成和减弱阶段很弱,以未碰并或偶发碰并为主。(5)自转化率计算结果表明,在V≥200 m的雾中,碰并过程很少发生;在100 m≤V<200 m的强浓雾中,以未碰并或间断碰并为主;碰并过程主要出现在V<100 m等级的强浓雾和特强浓雾中;与V<50 m的特强浓雾相比,50 m≤V<100 m的强浓雾中碰并过程发生的概率更大、强度更强。(6)在济南冬季特强浓雾中含有大量的小雾滴,但各微物理量的最大值、最大的起伏变化并未出现在特强浓雾中,而是出现在50 m≤V<100 m强浓雾中,这可能与强浓雾中较强的碰并过程有关,碰撞过程中产生的并合和破碎可能是微物理量起伏变化最大的主要原因。(7)利用雾滴谱资料计算的能见度与实测值在变化趋势上具有较好的一致性,但比实测值大1~2个数量级,这可能主要与雾中大量的气溶胶粒子有关,对于污染大气,基于雾滴谱仪观测资料来估算雾中的大气能见度是不够的,必须同时考虑气溶胶粒子对能见度的影响。

As affected by stable atmosphere,there had been ten fog events in Jinan from 19 December 2016 to 9 January2017,during which the minimum value of visibility was below 50 m.The low visibility caused by the continuous fog events brought serious negative impacts to industrial and agricultural activities as well as to people’s daily live.In this paper,based on the measurements from fog drop spectrometers,automatic weather stations,and conventional meteorological instruments during the 10 fog events,the fogs’microphysical characteristics were analyzed,microphysical processes and intensity were deduced,and their effects on visibility were discussed.The results were as follows:(1)The droplet spectrum distribution was different with different thicknesses of the winter fogs,changing from mono-modal to multi-modal as fog became thicker.(2)The droplet number concentration(Nc)had a close inverse correlation with visibility(V),while the liquid water content(LWC)and relative dispersion of the droplet size distribution(S)did not have a definite inverse relationship with V.(3)The temperature of air had an impact on the microphysical processes.The activation and condensational growth(or droplet evaporation)processes played leading roles in the whole winter fog lifecycle.(4)The collection processes arose in the development and maturation stages of the fogs,but not or very infrequently in the formation and weakening stages of the fogs.(5)The results from the autoconversion rate show that the collection rarely occurred in the fogs with V>200 m,while it was very weak or intermittent in the heavy fogs with 100 m≤V<200 m.The collection mainly occurred in the extremely dense fogs with V<50 m and heavy fogs with 50 m≤V<100 m,while it was more frequent and stronger in the heavy fogs with 50 m≤V<100 m than in the extremely dense fog with V<50 m.A larger number of small fog droplets led to poorer visibility in the extremely dense fog with V<50 m,however the maximum values of the droplet properties such as Nc,LWC appeared in the heavy fogs with 50 m≤V<100 m,in which these quantities showed the biggest variations as well.(6)The variations may be related to the more frequent and stronger collection in the heavy fogs with 50 m≤V<100 m,in which collision-coalescence and collision-fragmentation played a role in the biggest variations of microphysical quantities.(7)The trend values of calculated visibility based on the observational data from fog drop spectrometers agreed well with the actual trend values,however the calculated visibility values themselves were much greater than the actual ones,mainly caused by the large amount of aerosol particles in the fogs.In polluted fogs,observational data from fog drop spectrometers are not enough to estimate the visibility of fogs.The influence of aerosol particles on visibility must be considered at the same time.