南京一次持续性浓雾天气过程的边界层特征及水汽来源分析

Analysis on Boundary Layer Features and Sources of Water Vapor of One Continuous Dense Fog in Nanjing

本文选取2006年12月24—27日(平流辐射雾)南京大雾的外场观测资料及NCEP的2.5°×2.5°NC再分析资料和GDAS全球1°×1°气象资料,结合天气形势、气象要素、物理量场,并利用轨迹分析方法,对这次浓雾的边界层特征及水汽输送进行分析,探讨这次浓雾形成和持续的主要边界层物理和天气学成因。分析表明:(1)这次浓雾过程期间始终存在深厚的逆温层结,甚至出现多层逆温。浓雾过程中,在中上空不同逆温层顶温度比地面温度高出2~5℃。逆温层厚多在200 m以上,26日08时逆温层厚达500 m。逆温层的存在,使大气层结更加稳定,在雾形成前期利于低层水汽聚集,雾形成后又抑制水汽的扩散,利于雾体的发展和维持,是这次浓雾能持续约64 h,强浓雾时段(能见度<50 m)持续约37 h的重要因素。(2)这次平流雾过程低层水汽通量散度呈负值,上空持续出现水汽辐合,最强水汽辐合出现在25日02时左右,为-30×10^(-7)g·s^(-1)·cm^(-2)·hPa^(-1)。低层辐合利于水汽的聚集,雾得以形成和发展,而雾过程后期水汽辐散则加快雾的消亡。贯穿整个雾过程的水汽辐合是这次平流辐射雾长时间维持的重要条件。(3)这次平流辐射雾过程中水汽输送路径是自中国东部沿海抵达南京;雾期间,水汽又来自海上源源不断的输送,最大时南京上空水汽通量达到2 g·s^(-1)·hPa^(-1)·cm^(-1)。水汽的供应和后期补充量,决定了浓雾的持续时间。

Based on the fog observation data during 24-27 December 2006 （advection radiation fog）, NCEP NC reanalysis data （2.5°×2.5°） and GDAS global meteorological data （1°×1°）, detailed trajectory analysis of the boundary layer characteristics and water vapor transport of the fog is investigated, combined with the weather condition, meteorological elements and physical quantity field. The results show that： （1） there is thick inversion layer, even multi layer inversion throughout the dense fog event. Temperatures of different inversion tops in the middle and high levels are 2-5℃ higher than the surface temperature. The thickness of inversion layer is more than 200 m, and it gets to 500 m at 08：00 BT 26 December, indicating the atmosphere is very stable and conducive to the convergence of water vapor before the fog forms. However, it is not favorable for the divergence of water vapor after the formation of fog, which helps the development and maintenance of the fog, causing the fog to last about 64 hours with dense fog （visibility 〈50 m） about 37 hours; （2） The divergence of water vapor flux in low level is negative in the advection fog event. The upper air has persistent moisture convergence and the strongest moisture convergence appears at 02：00 BT 25 December, being -30×10^-7 g·s^-1·cm^-2·hPa^-1. The accumulation of low level water vapor makes fog form and develop while the divergence of water vapor flux speeds up its dissipation. （2）The long lasting advection radiation fog is mainly caused by the continuous water vapor convergence; （3） The water vapor path is from the coastal area in easten China to Nanjing. The water vapor is continuously supplied from sea during the fog event, with the water vapor flux maximum getting to 2 g·s^-1·hPa^-1·cm^-1. The sufficient supply and supplementary of water vapor determines the duration of the fog.