QX气象Meteorological Monthly1000-0526气象编辑部中国北京qx-43-1-5610.7519/j.issn.1000-0526.2017.01.006P458论文对流移入杭州湾后飑线发展机制分析Analysis on Influence of Convection After Moving into Hangzhou Bay on the Development of Squall Line高梦竹GAOMengzhu
To enhance the understanding of the squall line maintaining mechanism under influence of Hangzhou Bay, the diagnosic analyses are made on the squall line in northern Zhejiang on 27 July 2014, using Doppler radar data, convectional and densely-observed surface data and NECP GFS data. The focus is on the evolution of convection that moved into Hangzhou Bay from the north and the south coasts and its effect on the overall development of squall lines. The result indicates that the north-south convergence lines and moderate strength of deep vertical wind shear are the key environmental factors of the generation and development of the squall line event. Convections are strengthened after moving into Hangzhou Bay from the north and the south coasts, leading to squall line maintaining. The better moisture conditions and unstable energy conditions in Hangzhou Bay result in the further strengthening of convection from the north shore of Hangzhou Bay, and the connection of two linear convections in southern Jiangsu, northern Zhejiang as well as the sustainable development of the squall line. The temperature and humidity on the surface of Hangzhou Bay is equal to the surface of south coast, and the strong wind shear in Hangzhou Bay leads to the maintaining strength of the convection into Hangzhou Bay from the south coast. Under the interaction of cold pool and rear inflow, bow echo develops on the sea, becoming an important factor for the maintenance of the squall line.
飑线是一种线状排列的中尺度对流系统,常伴有雷雨大风、冰雹等强对流天气,具有发展迅速、时空分布不均匀、破坏力强等特点(朱乾根等,2007)。针对陆地上飑线,国内外学者从结构特征、环境条件、组织方式、生命史、维持机理等方面已经开展了不少研究(Bluestein and Jain, 1985;丁一汇等,1982;Ogura and Liou, 1980; 陈涛等,2013;李娜等,2013;马中元等,2014;潘玉洁等,2008;孙建华等,2014;许爱华等,2014;李延江等,2013;刘莲等,2015;郑丽娜和刁秀广,2016)。
近几年来,随着气象观测资料种类和数量的增加以及中尺度模式的发展,飑线从陆地移入大范围水体的入海过程和相关机制也开始逐步被讨论和研究,探讨了海岸线附近陆地与海面的温度差异,以及冷池与垂直风切变强度对入海飑线发展的影响(Lericos et al, 2007;沈杭锋等,2010;Murray and Colle, 2011)。Lombardo and Colle (2010;2011;2012;2013)针对美国东北沿海地区入海飑线的研究表明,除了受到周围环境因子的影响,入海后飑线本身与环境场之间的相互作用,也对其形态、强度变化起着重要作用。
根据雷达图像特征,按照对流的不同形态可以将对流分为对流单体、线状对流和非线状对流,线状对流与其他两类相比较,最主要区别在于:长度至少为75 km,长宽比大于3: 1,这些特征至少维持2 h (Gallus et al, 2008)。飑线是一种比较典型的线状对流,其形态、强度和持续时间等有较为严格的规定(Lilly et al, 1979; Maddox et al, 1982)。在以往的研究(Meng and Zhang, 2012; Chen and Chou, 1993; Geerts, 1998; Parker and Johnson, 2000)中,飑线定义为:满足雷达回波强度在40 dBz以上,回波带长度超过100 km,长宽比大于5: 1,持续时间≥3 h,整个回波带具有共同的前边界等条件的线状对流。
The radar composite reflectivity of squall line in its forming, developing and demising stages (unit: dBz)
图 1k中的蓝色箭头表示飑线断裂处;其中图 1a~1i为杭州雷达,图 1j~1l为宁波雷达;宁波雷达与杭州雷达的组合反射率因子相比,对流强度偏弱,但杭州湾上弓形回波的形态更明显)]]>Fig. 1k represents the break of squall line; Figs. 1a-1i are for Hangzhou Radar and (1j-1l) for Ningbo Radar; compared with Hangzhou Radar, the convection by Ningbo Radar is weaker, but the shape of bow echo is more obvious)]]>
The moving path of squall line in its peak and sustained periods (a), the distribution of severe precipitation at 17:00 BT (b), 18:00 BT (c), 19:00 BT (d) with 1 h precipitation more than 20 mm·h-1 and wind speed faster than 17 m·s-1 on 27 July 2014
The 500 hPa (a) and 850 hPa (b) weather maps based on observation and the schematic figure of 500 hPa (c), 850 hPa (d), ground (e) circulation
Fig. 3c, the black reticular arrow refers to the southwest flow at the edge of subtropical high, black shaded arrow is the northwest flow after trough; in Figs. 3d and Fig. 3e, reticular shadow is the squall line position)]]>
Mesoscale analysis of circulation based on observation and GFS data at 08:00 BT 27 July 2014 (a), NOAA high resolution visible cloud image at 13:30 BT (Dashed line represents the leading edge of the convection A3 and A4) (b), the ground CAPE based on GFS data (shaded area, unit: J·kg-1) and 0-6 km vertical wind shear (contour, unit: m·s-1) at 14:00 BT (c)
The moving path of convective cell X7 in Hangzhou Bay and the radar composite reflectivity at 17:54 BT (a), the location, velocity and the occurrence time of maximum wind in north shore of Hangzhou Bay and the moving path before and after convective cell X7 into the sea (b)
Fig. 5a, black points and line mean the location and path of the convection, the blue line mean the Hangzhou Bay shoreline)]]>对流单体入海后强度变化及原因分析
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