新疆地区一次对流性降水的三维中尺度风场研究
Study on Three-dimensional Wind Fields of Mesoscale Convective Systems in Xinjiang
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摘要: 利用2004年外场试验获得的乌鲁木齐和五家渠C波段双多普勒雷达资料, 分析了双多普勒雷达风场反演方法和资料的可靠性, 研究了2004年8月8日发生在乌鲁木齐和五家渠的一次强对流性降水的回波和风场中尺度结构及演变过程。结果表明:这两部雷达观测的回波强度相关很好, 雷达基线上的径向速度基本一致, 资料可靠, 适合进行双多普勒雷达观测; Cressman插值的影响半径的变化对风场的中尺度结构基本没有影响, 径向速度误差引起的风场反演误差与该点所处的位置有关, 1 m/s径向速度误差也不会改变风场的中尺度结构。该过程为对流单体发展为对流带状回波的过程, 在对流单体的左侧生成新的对流单体, 逐步发展为长度约90 km范围的带状对流系统, 该系统恰与较强的东北风和较弱的西风形成的辐合相对应, 上升气流与强对流回波相对应, 不同对流单体有各自相独立的风场结构。用双多普勒雷达观测得到对流系统的内部风场有利于了解对流系统的内部动力过程, 从而探讨降水的形成和演变机理。Abstract: The dual Doppler radar system constructed by two C-band Doppler radars in Urumqi and Wujiaqu is used to detect convective systems in Xinjiang Uighur Autonomous Region in 2004, it is supported by the project of National Natural Science Foundation of China (NSFC) "Study on Meso-scale Kinetic and Thermodynamic Feature of Precipitation System with Dual Doppler Radar in Xinjiang". It focuses on the three-dimensional wind field retrieved with dual Doppler radar technique in the strong mesoscale convective systems that produces relatively heavy rainfall within the observational network from 22:30 (BT) 7 August to 02:30 8 August 2004, and the three-dimensional wind structure of these systems as well as their evolution processes are analyzed. Moreover, the radar data quality and reliability of the wind retrieval algorithm with dual Doppler radar are also examined.The positions and structures of radar echoes captured by the two radars are well matched, the radial velocities observed by the radars above the radar baseline are mostly the same, the dual Doppler radar technique is reliable and suitable for the observations of mesoscale convective systems. The sensitivity analysis of the interpolation algorithm on wind retrieval framework demonstrates that the influence radius of Cressman interpolation does not play a significant role in determining the wind structures of mesoscale convections. Even though the wind retrieval errors resulted from the radial velocity measurement errors are associated with their relative positions to the radars, however, such type of errors within 1 m/s on redial velocity would not change the major shapes of the mesoscale wind structures.According to the evolutions of these mesoscale systems, the several convective cells develop into convective precipitation band. Some new cells are generated along the left side of the old ones, whose reflectivity vary from 50 to 55 dBz with the maximum 8 km top height, and finally develop as a band-shaped convective cluster (with 90 km length) after 4 hours. The convective band is corresponding with a strong convergence produced by a branch of strong northeasterly wind and a weak westerly wind, and the updraft is just located at the maximum reflectivity center. The different convective cells actually have their independent wind structures respectively, but they would subsequently interact with each other to engender heavy rainfall weathers. Besides, there are limits to the measurements on this type of mesoscale convective systems in the conventional observations, such as radio sounding, whose spatial and temporal resolutions are limited at hundreds kilometers and 12 hours respectively. Consequently, it is not sufficient to determine their dynamic structures, which develop quickly and have relatively short lifecycle. Fortunately, due to the retrieved wind fields of these mesoscale convections with dual Doppler radar observations, it would be easier to analyze their kinetic features and therefore study the initiation and development mechanism of the mesoscale rainfall systems.
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Key words:
- dual Doppler radar;
- wind structure;
- mesoscale convective system
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图 2 2004年8月8日05:02在2 km等高面上乌鲁木齐与五家渠两雷达径向速度误差都为随机误差 (a) 和系统误差 (b) 时的水平风场大小的反演误差
(五家渠雷达坐标为 (13.25), 其他说明同图 1)
图 4 2004年8月7—8日不同时刻乌鲁木齐雷达径向速度的演变过程
(说明同图 3)
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