弱天气系统强迫下北京地区对流下山演变的热动力机制

肖现; 陈明轩; 高峰; 王迎春

doi:10.3878/j.issn.1006-9895.1403.13318

弱天气系统强迫下北京地区对流下山演变的热动力机制

doi: 10.3878/j.issn.1006-9895.1403.13318

1.
中国气象局北京城市气象研究所, 北京100089;中国科学院中层大气与全球环境探测重点实验室, 北京100029;中国科学院大学, 北京100049
2.
中国气象局北京城市气象研究所, 北京100089
3.
北京市气象局, 北京100089

基金项目: 公益性行业(气象)科研专项项目GYHY201306008,国家自然科学基金项目41305041

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出版历程
- 收稿日期: 2013-11-25
- 修回日期: 2014-03-31

A Thermodynamic Mechanism Analysis on Enhancement or Dissipation of Convective Systems from the Mountains under Weak Synoptic Forcing

1.
Institute of Urban Meteorology, China Meteorology Adminstration, Beijing 100089;Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing 100029;University of Chinese Academy of Sciences, Beijing 100049
2.
Institute of Urban Meteorology, China Meteorology Adminstration, Beijing 100089
3.
Beijing Meteorological Bureau, Beijing 100049

摘要

摘要: 利用三维数值云模式和雷达资料四维变分(4DVar)同化技术,通过对京津冀地区4部新一代多普勒天气雷达观测资料进行快速更新同化和云尺度模拟,初步分析了弱天气系统强迫下两次发生在北京地区对流风暴的低层动力和热力影响机制。这两次风暴过程处于弱天气系统强迫和弱层结背景下,局地冷池和环境风场的相互配合是造成山上对流风暴是否能够顺利传播下山的关键机制。起初,两个个例平原局地热、动力不均衡形成平原冷池,而冷池的“障碍物”作用进而阻碍环境风场的传播配置。在此机制下,导致在冷池东南边缘形成较强的辐合上升、垂直风切变和螺旋度。在6月26日个例中,由于冷池强度较强且位置偏南,因此阻断了东南暖湿气流向山区的输送,形成由平原至山区的辐散区使得山区的对流风暴不断减弱。但是,随着已经消散的对流风暴下沉气流,覆盖至冷池边缘东南气流上空形成了较强的风切变和垂直螺旋度,进而促使在冷池边缘形成新的对流风暴。而且,在新对流风暴生成后,由于平原地区整体切变强度较弱,因此形成了冷池扩张强度大于对流风暴传播速度的态势。这种配置会切断暖湿入流,从而导致对流风暴快速消亡。对于8月1日个例,冷池位置偏北,因而不受冷池阻挡作用的偏南风在山脚形成较强的辐合上升,同时与下山的偏西风形成明显辐合上升区,有利于山区对流风暴的不断增强;进而,受此影响,山上风暴降水产生若干冷池,新生冷池和原有冷池的相互挤压,在迫使中、北部风暴增强的同时,最终也导致这些风暴互相靠近,最终合并组织成带状对流系统。同时,北部冷池边缘形成的辐合带也为对流风暴向山下传播提供有利条件,而回波产生的冷池进一步增强,并明显扩展。低层风场指示冷池出流(阵风锋)更加强烈且存在明显的“前冲”特征,显现出部分飑线系统的热动力特征。但是由于此时平原地区处于弱切变环境中,风切变强度不能与冷池出流强度相平衡,同样冷池扩展将领先于对流风暴移动,切断东南暖湿入流,导致原有风暴快速减弱。在文章的最后,基于观测和模拟结果,对比分析这两个个例,初步得出了与对流风暴传播下山发展演变密切相关的低层热、动力配置概念模型。
- 雷达 /
- 同化 /
- 冷池 /
- 螺旋度 /
- 切变 /
- 风暴
Abstract: A preliminary analysis of thermodynamic mechanisms of two well-defined convective systems over Beijing area and its vicinity was studied with a three-dimensional cloud-scale numerical model and a rapid-update cycling 4DVar assimilation technique using data from four of China's new-generation weather radar stations (CINRADs). The two convective systems were both under weak synoptic forcing and precipitation stratification in the low-middle layer. The cooperation of the cold pool and wind field act as trigger and strengthening mechanisms for the storm, which could propagate from the mountains to the plains. Originally, the cold pool was generated due to the uneven distribution of the thermodynamic field, and blocks wind propagated at the southern edge of the cold pool. The mechanism resulted in relatively high convergence, relatively vertical wind shear and helicity. In the first case, which occurred on June 26, 2009, a relatively strong cool pool located to the south, cut off the warm southeastern inflow that caused distinct divergence from the plains to the mountains, causing storms to continually weaken on the mountains. However, outflow from the dissipating storms moved over southeast winds, resulting in high shear and helicity, and therefore new storms formed at the edge of the original cool pool. Due to low shear over the plains, the cool pool extended more quickly than the storms, causing the storms to dissipate. For the second case on August 1, 2009, the cold pool was located to the north. Veering winds that were forced and blocked by the cool pool and mountains formed distinct and strong convergences via. When storms reached the foothills, the original long-term cool pool still provided relatively high convergence, shear and helicity for the storm spreading from the mountains to the plains. New and original cool pools squeezed each other, resulting in an intensified northern storm. Storms drifted toward each other, eventually leading to linearly organized echoes. As linear echoes spread over the plains, the perturbation temperature shows the cold pool further intensified and expanded. Gust fronts intensified, tilted forward, and moved away from the storm. Thermodynamics of the linear echoes showed some characteristics of a squall line. However, the weak wind shear in the path of storm propagation resulted in disequilibrium with the cold pool. The gust front blew out the convergence line away from the original storm, which became weaker. These data, combined other investigations, imply that simulated helicity and shear are useful to indicate development of the storms. Finally, a conceptual model was developed using observed data and simulation results, showing low-level dynamic and thermodynamic collocation significantly affects development and evolution of these storms from the mountains to the plains.
- Radar /
- Assimilation /
- Cool pool /
- Helicity /
- Shear /
- Storm

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