天山北坡三明治焚风对城市群大气污染的影响研究

The foehn winds occur in the mountains in the world, resulting in all kinds of disasters including the flood, avalanche and air pollution, and the damages to the traffic and aviation. Therefore, the study focusing on foehn has been ongoing for over one century. The sandwich foehn is the subcategory of the shallow foehn and has received a little attention so far. The Urumqi urban agglomeration is situated at the north exit of the middle Tianhan Mountains valley, frequently suffering from the air pollution events during winter. Also, the boundary layer structure appears a three-layer structure of sandwich foehn after the airflow across over the cliffy terrain of the Tianshan Mountains during winter. What is the relationship between sandwich foehn and the atmospheric environment in urban agglomeration? It has not been clear up to now. In combination of numerical simulation and various observations consisting of the operational radiosonde, four wind profile radars, microwave radiometer, 166 automatic weather stations, eight meteorological towers and lidar, the synopic and climatic characteristics of sandwich foehn in the lee side of the Tianshan Mountains will be analyzed, also the delicate boundary layer structure of sandwich foehn and the urban agglomeration, and their evolution will be revealed. Through analysis on the upwind atmospheric stratification and the wave patterns in the lee side of mountain, we will illustrate the thermodynamic impacts of the sandwich foehn on the boundary layer structure over the urban agglomeration. Furthermore, the conceptual model of the heavy air pollution of the Urumqi urban agglomeration will be built up after analysis of the simulation of the transport and dilution of the air pollutants using the regional atmospheric numerical model. In this case, it will provide the necessary and scientific supports to the sustainable development of the Urumqi urban agglomeration.

焚风遍布于世界各大山系，造成洪水、雪崩、大气污染等灾害，威胁陆路交通和民航安全，因此百年来研究持续不断。三明治焚风隶属于浅薄型焚风，至今关注不多。乌鲁木齐城市群地处中天山峡谷北端，冬季气流经过陡峭山体后，常使城市群边界层呈现三明治焚风结构，同时冬季该城市群还频繁遭遇重污染事件，三明治焚风与城市群大气环境存在何种联系？目前不甚清楚。本项目基于城市群及其周边常规探空、4部风廓线雷达、微波辐射计、166个地面自动气象站、8座百米气象塔及激光雷达等多种观测数据，结合模拟，深入分析天山北坡三明治焚风的天气气候规律，揭示三明治焚风及城市群三维边界层精细结构和变化规律；基于过山气流层结和背风坡波动形态阐释三明治焚风过程对城市群边界层结构的动力热力影响；进一步利用区域大气化学模式模拟，揭示三明治焚风影响下的城市群污染物传输扩散规律，构建乌鲁木齐城市群的重污染概念模型，为城市群可持续发展提供必要的科学依据。

乌鲁木齐城市群冬季三明治焚风（即低空型焚风）与重污染事件频繁发生，一直以来都欠缺对这二者之间内在联系的研究。本项目基于多种探测数据和数值模拟，开展了以下研究：①天山北坡低空型焚风气候特征及天气概念模型；②低空型焚风的三维边界层结构特征及其影响因素；③低空型焚风与接地型焚风发生机制的差异；④低空型焚风对下游地区边界层结构的影响；⑤低空型焚风对污染物输送扩散的影响。研究发现，乌鲁木齐的低空型焚风一年可出现128.5天，春、夏、秋、冬的日数和频率分别是26.5 d(28.8%)、 18.0 d(19.6%)、 37.0 d(40.7% )、47.0 d(52.2%)。低空型焚风典型天气环流背景是：500 hpa新疆被高压脊控制，850 hPa被槽前西南气流主导，地面在新疆东西向的天气尺度气压梯度的驱动下，吐鲁番盆地~乌鲁木齐地区形成了南北向中尺度气压梯度，由此激发低空型焚风。焚风发生前后，天山南北坡存在逆温；南坡偏东风持续输送冷空气，维持了峡谷两端气压梯度。模拟表明：达坂城南部山脉对重力波的形成作用很大；微型焚风锋面最北在乌鲁木齐南部市区，最南可达峡谷柴窝堡附近，锋面同时带动着污染物在市区和南郊南北运移。低空型焚风的出现促使乌鲁木齐上空的逆温增强、混合层高度降低。接地型焚风发生的必要条件：吐鲁番盆地~乌鲁木齐气压差偏大、天山南坡偏冷和北坡偏暖的热力条件。乌鲁木齐70次重污染天气过程中有焚风出现的次数多达50次（71.4%）。CUACE模拟分析发现，冬季乌鲁木齐出现焚风，城市群的污染物就随着山谷风、焚风的较量而南北向移动。焚风偶尔可以将污染物输送到乌鲁木齐西北下游地区。较强西北冷空气对城市群污染物的清除能力最强，城市群污染物沿天山北坡向东和沿中天山峡谷走向东南2种途径向下游区域传输扩散。本项目对于城市群空气质量预报、焚风预报、大气污染防治和城市规划提供了科学支撑。