兰州大学机构库 >大气科学学院
塔克拉玛干低空急流特征与形成机制及其对沙尘活动影响的模拟研究
Alternative TitleModeling the characteristics and mechanism of Low-level jets and their effects on dust activity in Taklimakan desert
韩梓航
Subtype博士
Thesis Advisor付强 ; 葛觐铭
2023-05-29
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name理学博士
Degree Discipline大气物理学与大气环境
Keyword塔克拉玛干沙漠 Taklimakan Desert 低空急流 LLJs 沙尘活动 Dust activities 日变化 Diurnal variation
Abstract

低空急流广泛分布于塔克拉玛干沙漠上空,不仅影响塔里木盆地的降水和污染物扩散等天气现象,还会破坏风电设备、影响飞行安全和石油开采,给人类的生产和生活带来巨大危害。塔克拉玛干沙漠是中国乃至东亚最主要的沙尘源区,其释放的沙尘气溶胶不但能造成大气能见度降低和呼吸系统疾病大规模爆发等直接环境后果,还可以通过影响能量收支、云和降水等过程对区域甚至全球气候变化产生重要影响。以往关于塔克拉玛干沙漠上空低空急流的研究,主要针对低空急流强度、高度和时空分布等基本气候特征及其对南疆降水的影响等方面进行展开,缺少对日变化特征与形成机制的讨论;虽然已经对塔克拉玛干沙漠的沙尘活动开展了全面且深入的研究工作,但是低空急流对沙尘活动的影响并未引起足够的重视,关注点也仅限于对沙尘气溶胶的传输作用。针对上述存在问题,本文在前人研究的基础上,结合多种卫星遥感资料、常规地面观测资料、探空资料、再分析资料,使用中尺度数值气象模式WRF/WRF-Chem,系统研究了塔克拉玛干地区低空急流不同季节的日变化特征与形成机制,探讨了不同配置下模式对低空急流的模拟效果,明确了低空急流日变化机制及其对沙尘活动的影响,并量化了低空急流以及锋面气旋活动对夏季塔克拉玛干地区沙尘排放的贡献。其主要结论如下:

1、揭示了塔克拉玛干地区上空不同季节低空急流的日变化特征及形成机制。塔克拉玛干沙漠上空低空急流仅在日时间尺度上存在显著的周期性,不同季节强、弱低空急流风速和高度的日变化趋势基本相同,但在风速大小上存在差异,表现为春、夏强于秋、冬。强低空急流主要分布在沙漠腹地,存在明确的东-东北主导风向,而弱急流则主要发生在沙漠边缘,风向分布比较均匀,没有明显的主导风向。塔克拉玛干地区上空低空急流的形成是(i)高原-盆地产生的地形斜压性,(ii)沿库鲁克塔格山脉形成的下山气流,(iii)沙漠与盆地东面山地间的热力差异和(iv)湍流扩散系数的日变化四种机制共同作用的结果。

2、评估了不同配置对WRF模拟塔克拉玛干地区低空急流及近地面风速的影响。 使用NCEP-FNL再分析资料作为初始场和边界条件用于WRF动态降尺度研究塔克拉玛干地区的低空急流的垂直结构和日出后的近地面风速有最好的效果。十几公里的水平分辨率和默认的垂直网格足够模拟塔克拉玛干低空急流的主要特征,但模拟低空急流在日出后对近地面风速的影响时,更精细的水平分辨率以及将距离地面1000m以下垂直分辨率加密为20层可以获得更准确地近地面风速。只针对塔克拉玛干地区的低空急流模拟时,YSU非局地边界层参数化方案好于MYNN局地边界层参数化方案。通过地形敏感性实验和地表热通量实验发现,边界层湍流黏度的日变化对低空急流形成的贡献率高于地形作用。

3、明确了塔克拉玛干沙漠夜间低空急流日变化机制和对沙尘起沙和传输的影响。基于WRF-Chem模式成功地模拟了塔克拉玛干地区夜间低空急流的形成和日变化,分别从现象、量化分析和环境因素三方面明确了日出后夜间低空急流动量通过湍流混合向地表传递是清晨塔克拉玛干沙漠沙尘排放的直接原因,夜间低空急流的风速变化主要是由非地转风的惯性震荡所引起的,湍流的日变化很大程度上解释了低空急流的日变化。夜间悬浮在残留层中的沙尘气溶胶可以在夜间低空急流的作用下直接输送至西部的帕米尔高原或在西风带的作用下传输到更广泛的下游地区。

4、量化了低空急流和锋面气旋活动对塔克拉玛干沙漠沙尘排放的贡献。塔克拉玛干沙漠沙尘排放的日循环中,分别在日出后和下午存在两个沙尘排放峰值,发生在上午的沙尘排放贡献了塔克拉玛干地区总沙尘排放量的41%。塔克拉玛干地区有约38%的沙尘排放与低空急流有关,仅有约2%的沙尘排放来自于与气旋有关的锋面活动,剩余约60%的沙尘排放主要发生在下午,有极大的可能性来自于边界层内的干、湿对流、山谷风环流等未被定义的过程。

Other Abstract

The Low-level jets (LLJs) widely exist over the Taklimakan Desert (TD), affecting weather phenomena such as precipitation, pollutant dispersion, flight safety, wind power equipment, and human activities in the Tarim Basin (TB). TD is the most significant area of the contribution to dust aerosols in China, and the dust aerosols have triggered huge environmental impacts for the whole East Asia under the combined effects of topography, thermal conditions, and background circulation, including reduced atmospheric visibility and the outbreak of human respiratory disease. Previous studies on LLJs in the TD mainly focused on the climatic characteristics of the LLJs in different seasons, such as wind speed and height. However, these studies lacked discussions on the characteristics of diurnal variations and the formation mechanisms, as well as the effect of LLJs on dust activity, which has not received sufficient attention.

Based on previous studies, this paper systematically studys the diurnal variation characteristics and formation mechanisms of LLJs in the Taklimakan region in different seasons, explores the simulation effect of WRF model on LLJs with different configurations, clarifys the daily variation mechanism of LLJs and its influence on dust activity, and quantifys the contribution of the LLJs and frontal cyclone activity to dust emission in the Taklimakan region in summer, using a combination of multi-source remote sensing datasets, various ground sites observation datasets and reanalysis datasets, as well as the mesoscale Weather Research and Forecasting (WRF)/Chemical model (WRF-Chem). The main conclusions are as follows:

(1) The characteristics of diurnal variation and the formation mechanisms of LLJs over the TD in different seasons are revealed. LLJs over the TD only exhibit significant periodicity on a diurnal time scale, and the diurnal trends of wind speed and height of strong and weak LLJs are similar in different seasons, but there are differences in the magnitude of wind speed, which are stronger in spring and summer than in autumn and winter. Strong LLJs mainly exist in the hinterland of TD, with a clear east-northeast dominant wind direction, while weak jets mainly exist in the edge of TD, with a discrete wind direction. The formation of LLJs over the Taklimakan region is the result of four mechanisms: (i) the oblique topographic compressibility generated by the plateau-basin, (ii) the downhill flow formed cross the Kuruqtag Mountains, (iii) the thermal difference between the desert and the mountains to the east of TB, and (iv) the diurnal variation of the turbulent diffusion coefficient.

(2) The effects of different configurations on the simulation of LLJs and near-surface wind in the Taklimakan region with WRF model are evaluated. Results show that the use of the National Centers for Environmental Prediction Final (NCEP-FNL) reanalysis data as the initial field and boundary conditions for WRF dynamic downscaling can yield the best results for studying the vertical structure of LLJs and near-surface wind after sunrise in the TD. A horizontal resolution of a few dozen kilometers and a default vertical grid are sufficient to capture the main features of the LLJs over Taklimakan, but a finer horizontal resolution and vertical resolution encryption to 20 layers below 1000 m from the ground can yield more accurate near-surface wind speeds when simulating the effects of LLJs on near-surface wind speeds after sunrise. The Yonsei University (YSU) non-local boundary layer parameterization scheme is found to be better than the Mellor-Yamada-Nakanishi-Niino (MYNN) local boundary layer parameterization scheme for LLJs simulations in the Taklimakan region only. Through topography sensitivity experiments and surface heat flux sensitivity experiments, it was found that the diurnal variation of turbulent viscosity in the boundary layer contributes more to the formation of LLJs than the role of topography.

(3) The mechanism of the diurnal variation of Nocturnal Low-Level Jets (NLLJs) over the TD and its influence on dust emission and transport are clarified. By using the WRF-Chem model, the formation and diural variation of NLLJs in the Taklimakan region are successfully simulated. The study finds that the downward transfer of NLLJs momentum to the surface through turbulent mixing after sunrise is the direct cause of dust emission over TD in the early morning. Moreover, the wind speed variation of NLLJs is mainly induced by the inertial oscillation of non-geostrophic wind. The diurnal variation of turbulence viscosity largely accounts for the daily variation of NLLJs. The dust aerosols suspended in the residual layer during the night can be transported to the Pamir Plateau in the west by the NLLJs directly or to a wider downstream area by the westerly wind in the upper air.

(4) The contribution of LLJs and frontal cyclone activity to the dust emission over the TD are quantified. There are two peaks of dust emission in the diurnal variation in the TD, one appears after sunrise and the other occurs in the afternoon. The dust emission occurring in the morning contribute 41% of the total dust emission in the Taklimakan region. About 38% of the dust emission in the Taklimakan are associated with LLJs, while only about 2% of the dust emission come from cyclone-related frontal activity. The remaining approximately 60% of the dust emissions occur mainly in the afternoon, with a high probability of originating from undefined processes such as dry and wet convection and mountain-plain wind circulation within the boundary layer.

Subject Area大气边界层与大气湍流
MOST Discipline Catalogue理学 - 大气科学 - 大气物理学与大气环境
URL查看原文
Language中文
Other Code262010_120190900421
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/538986
Collection大气科学学院
Affiliation
兰州大学大气科学学院
Recommended Citation
GB/T 7714
韩梓航. 塔克拉玛干低空急流特征与形成机制及其对沙尘活动影响的模拟研究[D]. 兰州. 兰州大学,2023.
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