Collaborative Research: A Climatological Analysis of the Variability of Southerly Low-Level Jets and Their Relationship to Synoptic-Scale Disturbances

低空喷流（low - low jets, LLJs）是在地球表面上方约300至3000米的低层大气中发现的相对强劲的气流。llj在美国中部特别频繁，并强烈影响该地区的降水和强风暴气候学。偏南的llj将水汽和感热从墨西哥湾向北输送，并帮助破坏低层大气的稳定，从而导致云和降水的形成。主要研究者最近的研究表明，llj的频率变化更大，llj的特征（即空间格局、方向、高度和深度）比以前描述的更复杂。这些变化对大平原和中西部地区降水的频率、时间和位置有重要影响。该研究项目将明确llj频率和特征的变化程度，改进llj结构和演变的概念模型，并评估当前关于llj与天气尺度天气干扰（特别是中纬度气旋和高层极大风）之间联系的理论。本文将使用气候学和案例研究方法，1)根据气流结构对1991-2000年期间发生的急流事件进行分类；2)将不同类型急流的频率变化与大尺度环流异常联系起来；3)探索不同空间结构下llj的持续、演变和垂直结构。4)更好地定义llj与高空急流之间的联系；5)研究llj与中纬度气旋内气流之间的关系。将使用三个数据集。这些是国家气象局的探空仪观测数据，NOAA的剖面网络观测数据，以及NCAR/NCEP再分析数据。本研究项目是为数不多的对llj进行大规模、多事件气候分析的项目之一，也是唯一明确研究llj在多个时间尺度上的变异性的研究。这项研究将有助于应用科学和基础科学，并将使人们更好地了解美国大平原和中西部地区的降水变化。更好地理解llj与异常大尺度环流模式之间的联系，对于评估在扰动气候下llj的频率和特征以及相关降水的可能变化具有重要意义。经改进的热带气旋结构概念模式，有助解释数值天气预报模式的输出，并有助进行短期天气预报。这项气候学研究的发现也将为未来的机制研究和建模工作建立一个急需的基线。此外，研究结果将有助于验证或反驳我们目前对llj与高层喷气机之间联系的理解，以及llj与中纬度气旋内气流之间关系的理解。该项目的教育效益包括培训研究生，让代表性不足的本科生参与研究过程，以及开发改进的llj和中纬度气旋教学模块。

Low-level jets (LLJs) are currents of relatively strong winds found in the lower atmosphere between approximately 300 and 3000 meters above the earth's surface. LLJs are particularly frequent in the central United States and strongly influence the precipitation and severe storm climatology of this region. Southerly LLJs transport moisture and sensible heat northward from the Gulf of Mexico and help destabilize the lower atmosphere which leads to the formation of clouds and precipitation. Recent research by the principal investigators suggested that the frequency of LLJs is more variable, and that the character (i.e., the spatial pattern, orientation, height, and depth) of LLJs is more complex, than previously portrayed. These variations have important consequences for the frequency, timing and location of precipitation in the Great Plains and Midwest. This research project will specify the degree of variability in the frequency and character of LLJs, improve conceptual models of the structure and evolution of LLJs, and evaluate current theories regarding the linkages between LLJs and synoptic-scale weather disturbances, specifically mid-latitude cyclones and upper-level wind maxima. Climatological and case study methodologies will be used to 1) classify jet events that occurred during 1991-2000 according to their airflow configuration, 2) relate variations in the frequency of the different jet types to large-scale circulation anomalies, 3) explore the persistence, evolution, and vertical structure of LLJs with different spatial configurations, 4) better define the linkage between LLJs and upper-level jets and 5) investigate the relationship between LLJs and airstreams within mid-latitude cyclones. Three data sets will be used. These are the National Weather Service rawinsonde observations, observations from the NOAA Profiler Network, and the NCAR/NCEP reanalysis data.This research project is one of very few large-scale, multi-event climatological analyses of LLJs and is the only study to explicitly investigate the variability of LLJs at multiple time scales. The study will contribute to both applied and basic science, and will lead to a better understanding of precipitation variability in the Great Plains and Midwest of the United States. A better understanding of the linkages between LLJs and anomalous large-scale circulation patterns has significance for assessing possible changes in the frequency and character of LLJs and associated precipitation in a perturbed climate. The improved conceptual models of the structure of LLJs will aid in the interpretation of the output from numerical weather prediction models and will assist with short-range weather forecasting. The findings of this climatological study will also establish a much-needed baseline for future mechanistic studies and modeling efforts. Additionally, the results of the study will help validate, or disprove, our current understanding of the linkages between LLJs and upper-level jets, and the relationship between LLJs and airstreams within mid-latitude cyclones. Educational benefits of the project include the training of graduate students, involvement of underrepresented undergraduate students in the research process, and development of improved teaching modules on LLJs and mid-latitude cyclones.