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

低空急流（Low-level jet，LLJ）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风。 LLJ在美国中部特别频繁，并强烈影响该地区的降水和严重风暴气候。 南风LLJ将水分和感热从墨西哥湾向北输送，并有助于破坏低层大气的稳定，从而导致云和降水的形成。 主要研究人员最近的研究表明，LLJ的频率变化更大，而且性格（即，LLJ的空间模式、方向、高度和深度）比先前描绘的更复杂。 这些变化对大平原和中西部降水的频率、时间和位置有重要影响。 这一研究项目将具体说明低空急流的频率和特征的变化程度，改进低空急流结构和演变的概念模型，并评价当前关于低空急流与天气尺度天气扰动，特别是中纬度气旋和高层风极大值之间联系的理论。 将采用气候学和个例研究方法1）根据气流结构对1991-2000年发生的急流事件进行分类，2）将不同急流类型频率的变化与大尺度环流异常联系起来，3）探索具有不同空间结构的LLJ的持续性，演变和垂直结构，4）更好地定义LLJ和高层急流之间的联系; 5）研究LLJ和中纬度气旋内气流之间的关系。 将使用三个数据集。 这些都是国家气象局rawinsonde观测，从NOAA廓线网络的观测，和NCAR/NCEP再分析数据。这项研究项目是极少数的大规模，多事件的气候分析LLJ之一，是唯一的研究，明确调查LLJ在多个时间尺度上的变化。 这项研究将有助于应用科学和基础科学，并将有助于更好地了解美国大平原和中西部的降水变化。 更好地了解LLJ和异常大尺度环流模式之间的联系，对于评估扰动气候中LLJ的频率和特征以及相关降水的可能变化具有重要意义。 LLJ结构的改进概念模型将有助于解释数值天气预报模型的输出，并将有助于短期天气预报。这项气候研究的结果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，这项研究的结果将有助于验证或反驳我们目前对LLJ和高层急流之间联系的理解，以及LLJ和中纬度气旋内气流之间的关系。 该项目的教育效益包括培训研究生，使代表性不足的本科生参与研究过程，以及开发关于LLJ和中纬度气旋的改进教学模块。

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.