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

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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.