A Long-Term Climatology of Southerly and Northerly Low-Level Jets for the Central United States

Abstract

A forty-year (1961–2000) climatology of southerly and northerly low-level jets (LLJs) was prepared from twice-daily rawinsonde observations for thirty-six stations in the central United States. Although there is extensive literature on southerly jets, their spatial variability has received little attention. Furthermore, there is a paucity of studies examining any aspects of northerly LLJs. Thus, the motivation for developing the climatology was to (1) examine in greater detail the spatial variation of LLJ characteristics across the central United States, (2) better document the nature of northerly LLJs, (3) provide a baseline for evaluating potential future changes in LLJ frequency and characteristics, and (4) assess through comparisons with previous studies what aspects of LLJs in the central United States remain poorly understood. The climatology is a substantial temporal and spatial expansion of previous studies and provides a more thorough description of the spatial variability of LLJs in terms of frequency, direction, speed, and elevation at annual, seasonal, and daily time scales. The complex spatial variations of LLJ characteristics in the central United States are summarized by seven objectively defined regions within the central United States with similar jet characteristics. These regions (Central Plains, Upper Midwest, East Central, Southeast, South Texas/Western Gulf, Foothills, and High Plains) vary in shape and extent by season and are indicative of the varying influence of different forcing mechanisms for LLJ formation. Comparison of the findings of this study with those of previous climatological analyses suggests that many of the characteristics of LLJs in the central United States are still incompletely documented and understood.