Discriminating Factors that Favor the Development of High-Impact Weather Events in Association with Polar–Subtropical Jet Superpositions

Abstract

Abstract A vertical superposition of the polar and subtropical jet streams constitutes a unique synoptic-scale environment with the potential to induce high-impact weather, including anomalously strong surface cyclones that are accompanied by heavy precipitation and strong winds. Jet superpositions are not always a sufficient condition for the occurrence of high-impact weather, however, so understanding the dynamical and thermodynamic environments that favor the development of high-impact weather in association with jet superpositions is essential for improving sensible weather forecasts during these events. In this study, we pair a climatology of jet superpositions with climatologies of atmospheric rivers and surface cyclones to determine the frequency with which these features accompany jet superpositions. We subsequently construct two subsets of jet superpositions for further analysis. “High-impact” jet superposition cases are defined as those that feature an atmospheric river and a highly anomalous surface cyclone relative to climatology, which can potentially support extreme near-surface winds and precipitation. In contrast, “null” cases are defined as jet superposition cases that are not associated with a surface cyclone and are therefore less likely to support widespread high-impact weather. Composite analyses are then performed to identify discriminating environmental factors between high-impact and null cases, and how these factors influence jet superposition dynamics. We find that stronger environmental baroclinicity and a sufficient moisture supply within the near-jet environment are common characteristics of high-impact cases. These characteristics subsequently support the development of a more vigorous ageostrophic transverse circulation beneath the superposed jet’s exit region during high-impact cases and more intense surface cyclogenesis. Significance Statement A jet superposition event occurs when the normally separate polar and subtropical jets combine to form a single jet. This study aims to understand what factors differentiate jet superposition events that coincide with strong winds and heavy precipitation, or “high-impact weather,” from those that are less likely to coincide with such weather conditions. We identified several important environmental characteristics that tend to precede jet superposition events with a large potential to induce high-impact weather, including increased moisture and a strengthened pole-to-equator temperature gradient. These results provide indicators forecasters may consider when predicting the impacts of a jet superposition event.