Storm-Scale Dynamical Changes of Extratropical Transition Events in Present-Day and Future High-Resolution Global Simulations

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  • 1 Department of Geographic and Atmospheric Sciences, Northern Illinois University, DeKalb, IL
  • 2 Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC
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Abstract

Tropical cyclones (TCs) propagating into baroclinic midlatitude environments can transform into extratropical cyclones, in some cases resulting in high-impact weather conditions far from the tropics. This study extends analysis of extratropical transition (ET) changes in multi-seasonal global simulations using the Model for Prediction Across Scales-Atmosphere (MPAS-A) under present-day and projected future conditions. High-resolution (15 km) covers the Northern Hemisphere; TCs and ET events are tracked based on sea-level pressure minima accompanied by a warm core and use of a cyclone phase space method. Previous analysis of these simulations showed large changes in ET over the North Atlantic (NATL) basin, with ET events exhibiting a 4–5° northward latitudinal shift and a ~6 hPa strengthening of the post-transition extratropical cyclone. Storm-relative composites, primarily representing post-transformation cold-core events, indicate that this increase in post-transition storm intensity is associated with an intensification of the neighboring upper-level trough and downstream ridge, and a poleward shift in the storm center, conducive to enhanced trough-TC interactions after ET completion. Additionally, the future composite ET event is located in the right-jet entrance of an outflow jet that is strengthened relative to its present-day counterpart. Localized impacts associated with ET events, such as heavy precipitation and strong near-surface winds, are significantly enhanced in the future-climate simulations; 6-hourly precipitation for NATL events increases at a super-Clausius-Clapeyron rate with area-average precipitation increasing over 30%. Furthermore, intensified precipitation contributes to enhanced lower-tropospheric potential vorticity and stronger upper-tropospheric outflow, implying the potential for more extreme downstream impacts under the future climate scenario.

Corresponding author address: Allison C. Michaelis, Department of Geographic and Atmospheric Sciences, Davis Hall 118, Northern Illinois University, DeKalb, IL, 60115. E-mail: amichaelis@niu.edu

Abstract

Tropical cyclones (TCs) propagating into baroclinic midlatitude environments can transform into extratropical cyclones, in some cases resulting in high-impact weather conditions far from the tropics. This study extends analysis of extratropical transition (ET) changes in multi-seasonal global simulations using the Model for Prediction Across Scales-Atmosphere (MPAS-A) under present-day and projected future conditions. High-resolution (15 km) covers the Northern Hemisphere; TCs and ET events are tracked based on sea-level pressure minima accompanied by a warm core and use of a cyclone phase space method. Previous analysis of these simulations showed large changes in ET over the North Atlantic (NATL) basin, with ET events exhibiting a 4–5° northward latitudinal shift and a ~6 hPa strengthening of the post-transition extratropical cyclone. Storm-relative composites, primarily representing post-transformation cold-core events, indicate that this increase in post-transition storm intensity is associated with an intensification of the neighboring upper-level trough and downstream ridge, and a poleward shift in the storm center, conducive to enhanced trough-TC interactions after ET completion. Additionally, the future composite ET event is located in the right-jet entrance of an outflow jet that is strengthened relative to its present-day counterpart. Localized impacts associated with ET events, such as heavy precipitation and strong near-surface winds, are significantly enhanced in the future-climate simulations; 6-hourly precipitation for NATL events increases at a super-Clausius-Clapeyron rate with area-average precipitation increasing over 30%. Furthermore, intensified precipitation contributes to enhanced lower-tropospheric potential vorticity and stronger upper-tropospheric outflow, implying the potential for more extreme downstream impacts under the future climate scenario.

Corresponding author address: Allison C. Michaelis, Department of Geographic and Atmospheric Sciences, Davis Hall 118, Northern Illinois University, DeKalb, IL, 60115. E-mail: amichaelis@niu.edu
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