A 22‐Year Evaluation of Convection Reaching the Stratosphere Over the United States

Abstract

Stratosphere‐reaching moist convection can significantly alter the dynamics, chemistry, and climate of the Earth system. This study seeks to add to the emerging understanding of the frequency, depth, and stratospheric impact of such events using 22 years (1996–2017) of ground‐based radar observations in the contiguous United States. While most prior studies identify such storms using the temperature lapse‐rate tropopause (LRT) as a troposphere‐stratosphere boundary, this study is the first to identify convection that reaches into stratospheric air below the LRT (tropopause depressions, excluding folds) as well. It is found that tropopause depression (TD) overshooting and LRT overshooting occur at similar frequency over the United States, with TD overshooting being more episodic in nature than LRT overshooting. TD overshooting is also found more often throughout the cooler months of the year, while LRT overshooting dominates all overshooting in the summer months. Stratospheric residence of overshoot material, as estimated using trajectory calculations driven by large‐scale winds, suggests that the vast majority of TD overshoot material does not remain in the stratosphere within 5 days downstream and rarely impacts altitudes more than 1 km above the LRT. Conversely, the majority of LRT overshoot material remains in the stratosphere downstream and routinely impacts altitudes >1 and >2 km above the tropopause.

Sensitivities of Cross‐Tropopause Transport in Midlatitude Overshooting Convection to the Lower Stratosphere Environment

“…These studies also show that overshooting convection has pronounced annual and diurnal cycles, with maximum overshooting occurring near the time of peak daily surface temperature and during times when the tropopause altitude is low. Approximately half of tropopause‐overshooting convection in the United States reaches the stratospheric overworld (potential temperature >380 K; Cooney et al., 2018), suggesting these events can have significant, long‐lasting impacts on stratospheric composition since air extends deeper into the stratosphere and thus takes longer to slowly descend and return to the troposphere (Holton et al., 1995; Homeyer & Bowman, 2021). While trends in overshooting frequency in observations or those projected for future climates have not been explicitly studied, the frequency of such thunderstorms may increase with a warming climate given that the frequency of favorable severe thunderstorm environments is projected to increase (Del Genio et al., 2007; Lepore et al., 2021; Trapp et al., 2007).…”

“…Tropopause‐overshooting convection occurs throughout the world, with higher frequencies in the midlatitudes and over land, especially within North and South America (Liu & Liu, 2016; Liu et al., 2020). In the Northern Hemisphere, overshooting is most common in the Great Plains of the United States and peaks in the warm season (March‐August: Cooney et al., 2018; Homeyer & Bowman, 2021; Solomon et al., 2016). These studies also show that overshooting convection has pronounced annual and diurnal cycles, with maximum overshooting occurring near the time of peak daily surface temperature and during times when the tropopause altitude is low.…”

Extreme Altitudes of Stratospheric Hydration by Midlatitude Convection Observed During the DCOTSS Field Campaign

“…The GridRad domain spans 24–50°N latitude and 235–294°E longitude. The radar reflectivity at horizontal polarization Z H is used to identify tropopause‐overshooting convection as Z H = 10 dBZ echo‐top altitudes above the tropopause, applying a few quality‐assurance criteria consistent with past GridRad overshoot climatologies (Cooney et al., 2018; Homeyer & Bowman, 2021; D. L. Solomon et al., 2016). As has been demonstrated in previous studies, GridRad echo‐top altitudes are unbiased and have an uncertainty of ±1 km.…”

“…All data analyzed are sourced from the airborne, remote sensing, and model collections of the DCOTSS archive (NASA, 2023a(NASA, , 2023b(NASA, , 2023c. DCOTSS was a multi-year mission based predominantly out of Salina, Kansas in the central U.S., near the climatological maximum in the frequency of tropopause-overshooting convection (Cooney et al, 2018;Homeyer & Bowman, 2021). DCOTSS employed the NASA ER-2 aircraft, equipped with 12 instruments to measure atmospheric trace gas and aerosol composition and additional meteorological variables, spanning altitudes from the surface up to ∼21.25 km above mean sea level (AMSL).…”

Climatology, sources, and transport characteristics of observed water vapor extrema in the lower stratosphere

“…Tropopause-overshooting convection typically results in the most extreme stratospheric hydration. Both regional and global climatologies of deep convection show that convection overshoots the extratropical tropopause (and occasionally reaches the stratospheric overworld) relatively frequently over land, especially in the Americas (Solomon et al, 2016;Cooney et al, 2018;Liu and Liu, 2016;Liu et al, 2020;Homeyer and Bowman, 2021). While some studies identify a minimal role of convective contributions to stratospheric water vapor (∼10%), these are typically restricted in focus to tropical convection and impacts on the stratospheric overworld (e.g., Dauhut and Hohenegger, 2022;Ueyama et al, 2023, and references therein).…”