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

Abstract

Tropopause‐overshooting convection in the midlatitudes rapidly transports lower troposphere air and cloud material to the upper troposphere and lower stratosphere (UTLS), with notable events resulting in the formation of above‐anvil cirrus plumes (AACPs). However, there is limited understanding of how transport driven by overshooting convection and AACP properties are modified by variations in UTLS environments, especially the extent of stratosphere‐to‐troposphere (i.e., downward) transport. Here, AACP development and UTLS transport sensitivities to lower stratosphere stability and the UTLS wind environment are evaluated. The Bryan Cloud Model is used to simulate midlatitude supercell convection and evaluate resulting UTLS composition changes within two common midlatitude thermodynamic environments: a single tropopause and a double tropopause, and two UTLS wind environments: one with a stronger stratospheric wind profile supportive of frequent gravity wave breaking and AACP development and the other with a weaker stratospheric wind profile to suppress gravity wave breaking and discourage AACP formation. Multiple passive tracers and water vapor concentrations are used to evaluate the exchange of air between the troposphere and stratosphere and transport confined to each layer. It is found that greater troposphere‐to‐stratosphere transport (TST) and stratospheric hydration occurs in storms with AACPs, while stratosphere‐to‐troposphere transport (STT) is greater in storms without AACPs. This STT is facilitated by a mechanical oscillation induced by the overshoot. AACP‐producing storms have increased downward transport of stratospheric overworld air to the lowermost stratosphere, which is enhanced within a double tropopause environment. More expansive AACPs, deeper overshoots, and greater TST also occurs in double tropopause environments.

Deep Convective Cloud Top Altitudes at High Temporal and Spatial Resolution

“…For land‐based convection, where vertical velocities are stronger and turbulence is presumably enhanced, larger fractions of environmental air will be entrained into the plume. For cases where there is strong shear at the tropopause (Gordon & Homeyer, 2022), this entrainment will be even stronger. A refinement of the method would entail trying to incorporate these effects by having mixing vary according to convective strength (e.g., land vs. ocean) and vertical shear.…”

“…For land-based convection, where vertical velocities are stronger and turbulence is presumably enhanced, larger fractions of environmental air will be entrained into the plume. For cases where there is strong shear at the tropopause (Gordon & Homeyer, 2022), this entrainment will be even stronger.…”

Convectively Transported Water Vapor Plumes in the Midlatitude Lower Stratosphere

“…The smaller water vapor mixing ratio in the higher‐altitude moistening segments could also be limited to the strength of wind shear (Figure S3 in Supporting Information ) in our GEM simulation. Previous studies have shown that when the storm‐relative wind in the stratosphere is stronger, hydraulic jump occurs and AACP is produced, so a larger amount of water vapor is transported to higher altitudes (Gordon & Homeyer, 2022; Homeyer et al., 2017; O’Neill et al., 2021).…”

“…(2017). Since convective moistening can be produced by storms both with and without AACPs (Gordon & Homeyer, 2022; Homeyer et al., 2017; O’Neill et al., 2021), we analyze the water vapor plume regardless of the existence of the AACP. $$${{\mathrm{H}}_{2}\mathrm{O}\_\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{y}(t,z,\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{d}\mathrm{e},\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{d}\mathrm{e})={\mathrm{H}}_{2}\mathrm{O}(t,z,\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{d}\mathrm{e},\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{d}\mathrm{e})-\overline{{\mathrm{H}}_{2}\mathrm{O}}}_{0}(t0,z,\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{d}\mathrm{e})$$$ …”

“…However, a portion of mass can be irreversibly transported into the stratosphere driven by gravity wave breaking, sometimes forming an above anvil cirrus, often referred to as a “jumping cirrus” (Dauhut et al., 2018; Hanisco et al., 2007; Hassim & Lane, 2010; Homeyer et al., 2017; Qu et al., 2020; Wang, 2003). More recent studies have shown that the irreversible transport is attributed to the hydraulic jump and mechanical oscillation near the overshooting top (Gordon & Homeyer, 2022; O’Neill et al., 2021). Cold tropospheric air mixes with warmer stratospheric air during the transport, therefore ice rapidly sublimates, and water vapor plumes are formed, hydrating the subsaturated midlatitude stratosphere.…”