Quantifying the Direct Convective Transport of Tropospheric Air to the Overworld Stratosphere

Abstract:

The Harvard Water Vapor (HWV) instrument has repeatedly observed moist layers in the extra-tropical stratosphere, with some present well into the stratospheric overworld (>380 K) over the continental United States during the summer. Gravity wave breaking and/or turbulent mixing associated with deep overshooting convection can lead to the irreversible deposition of water as ice in this region, which rapidly sublimates in the undersaturated environment. This convective mechanism provides a means of delivering water vapor to the stratosphere that bypasses the strict thermal control of tropical cold-point tropopause temperatures. This mechanism may also efficiently deliver boundary-layer air, potentially rich in very short lived halogen species, directly to the overworld stratosphere and thus have implications for the future of ozone recovery. The present analysis examines in situ water vapor and long-lived tracer data acquired during encounters with convectively sourced plumes in the overworld stratosphere to quantify the amount of tropospheric air that is delivered via this mechanism. The trace species considered include CO, CH₄, CO₂ and O₃. The data were acquired aboard NASA’s WB-57 and ER-2 aircraft during multiple summertime missions, including the SEAC4RS mission, staged out of Houston, TX, over the past decade. A simple mixing model is used to constrain the convective contribution to each of the trace species. Additionally, we will discuss how the magnitude convective influence decreases with altitude above the tropopause, and what conditions favor convective penetration of the 380 K surface. Given the potential for dramatic changes to convective frequency and strength in response to climate forcing, it is imperative that this term be well characterized for incorporation into prognostic chemistry and climate models.