Transport from convective overshooting of the extratropical tropopause and the role of large-scale lower stratosphere stability
Dr. Cameron R. Homeyer
National Center for Atmospheric Research, Boulder, Colorado
23 January 2014, 4:00 PM
National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Reception at 3:30 PM
Deep convection that overshoots the altitude of the extratropical tropopause is an atmospheric phenomenon that has direct implications for global climate change. In particular, transport of tropospheric and stratospheric air across the tropopause affects the distribution and concentration of radiatively important (greenhouse) gases in the upper troposphere and lower stratosphere (LS) such as water vapor and ozone. In contrast to large-scale transport processes, transport in deep convection has been inadequately sampled and is not resolved in global climate models. In addition, the frequency, vertical extent, irreversibility, and environmental conditions conducive to deep overshooting convection are not well understood. Because the climate sensitivity to changes in LS water vapor in the extratropics is large, understanding and quantifying the impact of convective transport is a critical step for predicting chemistry-climate interactions.
In this talk, I will present direct observations of convectively injected water vapor in the LS from instruments aboard two aircraft operated during the Deep Convective Clouds and Chemistry (DC3) experiment. These observations were taken downstream of convective systems that were overshooting up to 4 km above the tropopause within an environment of reduced LS stability (a double tropopause). Motivated by these observations, I will also present simulations of observed convective systems with the Advanced Research Weather Research and Forecasting (ARW-WRF) model that are used to test the influence of the large-scale LS stability environment on the vertical extent of convective overshooting and transport. Three unique environments are identified: a double tropopause (largest reduction in LS stability), stratospheric intrusion, and single tropopause (no reduction in stratospheric stabiltity). Representative cases for each environment that have comparable magnitudes of convective available potential energy (CAPE) and vertical wind shear were selected for simulation. These simulations show that overshooting and transport is sensitive to changes in LS stability, with the deepest transport above the tropopause found in the double tropopause case.
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