Daniel Phoenix-April 17- Weather and Climate

Weather and Climate Systems Seminar

Impacts of Tropopause-Penetrating Convection on the Chemical Composition of the Upper Troposphere and Lower Stratosphere

Dan Phoenix

Wednesday, April 17

3pm/NWC 5600

Tropopause-penetrating convection is capable of rapidly transporting air from the lower troposphere to the upper troposphere and lower stratosphere (UTLS). Since the vertical redistribution of gases in the atmosphere by convection can have important impacts on the chemistry of the UTLS, the radiative budget, and climate, it has become a recent focus of observational and modeling studies. Despite being otherwise limited in space and time, recent aircraft observations from field campaigns such as the Deep Convective Clouds and Chemistry (DC3) experiment have provided new high-resolution observations of convective transport. Modeling studies, on the other hand, offer the advantage of providing high-resolution spatially and temporally continuous output related to the physical, dynamical, and chemical characteristics of storms and their environments.

To examine the impact of tropopause-penetrating convection on the chemical composition of the UTLS, two 10-day periods of high frequency, tropopause-penetrating convection over the United States were simulated using the Weather Research and Forecasting model with Chemistry (WRF-Chem). During this period, convection routinely injects high concentrations of water vapor (greater than 50 ppmv) into the lowermost stratosphere. Changes in water vapor are most sensitive to the height of the tropopause, the tropopause temperature, and the overshooting depth of the storm with gravity wave breaking being the dominant mechanism responsible for irreversible transport. Convection also has a net effect of decreasing (increasing) upper tropospheric concentrations of ozone (carbon monoxide).

To evaluate the mechanisms responsible for stratosphere-to-troposphere transport (STT) of ozone-rich air, high-resolution simulations of a case with observed STT around the anvil of a mesoscale convective system (MCS) were performed. Several hypotheses, which include dynamic instabilities, mass conservation, and ageostrophic circulations driven by pressure perturbations are evaluated. Model results suggest that this transport pathway occurs as a two-step process: (1) downwelling that is driven by mass conservation as the MCS deposits air into the UTLS and (2) differential advection of outflow air in the upper troposphere, which wraps high ozone air around and under the MCS anvil. Dynamic instabilities are not a leading contributor to this transport process. Although WRF-Chem appears to adequately simulate this transport, trajectory calculations indicate that the transported air does not originate above the lapse-rate tropopause. Since observations showed ozone concentrations in excess of 200 ppb (typical of the lower stratosphere), this suggests that the model did not fully represent this transport process.