Modeling Extremely Deep Convection over North America as a Source of Stratospheric Water Vapor

Abstract:

We have run the Advanced Research Weather Research and Forecasting Model (ARW) at scales that numerically resolve convection over a broad swath of the north central U.S. Our intentions were to simulate convective events that generated stratospheric water vapor plumes observed during the SEAC4RS mission, to quantify the amount of water vapor injected into the stratosphere by extremely deep convection, and to investigate ARW as a potential tool to forecast multi-decadal trends in extremely deep convection over North America. We have run ARW for five and a half days beginning at 12 UTC on 26 August 2013 on a 3-km horizontal grid with 50 vertical levels. We used MERRA for the initial conditions and boundary conditions because of its skill in reanalysis of water vapor. ARW was able to simulate many of the fundamental features of deep convection over North America, including specific events. We have shown that the convection simulated by ARW bears many of the features of mesoscale convective systems, including the flow of cold air over warm moist air, cold downdrafts and gust fronts, mid-level inflow, and wedges reminiscent of squall lines. The source of water vapor for the convection is low-level eastward transport into the ARW domain. Convection is initiated where local maxima in equivalent potential temperature of surface air form. Convection regularly penetrates to the level of neutral buoyancy of the surface air and can even influence the concentration of water vapor above. A few convective events inject water vapor above the 400 K potential temperature surface. Surprisingly, deep convective events can also desiccate the upper air, even in the stratosphere. There is clear evidence of convection generating ducted internal gravity waves that propagate upstream to trigger more deep convection. We will present a quantification of the amount of water vapor injected into the stratosphere by extremely deep convection, the causes of desiccation, and the mechanisms responsible for triggering extremely deep convection.