Tuesday, 8 November 2016
Broadway Rooms (Hilton Portland )
The 2015 Plains Elevated Convection At Night (PECAN) experiment focused on the development, structures, and dynamics of nocturnal convective systems. It is frequently assumed that nocturnal MCSs are “elevated”, that is, entirely decoupled from the nighttime stable boundary layer. Perhaps the most confounding and important aspect of this mystery is the creation and fate of convective downdrafts and outflow in cases with a nocturnal stable layer. Knowing if MCSs are elevated is important because convection that is coupled to surface air to some degree is far more likely to produce severe weather. Additionally, knowing what controls nocturnal MCS longevity, strength, and motion is key to diagnosing both severe weather and heavy precipitation threats. The following central questions help to motivate the present study: (i) what determines whether MCS outflow is manifest as a cold pool vs. a bore or gravity waves, (ii) what are the characteristic cold pool, bore, and gravity wave motions, and (iii) what is the geometry of the induced upward parcel displacements for each mode?
This study incorporates simulations in both an idealized framework with CM1 (using mobile sounding observations from PECAN) and a real world framework with WRF (using NAM or RAP initial and boundary conditions). We use four well-simulated cases to address the specific aims from above: an elevated “bow and arrow” MCS from 24 June that produced both marginally severe surface winds and regional flooding; an initially elevated QLCS from 25 June that produced increasingly severe surface winds as the evening progressed; a likely surface-based QLCS from 5 July that produced severe winds and a weak observed tornado; and, a bow echo from 12 July that occurred in an environment with an extremely shallow nocturnal stable layer. We will present comparisons among these MCSs’ outflows, surface winds, inflow parcel trajectories, and structural evolution over time.
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