Monday, 3 August 2015
Back Bay Ballroom (Sheraton Boston )
The occurrence of deep convection over southern Quebec exhibits strong mesoscale variability. According to the 9-year radar climatology of deep convection by Bellon and Zawadzki (2003, or BZ03), a meridional axis of low convective activity extends from the Adirondack mountains of New York north through Montreal and into the Laurentian mountains. To the east, regional enhancements in convection within the Champlain Valley of New York extend northeastward through the eastern suburbs of Montreal. While these findings suggest strong topographic controls on the regional convection signature, BZ03's 9-year data analysis may have been insufficient to obtain a robust climatological signature. Moreover, they did not physically explain the mechanisms that might give rise to the observed deep-convection signature. Herein we build on BZ03 by extending the radar climatology to the 1993-2014 period and performing a suite of quasi-idealized, convection-permitting numerical simulations of convection events over the region. The extended climatology confirms that BZ03's results are robust, and that the regional convection patterns strongly depend on the low-to-mid-level geostrophic winds. To physically interpret these findings, we conduct quasi-idealized simulations that use the real terrain and land use of the region but are initialized horizontally homogeneously using composite soundings based on the climatology. This model setup is designed to directly test our hypothesis that topographic variations play a key role in the regional convection climatology. The simulations reveal that pockets of orographically induced convergence to the lee of larger terrain features explains most of the observed regional variability in convection occurrence.
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