Monday, 3 August 2015
Back Bay Ballroom (Sheraton Boston )
During the summer of 1993 the upper Mississippi River basin experienced its worst flood situation on record, causing damage estimated at $15-20 billion. This flood affected nine states and lasted for 2-3 months with extreme rainfall amounts corresponding to a return frequency of <1/100 year at many stations. We investigated the effect of quasi-stationary large-scale anomalies and terrain-induced low-level jets (LLJ) on mesoscale factors contributing to the flood by performing 30-day simulations with a mesoscale model. The simulations found: (1)The large-scale anomalies produced environmental conditions that favored a generation of MCC-like systems rather than individual convective cells or squall lines that would have produced less precipitation over large areas. Ambient conditions favoring mesoscale convective complex (MCC) include an intensified east-west orientation of quasi-stationary fronts, weaker upper tropospheric inertial stability, and a strong low-level jet (LLJ). (2) The anomalous ageostrophic wind component to the far south of the flood region (Texas area) and geostrophic wind component over the flood region bridged together to form an extended moisture conveyer (LLJ) that sustained the long-lasting 1993 flood. (3)The strong LLJs during the period were enhanced by dynamic effects (blocking and lee-side cyclonegenesis) and slope thermal effects exerted by the Rocky Mountains.
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