75 Modeled Response of Midlatitude Cyclones to Projected Changes in North American Snow Cover

Monday, 7 January 2019
Hall 4 (Phoenix Convention Center - West and North Buildings)
Ryan Clare, Univ. of Wisconsin–Madison, Madison, WI; and A. R. Desai, M. Notaro, J. Martin, and S. J. Vavrus

It has long been hypothesized that snow cover and snow extent have an influence on the development or steering of synoptic mid-latitude cyclones (MLCs). Rydzik and Desai (2014) showed a robust statistical relationship among snow cover extent, generation of low-level baroclinicity, and MLC trajectories. Though snow cover extent is highly variable year to year, the changing global climate is expected to continue an already observed pattern of poleward retreat of mean snow cover in North America, particularly in late winter and spring. Modelling experiments have also shown a poleward shift in the tracks of MLCs in many regions of the Northern Hemisphere. For this experiment, large ensemble simulations with the Weather Research and Forecasting model (WRF) were forced with projected late twenty-first century extents of snow from models of the CMIP5 project to test the effect contributed solely by the snow cover of a changing climate. Our experiment induces an adjustment to the extent of snow cover in North America according to tenth, fiftieth, and ninetieth percentile reductions in winter and spring snow cover for the late twenty-first century (2080-2099) in the RCP4.5 and RCP8.5 experiments initialized zero to four days prior to cyclogenesis. The effects on 15 individual MLC cases across 375 distinct simulations are analyzed. We demonstrate that snow cover reductions directly induce the intensification of MLCs and cause them to deviate from their original trajectory while increasing the amount of precipitation and reducing the amount of that precipitation which falls as snow. It is shown that these changes correspond with the shifting of low-level baroclinicity and enhanced moisture flux into the boundary layer.
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