Impact of Dynamical Downscaling Two CMIP5 Models on the Historical and Future Changes in Winter Extratropical Cyclones Along the East Coast of North America

This study investigates the impact of dynamical downscaling on the historical and future projections of winter extratropical cyclones over eastern North America and the western Atlantic. Six hourly output from two global circulation models (GCMs), CCSM4 and GFDL-ESM2M, from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are used to create the initial and boundary conditions for 20 historical (1986-2005) and 20 future (2080-2099) winters using Weather Research and Forecasting (WRF) model. Two sets of WRF grid spacing (1.0 degree and 0.2 degree) are examined to determine the impact of model resolution. Although the cyclone frequency in the WRF runs is largely determined by the GCM predictions, the higher-resolution WRF reduces the cyclone intensity underprediction. There is an increase in late 21^st Century cyclone activity over the East Coast of North America in the CCSM4 and its WRF, while there is little change in the GFDL-ESM2M and WRF due to a larger decrease in the temperature gradient in this region. There is a future increase in relatively deep cyclones over the East Coast in the high-resolution WRF forced by CCSM4. These storms are weaker than the historical cases early in their life cycle, but then because of latent heating they rapidly develop and become stronger than the historical events. This increase does not occur in the low-resolution WRF or the high-resolution WRF forced by GFDL since the latent heat increase is relatively small. This implies that the diabatic processes during cyclogenesis may become more important in a warmer climate, and these processes may be too weak in existing coarse-resolution GCMs.