Representation of the Extratropical Cyclone Wind Field and Warm Conveyor Belt in Climate and Weather Models

Intense precipitation and strong winds from extratropical cyclones impact millions of people along the Eastern United States each year. Climate models have been used to assess the changes in these storms, but the horizontal resolution of these models is typically 1 degree or more. Thus, it is not clear how well these climate models simulate the processes associated with the significant weather around these storms (strong winds and heavy precipitation).  The wind and precipitation structures in these models need to be compared with the spectrum of observed structures. The warm conveyor belt (WCB) is an important airstream for this heavy precipitation in the cyclone comma through moisture transport and vertical motion. This study will examine how well models at various model horizontal resolutions can simulate the surface wind field and WCB, and how any variations in the simulated WCBs can lead to precipitation and cyclone deepening differences.  This study uses the Weather Research and Forecasting (WRF) Model run at ~100 km and ~20 km grid spacing for a 10-year historical period (1996-2005) just during the cool season (January to March). One set of WRF runs is forced (initial and boundary conditions) using the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data, and another set using the CCSM4 6-hourly output from the Coupled Model Intercomparison Project Phase 5 (CMIP5). This study will focus on the strongest analyzed storms (< 990 hPa) along eastern North America and the western North Atlantic (20 to 60 N and 50 to 90 W). The time of maximum surface wind intensity (based on 10 m winds) will be used to compare the wind field between the analysis and the two different WRF resolutions. The time of maximum precipitation in the cyclone comma head will be used to evaluate the WCBs at the two resolutions. The origin, vertical displacement, and final locations of the WCB will be evaluated. The WRF at the two resolutions will be compared to the CFSR analysis structures. Lastly, the course resolution WRF results will be compared with the CCSM4 wind and WCB structures using the CCSM4 output directly for the historical period.