Hurricane Force Winds in Extratropical Cyclones: The Role of Frontogenesis and Frontolysis
In this study we examine two intense North Atlantic storms, the first from early February 2011 and second from January 2013. Both storms were sampled during the rapid intensification phase by the NOAA WP-3D research aircraft as part of the Ocean Winds Winter Experiment. In each case GPS dropsondes and the Stepped Frequency Microwave Radiometer (SFMR) observed winds of hurricane force intensity. The Weather Research and Forecasting (WRF) model, version 3.4.1, was used to simulate the complex evolution of these cyclones and associated wind maxima. NCEP Global Forecast System Final (GFS FNL) analysis data at 0.5 degree resolution was used to initialize the 12 km resolution WRF simulation. Hourly output from the WRF simulations produced hurricane force winds (33 m s-1 and up to 37 m s-1 in some instances) during the rapidly deepening phases with the 2011 and 2013 storms having 43 and 38 hPa pressure falls in 24 hours, respectively. The 2011 cyclone showed a very complex evolution of the bent-back front and intense winds with multiple occurrences of hurricane force winds in the vicinity of the comma head. Satellite imagery confirmed the evolution of multiple comma heads. Although the WRF successfully simulated the evolution, aircraft and satellite data showed a persistent displacement of the area of high winds as compared to the WRF output. Wind field results from the 2013 storm showed excellent agreement with the WP-3D SFMR and dropsonde data and scatterometer winds from ASCAT.
Simulations of the storms showed hurricane force winds being produced by both cold conveyor belt jets and sting jets, sometimes simultaneously. We use Petterssen frontogenesis (frontolysis) to diagnose the mechanisms for momentum transfer of high winds to the surface. The concurrent evolution of the high winds and areas of frontogenesis/frontolysis will be shown, including instances where the winds and frontogenesis/frontolysis weaken significantly only to regenerate again at a newly formed strong area of frontogenesis/frontolysis. For example, in one instance in the 2011 storm, very high frontogenesis values were within 150 km of high frontolysis values, all within an area of hurricane force winds. Visualizations will include horizontal displays and cross sections of vertical motion and potential vorticity, as well as show the relationship between the wind fields and frontal evolutions.
Future work will include higher resolution simulations, investigation of the early evolution of the larger scale leading to the development of the bent-back front and high winds, trajectory analysis on the airflow through the low level jet and bent-back front to try and determine the origin and evolution of the flow, and sensitivity studies to determine the key parameters/contributions. The goal of this research is to help forecasters better understand and anticipate the evolution of extreme winds in extratropical cyclones.