Hurricane Force Winds in Extratropical Cyclones: A Trajectory Analysis of the Wind Maximum in the Vicinity of the Bent-back Front

Satellite scatterometer derived ocean vector winds from the NASA QuikSCAT, EUMETSAT ASCATs, and Indian OSCAT instruments have shown that winds of hurricane force occur relatively frequently in cold season maritime extratropical cyclones. Extreme winds occur in the rapid deepening to mature phase of the cyclone life cycle, typically, on the cold side and downstream of the intense baroclinic zone of the bent-back front. Maritime extratropical cyclones can be very large, rapidly moving, and quickly evolving. Although these storms are rather common during the cold season, each one has the potential to pose multiple threats of high winds, extreme waves, and storm surge to waterborne activities such as transportation, fishing, the energy industry, and to coastal communities. Understanding the evolution of extreme winds in oceans storms is key to predicting the threats associated with maritime extratropical cyclones.

We examine two intense storms in the North Atlantic basin, 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 on the cold (south) side of the intense bent-back frontal zone. The Weather Research and Forecasting (WRF) model, version 3.4.1, was used to simulate the complex evolution of each cyclone 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. The WRF produced hurricane force winds (33 m s-1 and up to 37 m s-1 in some instances) in both storms during the rapidly deepening phases with the 2011 and 2013 storms having 43 and 38 hPa pressure falls in 24 hours, respectively. The WRF simulation of the 2011 cyclone showed 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. The Read/Interpolate/Plot (RIP) program was used on the WRF output to produce back trajectories to analyze the sources and paths of air parcels within the high wind region in the bent-back fronts.

Model simulations of the two case studies as well as back trajectories of air parcels within the high wind regions tell a different story of the evolution of the high winds as has been discussed in the current literature. Evidence in the high wind regions within the bent-back front of these two case studies do not support the transfer of high momentum air of a sting jet originating in the mid-levels to the surface or of a continuous cold conveyor belt jet wrapping completely around the cyclone, two of the most common explanations for extreme winds in the current literature. For example, in the 2013 case, back trajectories at the 850 hPa and 925 hPa levels show air that streams in from the west and decelerates until the cyclone center passes to the south of the parcels causing the air to rapidly accelerate through the low level jet that developed in the bent-back front region. A third back trajectory at 700 hPa shows a parcel that originates to the south of the cyclone center and is drawn northward and wraps around the center of the storm cyclonically while rapidly ascending over the bent-back front. Multiple trajectories will be shown for both case studies and overlaid on other imagery and appropriate WRF fields. The forces acting on the parcel trajectories will be presented to help determine the evolution of the extreme winds within the bent-back frontal region.