Hurricane Force Winds in Extratropical Cyclones: a Modeling and Observational Study

The mission and responsibility of the NOAA/NWS Ocean Prediction Center (OPC) is to protect life and property at sea, provide safety, and enhance economic opportunity. Maritime extratropical cyclones, particularly in the winter months, can produce extreme winds and waves, therefore, can pose a significant threat to shipping, fishing, and oil exploration. Waves generated by these storms impact coastlines far removed from the storm system. These storms can be of very large sizes, with radii of gale force winds in excess of 800 n mi or more in some cases. Ocean surface winds measured from NASA's QuikSCAT satellite demonstrated that hurricane force winds occur more frequently in maritime extratropical cyclones than previously thought. Based on ten years of QuikSCAT data, 1999-2009, we know that extreme winds occur in the rapid deepening to mature phase of the life cycle, typically, on the cold side of the bent back portion of the occluded front. Because of the rapid intensification, the development of dangerous conditions, and inability of global numerical models to accurately predict the timing, structure and intensity of these storms, it is necessary to better understand the evolution of these storms and diagnose the causes of extreme winds with the goal of producing better forecasts.

This study follows a particularly strong North Atlantic storm during the period from 1200 UTC 31 January 2011 to 1200 UTC 2 February 2011. The Weather Research and Forecast Environmental Modeling System (WRF EMS) was used to model the evolution of this cyclone. This storm was chosen for examination because of its strength, rapid intensification, and high resolution in-situ data acquired by the NOAA WP-3D research aircraft as part of the Ocean Winds Winter Experiment. The NOAA WP-3D N43RF flew within the storm from 1605-1851 UTC on February 1 during rapid intensification and sampled hurricane force winds via the Stepped Frequency Microwave Radiometer (SFMR) and GPS dropsondes. The WRF EMS was used to downscale model data from the NCEP Global Forecast System (GFS), which was used to initiate the WRF EMS run. Input was every three hours at one degree resolution and output from the WRF EMS was hourly at 12 km resolution. The output showed a 1007 hPa low at 1200 UTC 31 January 2011 that rapidly deepened to 966 hPa at 1200 UTC 1 February 2011, a 41 hPa drop in 24 hours. The WRF EMS produced hurricane force winds three separate times over the 48 hour period to the south of the low. Omega, potential vorticity, and theta cross sections were generated throughout the storm evolution to better understand the evolution of the thermal and wind field structures.

Future work on this storm will include trajectory analysis of the airflow through the low level jet and bent back front to try and determine the origin and evolution of the flow. Dynamic diagnostic tools will also be applied to the storm and bent back frontal region in an effort to understand the forcing and resultant evolution. All of this is being done with the ultimate goal to help OPC forecasters better understand, anticipate, and predict these dangerous storms.