Extended Abstract (92K)4A.3
Hurricane force extratropical cyclones
Joseph M. Sienkiewicz, NOAA/NWS/NCEP, Camp Springs, MD; and J. M. Von Ahn and G. M. McFadden
Unlike tropical cyclones, extratropical cyclone intensity has typically been related (within the meteorological community) to the minimum central pressure and not the maximum wind speed. This, in large part, is due lack of consistent observations and the complexity of wind field. Satellite derived ocean vector winds from the NASA Seawinds scatterometer onboard the QuikSCAT satellite have given forecasters at the NOAA Ocean Prediction Center (OPC) the ability to routinely and consistently observe winds in excess of Hurricane Force (winds 32.7 ms–1 and higher) in association with extratropical cyclones. OPC marine forecasters are now able to differentiate between common STORM FORCE (winds 24.5 to 32.6 ms–1) and HURRICANE FORCE cyclones. Therefore forecasters are able to confidently categorize cyclone intensity by warning category (GALE, STORM, HURRICANE FORCE) and maximum wind speed.
Hurricane Force (HF) cyclone activity has been investigated for four winter seasons (October through April) over the North Atlantic and North Pacific for the years 2001 through 2005. HF cyclones were observed to be most frequent over the North Pacific in December and in January over the North Atlantic. For the first three years of the study, on average 20 HF cyclones were observed each season for each ocean basin. During the last season the number of observed cyclones increased to over 30 in the Atlantic and 25 in the Pacific. This increase is due to the OPC forecasters relying on the higher resolution 12.5 km QuikSCAT winds. There is less horizontal averaging between wind vector cells with the 12.5 km QuikSCAT thus a higher frequency of observed HF conditions.
Extreme conditions of HF are short lived (on average less than one day). Composites of the maximum wind for several cyclones show that the preferred location for HF winds is south of the cyclone center in a crescent shape wind maximum. These cyclones tend to be meteorological bombs having maximum deepening rates of one Bergeron or more. There are preferred tracks for occurrence for each ocean basin. Composites of 250 hPa winds, 500 hPa heights, and sea-level pressure for the preferred tracks illustrate the large-scale environment for extreme cyclone development. OPC forecast skill for day 2 and day 4 will be discussed. Some skill is evident for day 2 for both oceans and for day 4 for the North Atlantic. Forecast skill at day 4 over the North Pacific for these extreme cyclones is nearly non-existent.
Session 4A, Extratropical Cyclones and Fronts
Monday, 1 August 2005, 3:30 PM-4:45 PM, Ambassador Ballroom
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