Session 13A.5 The extratropical transition of Hurricane Helene (2006): Observation of structural evolution and operational model evaluation using QuikSCAT

Friday, 29 June 2007: 11:30 AM
Summit A (The Yarrow Resort Hotel and Conference Center)
Michael J. Brennan, UCAR/National Hurricane Center, Miami, FL; and J. R. Rhome and R. D. Knabb

Presentation PDF (1.4 MB)

The evolution of the horizontal wind field of a tropical cyclone (TC) as it undergoes extratropical transition (ET) determines the distribution of hazardous conditions around the cyclone as it moves to higher latitudes. While the scarcity of in-situ observations over the open oceans makes analysis of this evolution difficult to observe, ocean surface vector wind (OSVW) data from the NASA QuikSCAT scatterometer can provide spatially consistent snapshots of the cyclone's wind field during the ET process. QuikSCAT retrievals are usually more reliable in the periphery of TCs, outside areas of heavy precipitation. However, as convection diminishes near the center of the TC during ET, QuikSCAT wind retrievals become more reliable near the cyclone center, making them more useful in the estimation of cyclone intensity (maximum sustained surface wind). This evolution is examined in Atlantic basin Hurricane Helene (2006).

Helene had reached a peak intensity of 105 kt on 18 September, but weakened to 70 kt as the storm turned toward the northeast on 22 September. During the early stages of ET (22–24 September), Florida State University (FSU) cyclone phase space diagrams indicate that Helene maintained a deep warm core while the circulation became more asymmetric. Early on 23 September, Helene attained a secondary peak in intensity of 80 kt, as determined by QuikSCAT data in a convection-free area to the southwest of the center. As Helene intensified, the azimuthally averaged radius of 34-kt winds around the cyclone increased by ~40%. The wind radii continued to expand as Helene gradually weakened to 65 kt early on 24 September.

Several QuikSCAT passes over Helene during the early stages of ET are examined, and the evolution of the 34-kt wind radius in each quadrant of the cyclone is documented. Additionally, operational numerical weather prediction (NWP) model forecasts of the outer wind field of Helene during this period are evaluated. In particular, this study examines the sensitivity of model surface wind field forecasts of Helene during ET to (i) the initial vortex structure in the model analysis, and (ii) the varying degree of interaction of Helene with a mid-latitude upper-level trough in model forecasts. It is hypothesized that forecasts of Helene by models utilizing a symmetric TC vortex bogus struggled to capture the development of asymmetries in Helene's wind field during ET, particularly in short-range forecasts (12–24 hours).

Implications of these results for operational forecasting of wind radii during ET are discussed, along with the potential utility of future OSVW missions for observing these wind field evolutions.

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