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Operational Implementation of an Objective Annular Hurricane Index

A n d r e a B . S c h u m a c h e r 1 , J o h n A . K n a f f 2 , T h o m a s A . C r a m 1 , M a r k D e M a r i a 2 , J a m e s P . K o s s i n 3 (Presenter email address: schumacher@cira.colostate.edu)

1CIRA, Colorado State University, Fort Collins, CO 2NOAA/NESDIS, Fort Collins, CO 3CIMSS, University of Wisconsin-Madison, Madison, WI

Annular hurricanes are a special class of storms characterized by a large, symmetric eye and very little rain outside the eyewall. Annular hurricanes pose a distinct challenge when forecasting hurricane intensity, as they tend to maintain their intensity for longer than average storms. For this reason, an objective Annular Hurricane Index (AHI) was developed and implemented at the National Hurricane Center at the end of the 2006 season for use in 2007. Beginning in 2007, the AHI will be available to the NHC forecasters as part of the output from the SHIPS intensity model.

Knaff et al. (2003) determined that annular hurricanes exist in a specific set of environmental conditions. Hence, the AHI uses environmental factors such as vertical wind shear, 200-hPa zonal wind, and sea surface temperature as well as storm structure characteristics to determine the existence of annular structure. The predictor values were determined by examining NCEP environmental fields and GOES IR imagery over the period of 1995-2003 for annular and non- annular hurricane cases. The algorithm’s development uses a two step process. The first step screens the cases using environmental (i.e., within 3 standard deviations of annular cases) and IR imagery characteristics (e.g., large circular eyes, limited rainbands etc.). The remaining cases are then used to develop a linear discriminant function.

In real time, the AHI product acquires large-scale environmental values from the SHIPS model and information on structure from GOES IR imagery. If a real-time case does not pass the screening, the AHI is set to 0. If, on the other hand, a real-time case passes the screening, the discriminant function is then run to create the AHI. The AHI has a value ranging from 0 to 100, with 0 indicating no annular structure, 1 indicating the worst match to annular structure, and 100 indicating the best match to annular structure. Post-analysis of the 2004-2006 Atlantic and E. Pacific hurricane seasons has provided positive results that suggest the AHI will be a skillful tool in the objective identification of annular hurricanes. Product validation will continue throughout the 2007 season.

The views, opinions, and findings in this report are those of the authors and should not be construed as an official NOAA and/or U.S. Government position, policy, or decision.


Knaff, J.A., J.P. Kossin and M. DeMaria, 2003: Annular hurricanes. Wea. Forecasting, 18, 204-223

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