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A TECHNIQUE TO PREDICT THE EXTRATROPICAL TRANSITION OF TROPICAL CYCLONES.

By

Elizabeth A. Ritchie1, J. S. Tyo2, and O. Demirci3

An abstract submitted to the 61st Interdepartmental Hurricane Conference New Orleans, 5–9 March, 2007. March

Extratropical transition (ET) is a process that a tropical cyclone can undergo as it moves to higher latitudes over colder seas and interacts with the midlatitude regime. The resulting extratropical cyclone can be powerful with heavy precipitation to the left of track and a large extent of gale-force-winds to the right of track producing potential hazards to coastal settlements and maritime activities. The whole transition process typically takes 18-48 hours to be completed with two basic possible end results, either dissipation or intensification of the combined system. These two classes of tropical cyclones behave very similarly during the early stages of extratropical transition. Thus, the end result of this transition is very hard to predict accurately even with full-physics atmospheric prediction systems such as the Navy’s Operational Global Assimilation and Prediction System. Although there are many factors involved in the process, research has shown that the reintensification of the extratropical cyclone results primarily from its interaction with a midlatitude trough. Thus, the end result of extratropical transition may depend more on the phasing between the tropical cyclone and the trough it moves into rather than the details of the tropical cyclone structure.

Here we present results from a multi-stage technique to predict the simplest of ET outcomes: whether an ETing system will reintensify or dissipate after ET. To date the system uses NOGAPS analyses as input to a statistical technique that separates the discriminating large scale factors associated with reintensifying or dissipating ET cases. Using these data we have achieved a best performance of 88% detections with 27% false alarm rate on test storms from 2003 and 2004 with a limited training set from 1997 - 2002.

The system is not yet fully operational-capable: we are working on the last few steps needed for it to run in a full forecast mode. In addition, we are looking at creative ways to increase the training set so that more useful classes of ET can be discriminated by this system. These classes include: dissipation, weak reintensification; moderate reintensification; strong reintensification; and timing to minimum pressure (e.g., 24 hours, 48 hours, > 72 hours).

1 2 3

University of Arizona, Tucson AZ. ritchie@atmo.arizona.edu University of Arizona, Tucson AZ. tyo@optics.arizona.edu University of New Mexico, Albuquerque, NM. demirci@ece.unm.edu

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