Seventh Annual AMS Student Conference


Late season tropical cyclone formation in the northeastern Atlantic Ocean: 1975-2005

Rachel G. Mauk, Ohio State University, Columbus, OH; and J. S. Hobgood

In the past thirty years, twenty named tropical cyclones formed in the northeastern Atlantic Ocean in the months of October, November, and December. By the accepted definition of a favorable environment for tropical cyclone formation, most of them should not have developed. The past seven seasons produced ten of the twenty, with 2001 and 2005 spawning three and four systems, respectively.

The study period begins in 1975, the year in which the National Hurricane Center (NHC) implemented the Dvorak and Hebert-Poteat techniques for satellite identification. The northeastern Atlantic is defined as the portion of the Atlantic north of 20N and east of 60W. Purely subtropical storms are excluded from the study to focus on the conditions for tropical transformation.

The climatology of the twenty late-season tropical cyclones (LSTCs) is discussed including the peak development periods, location, maximum strength, and type of non-tropical origin. LSTCs in this region arise from four unique origins, providing a convenient way to classify them. Type I originate as pre-existing, non-frontal and non-tropical cyclones. Type II systems develop along dissipating frontal systems. Type III develop directly from an occluded frontal cyclone, while Type IV have tropical origins. Thirteen of the twenty systems are Types I and II, and the remaining systems are split relatively evenly between Types III and IV.

The incipient environment is examined in the twenty-four hours leading up to the system's naming. Wind fields are calculated on a 13x13 Lagrangian grid. Wind shear is calculated for the 850-200hPa, 850-300hPa, and 850-500hPa layers. The shear values are averaged on 3x3 and 1x1 grids for quantitative comparison of systems. The synoptic environments are also visualized on a 13x13 Lagrangian grid; of particular interest are the locations of upper-level troughs, ridges, and upper-level cold lows. Using this data, the wind conditions favorable for the formation of LSTCs are analyzed.

Poster Session 1, Student Conference General Poster Session
Sunday, 20 January 2008, 5:30 PM-7:00 PM, Exhibit Hall B

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