JP3.44 Analysis of the Wind Field Evolution Associated with the Extratropical Transition of Bonnie (1998) (2008

JP3.44

Analysis of the Wind Field Evolution Associated with the Extratropical Transition of Bonnie (1998)

Clark Evans, Florida State University, Tallahassee, FL; and R. Hart

The extratropical transition (ET) of tropical cyclones is a process that

both contains and introduces uncertainty into the synoptic-scale weather

pattern. Inherent to this is the meso- and synoptic-scale wind field

evolution associated with the transitioning cyclone. On the mesoscale,

this wind field evolution is manifest through the outward movement of the

radius of maximum winds; on the synoptic scale, it is manifest through

the acceleration of the wind field at outer radii. To date, no studies

have considered the mechanism(s) by which this evolution occurs, with

previous informal theories failing to capture the observed evolution.

This leads to uncertainties in operational observations and forecasts of

the wind field during ET.

In this study, the wind field evolution associated with a representative

cold-core ETing cyclone, North Atlantic Tropical Cyclone Bonnie (1998),

is analyzed. The synoptic-scale wind field evolution is shown to be a

function of the increasingly asymmetric nature of the cyclone leading to

both responses in the height and temperature gradients driving the wind

field as well as enhanced momentum transport. The mesoscale wind field

evolution is shown to be a function of the transition of the cyclone from

a warm-core to cold-core vortex. Results from spatial analyses of the

wind field and related meteorological fields, a momentum budget analysis,

and a balanced secondary circulation model are shown to complement the

findings, ultimately providing insight into a previously uncertain

process.