Extended Abstract (4.6M)10B.5
An investigation into the potential effects of recurving western Pacific tropical cyclones on the large-scale circulation
Randell J. Barry, University at Albany/SUNY, Albany, NY
Observations of tropical cyclones in the Western Pacific show that recurving storms may affect the large-scale flow in two ways. The first effect is a significant increase in windspeed at jet level often occurs within 24 hours after recurvature. The second effect is Rossby wave development downstream subsequent to recurvature. To document and begin to investigate the potential role the cyclones are playing in forcing these phenomenon, a composite analysis of selected storms over a ten year period of record has been conducted. Composites were developed based on each storm's impact on the large-scale flow. These impacts (i.e., jet acceleration or wave development) were categorized as strong, moderate or weak using objectives indices. The jet acceleration index used total windspeed increase at jet level downstream of recurvature while meridional windspeed increase at jet level downstream of recurvature was used for the Rossby wave development index.
Based on the index values, composites of selected meteorological parameters and diagnostics were produced for each category - strong, moderate, and weak jet acceleration and strong, moderate, and weak Rossby wave development. Results indicate that when there is a strong acceleration of the jet, the storm interacts with the midlatitude baroclinic westerlies in a manner that allows continuous diabatic heating in the region of storm but also allows for the downstream advection of this warm air on the warm side of the upper level front. This leads to an increase in the thermal gradient associated with the front in the region downstream of the disturbance. The increased thermal gradient, in turn, forces the jet acceleration. In the case of weak acceleration, results indicate that the midlatitude flow is initially too strong such that it rapidly weakens the storm through shearing effects and limits the aforementioned frontogenesis. Strong wave development occurs when two conditions exist. The first is the existence of a potential temperature gradient on the dynamic tropopause for a significance distance downstream from the point of recurvature (i.e., a potential vorticity gradient to serve as a waveguide). The second condition is midlatitude flow that allows concentrated diabatic heating in the region of the storm as it recurves. This heating rapidly builds an upper level ridge which in turn excites downstream Rossby waves.
Session 10B, Large-Scale Dynamics and Convection V (Parallel with Sessions 10A, 10C, and 10D)
Wednesday, 1 May 2002, 2:00 PM-3:30 PM
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