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The Role of Subtropical Intrusion in the Development of Typhoon Usagi (5W) 2007

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Master's thesis

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During July 2007, fields from both the NCEP GFS final analyses and ECMWF model of OW and SF analyses suggests the development of two distinct areas with sub-tropical intrusion from remnants of a decaying baroclinic system in the WNP. This analysis of the formation of Usagi points to sub-tropical intrusion of a strong lower-tropospheric baroclinic system undergoing decay as potential seedlings for typhoon formation in areas of high sea surface temperatures, weak low-level vertical wind shear, and persistent convection. As the PV anomalies is stretched and detached from the baroclinic source region, it is wrapped around a strong tropospheric anticyclone in the subtropics. This constitutes a different type of baroclinic initiation process than has been previously identified in Atlantic cyclone formation events associated with TT, which are induced by upper-level troughs. The area of high values of OW at the tip of a PV streamer favors sustained deep convection, which will enhance the low-level vorticity and moisten the mid-level thereby producing high values of SF. The area of strong vorticity at the tip of the second PV streamer possessed both high OW and SF, favoring deep convection and cyclonic vortex tube stretching that appeared to culminate in an enhancement of lower tropospheric cyclonic vorticity. Although this analysis was originally motivated by initial analyses suggesting that ex-hurricane Cosme underwent a direct vorticity interaction with the second PV streamer, our revised hypothesis on the role of Cosme is that it may have enlarged the wave pouch and helped preserve the mid-tropospheric circulation from hostile outside influences. In this sense, the Cosme wave may have played an important indirect role in the formation of Usagi. This hypothesis requires further investigation.

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  • Meteorology
  • Physical and Dynamic Oceanography

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