Structure of an Atypical Midsummer Tropical Cyclone in the East China Sea and the Environmental Effects on It.

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Some tropical cyclones in the western North Pacific have particular characteristics, classified as the SH pattern by the Japan Meteorological Agency (1990). One such cyclone, Tropical Storm Bolaven (0006), which took an irregular track in the East China Sea in midsummer, was examined. It did not attain typhoon intensity, and had associated deep convective clouds only in the south and dry air in the middle and upper troposphere in the north of the cyclone. The northern dry region was warmer than the southern moist region, and therefore the cyclone was associated with baroclinicity in the middle troposphere. However, the surface wind field was similar to that of a typical tropical cyclone. We also examined the relation of the structure of Bolaven and its environment using the objective analysis dataset of JMA. Bolaven was located southeast of a prominent Tibetan high in the early stage of its lifecycle. In the upper troposphere, a strong northerly wind on the eastern fringe of the Tibetan high converged in the north of Bolaven, and an easterly flow diverged in the south of Bolaven. It is inferred that regional differences in the curvature of the geopotential height field associated with the Tibetan high and the cyclone, and the consequent inertial stability, contributed to the flow and the distribution of horizontal divergence. In the middle troposphere, zonal temperature gradients were generated by the warm continental air mass in the western North Pacific, where the circulation of Bolaven generated horizontal temperature advection. These contributed to the asymmetric distribution of the vertical motion near the cyclone and to the irregular movement of the cyclone. In the later stage of its lifecycle, Bolaven had the structure of a meridionally reversed baroclinic disturbance in the middle troposphere, although the large-scale environment changed significantly in the upper troposphere. This suggests that an asymmetric tropical cyclone organized in a particular environment could be maintained by midtropospheric frontogenesis through the tilting effect for some period.