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Ying-Hwa Kuo and George T.-J. Chen
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Han-Ru Cho and George T. J. Chen

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A hypothesis concerning the development of the western sector of a Mei-Yu front is proposed. An observational evidence showing the presence of a positive low-level potential vorticity (PV) anomaly along the western sector of a Mei-Yu front is first presented. It is then proposed that the frontogenetic process in this sector is maintained through the interactions between the PV anomaly and the cumulus convection induced by Ekman-layer pumping. The scale contraction produced by the convergence flow of the convection provides the basic frontogenetic forcing. In addition to the linear CISK process, the front also intensifies through a nonlinear feedback mechanism due to the fact that the effect of condensation on potential vorticity is proportional to the value of absolute vorticity. Nonlinear semigeostrophic model results indicate that due to this nonlinear feedback process, the fronts intensify and the cross-frontal horizontal scale contracts at a rate faster than the exponential growth. The nonlinear feedback process also causes the western sectors of Mei-Yu fronts to develop in such a way that a discontinuity would form in physical space at a finite time if there were no limit to the intensity of cumulus convection. The properties of Mei-Yu fronts forced by these processes are compared with those of the classical type of fronts observed in midlatitude cyclones.

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George T. J. Chen and H. C. Chou

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Six cases of prefrontal squall lines were observed over the Taiwan Strait and western Taiwan during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The mean propagation speed was 10 m s−1, and the mean life span was 11.4 h for the six squall lines. All the lines occurred ahead of the Mei-Yu front and moved away from the front with time. The mean environmental conditions associated with the squall lines were analyzed by compositing the six cases. The environmental conditions observed during the TAMEX squall lines were found with characteristics between tropical and midlatitude squall lines. The steering level was near 7 km during the mature stage. A low-level jet at 3–4 km was present, with strong vertical shear in the presquall environment below 700 hPa. The squall lines were oriented 45° to the shear in the 1–3-km layer, like midlatitude cases. The CAPE, however, is similar to the tropical squall lines. The inflow ahead of the squall lines was deeper and stronger below 400 hPa, and the CAPE was higher during the mature stage as compared to the intensifying stage. The squall-line collapse is correlated with decreasing CAPE and low-level inflow ahead of the lines.

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C-P. Chang and George T-J. Chen

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The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981–86 are used to study the tropical circulations associated with the development of low-level westerlies during both events.

Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough–front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is the only Asian monscon region where midlatitude fronts can move into the Tropics without having to overcome significant terrain barriers.

Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres. During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over.the Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection and propagates westward to dominate the flow changes over.the Bay of Bengal and the southern Indian Ocean. The steady 4–5 m s−1 westward speed and near-perfect symmetry with respect to the equator indicate the possibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northern South China Sea.

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Hung-Chi Kuo, George T-J. Chen, and Chung-Hsien Lin

Abstract

The merger of tropical cyclones Zeb and Alex is described. The process includes mutual cyclonic rotation, followed by merger of Zeb and Alex. The cyclonic rotation of Alex around Zeb accelerated as the separation distance decreased to 850 km. During the merger process, Alex was quickly elongated and wrapped cyclonically around Zeb to become a spiral band of Zeb. The final merger occurred at a distance of 450 km. The observed merger processes appear to be in good agreement with the potential vorticity theories of vortex interaction and the formation of spiral bands. Despite the presence of moist convections, the straining-out regime of Dritschel and Waugh appears to be applicable to the interaction between Alex and Zeb.

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