Numerical Study on the Baiu Front Genesis by Heating Contrast between Land and Ocean
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The formation mechanism of the Baiu front, which appears in early summer and gives a typical rain band around Japan, was reproduced using a regional atmospheric model. The initial and boundary conditions of the model were derived from the European Center for Medium-Range Weather Forecasts (ECMWF) data. In the "realistic simulation", temporary variable boundary conditions derived from twice-daily ECMWF data were utilized. In the "zonal mean simulation", zonally uniform and temporally constant atmospheric fields obtained from ten-day averaged global zonal mean ECMWF data was utilized as the initial and boundary conditions. The simulated Baiu front in the zonal mean simulations, as well as that in the realistic simulations, has similar structures to the real Baiu front, i.e., the Low-Level Jet (LLJ) runs parallel to both the precipitation zone and the upper-level jet in the eastern part. Basically, the simulated Baiu front is formed by the deformation of the zonal mean field due to the Land/Sea contrast and topography. Although the distribution of the simulated rainfall over the Baiu front depends on the cumulus convective parameterizations, the fundamental structure of the Baiu front does not depend on them. A comparison between the zonal mean simulations of early and late June indicates that the Baiu front is formed at a higher latitudes in late June, when the upper-level jet is weak and located northward. The location of the Baiu front is quite sensitive to the zonal mean field. The Baiu front accompanied by the LLJ is also represented by numerical experiments without topography, which suggests that the Baiu front could be reproduced by two factors alone, the zonal mean field and the Land/Sea contrast. The orography, including the Tibetan plateau, intensifies the LLJ and the precipitation over the Baiu front. The LLJ also appears in the zonal mean simulation without a condensation process. However, the LLJ is formed along the eastern coast of the Eurasian continent and locates in the northern side of the upper-level jet eastward of Japan, which is a different feature from the zonal mean simulations with a condensation process. Accordingly, it is speculated that the condensation process acting on the atmospheric field is necessary to keep the LLJ in the southern side of the upper-level jet as in the real Baiu front.