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Numerical Study on the Baiu Front Genesis by Heating Contrast Between Land and Ocean

 

F. Kimura

Frontier Research System for Global Change (FRSGC)

Tokyo, Japan

T. Yoshikane (University of Tsukuba) and S. Emori (NIES)

 

The formation mechanism of the Baiu front, which appears in early summer giving a typical rain band around Japan, was reproduced using a regional atmospheric model. The twice-daily data of the ECMWF objective analysis was utilized as the initial and boundary conditions for the realistic simulation. A zonally uniform and temporally constant atmospheric field obtained by ten-day averaged zonal mean ECMWF data was also utilized as the initial and boundary conditions (zonal mean simulation).

The simulated Baiu front in the zonal mean simulations, as well as 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. Fundamentally, the simulated Baiu front is formed by the deformation of the zonal mean field due to the Sea/Land contrast and the topography. Although the distribution of the simulated rainfall in the Baiu front depends on the cumulus convective parameterizations, the fundamental structure of the Baiu front does not depend on them.

In comparison between the zonal mean simulations of early and late June, the Baiu front is formed in the higher latitude in late June, when the westerly is weak and the upper level jet is 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, suggesting that the Baiu front could be reproduced only by the two factors, the zonal mean field and the Sea/Land contrast. The orography including Tibetan plateau intensifies the LLJ and the precipitation over the Baiu front.

The LLJ also appears in the zonal mean simulation without the condensation process. However, the LLJ is formed along the eastern coast of Eurasian continent and locates in the northern side of the Upper Level Jet in the eastward of Japan, which is a different feature from those in the cases with the 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.

 

 

 

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