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Yafei Wang, Koji Yamazaki, and Yasushi Fujiyoshi

Abstract

This study deals with two teleconnection patterns and the subsequent wave train propagations during an East Asian summer. Diagnostic results are as follows: 1) a stationary wave ray with zonal wavenumber 5 approximates the arc path linking the correlation centers originating from the Caspian Sea via Lake Baikal to the sea off the southeast coast of Japan (i.e., the OKJ arc path as a focus area) in a pentad correlation map between 500-hPa geopotential height (Z500) and outgoing longwave radiation (OLR) at 30°N, 150°E in June 1979–98. Ray tracing shows that it took 8–10 days for this stationary wave to propagate from an initial position around the Caspian Sea to the focus area, which roughly coincides with the observed case in July 1998. 2) A wave train pattern (P-Ja) observed in the boreal summer propagated along the arc line in the same way as the normal poleward Rossby wave train originating from the Philippines across the North Pacific (P-J), but with a phase shift northeastward of about 90°. 3) Further correlation analyses showed that the P-J-like waves belong mainly to intraseasonal propagating ones while OKJ waves belong mainly to intraseasonal stationary ones. 4) Propagation of the newly observed wave train pattern (P-Ja) occurred following another wave train along the OKJ arc path in mid-July 1998. Both northeastward and southeastward wave propagations merged off the east coast of Japan. 5) The northeastward-propagating wave train observed in mid-July 1998 was triggered by the southeastward-propagating (OKJ) wave train that produced a deep cyclonic circulation and a strong convective activity in the focus area. The link of wave forcing and deep convection was made solely because of a strong upper-level divergence in the focus area.

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Carlos R. Mechoso, Koji Yamazaki, Akio Kitoh, and Akio Arakawa

Abstract

The predictability of the stratospheric warming events during the winter of 1979 is investigated by performing a series of 10-day forecasts using the UCLA general circulation model. In general, those events are predictable from several days in advance. The accuracy of the prediction, however, can be sensitive to the starting date and such model characteristics as the horizontal resolution. This sensitivity seems to arise because relatively small errors in the predicted tropospheric zonal mean wind can produce large differences in the characteristics of upward wave propagation and thereby large errors in the stratospheric forecast.

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Carlos R. Mechoso, Max J. Suarez, Koji Yamazaki, Joseph A. Spahr, and Akio Arakawa

Abstract

The impact of an upper boundary on numerical forecasts is studied by comparing the results of a nine-layer model with a top in the lower stratosphere, to those of a 15-layer model with a top near the stratopause. A single case is considered for which initial conditions are taken from a climatologically adjusted winter simulation produced by the 15-layer model. It is found that, as a result of the lowered upper boundary, them is a marked equatorward shift of upper-level westerlies. Significant errors in the ultra-long waves appear at SW mb within the first five days. Errors at 500 mb then spread to progressively shorter waves with large errors in cyclone-scale waves by day 12. Large errors in an ultra-long, wave (wavenumber 3) after day 10 am associated with the climatological adjustment of the stationary flow to the lowered boundary.

Two different assumptions in the radiation calculation in the nine-layer model am used. Results indicate that radiative effects are of secondary importance to the predictability of waves in the middle troposphere.

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