The Jet Stream and the Origin of the Meiyu-Baiu Rainband.
Takeaki Sampe and Shang-Ping Xie
Meiyu/Baiu rain band
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Upward motion (warm colors) follows closely warm
horizontal temperature advection in the midtroposphere (contours) during the Meiyu-Baiu season.
Meiyu-Baiu is the single most important climate
phenomenon for highly populated East Asian regions
from east China through Japan, but what maintains
this climatological rainband has been a long-standing
mystery. The difficulty lies in the strong interaction
between convection and the low-level circulation. The
latter supplies the moisture, but the rainband itself
creates heating and can cause the low-level
circulation. Although moist, low-level southerly winds
blow throughout the summer, the Meiyu-Baiu
rainband appears only from June to mid-July.
This study avoided the circularity problem by
analyzing events in the mid-troposphere. At 500 hPa,
a characteristic large-scale circulation is seen from
June to mid-July, in which a band of warm air is
advected eastward along the southern flank of the jet
stream and downstream of the temperature
maximum anchored by the Tibetan Plateau. Along
this warm advection band, low-level air is sucked
upward, and the induced upward motion triggers the
heavy Meiyu-Baiu rainfall and diabatic heating.
Thus, while the summer southerly winds provide the low-level moisture, the jet stream’s advection
of the mid-level warm air determines the location and timing of the Meiyu-Baiu rainband. This
more complete, dynamical picture should help in making better regional rainfall predictions.