Abstract
Many previous papers stressed that the Meiyu-Baiu rain zone exhibits the quasi-stationary nature, except the abrupt transitional period. This is true only for the 10-day averaged large-scale field. The actual Meiyu-Baiu front indicates complicated variations even in “the quasi-stationary period”. The present paper will study a large λ-shaped cloud zone seen around July 6, 1991 as one of the cases of significant variations of the Meiyu-Baiu front. This λ-shaped cloud zone is formed when a north-south oriented cloud zone elongated from the intense rainfall area of Meiyu-Baiu front with pole-ward moisture transport. This process is quite different from “the abrupt transition of the Meiyu-Baiu front”, since the Meiyu-Baiu frontal zone itself remains in almost the same latitude belt. The λ-shaped cloud zone is associated with a north-south extending trough, which resulted from coupling of the short-wave troughs in northern (40-50°N) and southern (30-40°N) latitude. When the extending trough passed over the Tibetan Plateau and approached a quasi-stationary ridge over ∼120°E, the narrow north-south elongated cloud zone, which is associated with southerly wind and ascending motion, is formed in front of the trough. Convective precipitation occurs within the north-south elongated cloud zone. However, subsidence to the west of the trough and that to the east of the ridge block the northward shift of the Meiyu-Baiu front. The north-south elongating cloud zone disappears when the eastward propagating southern short-wave trough is de-coupled from the northern short-wave trough by the blocking ridge. The λ-shaped cloud zone occurs only under the specific phase relations among the short-wave troughs in northern and southern latitudes, quasi-stationary ridge and intense rainfall area in the Meiyu-Baiu front. Typical large λ-shaped cloud zone appears only a few times in a Meiyu-Baiu season.
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Schedule a callMore From: Journal of the Meteorological Society of Japan. Ser. II
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