Abstract
Using a re-analyzed FGGE IIIb data set, we study the large-scale aspects of the East Asian monsoon of early summer 1979. Specifically, we analyze the structure of the quasi-stationary rainy band (the Baiu front) existing in East Asia and study its relationship to environmental large-scale fields for the SOP2 period (May 5 to July 5, 1979). A composite technique is employed to obtain a reliable structure of the Baiu front. In the analysis, spatially- and/or temporarily-averaged fields are used to filter out small-scale features.
The location and strength of the southwesterly moisture flux correlate with those of the Baiu front. Moreover, the Baiu front is always located between the upper-level westerly jet and the lower-level anticyclonic shear line.
Two contrasting features are found in the Baiu front. Near or to the south of the front axis, we find strong convergence (divergence), cyclonic (anticyclonic) circulation, and an enhanced westerly (easterly) in the lower (upper) level with weak vertical wind shear and small temperature gradient. To the north (-10 degrees) of the front, we find a strong upper-level jet, strong vertical wind shear, and large temperature gradient.
We find that all fields in the Baiu front slope upward and northward, and thus fields at all levels from 1000mb to 200mb are closely related. In particular, the enhanced westerly is seen both over the axis of the front at low levels and in the upper north side of the front.
Geostrophic approximation and homogeneity along the front axis are fairly good when appropriate averaging is done. A simple calculation based on a quasi-geostrophic approximation shows that the diabatic heating due to condensation can reproduce patterns and magnitudes comparable with the observed geopotential anomaly. These results suggest that large-scale features of the Baiu front can be explained consistently and qualitatively in terms of quasi-geostrophic dynamics. These furthermore suggest that the diabatic heating is a driving force to maintain the Baiu front and it causes an enhanced westerly over the front axis and an intensification of the anticyclonic shear to the south of the front.
