Volume 92 (2014) Issue 6 Pages 519-541
This study provides an overall understanding of the summertime synoptic variability of precipitation and moisture transport at mid-latitude from the eastern coastal region of China to the northwestern Pacific. Using satellite precipitation and reanalysis data, a clear relationship is found between upper tropospheric disturbances (Rossby waves), surface precipitation, and lower tropospheric humidity through July and August. The upper tropospheric disturbances are characterized by the undulation of the 1.5 PVU contours of potential vorticity (PV) on the 350 K isentropic surfaces. Case studies suggest that a precipitation band of several hundred kilometers wide and a thousand to several thousand kilometers long is formed very frequently on the equatorward and low-PV side of the northernmost 1.5 PVU contours, which meander together around 40°N. Lower tropospheric specific humidity is also enhanced there, and it falls sharply to the north of these contours. The synoptic situations associated with it include, but are not limited to, a common situation in which moist convection is enhanced ahead of upper-level troughs. These results are confirmed by a composite analysis over the 12 summers from 2001.
A novel method of analyzing the forcing of the quasi-geostrophic potential enstrophy, in which boundary contributions are incorporated, reveals that upper tropospheric disturbances in the area are propagated predominantly from the west along the Asian jet, and that they exert a significant forcing onto near-surface levels, while the upward forcing from near-surface levels to upper tropospheric disturbances is weak. A Q-vector analysis shows that the upwelling associated with the precipitation bands is forced predominantly by confluence. This process is frontogenetic, and surface fronts are often formed therein. The upwelling is enhanced by latent heating. The latitudinal extent of humid air masses is affected not only by this circulation but by low-level flows induced by upper-level disturbances in a cooperative manner.