Synoptic Variability of Moisture Transport and Precipitation over Summertime East Asia and the Northwestern Pacific in Association with Upper Tropospheric Disturbances

Abstract:

This study aims to provide overall understanding of the summertime synoptic variability of precipitation and moisture transport at midlatitude over East Asia and 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. 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 belt of several hundred kilometers wide and one 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 40N. 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, the familiar 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 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 opposite upward forcing is weak. A Q-vector analysis shows that the upwelling associated with the precipitation bands is forced predominantly by confluence. This circulation is frontogenetic, and surface fronts are often formed therein. 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. Latent heating in the precipitation bands is significant, but its role is passive to reduce the effective static stability.

Results on the Atlantic is briefly touched.