Sources and Transfer of High Isentropic Potential Vorticity During Meiyu Period

The climatological mean sources and evolutions of isentropic potential vorticity(IPV)during Meiyu period are studied. Compared to the factors in the isobaric coordinate system during the Meiyu period, potential vorticity on the isentropic surface shows clearer "trough" and "ridge", and it is found that there are two high IPV "tongue" areas near the east of Lake Baikal and the southeast of Karafuto in the lower troposphere during the Meiyu period. And pentad IPV evolutions show that the regions where the meridional IPV gradient(MIPVG)weakens in the upper troposphere could be the entrance of high IPV air invading. Before the Meiyu rainy season the MIPVG obviously weakens in upper troposphere over East Asia, then high IPV contours begin to extend equatorward forming "tongue" area. Simultaneously, in lower troposphere MIPVG appears to reverse, shaping local north south dipole between low and high MIPVG. Evident potential vorticity transports and mass exchanges exist at tropopause near 40°N, 120°E during Meiyu period where the tropopause easily folds. By using 10—90 day bandpass and lead/lag correlation analysis, the sources and paths of high IPV anomalies are further investigated and traced. The results show that high IPV anomalies originate from the lower stratosphere and upper troposphere of the high latitude, and the maximum correlation coefficients between IPV anomalies and rainfall anomaly on 345 K isentropic surface before, during and after Meiyu period appear when the former leads the latter by about 10 days, near 55°N, 130°E in the east of Lake Baikal which is an important source of high IPV influencing the rainfall during Meiyu period. On June 10 before Meiyu, the high IPV air is transferred primarily southward along the NE—SW direction at 2 PVU surface from the high IPV source and accumulates near the steepest areas of this surface, developing upward, then crosses downward to the tropopause, and partly invades the south of 40°N when the IPV anomalies fields are typical longitudinal mode. However, in the troposphere it becomes a distribution of latitudinal mode spreading out like a fan. On 315 K isentropic surface, high IPV anomalies invade later than those in the upper troposphere, and are also transferred southward along the NE—SW direction from the high IPV source region near 45°N in the lower troposphere to the Meiyu areas since about June 18. As a result, the region on the east of Lake Baikal is possibly a main source area of cold air influencing Meiyu and key region to make the medium term forecast of Meiyu precipitation.