| By using a dataset during1960-2009including the reanalysis daily data provided by NCEP/NCAR, Meiyu data in the lower-middle reaches of the Yangtze River and24h (from20:00to the next20:00Beijing time) total precipitation data collected from714fiducial stations in China, this paper aims to investigate impact of the east Asian subtropical westerly jet (EASWJ)to middle-range forecasting of Meiyu in the lower-middle reaches of the Yangtze River. The vertical structure and spatial pattern of the EASWJ are analyzed systematically in its climatic state. Objective and quantificational methods have also been found to characterize the EASWJ and as a result long time series EASWJ dataset has been built. Besides, influence mechanism of the EASWJ on abnormal Meiyu and activity of corresponding large scale circulation systems is discussed, by analyzing relationship between Meiyu and meridional activities, intensity variation, zonal abrupt change of center and morphological characteristics of the EASWJ with daily and Pentad data. At last, the EASWJ is studied in an aspect of its propagation characteristics of baroclinic wave energy. The major results are as follows:(1) Study on climatic character of the EASWJ shows that its space distribution during Meiyu period is different from mean character in the whole summer, featuring with its southward location, stronger intensity, wide coverage and big variable rate. The Meiyu period is an important stage for the EASWJ transiting from winter to summer, when the EASWJ has unique middle-range evolution character in aspects of its meridional activity, zonal abrupt change of jet center and transient eddy of jet intensity. Generally, there are two stable stages and three northward shift stages of jet position when the EASWJ seasonally moves to the north continuously. The two stable stages of the EASWJ are corresponding to the pre-flood season in south China and Meiyu over the lower-middle reaches of the Yangtze River valley respectively. The second and third seasonal north shifts of the EASWJ are corresponding to beginning and ending of the Meiyu respectively. The second north jump of the EASWJ on June7th is10days ahead of Meiyu beginning day, which has foreboding sense to forecast the beginning of Meiyu; the third jet north jump and jet center disappearing above the west Pacific are related to the ending of Meiyu. (2) Using daily data of position, intensity and center of the EASWJ during Meiyu period from1960to2009, relationship between middle-range variation of the EASWJ and Meiyu is discussed. The result shows daily evolution of the EASWJ during Meiyu period indicates a negative relationship between jet position and Meiyu intensity, that is to say, Meiyu intensity is weaker (stronger) when index of jet position is northward (southward). Jet intensity is positively related to Meiyu intensity, namely, Meiyu intensity is stronger (weaker) when jet intensity is stronger (weaker), but relation of jet intensity to Meiyu is not as close as that to jet position. Regional rainstorm in heavy Meiyu years mainly appears right and behind the EASWJ with zonal and northeast-southwest pattern. West jump day of the EASWJ is closely related to the end date of Meiyu, difference between west jump day of the EASWJ and the end date of Meiyu is less than5days in most years. Position index of the EASWJ varies periodically in a timescale of one week or double week. Intensity index of the EASWJ varies periodically in one week. Period of jet position and intensity indicates that there are jet cores propagating to the east with higher frequency in the relatively stable jet band.(3) Middle-range variation of the EASWJ is remarkablly different in abnormal heavy Meiyu years from that in light Meiyu years. The composite analysis shows that the jet intensity is stronger, jet band is more narrow and mass and momentum of the EASWJ are more centralized in heavy Meiyu years than that in light Meiyu years. From perspective in daily evolution of the EASWJ, in heavy Meiyu years, the second north jump day of the EASWJ is usually earlier than the beginning date of Meriyu, the third EASWJ north jump day and the ending date of Meiyu is almost synchronous; after the beginning day of Meiyu, the EASWJ swings stably around its climatic longitude, zonal wind is stronger in key region (110°～130°E,30°～37.5°N), the jet centeris frequently around125°E which is near and in leeward side of Chinese mainland. In light Meiyu years, the EASWJ usually doesn't happen the second north jump and moves to north of40°N directly; jet position is northward and moves with bigger amplitude along longitude, jet intensity is weaker, jet center is mainly present over the Japanese islands and west Pacific which is far away from Chinese mainland.The physical mechanism of influence of the EASWJ on abnormal Meiyu shows that, in heavy Meiyu years,200hPa jet axis, zero line of divergence and zero line of divergence departure overlap at37.5°N over east China, divergence center in upper level also coincides with center of divergence departure over the lower-middle reaches of the Yangtze River. Strong divergent flow in upper levels cooperating with strong convergent flow in lower levels and deep vertical ascending movement from ground level to upper levels can provide favorable dynamic condition for Meiyu to happen; Coupling between upper and lower jet streaks is in favor for lower southwesterly to strengthen, as a result southwest water vapor flux departure exists in the south of lower-middle reaches of the Yangtze River, which could provide favorable warm vapor transport condition for durative rainfall. Upper frontal zone and typical steep Meiyu frontal zone are helpful for deep convection to develop by favoring mass and momentum of upper jet to maintain. In light Meiyu years, situation is opposite.(4) In abnormal heavy Meiyu years, the EASWJ is closely related to west pacific subtropical high and monsoon surge. If the EASWJ at200hPa is characterized with stronger intensity, comparably stable meridional movement and westward main part, the west pacific subtropical high shows the same character at500hPa and the monsoon surge is stronger. In light Meiyu years, situation is opposite. Position of the EASWJ is closely related to position of the west pacific subtropical high, intensity of the EASWJ is closely related to zonal intensity index of monsoon surge in both heavy and light Meiyu years, and however, such relationship is closer in heavy Meiyu years than that in light Meiyu years.Study in influence mechanism on activity of the west pacific subtropical high and monsoon surge shows that, in heavy Meiyu years, intensity of the EASW is stronger, negative vorticity departure exists in the whole levels from bottom level to the upper levels expanding from north of ridge line of subtropical high to south of westerly jet axis, which is helpful for subtropical high to be stronger. Subtropical monsoon circulation (STMC) which is accompanying around ridge line of subtropical high and upper convergent departure are both helpful for subtropical high to be stronger. Ascending part of the STMC is in south of the EASWJ and north of southwest jet in lower level, while submerging part of the STMC is in north of tropical easterly. In that way, the STMC enhances westerly speed of lower southwesterly jet by Coriolis force though its meridional movement to the north, easterly speed has also been enhanced though meridional movement from north of easterly jet to the south. As a result, westerly jet in lower level and easterly in upper level are enhanced, monsoon circulation is holden by means of mass-momentum adjustment. In heavy Meiyu years, latent heat of condensation releasing from deep convective precipitation leads to increasing and maintaining of north-south temperature gradient, thus jet intensity becomes stronger and keeps maintaining, this effect coordinating dynamic effect make intensity of subtropical high to be strengthened and west ridge point of subtropical high to be westward, so that monsoon surge can be enhanced also. In light Meiyu years, situation is opposite.(5) Analysis on propagating character of baroclinic wave packets of the EASW in abnormal Meiyu years shows that center of wave packets with great values is inclined to the westward and southward at200hPa in heavy Meiyu years. However, center of wave packets with great values is inclined to the eastward and northward in light Meiyu years. Band of baroclinic wave packets with great values is weaker in heavy Meiyu years than that in light Meiyu years. In heavy Meiyu years, propagating speed of wave packets is bigger than phase velocity, indicating jet wave has development effect to downstream area. There are three distinct great value zones of baroclinic disturbance wave packet persisting a period of2～7d. Intensity variation and different collocation of the center of these three wave packets with great values could represent the difference in heavy and light Meiyu years. This result could also be used to investigate stage characteristics of precipitation before Meiyu, during Meiyu and after Meiyu. That is, the Meiyu season begins in the Yangtze River basin when band I of wave packets with great values is located between40°N and50°N, band of wave packets with great values west of130°E enhances, center of wave packets with great values in exit region of85°～125°E moves to the south around38°～42°N, contemporary with band III of wave packets with great values weakens in west Pacific. The Meiyu season ends in the Yangtze River basin when band I of wave packets with great values especially band of wave packets with great values in exit region is inclined to the northward and be weaker, band II of wave packets with great values enhances observably in the Bay of Bengal. Different collocation and maintenance of these three wave packets zone with great values may reflect the course of maintenance between subtropical upper-level jet and monsoon circulation.
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