Moisture Sources And Possible Large-scale Mechanism For Severe Rainfall Over Some Regions In China During Summer

Features of severe rainfall over the upper reaches of the Yangtze River, northern China and northeastern China in summer are firstly investigated using the observation rainfall data in China for the 1980-1997 period in this paper. Water-vapor transport and moisture sources contributing to severe rainfall over above regions are discussed with particular reference by synthetic analysis and correlation analysis using the daily NCEP/NCAR reanalysis dataset during 1980-1997. In addition, from the angle of low frequency oscillation the possible large-scale mechanism of moisture transport for severer rainfall over all reaches of the Yangtze River and northeastern China in 1998 is further explored by using of daily ECMWF reanalysis data. The main conclusions to be drawn in this paper are as follows:I Severe rainfall is the major part of the total rainfall during summer over the upper reaches of the Yangtze River, northern China and northeastern China. And heavy rain occurs locally over the above regions. Severe rainfall can be composed of climate rainfall and anomalous one, and the latter accounts much for severe rainfall.II As far as daily evolvement of moisture transport during rainfall period, the variation of moisture transport from the west Pacific via the South China Sea (SCS) and that from the north of severe rainfall region exerts noticeable influence on severe rainfall over the upper reaches of the Yangtze, northern China and northeastern China while the eastward moisture transport from the Bay of Bengal (BOB) decreases during the severe rainfall period. The correlation analysis of moisture transport show that the severe rainfall over the upper reaches of the Yangtze River has closer relation with the moisture transport from the west Pacific passing SCS and that from the northwest than with that from the BOB. Therefore, the effects of the former on severe rainfall can't be neglected and may be more important than the latter to some extent. Similarly, for the severe rainfall over northern China or northeastern China, daily variation, anomaly of moisture transport and correlation analysis all indicate that the moisture transport responsible to severe rainfall comesfrom the west Pacific via SCS and high latitudes. Nevertheless, the moisture transport stemming from the Bay of Bengal or from the across-equatorial flow over the Australia-Asia region exerts comparatively less distinct impact on the occurrence of severe rainfall.HI, Variations of rainfall amount and regional mean moisture transport exhibit notable 30-60d low frequency oscillation (LFO over all reaches of the Yangtze River and northeastern China during summer of 1998 and over northern China during summer of 1981.IV During summer of 1998, for zonal moisture transport along 20° N, obvious westward LFO from the west Pacific and eastward slow LFO from BOB are divided by 105° E. LFO of the meridional (zonal) moisture transport along 32.5° N (25° -30° N) may stem from the north-middle Pacific and propagates to 100° E, but the propagation from the north-west Pacific to southeastern China has important impact on the violent LFO and its relating rainfall over the region. Over 110° -120 ° E, LFO of moisture transport flux convergence spreads northward from the South China Sea to 40° N and is accompanied by the northward shift of the rainfall belt over eastern China.V During the middle and last ten-day of June in 1998, heavy rainfall occurs over the middle and lower reaches of the Yangtze River has close association with the northward shift of the moisture transport LFO with anti-cyclonic circulation centered over the west Pacific-SCS and the cyclonic one north of it. Severe rainfall over the upper reaches of the Yangtze during the end of June and August relates to the westward movement of the moisture transport LFO with anti-cyclonic circulation and its west part stagnating over the upper reaches of the Yangtze. The low-frequency moisture transport from the west Pacific via SCS does contribution to the occurrence of severe rainfall. For the heavy rainfa...