Activities Of Wave Packets In Association With Intra-seasonal Oscillation Of Circulation And Their Impacts On The Persistent Strong Precipitation Events Inyangtze-Huaihe River Valley During Boreal Summer

The connection of heavy rainfall processes with different periods (10-30d, 15-45d,30-60d) of low frequency oscillation in Yangtze-Huaihe River Valley (YHRV, hereafter) are analyzed, by employingthe wavelet analysis, Lanczos filter and time-lagged one-point correlation maps, using data NCEP/NCAR reanalysis along with daily observed precipitation from 753 stations of China in 2003,2007, 2011. For different periods of low frequency, the features of low frequency oscillation in precipitation and circulations along with the related waves and the Rossby wave packets in mid-latitudes have been investigated in the present paperto find clues to better forecasting and understanding the heavy rainfall epochs in YHRV. The results are summarized as follows.The heavy rainfall events is closely related to the low frequency precipitation, but the intra-seasonal variation of precipitation is dominated by different periods of low frequency oscillation in different years. The low frequency perturbations in circulations show wave train structures in both the upper and lower troposphere, but the wave packets with different periods exhibit different propagation characteristics in the zonal and meridional directions.The low frequency component of precipitation plays an important part in precipitation disturbances in summer of 2007. The main period of low frequency in precipitation is 10-30 day. The time series of 10-30d precipitation component is significantly and strongly correlated with the daily rainfall anomaly with the correlation coefficient of 0.44. The low frequency circulation shows baroclinic structure during active phase of precipitation, and wave shape in the higher and lower troposphere. The low frequency wave transported eastward slowly in high level with phase speed of approximately 2-3°per day. Wave trains propagate downstream areas in westerlies with energy velocity 14°per day in the west of 120°E, which demonstrated that the low frequency wave packets dispered into downstream regions distinctly. Five days before the heavy rainfall events in YHRV, the wave packets of low frequency oscillation originate from the Urals, and then propagate along a path that extends from the Urals to YHRV. That is, wave packets move from Urals, via Tianshan Mountains, then across Qinghai Lake, and finally into YHRV. The low frequency fluctuations are important in the low frequency precipitation.Four stronger precipitation events happened during Meiyu period of 2011 in YHRV, and the low frequency component of precipitation also plays an important part in precipitation disturbances. The main period in precipitationis found to be 15-45 days oscillation. This low frequency component of precipitation accounts for 12% of the total variance of rainfall in most part of YHRV. The structure of low frequency circulation is similar to 10-30d one in 2007.The low frequency wave propagates eastward very quickly,and it propagates southward slowly and significantly at a phase speed of approximately 1.5 latitude per day.The wave packets, however, propagates southward much more quickly at a speed of 3-4 latitude per day. The wave packets of low frequency oscillation originates from 50°N four days before the heavy rainfall events in YHRV.The total summer precipitationis is more than normal in 2003 in YHRV because of 2002/2003 strong El Nino events as the climate background. The results demonstrate that the 30-60d low frequency oscillation is important in inducing the heavy rainfall events in YHRV during summer of 2003. The time series of 30-60d precipitation component is significantly and strongly correlated with the daily rainfall anomaly with the correlation coefficient of 0.50. The low frequency circulation also shows a baroclinic structure with the positive vorticity in the lower troposphere, and the negative in the upper troposphere, which is conducive of the heavy rainfall events. The low frequency wavespropagatesouthward slowly at 300hPa with phase speed of approximately 1 latitude per day. Thirteen days before the heavy rainfall events in YHRV, the wave packets of low frequency oscillation originate from 55°N, and then propagate to YHRV ahead of the events. The group velocity is faster than phase speed with about 2-3 latitude per day.The mechanism of the energy dispersion of low frequency fluctuation in meridional rather than in zonal in 2011,2003 can be described using Rossby wave dispersion formulae for cases of slowly varying basic flow. The ratio of the zonal wave number to the meridional wave number is different for different low frequency periods.