Study of the extended range forecast of the principal 20-30 day oscillation pattern of the circulation over East Asia in the summer of 2002

A series of the principal oscillation pattern (POP ) models are set up based on the low-frequency meridional wind of 850 hPa over East Asia for March September in 2002 on the time scale of the 20-30 day, and the independent forecast experiments of POP models are performed for investigating the propagation of the low-frequency oscillation associated with the heavy precipitation process over the lower reaches of the Yangtze River valley in terms of the extended range of the 10-30 days. The results show that the skill score of the correlation prediction up to 20 days ahead of time could reach more than 0.50 in the 135 forecast runs, with the corresponding low frequency variations of the positive meridional wind to three severe precipitation events in summer well forecasted. Based on the forecast experiments against the years of distinct 20-30 day oscillation, it is shown that this POP model is a useful tool to predict the spatial and temporal evolutions of the low frequency oscillation. Hence, it is important for improving accuracy of forecasting the severe precipitation process over the lower reaches of the Yangtze River valley for future 3-4 weeks.