Abstract:

 Based on daily precipitation data at 78 weather stations of Beijing-Tianjin-Hebei region during 1961-2012, spatial and temporal evolution of precipitation day at different levels were analyzed using methods of a trend analysis, a Morlet wavelet and an empirical orthogonal function (EOF). The results show that it has not a significant trend for precipitation day at different levels in the recent 52 years in the study area. Spatial distribution of the total rain and light rain days is in a decreasing trend from northwest to southeast. The moderate and heavy rain days have two similar centers of high values, and both are located in the northeast of Beijing-Tianjin-Hebei region and in the west of Taihangshan mountain region, respectively. However, the spatial distribution of torrential rain day has no significant feature and it is random. Both total and light rain days have two time-scale oscillation periods i.e. 13 years and 6 years, so are moderate, heavy and torrential rain days i.e.15-18 years and 8-11 years. The first mode of EOF analysis suggests that the number of precipitation day in all levels has a better consistency in the region. The second mode of EOF analysis suggests that total rain day, light, moderate and heavy rain days have an opposite phase in the north and south, and torrential rain day is more (less) in the northeast and east of study area, less (more) in middle and west of study area.

Key words: Number of rain day, Precipitation level, Oscillation period