Is the interannual variability of summer rainfall in China dominated by precipitation frequency or intensity? An analysis of relative importance

The summer rainfall in China has a large interannual variability, which results from the concurrent variations of precipitation frequency and intensity. Using the observed daily precipitation in the 194 stations during recent 62 years, we examine the relative importance of the frequency and intensity in the variability of the rainfall. A simple method, based on linear regression, is used to estimate the relative importance. The products of the change rates of rainfall with respect to frequency and intensity, determined from the regression, and the corresponding standard deviations of the two variables, which reflect their variation scales, are defined to measure the importance of frequency and intensity. To determine the frequency, rainfall amount, and intensity from daily precipitation, we need a threshold to define the “rainy day”. In this study, we use a series of thresholds, ranging from 1 to 30 mm/day. So, while presenting the result of relative importance for each threshold, we also examine how the relative importance varies with the threshold. Results show that for the threshold of 1 mm/day, with which the rainfall may include even the light rains, the variabilities of summer rainfall in most stations are dominated by intensity. With the increase in threshold, the importance of frequency increases, while the importance of intensity decreases. When the threshold reaches 30 mm/day, with which the rainfall includes only moderate-to-heavy rains, the variabilities of the rainfall in all stations are dominated by frequency. Analysis suggests that such a change, in the dominance with the threshold, is reasonable. This reasonability, in turn, supports the reliability and robustness of the method.