| Under the background of global climate change, the spatiotemporal characteristics of precipitation and its mechanism are hot topics in weather and climate researches, and it is benefit to further understanding the flood events since flooding is usually associated with heavy precipitation. Based on a newly developed intensive hourly precipitation observational dataset from the China Meteorological Administration(CMA), the changes in precipitation amount, frequency, intensity and duration during the warm season from 1982 to 2012 were investigated over eastern China, which is divided into ten subregions. Ten intensity categories were used to reveal the contributions of precipitation frequency and intensity to the variation of rainfall amount. The possible mechanism accounting for the precipitation changes is studied in South China, since the increasing trend is the most obvious in this region. Then the spatial and temporal variations of precipitation, especially the diurnal variations, were compared during the first and second flood season over South China. Moreover, to explore the possible causes of diurnal rainfall variations, the reanalysis data and diurnal precipitation cycle numerical experiments were applied. At last, based on the understanding of heavy rain, this paper introduces a new physical parameter, which contains atmospheric thermodynamics, dynamics and moisture factors. The main contents and results are as follows:(1) Since the hourly data stations are mainly located in eastern China, the precipitation features and regional trends in both rainfall amount and frequency were investigated over eastern China. The results show that, generally speaking, the extreme heavy rainfall showed a significant increasing trend while the light rainfall showed a significant decreasing trend. Nevertheless, the spatial unevenness was detected and in South China, significant increasing trends were found(so South China is the key area in this paper). The trends of precipitation amount were mainly caused by the variations of precipitation frequency, which had a contribution rate of greater than 95%. However, for the heavy rainfall category, intensity changes were also very important, which may increase the risk of flooding remarkably.(2) In order to study the underlying mechanisms of precipitation changes in South China, a total of 20 factors that have been proven to determine/influence the precipitation by previous case studies were evaluated. The results reveal that the lower-level vorticity(VOR) at 850 h Pa was the key factor to influence the hourly precipitation. The vorticity budget was used to understand the variation of VOR, and term STR(stretching effect) which is determined by the divergence was found to be the dominant factor. A possible mechanism accounting for the increasing trend of the hourly precipitation over South China is proposed. The Western Pacific subtropical high stretched more westward during the last 31 years, which favored the divergence at higher levels over South China. Thus, lower-level convergence was enhanced, which rendered an increasing trend of VOR in the lower-level through term STR. This increasing trend in the lower-level vorticity was corresponding to the increasing and/or intensifying of the weather systems that triggered the precipitation directly. Thus the precipitation over South China shows an increasing trend.(3) In South China, the precipitation-concentration period contains the first and second flood season, and there are many differences in hourly precipitation features during these two periods, in particular the diurnal variations of rainfall and its duration. In the first flood season, the rainfall amount shows an obvious two-peak pattern, and the first peak is in the early morning, the other one is in the late afternoon. The early morning peak mainly appears in the western part of South China, which is a unique climatology of the first flood season over South China. The mean duration of rainfall events is relatively long(longer than 6 hours), while the mean intensity is small. Moreover, the early morning peak has an eastward delay of the diurnal phase in the west of 112°E. In sharp contrast to the western part, the afternoon peak rainfall V becomes a remarkable feature over the eastern plain of southern China, and the mean duration of rainfall events is short(shorter than 4 hours), but the intensity is relatively strong. In the second flood season, the precipitation presents a dominated late afternoon peak both in western and eastern part of South China. Even though there is still morning peak and an eastward delay in the west of 110°E, it is significantly weaker than the first flood season.(4) To further understanding the possible causes of the out-of-phase diurnal variations in the west and east of South China, the 3-hour-interval reanalysis data from ERA-20 C and the Weather Research and Forecast(WRF) model were used. The diurnal cycle numerical experiments by WRF is a 10-day simulation, and the initial condition is the average at 0000 UTC of April to June in five years(2006-2010) of the FNL 1°?1° data, while the cyclic boundary conditions are the averages of 0000, 0600, 1200 and 1800 UTC. The results show that the WRF model successfully simulated the main features of the diurnal variation of rainfall in the study area. In the western and eastern part of South China, the mechanisms of diurnal variation of precipitation are different. In the western part, the diurnal variation is closely related to the development of convective activity from the Yunnan-Guizhou Plateau to the Guangxi basin and the low-level jet. The southwesterly low-level jet mainly located in the 900-850 h Pa over this region, and it occurs mainly at late night and early morning(1500-2300UTC). In contrast, in the eastern plain, it is mainly affected by the land-ocean property circulation. The development of heat convection is remarkable in afternoon over land, so the diurnal peak of precipitation is in late afternoon(0600-1200UTC).(5) As the atmosphere is an organic combination of dynamic and thermal fluids, and precipitation is the result of many factors in atmosphere, so the key point of precipitation forecasting is data processing and mining strong signals from the various early physical quantities. Base on the Ingredients-Based Methodology(IM), this paper introduces a new physical parameter THP(Temperature, Helicity and Precipitable water), which contains atmospheric thermodynamics, dynamics and moisture factors, so the stability of THP is superior than the method of using single physical parameter in forecasting analysis. From the analysis and comparison, we found that, the THP index can provide good direction to the distribution and the movement of heavy precipitation in South China, and heavy rainfall is located in the high value area of THP. Moreover, judging from the inspection effect, the THP index has a good applicability during summer over eastern China. The variation of THP is ahead of changes in observational precipitation, so THP index can also give a good reference to the beginning and ending time of heavy rain. |
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