| Diurnal rainfall variation is the basic feature of regional climate,which has important scientific connotation and is an important part of precipitation refinement research.Most of the existing researches used the "top-down" method to study diurnal rainfall variation,in which the areas of interest were predefined by region boundaries and regional average diurnal variation curves were used to represent the diurnal variation characteristics of the region.However,region boundaries used in most researches were empirical and subjective,which makes it difficult to describe the spatial distribution of diurnal rainfall variation characteristics of the inner region in detail.In order to improve the objectivity and accuracy of the results,we use the "bottom-up" data mining method to deeply mine the spatial patterns of diurnal variation of different rainfall duration events from gridded rainfall data over China and analyze the control factors of diurnal rainfall amount to deepen the understanding of characteristics of diurnal rainfall variation.The main conclusions are as follows:(1)There are significant regional differences in the diurnal rainfall variation in summer over China.Regions with afternoon rainfall peak are mainly distributed in the coastal areas of southeast China,northeast China,Inner Mongolia Plateau and Loess Plateau.Regions with nocturnal rainfall peak are mainly distributed in the Hengduan Mountains,Sichuan Basin,east plain of Yanshan-Taihang Mountains,valley of the Yarlung Zangbo River,north of Kunlun Mountains and Tianshan Mountains.These regions are affected by topography,which have significant mountain-plain solenoid circulation(MPS)and result in the prevailing nocturnal rainfall events.In desert of northwest China,midday rainfall peak is the dominant rainfall type and regions with midday and evening rainfall peaks scatter around.The middle and eastern part of the Tibetan Plateau is dominated by afternoon and evening rainfall peaks,while the middle and lower reaches of the Yangtze River are characterized by morning and afternoon peaks.(2)Diurnal rainfall variation types of different rainfall duration have different spatial distribution pattern.Short duration rainfall events usually have midday or afternoon rainfall peaks,which are widely distributed in south of China,central eastern China and northeast China.Nocturnal rainfall events are mainly distributed in the east of Hengduan Mountains,around the Sichuan Basin,east of Yanshan-Taihang Mountains and north of the Kunlun Mountains and Tianshan Mountains.The diurnal rainfall variation of long-duration rainfall events mainly happens from night to morning.From the Hengduan Mountains to the Sichuan Basin and then to the middle and lower reaches of the Yangtze river,the diurnal rainfall variation of long-duration precipitation has eastward phase time delay from night to morning(night->midnight->early morning-> morning).The diurnal rainfall peak of southeast coastal area is still in the afternoon and the Northeast Plain,the Sanjiang Plain and the North China Plain have large areas of no-peak areas.(3)The total diurnal rainfall variation is made up of superimposition of the diurnal variation for different duration rainfall events.For instance,in the region of North China plain nearby Yanshan-Taihang Mountains,short and medium duration rainfall events present characteristics of nocturnal rainfall peak,and long-duration rainfall events show no-peak characteristics,and the near-Bohai plain has early morning rainfall peak diurnal variation characteristics.However,the spatial distribution of total rainfall variation shows nocturnal rainfall peak near the mountain(dominated by shortduration rainfall events)and early morning rainfall peak near the Bohai plain(dominated by long-duration rainfall events).Double peak characteristics of night and early morning in the North China Plain area obtained by using the regional average data in the previous study are the result of combining the diurnal rainfall variation in the near-mountain plain and the near-Bohai plain.Obviously,diurnal rainfall variation in whole region can be analyzed by more detailed regional division and rainfall duration division,which is conducive to further diurnal rainfall variation mechanism research.(4)The consistency measure of diurnal variation of rainfall amount and frequency / intensity regularly varies with degree of relief.Consistency measure of rainfall frequency increases with the increase of the degree of relief,while the consistency measure of rainfall intensity decreases with the increase of the degree of relief.Rainfall frequency in high relief degree mountains plays a predominant function on the diurnal rainfall amount variation.In plains,basins and plateau areas with low degree of relief,rainfall intensity dominates the diurnal rainfall amount variation.With the increase of rainfall duration,the influence of rainfall frequency on diurnal rainfall amount variation gradually decreases,while the influence of rainfall intensity gradually increases.In this paper,data mining method was applied to study the spatial distribution characteristics of diurnal rainfall variation in summer over China.For the first time,detailed spatial distribution map of diurnal summer rainfall variation types of China was drawn and the controlling factors of diurnal rainfall amount variation were discussed,which could provide valuable clues for further study of precipitation mechanism. |
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