Fine-scale Analysis Of Hourly Precipitation Over The Complex Topographies In Northwest China

Orographic precipitation is important sources of water resources in Northwest China.Its fluctuations not only affect the local fragile ecosystems,but also affect the surrounding hydrology and agriculture.Using hourly rain-gauge precipitation data,high resolution satellite data and reanalysis products,this study carried out two aspects of research work for studying the precipitation characteristics over the complex topographies in Northwest China,including analyses of precipitation fine characteristics,detailed evaluation of performance in climate models.By using a series of new methods which are suitable for hourly precipitation analyses,this study firstly analyzed the hourly characteristics of precipitation over Northwest China,including hourly frequency and intensity,intensity-frequency distribution,duration and diurnal variation.It is found that there are obvious differences of precipitation over the Qilian Mountains and the Ili River Valley,where precipitation is large.Then focusing on the two regions,the fine features of precipitation and its relationship with local topography and circulation is analyzed.Finally,based on the characteristics of hourly precipitation over the topographies in Northwest China,the outputs from simulations of 21 Coupled Model Intercomparison Project Phase 6(CMIP6)models with different resolutions were evaluated.The bias characteristics of precipitation over the topographies were revealed and the added values of enhanced horizontal resolution were clarified.The main conclusions are summarized as follows:1.Hourly precipitation characteristics over the Northwest China.The largest precipitation amounts are located over the Qilian Mountains,the central Tian Shan Mountains and the Ili River Valley,and the regions also have high precipitation frequencies and strong precipitation intensities.In contrast,the small precipitation amounts over the basin areas are the results of low precipitation frequencies and weak precipitation intensities.Precipitation amounts over the Northwest China are closely related to the elevation of the topography.Precipitation amount increases with increasing elevations,which is contributed by the precipitation frequencies rather than precipitation intensities.Distinct regional features of intensity-frequency distribution in precipitation are observed over the Northwest China.Over the southeast Qilian Mountains,the central Tian Shan Mountains and the eastern Ili River Valley,the proportions of strong and weak precipitation are relatively large.Over the western Ili River Valley and Junggar Basin,the proportion of strong(weak)precipitation is relatively large(small).Over the Tarim Basin,the proportions of strong and weak precipitation are relatively small.The stations with different precipitation intensity-frequency distributions have different characteristics of diurnal variation.In stations with a relatively high proportion of heavy precipitation,precipitation tends to occur in the late-afternoon.While stations with a relatively high proportion of weak precipitation,precipitation during the night to early-morning periods also plays an important role.2.Characteristics of summer precipitation events over the Ili River Valley.The mean amount and frequency of hourly precipitation during summer show considerable spatial variabilities in terms of along-valley and across-valley directions.Along the valley,the rainfall gradually increases with increasing elevations.Across the valley,the rainfall also increases from the valley floor to the northern or southern slopes.Three distinct types of precipitation events are identified: western-local,eastern-local,and eastward-movement events.The eastern-local and western-local events are located over the east and west of the valley,respectively.The eastern-local events have more numbers and longer duration than the western-local ones,resulting in more precipitation in the east than in the west of the valley.In contrast,the eastward-movement events with the longest duration initiate from the west of the valley and gradually move to the east of the valley.For these events,the spatial distribution of the composite precipitation exhibits a similar pattern to that of the summer mean precipitation.These contribute to more than half of the summer total precipitation in most stations.It is found that the circulation structure accompanying the eastward-movement events exhibits a consistent feature with that of the Central Asia vortex/ trough systems.3.The regional differences and connections of the diurnal variation of precipitation over the Qilian Mountains.Distinct regional features of diurnal variations in precipitation are observed over the Qilian Mountains.Precipitation over the Qinghai Lake areas shows a single nocturnal peak.A dominant,late-afternoon peak of precipitation occurs over the mountain tops.Over the northeastern and southeastern slopes,a dominant diurnal peak appears in the late afternoon,and an evident second peak is found in the early morning,respectively.The early-morning peak is dominant across plains with low elevations.From the mountain tops to the plains,the diurnal peaks of precipitation gradually vary from the dominant late-afternoon peak to the dominant early-morning peak with the enhanced early-morning peak in concurrent with the decreasing gauge elevation over the northeastern and southeastern slopes.4.Evaluation of CMIP6 models with different resolutions in simulating precipitation.By dividing the 21 CMIP6 models into very-high,high and low resolution groups,the precipitation bias and its relationship with model resolution is investigated over the Northwest China.The three groups of climate models with different resolutions reproduce the spatial patterns of precipitation amounts and frequencies,but they overestimate the precipitation in mountainous areas and underestimate the precipitation in basin areas.The enhanced horizontal resolution of models improves the simulation performance of the spatial pattern of precipitation.Both of the high and very-high resolution models reproduce more realistic fine-scale features of precipitation tied to local topography.The relationship between the precipitation amount and elevations is also reasonably reproduced.Focusing on the precipitation intensity-frequency distribution,all the three groups of models overestimate the frequency of strong precipitation and underestimate the frequency of weak precipitation.The positive bias in mountainous areas is mainly contributed by strong precipitation,while the negative bias in basin areas mainly comes from weak precipitation.Focusing on the diurnal variations of precipitation,the spatial distribution of diurnal peaks is reproduced by the very-high models.However,the double peaks over the basin areas are not simulated,and the simulated late-afternoon peaks are earlier than observations.