| The rainfall amount of the presummer rainy season,also known as the first rainy season(April-June)of China,can reach 1000 mm,accounting for 40%-50%of the total annual rainfall amount of South China(SC).As large amount of rainfall during this period,the mechanis of the rainfall during the presummer rainy season are the research focuses for Chinese meteorologist.Based on the reaseach achievements obtained before,this study focus on the multiple-scale charactoristics of the extreme rainfall occurred during the presummer rainy season.Firstly,we have analyzed the atmospheric circulation and quantitatively evaluated the moisture sources and paths of the rainfall during this period.Secondly,we have revealed the multiple-scale charactoristics of the extreme rainfall occurred during this period based on the dense regional Automatic Weather Station data and radar data and establish several conceptual models for the extreme rainfall.At last,the nocturnal convection initiation and maintenance mechanisim of an extreme rainfall event during the presummer raimy season has been investigated by using the high temporal-spatial resolution observations obtanid during the South China Monsoon Rainfall Experiment(SCMREX)and convective-permitting simulations.The main results are as follows:Since the onset of South China Sea Summer Monsoon(SCSSM)separates the presummer rainy season into two periods,i.e.pre-and postmonsoon periods,the different features between the two periods are investigated.In terms of moisture contribution areas,the South China Sea is the most important one during both periods,and it accounts for about 35.3%and 31.1%of the total moisture during the two periods,respectively.The contribution of the Pacific Ocean ranks secondarily,accounts for about 24%of the total moisture,during the premonsoon-onset period while its contribution reduces significantly to 5.0%during the postmonsoon-onset period due to the northward jump of the Western Pacific Subtropical High.In contrast,the total contribution of Bay of Bengal and the Indian Ocean is 17.7%during the premonsoon-onset period,while it prominently increases to 43.2%during the postmonsoon-onset period which is associated with the establishment of the cross-equatorial flow after the onset of SCSSM.In addition,the contributions of Eastern China during both periods are comparable,are about 18.1%and 15.9%for pre-and postmonsoon periods,respectively.And the Eurasia contributes the rest 8.5%and 4.8%of the total moisture during the premonsoon-onset and the postmonsoon-onset periods,respectively.Using the dense regional automatic weather station(AWS)data,the features of hourly extreme rainfall(>60 mm/h)and their regional differences over SC are revealed.Three areas are identified with high occurrence frequency of extreme hourly rainfall,namely,the northern Guangxi,the southeastern Guangxi,and the central and coastal area of Guangdong.Distinct features of extreme hourly rainfall are found over the three regions.For the northern Guangxi,the diurnal variation of the occurrence frequency has only one peak around the middle night.The rainfall events associated with the hourly extreme rainfall records are usually characterized with a moderate duration(6-12h)and a growth type there.For the southwestern Guangxi,the diurnal variation of the occurrence frequency has no evident peak.And the extreme rainfall events there are usually characterized with a short duration(1-6 h)and an abrupt type.For the middle and coastal area of Guangdong,the diurnal variation of the occurrence frequency has a prominent peak in the early morning and a relative weaker peak in the late afternoon.The extreme hourly rainfall there also main occurred in warm-sector.The extreme rainfall events are often characterized with a moderate duration(6-12h)and a growth type there and the events with a long duration also can be found there.Four multiple-scale conceptual models have been classified for the extreme hourly rainfall observed during the presummer rainy season of SC by using the weather maps and radar images the radar images,the front-type,shear line-type,votex-type,and the warm-sector type.The five types account for 35.5%,16.4%,8.9%,and 38.1%of the total records,respectively.The front-type often can be observed over the north and middle Guangxi,in this type,the rainfall occurs near the surface front.For the shear line-type,the rainfall occurs at the southwest port of the shear line.And most of the rainfall-producing system is linear-shaped for the front and shear line types.The rainfall often occurs near the low level votex or at the south part(200 km)of the votex center,and the rainfall-producing system is usually the linear-shaped MCS and non-MCS for the votex-type.For the warm-sector type,the rainfall often occurs in the middle and coast area of Guangdong without the direct influence of the front,shear line,and votex.The rainfall-producing system over the coastal area is usually with multiple convective lines,while the system over middle Guangdong is often the non-linear convective system with a meso-? or meso-y scale.The extreme hourly rainfall occurs over the north part of Guangxi and the middle and coastal area of Guangdong is associated with the thermal and dynamic anomaly of the atmosphere in planetry boundary layer and low level troposphere.Moreover,the diurnal cycle of the occurrence frequency over the north Guangxi is also associated with the thermal and dynamic anomaly of the atmosphere in lower troposphere near the midnight of the rainy days,and the dynamic anomaly of the atmosphere in lower troposphere at early morning and afternoon of the rainy days is the large-scale factor for the diurnal cycle characteristics of the extreme hourly rainfall occurrence frequency over the middle and costal area of Guangdong.In addition,mechanisms of the nocturnal convection initiation of a heavy rainfall event(358.8 mm/d)occurred in warm sector over the inland of Guangdong Province on 23 May 2014 has been investigated using both the observations in high temporal-spatial resolution collected during the South China Monsoon Rainfall Experiment(SCMREX)and the convection-permitting ensemble simulations.This heavy rainfall event occurs under favorable large-scale settings.During the rainfall period,SC is dominated by southwesterly flows and a significant low-level jet(LLJ)can be observed in the lower troposphere and planetary boundary layer(PBL).And the rainfall region is with high precipitable water(64 mm)and large convective available potential energy(2000 J kg-1).The heavy rainfall is produced by a quasi-stationary and long lived MCS which is initiated around midnight.The MCS at the mature stage exhibites a MRB-type mode,i.e.multiple convective rain bands embedded in the stratiform precipitation area and with no well-organized cold outflows at surface.Both the observations and ensemble simulations reveal that the cold pool(about 1.5 km depth)produced by the previous convection(at the afternoon of 22 May)helps the initiation of nocturnal convection by lifting the high ?e air over the rainfall region brought by the LLJ.Furthermore,the flow brought by the LLJ provides the moisture and energy needed for the maintenance of the convection.And the MCS begins to dissipate with the weakening of the LLJ in the late afternoon. |
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