In order to improve the practical prediction ability of refined grid precipitation,the performance of ECMWF(EC),CMA-MESO,SXWRF and SCMOC precipitation forecast products in Shaanxi Province during the rainy season of 2021 are compared and evaluated,and the correction effect of Kalman dynamic frequency matching method on different models is discussed.Then,for the shortcomings of this method,based on the optimal TS scoring threshold method and SCMOC's judgment of weather process,the small-magnitude precipitation is revised for the second time.Finally,the heavy precipitation is revised by using the precipi-tation sub process modeling and the improved Kalman dynamic frequency matching method which is based on image similarity recognition technology.The results show that SCMOC has the highest accuracy of sun-ny and rainy forecast and the highest TS score of heavy precipitation,which are 81.60％and 0.30 respec-tively.The Kalman dynamic frequency matching method can significantly improve the accuracy of sunny and rainy forecast of EC,CMA-MESO and SXWRF precipitation forecast products,but the improvement effect of heavy rainfall forecast is unstable.The improvement ranges of the EC model sunny and rainy fore-cast accuracy and the TS score of rainstorm forecast are the largest,6.35％and 6.99％respectively.This correction method is more suitable for EC model.Compared with the EC model modified by Kalman dy-namic frequency matching method,the accuracy of sunny rain and light rain prediction of EC model after the second correction of sunny rain spaced elimination is improved by 0.51％and 0.64％,respectively.The correction of the precipitation sub process modeling can further improve the TS score of EC model heavy precipitation,which is 1.05％higher than the TS score of heavy precipitation without sub process correction.Other scoring indicators of heavy precipitation are also better.The improved Kalman dynamic frequency matching method can significantly further improve the TS score of EC precipitation of all magni-tudes,especially the TS score of heavy precipitation,improved by 2.79％.
PM2.5 and O3 which interact with each other have become the most important pollutants in the cities of Fenwei Plain.The pollution often occurs simultaneously in warm seasons and is closely related to meteorological conditions.Based on the daily PM2.5 and O3 concentration data,surface meteorological ob-servation data and ERA5 high-altitude reanalysis data of 12 cities in the Fenwei Plain from 2015 to 2021,the spatio-temporal characteristics of PM2.5 and O3 in the Fenwei Plain,as well as the relationship between PM2.5 and O3 when compound pollution occurs are analyzed.Besides,the impact of local meteorological conditions and synoptic situation on compound pollution is also studied.The results show that the average annual PM2.5 and MDA8 O3(maximum daily 8 h average ozone)concentrations in the Fenwei Plain began to decline continuously in 2017 and 2018,respectively,and the number of compound pollution days also be-gan to decrease continuously after 2019.Compound pollution mainly occurs in March to September,fre-quently seen in the cities of eastern Fenwei Plain,mostly in condition of high temperature and low humidi-ty.Finally,the synoptic circulation situation of compound pollution is divided into four types by principal component analysis with the T-PCA algorithm,featured with the northwest or westerly air flow in high-al-titude and the southerly wind or breeze in the low-level warm zone.These findings could provide some ba-sis for controlling pollution in the Fenwei Plain and are of great significance for cooperative governance of PM2.5 and O3 pollution.
Based on the EAR5 reanalysis data in June from 1979 to 2016,the moist thermodynamic advec-tion parameter,thermal helicity,divergence flux,moisture divergence flux and the thermodynamic wave-activity density are selected as five comprehensive factors.The probability prediction model of regional persistent rainstorm in Hunan is constructed by the means of nuclear density estimation and based on the optimal factor and weight combination which is established with the best TS score as the test standard.The results show that the average TS of independent samples from 2017 to 2019 reaches 29.9％,which is a positive skill relative to the European Centre for Medium-Range Weather Forecasts(ECMWF)fine grid forecast(with an average TS score of 22.4％).During the two regional persistent rainstorm operational experiments in the 2021 and 2022 flood seasons,the rainstorm forecast with a 24 h leadtime by the Hunan regional perisistent rainstorm probability prediction model is superior to the forecasts of ECMWF and CMA-GFS large-scale model as well as CMA-SH and CMA-GD regional mesoscale model.Therefore,the Hunan regional persistent rainstorm prbability prediction model has a strong ability to forecast the regional persistent rainstorm in Hunan.
Based on the hourly precipitation data in the wet season during 2011-2020 from the gradient ob-servation system between Cangshan Mountain and Erhai Lake of Dali National Climate Observatory,the diurnal variation characteristics of precipitation at the mountain top,hillside and dam area are analyzed.The results show that the diurnal variation of rainfall amount at the dam station presented one peak,while that at mountain side and hilltop stations presented two peaks.The diurnal variation of rainfall frequency at dam and hilltop stations displayed a single peak,while that at the hillside station was relatively stable.All hourly rainfall amount and rainfall frequency for a whole day basically increased with the rise of alti-tudes.The diurnal variation of rainfall intensity at hilltop station exhibited double peaks,while that the hillside and dam stations showed remarkable fluctuation with small values in the afternoon and large values over night.There were no significant differences in rainfall intensity from 14:00 BT to 17:00 BT among the three stations,but the rainfall intensities at the hillside and dam stations were obviously greater than that at the hilltop station during other periods.The rainfall amount with a duration of 2-16 h over night was a large value area,which maintained a longer duration with the rise of altitude.The rainfall amount with a duration of less than 6 h during the daytime was a large value area,and the occurrence of the large value was delayed with the increase of altitude.The rainfall frequencies at all the three stations had a large value area during the day(night)when the rainfall duration was less than 6 h,while that at the hilltop sta-tion only had a large value area at night when the duration was 7-18 h.The rainfall frequencies over night were greater than that during the day at the dam and hilltop stations,while there was little difference in the rainfall frequency during the day and night at the hillside station.The accumulated rainfall amount and frequency of long(medium and short)duration increased(decreased)with the increase of altitudes.The contribution of long duration rainfall amount(long duration rainfall frequency)to accumulated rainfall amount(accumulated rainfall frequency)at each station was the largest,but the contribution of short du-ration rainfall amount(medium duration rainfall frequency)was the least.
A large-scale extremely severe convective weather named"5·17"severe convection occurred in Shandong Province on 17 May 2020,and the hail coverage was the largest in the resent 10 years.The convective storms were highly organized.The regional supercell clusters and a strong squall line over 500 km in length caused this extremely severe convection.Based on ERA5 reanalysis data,automatic weather station data and Doppler weather radar data,the ambient conditions of this extremely severe convective weather are analyzed.The results show as follows.The cold vortex was located in the key area which was most conducive to the Shandong severe convection.The large-scale weather system forcing was strong,and the unusually strong cold air in the middle troposphere moved southward impacting the previously abnormally warm Shandong Province and resulting in the"5·17"extremely severe convection weather.The anomaly of weather system is more representative of the intensity of dynamic and thermal forcing,and when anoma-ly level is above 2σ,the extremely severe convection would be caused.When the intensity of the cold vor-tex weakened during its southward moving,accompanied by increasing anomaly,it may still cause ex-tremely severe convective weather in its southeast quadrant.The strong deep vertical wind shear was con-ducive to storms organization and development.The long axis of the squall line orientated the same direc-tion as the 0-6 km vertical wind shear vector.The area where new cells were generated,developed and merged was located in the front of the large value center of the wind vector difference.The low-level warm and moist advection continuously transported warm and humid air to Shandong,which was the mechanism of CAPE reconstruction and the main energy source for the long-term maintenance of supercell group and long squall line.
By using the object-oriented method of contiguous rain area(CRA),this paper investigates sources of the rainfall forecast error in deterministic forecasts by the ECMWF,and their changes with dif-ferent rainfall levels and forecast periods when typhoons influenced China in 2019.Then,the correlations between typhoon track errors and displacement errors of rainfall events are analyzed and the performance improvements of rainfall forecasts calculated with track correction or CRA shifting are compared.Finally,forecast errors of rainfall probability distribution,radial and asymmetric rainfall distribution are verified and analyzed.The results are as follows.In general,the main forecast errors come from displacement er-ror and shape error.Except for enormous amount of rainfall,track error is significantly correlated to the displacement error of CRA rainfall events.The improvement of rainfall forecasts made by track error cor-rection is less than that made by CRA shifting correction.The shape of probability density distribution for rainfall forecasts resembles the observed one,but the forecasted rainfall intensity in typhoon core area is stronger than the observed one.Before and after typhoons'making landfall or approaching the coast,the forecasted rainfall is much closer to typhoon center than the observed one,and it lags behind the observa-tion.The asymmetric structure of forecasted rainfall is significantly weaker than the observed one.
To promote the application level of hydrometeor classification for Chinese Doppler polarimetric radar,based on the hydrometeor classification algorithm(HCA)developed by the National Severe Storm Laboratory(NSSL)of the United States,we design an optimum algorithm,named HCA-Opt,by adding the amendatory identification for the hydrometeor in hail and three-body scatter area,using the tempera-ture of the numerical weather model analysis field to detect the location of the melting layer,and integra-ting restricted condition of vertical distribution for all hydrometeors.HCA-Opt is utilized to analyze the distribution characteristics of hydrometeor during a hailstorm cloud weather in Zhangqiu of Jinan City on 9 July 2021,and the location of the hail and the hydrometeor classification result are verified.The conclu-sions are as follows.The type of hydrometeor wrongly recognized as ground clutter by the HCA in the hail and the three-body scatter region can be correctly identified by the HCA-Opt.HCA-Opt can use the tem-perature from model analysis field to accurately identify the height of melting layer,solving the defect that the melting layer could not be detected by the algorithm in severe convective weather in HCA.Compared to HCA,the vertical distribution of hydrometeor identified by HCA-Opt is more reasonable.Besides,the classification result retrieved from the HCA-Opt can better describe the spatial distribution of hydrometeor at newborn and hailfalling stages of hailstorm,and initially reflect the characteristics of phase transforma-tion at different heights.Through the verification,the reliability of the moderate(light)rain and graupel identified by the HCA-Opt is the highest,while that of the crystal and wet snow is lower,and easy to be mixed with the dry snow.Overall,the HCA-Opt can improve the skill of hydrometeor identification to certain extent,provide indicative information for warning and detecting the hailfalling area and have a good operational application prospect.
In Taipusi Banner of Xilin Gol League,Inner Mongolia,a rarely-seen EF3 strong tornado oc-curred on 25 June 2021,resulting in 6 deaths and a large number of buildings'damage and other losses.Based on meteorological observation data,automatic weather station data,CB-Doppler radar observation data of Zhangbei,Hebei Province,FY-4 satellite data and NCEF FNL(1°× 1°)6 h reanalysis data,this paper comprehensively analyzes the process of the strong tornado.The results show that this tornado oc-curred in the background of unstable stratification environment of the forward-tilted trough.The strong conditional instability in the middle and low troposphere(the vertical temperature lapse rate at 850 hPa and 500 hPa was about 7.7 ℃·km-1),abundant water vapor in the lower layers,medium-strength convective available potential energy,and strong 0-6 km vertical wind shear provided favorable environmental condi-tions for supercell storms and tornadoes.In addition,the 0-1 km vertical wind shear was 8 m·s-1,and the lifting condensation height was 1.0 km,which provided relatively favorable environmental conditions for the occurrence of supercell storms and tornadoes.The convergence line accompanying the ground dry line triggered the parent storm that produced the tornado,and then evolved into the supercell.The radar reflectivity factor showed the characteristics of a typical hook echo,the inflow gap of warm and wet air at low level,the weak echo region at low level,the overhang echo at middle and high level,and the moder-ate-strength mesocyclone,etc.In the process of the formation and extinction of the tornado,three super-cell storms formed successively,and all of them appeared in the isolated convective storm form.The torna-do occurred at the top of one of the supercells,which was the junction of the front updraft and the rear downdraft.The possible start time and path of tornado based on the evolution of mesocyclone intensity an-alyzed by radar are in good agreement with the time of field investigation.Except for the strong tornado,this series of supercells also produced large hail and linear convection gale(thunderstorm gale),the strong echo center was significantly tilted from low to high,and the maximum reflectivity factor was as high as 65 dBz.In addition to the moderate-strength mesoclones,there was also obvious mid-level radial conver-gence in the diagram of radial velocity.At the time of supercell storm formation,the vertical integrated liquid water content(VIL)was as high as 73 kg·m-2,the VIL density was 4-5 g·m-3.These radar echo characteristics indicate the existence of large-sized hail,while the mid-level radial convergence was the radar echo characteristic of thunderstorm gale.
A climatology,which includes interannual variability,annual and diurnal cycles,intensities ac-cording to the"Enhanced Fujita Scale",geographic distribution,and environmental conditions derived from reanalysis data,is developed in this study for the tornadoes in Liaoning Province during 1971-2020 in order to understand the climatic characteristics of tornado.The tornado reports are obtained from the Chinese Meteorological Disaster Dictionary,Chinese Meteorological Disaster Yearbook and other associated data.The differences in environmental background between Liaoning Province in China and the tornado-prone areas in United States are compared,and the typical circulation conditions and atmospheric charac-teristics of Liaoning tornadoes are analyzed.The major findings are as follows.Over the 50 years(1971-2020),there were 105 tornadoes recorded(including 17 EF2 or stronger tornadoes)in 97 tornadic days in Liaoning Province,which suggests that Liaoning has 2.1 tornadoes per year with the annual generation density to be 1.4X10-5 km-2,approximately equal to 1/10 tornadoes in the United States.A total of 17 EF2 or stronger tornadoes are recorded,with an average annual occurrence of 0.3.The tornadoes occur mostly(95％)from May through September,and 67％of tornado genesis are between 14:00 BT and 19:00 BT.Significant tornadoes(EF2/EF3/EF4)mainly occur in the central and western part of Liao-ning,while weak tornadoes(EF0/EF1)happen more in the coastal region of the southeastern Liaoning.The tornadic season in the northwest of Liaoning is much earlier than in the southeast.CAPE and mid-level wind shear often change in opposite phases with seasons,and the appropriate configuration of the two is the prerequisite for the occurrence of tornadoes and other convective weather.Additionally,the lower low-level the storm relative helicity(SRH)is the major cause for the apparently lower tornado density in Lia-oning relative to the United States.About 87％of tornadoes are related to the cold vortices,which can be mainly divided into the short-wave trough category at the cold vortex bottom and cold vortex front category dominated by low-level frontogenesis(65％),the mid-level dry cold air forcing category dominated by mid-to-high level dry cold airflow(12％),and the cold vortex central area category dominated by mesoscale near-storm environment under severe thermal instability(6％).The environmental conditions in the high-incidence area of Liaoning tornadoes have the following characteristics:the mid-level impact system is cold vortex,the ground corresponds to frontal cyclones,tornadoes often appear in the northwest quadrant of the center of SRH and in the large-value zone of CAPE gradient,corresponding to the east side of cold front and the dry line at surface as well as the top of the temperature ridge.
Sub-seasonal to seasonal(S2S)climate and runoff prediction is of great importance for active dis-aster reduction.The prediction models of runoff anomaly and extreme drought for the future 40 days are developed based on CMA-CPS v3 climate model by National Climate Center,China Meteorological Admin-istration,and a hydrological model HBV.The performance of the models are evaluated with the indices of MSSS,ACC,AUC and BSS for hindcast,and verified for the runoff anomaly prediction over the Yellow River Basin in 2021.The results suggest that runoff mean prediction for the future 40 days is skillful,and the skill is higher in dry season than wet season.Moreover,the extreme drought prediction is skillful in later autumn(November)and the winter months(December,January and February).The direction of monthly runoff anomaly from May to October in 2021 are predicted correctly except in June and Septem-ber,but the anomaly degrees are different from observation.Finally,the analysis of skill variation with lead time and seasons,and the skill difference between runoff and precipitation reveals that the skill of pre-cipitation prediction by S2S climate model influences runoff skill,especially during wet season.However,there are also other factors than precipitation affecting the skill of runoff prediction.
The main characteristics of the general atmospheric circulation in August 2023 are as follows.The intensity of the polar vortex was slightly stronger in the Northern Hemisphere.The circulation over the mid-to-high latitudes of Asia at 500 hPa showed"two troughs"and"one ridge".The high-altitude trough affecting the northeastern and northern of China is stronger.The mean location of the ridge line in the western part of the Western Pacific subtropical high was more southward compared to that in normal years.In August,the monthly average temperature was 21.9℃,0.8 C higher than in the same period of normal years(21.1℃).The monthly average precipitation was 110.9 mm,which is 3.5％higher than normal(107.1 mm).During this month,the high temperature days were more than normal,with regional high temperature processes appearing successively in China.Meteorological drought continued to in the northwest and northeast of China.Six torrential rain events were reported in China.In August,six ty-phoons were active in the South China Sea and the Western North Pacific.The generation number of tropi-cal cyclones was higher than that in the previous years,but the number of landfall typhoons was lower than usual.Severe convective weather was frequent and local disasters were severe.In addition,one torna-do hit the Dafeng District,Yancheng of Jiangsu Province.
为提高精细化网格降水的实际预报能力,评估了 2021 年汛期 ECMWF(EC)、CMA-MESO、SXWRF 和 SCMOC 降水预报产品在陕西的表现,讨论了卡尔曼动态频率匹配方法对不同模式的订正效果,然后针对该方法不足,基于最优TS评分阈值法和SCMOC在天气过程判定中占优信息对小量级降水进行了二次订正,最后利用分类降水过程建模和基于图像相似识别技术改进的卡尔曼动态频率匹配法对暴雨进行了订正研究.结果表明:SCMOC晴雨预报准确率和暴雨TS评分均最高,分别为 81.60%和 0.30,表现最好;卡尔曼动态频率匹配法可明显提高EC、CMA-MESO和SXWRF模式降水预报产品晴雨预报准确率,对暴雨预报的改善效果不稳定,对EC晴雨预报准确率和暴雨TS 评分提升幅度均最大,分别为 6.35%和 6.99%,该订正方法更适合于EC模式;经晴雨消空二次订正后的EC模式晴雨和小雨预报准确率较一次订正后的EC模式均有提高,分别提高了0.51%和 0.64%;分类降水过程建模订正可进一步提高EC暴雨TS评分,较未分类过程订正后的暴雨TS评分提高了 1.05%,且暴雨其他评分指标也均变好;改进后的卡尔曼动态频率匹配法较改进前可进一步提高EC各量级降水TS评分,尤其是暴雨TS评分提高了 2.79%.
2021 年 6 月 25 日内蒙古锡林郭勒盟太仆寺旗发生了历史罕见的EF3 级强龙卷,导致 6 人死亡,大量建筑物等严重损毁.利用常规高空和地面观测、区域自动气象观测站、FY4 卫星云图、河北省张北 CB 型多普勒雷达等观测资料,以及NCEP(1°×1°)逐 6h再分析资料对这次强龙卷过程进行分析.结果表明:此次龙卷发生在前倾槽不稳定层结环境背景下,较强的对流层中低层条件不稳定(850 hPa与 500 hPa温度垂直减温率约为 7.7℃·km-1)、低层丰富的水汽、中等强度的对流有效位能和强的 0～6km垂直风切变为超级单体风暴形成提供了有利环境背景.此外,0～1 km风矢量差为 8 m·s-1,抬升凝结高度为 1.0 km,为超级单体龙卷的发生提供了相对有利的环境条件.与地面干线伴随的辐合线触发了产生龙卷的母风暴,随后演变为超级单体,其雷达反射率因子呈现典型的钩状回波、低层暖湿气流入流缺口、低层弱回波区和中高层回波悬垂,以及中等强度的中气旋等特征;龙卷的生成和消亡过程中有三个超级单体风暴相继形成,都呈现为孤立的对流风暴形态,龙卷发生在其中一个超级单体钩状回波的顶端,在前侧上升气流和后侧下沉气流交界处,雷达分析的基于中气旋强度演变的龙卷可能起始时间和路径与现场调查时间十分吻合.除了强龙卷,这系列超级单体还产生了大冰雹和直线型对流大风(雷暴大风),强回波中心自低到高明显倾斜,最大反射率因子高达 65 dBz,径向速度图上除了有中等强度的中气旋,还存在明显的中层径向辐合,超级单体风暴形成时垂直累积液态水含量(VIL)值高达 73 kg·m-2,VIL密度达到4～5 g·m-3,这些雷达回波特征指示大冰雹的存在,而中层径向辐合是雷暴大风的雷达回波特征.
针对 2019 年影响中国的 8 个台风,利用面向降水对象的CRA(contiguous rain area)方法研究了欧洲中期天气预报中心确定性预报的降水误差来源,及其在不同预报时段和降水量级下的变化趋势,分析了台风路径预报误差与降水对象的CRA位置误差之间的相关性,对比计算了台风路径修正与CRA shifting方法对于改进降水预报的作用,并评估了台风降水概率分布、径向分布和非对称分布的预报误差.结果表明:总体而言,台风降水预报的主要误差来自于位置误差和形态误差;除特大量级降水以外,台风降水对象的CRA位置误差与路径误差显著相关,通过修正台风路径能改进降水预报,但其效果要逊于CRA shifting方法;预报的台风降水概率密度分布形态与观测总体上较为一致,但台风核心区内的预报降水强度均大于观测;台风登陆或靠近我国沿海前后,预报降水较观测更靠近台风中心,且略滞后于观测,预报降水的非对称性明显弱于观测.
利用 1979-2016 年 6 月EAR5 再分析资料,选取湿热力平流参数、热力螺旋度、散度通量、水汽散度通量和热力波作用密度 5 个综合因子,采用核密度估计方法,基于TS评分最优为检验标准筛选确立最优因子和权重组合,构建了湖南区域持续性暴雨概率预报模型,并进行了独立样本检验与业务试用.结果表明:2017-2019 年独立样本回代检验,平均TS评分达到 29.9%,相比于欧洲中期天气预报中心(ECMWF)细网格(平均TS评分为 22.4%)为正技巧.在 2021 年、2022 年汛期两次区域持续性暴雨个例的预报试验中,提前 24h的暴雨预报优于 ECMWF、CMA-GFS 等大尺度模式和CMA-SH、CMA-GD等区域中尺度模式,对湖南区域持续性暴雨有较强的预报能力.
次季节气候和径流预测是主动减灾的一个关键.基于国家气候中心第三代气候模式系统的次季节到季节模式(CMA-CPS v3 S2S)的气候预测信息和 HBV水文模型,应用集合预测技术研发了未来 40d 时段平均径流量和时段内极端干旱概率预测模型,应用平均方差技巧评分、距平相关系数、相对操作特征曲线面积、布赖尔技巧评分开展了回报检验,并检验了 2021 年黄河流域径流异常预测效果.结果表明,所建模型能够以较高技巧预测黄河流域未来 40d 时段平均的径流量,且表现出枯季预测技巧高、湿季技巧低的季节差异;对秋末 11 月和冬季 3 个月(12 月、1 月、2 月)的极端干旱概率预测也有较高技巧.对于 2021 年 5-8 月黄河上中游干旱和 9-10 月的秋汛,该方法正确预测了除 6 月、9 月外的其他 4 个月的径流异常方向,但异常程度与实况存在差异.对径流预测水平影响因素的进一步分析表明,S2S降水预测能力影响径流预测水平,特别是丰水期的径流预测,但还有降水之外的其他因素影响径流预测技巧.
为提升我国多普勒双偏振雷达水凝物分类的应用水平,在美国强风暴实验室(NSSL)研发的水凝物分类算法(HCA)基础上,通过增加冰雹区与三体散射区水凝物类型的订正识别、选用数值模式温度分析场识别融化层、引入水凝物类型垂直分布限制条件等,建立了优化方法(HCA-Opt).利用 HCA-Opt 分析了 2021 年 7 月 9 日济南市章丘区一次雹暴云的水凝物分布特征,对冰雹落区与水凝物分类结果进行比对检验,得到如下主要结论:HCA-Opt可以正确识别冰雹与三体散射区的水凝物类型,修正了 HCA将其识别为地物的问题;HCA-Opt 利用模式温度分析场可准确识别融化层高度,解决了强对流天气下 HCA使用的融化层自动识别算法(MLDA)无法有效识别融化层的缺陷;与 HCA 相比,HCA-Opt 识别的水凝物在垂直分布上更加合理.HCA-Opt给出的水凝物分类结果较好描述了雹暴初生、降雹不同阶段的水凝物空间分布,初步揭示了不同高度水凝物粒子相态转化特征.HCA-Opt识别的水凝物分类中,中(小)雨与霰的可信度最高,冰晶与湿雪的可信度较低,且容易与干雪混淆;总体而言,HCA-Opt提高了水凝物分类识别技巧,对冰雹预警和落区判别具有较好的指示作用和业务应用前景.
2020 年 5 月 17 日,山东省出现大范围强对流天气(简称"5·17"强对流),冰雹范围之广为近 10 年之首.对流风暴高度组织化,区域性的超级单体群以及一条长度超过 500 km 的强飑线造成此次极端强对流天气.利用 ERA5 再分析、加密自动气象观测站、多普勒天气雷达等资料,剖析了此次极端强对流天气的环境条件.结果表明:冷涡位于最有利于山东出现强对流的关键区,大尺度天气系统强迫强,对流层中层异常强的冷空气南下影响前期异常增暖的山东地区,造成"5·17"极端强对流.天气系统的异常程度更能代表动热力强迫的强度,异常程度达到 2σ以上有可能造成极端强对流天气.当冷涡南下过程中强度减弱,但异常程度增加时,其东南象限仍能产生极端强对流天气.强的深层垂直风切变有利于对流风暴组织化发展,飑线的长轴走向与 0～6km垂直风切变矢量方向相同,新单体发生、发展、合并的区域位于风矢量差大值中心前沿.低层暖湿平流源源不断地向山东输送暖湿空气,是CAPE重建的机制,是超级单体群和长飑线得以长时间维持的主要能量来源.
基于大理国家气候观象台苍山-洱海梯度观测系统 2011-2020 年湿季小时降水资料,分析山顶、山腰和坝区 3 个站的降水日变化特征.结果显示:降水量日变化,坝区站呈现单峰型,山腰站和山顶站则是双峰型;降水频次日变化,坝区站和山顶站为单峰型,山腰站日变化比较平缓;各时次的降水量、降水频次基本随海拔高度的增加而增多;降水强度日变化,山顶站为双峰型,坝区站和山腰站波动较大,午后为小值区,夜间为大值区,3 个站在 14:00-17:00 的降水强度相差不大,而其他时段山腰站和坝区站的降水强度比山顶站大.夜间降水量在持续时间 2～16 h是大值区,随海拔的增加降水量大值区持续时间较长;白天降水量在持续时间小于 6h是大值区,随海拔的增加,大值出现的时间向后移.降水频次在持续时间小于 6h,3 个站在白天、夜间分别有一个大值区,而持续时间 7～18 h的只有山顶站夜间有大值区;坝区站和山顶站夜间降水频次大于白天降水频次,山腰站白天、夜间降水频次相差不大.长历时(中历时、短历时)的累计降水量、降水频次随海拔高度的增加而增大(减小);3 个站长历时降水量(长历时降水频次)对总降水量(总降水频次)的贡献最大,贡献最小的是短历时降水量(中历时降水频次).
2023 年 8 月大气环流的主要特征是:北半球极涡强度略偏强,500 hPa上亚洲中高纬地区环流形势为"两槽一脊",影响我国东北、华北地区的高空槽偏强,西太平洋副热带高压西段脊线的平均位置较历史同期偏南.8 月,全国平均气温为21.9℃,较常年同期(21.1℃)偏高 0.8℃,全国平均降水量为 110.9 mm,较常年同期(107.1 mm)偏多 3.5%.月内,高温日数较常年同期偏多,区域高温过程持续影响我国,西北、东北等地气象干旱持续发展;我国出现 6 次暴雨过程.8 月共有 6 个台风在南海和西北太平洋活动,生成个数较常年偏多,登陆个数较常年偏少.强对流天气多发,局地受灾严重,江苏盐城大丰区出现龙卷风.
