Maintenance and development of the Ural high and its contribution to severe cold wave activities in winter 2020/21

2020年底至2021年初,连续两次强寒潮入侵中国东部地区,导致大范围强降温.本文研究这两次寒潮的特征和形成机理.主要结果如下:从2020年12月中旬到2021年1月中旬,乌拉尔山地区维持一宽阔的高压脊.脊前北风的维持和加强导致局地斜压性增加,导致下游横槽槽底等高线梯度加大,冷平流加强,西伯利亚高压加强和向南扩张.准定...     

Twisted Film,or: How I Learned to Stop the Movie and Teach the Truth

Abstract

The 1996 blockbuster Twister both entertained and exasperated geographers. Misrepresentations and unsafe field practices resonated deeply; still, the film possesses relevance for educators. Science-based reviews illuminating on-screen inconsistencies and pseudoscience might surprise students accepting movies at face value. This article uses a disaster movie to teach beneficial concepts, definitions, and insight by identifying, elaborating on, and correcting important ideas, dialogue, and terminology. It includes experienced commentary on storm chasing. Chasers must work together using latest technologies to remotely sense environments of future tornadoes during minutes preceding formation. Unlike movies, the mantra for students chasing should be “safety first.”     

对一次局地短时强降水过程的集合预报研究

以上海区域降水集合预报系统为基础,对2007年6月23~24日发生在上海地区的一次短时强降水过程进行了集合预报研究。结果表明:该集合预报系统总体上对这次强降水过程作出了较好的预报,但对强降水发生的时间、地点的预报还有误差,集合预报提供的概率预报结果比集合平均预报结果更具有参考价值。模式物理过程、初值和侧边界的不确定性对降水预报结果的影响是不同的。对降水结果,模式对流参数化方案的影响非常大,并贯穿于整个积分过程;边界层参数化方案的影响较小;侧边界不确定性的影响在初始阶段较小,随后逐渐增大,积分一定时间后,其影响与对流参数化方案的影响相当;初值不确定性在初始阶段有一定的影响,随后逐渐减小。同时考虑3种不确定性的集合预报效果总体上好于没有考虑侧边界不确定性的集合预报效果,考虑模式物理过程和侧边界不确定性的集合预报效果总体上比考虑3种不确定性的集合预报效果更好。对于考虑模式物理过程和侧边界不确定性的集合预报,改进初值能有效地改进对有无降水的预报,但对强度稍大一些的降水预报没有改进。     

不稳定能量参数在一次强对流天气数值模拟中的应用

采用常规观测资料和1°×1°NCEP／NCAP再分析资料,运用MM5V3．7高分辨率中尺度数值模式,对2004年7月12日17时30分前后发生在江苏南通的一次强中尺度对流系统（龙卷）进行模拟分析。结果表明：南通上空处在上干冷、下暖湿的大气不稳定层结中。干冷空气侵入与边界层暖湿气流强烈辐合,大量能量释放造成了本次龙卷的发生。运用数值模拟结果,计算了最佳对流有效位能IBCAPE、对流抑制能量ICIN、归一化对流有效位能INCAPE、能量—螺旋度指数,刚和强对流天气威胁指数／SWEAT等不稳定能量参数。通过分析上述参数的空间分布和在南通上空的时间分布,发现它们对强对流天气的发生有一定反映。     

“07．7”鄂东南强对流天气的多普勒雷达资料分析

利用武汉多普勒天气雷达资料,对2007年7月27日发生在武汉及其周边部分地区的一次强对流天气过程,特别是引起武汉、洪湖的冰雹和地面大风灾害的2个强风暴（A、B）进行了详细分析,得到如下结论：（1）这次强对流天气的主要天气背景是,副热带高压西侧强烈的西南气流诱发了中小尺度扰动,强的低层垂直风切变,大的垂直不稳定,低层较干和中高层更干的水汽条件。（2）产生灾害性天气的对流系统最初是一条近乎南北向的断续型对流带,强风暴A和B在其成熟阶段都有低层弱回波和中高层悬垂回波结构,最大回波强度均大于60dBz;风暴A在其崩溃阶段,近地面径向速度迅速增大,随其北移．造成武汉市黄陂区大风灾害;风暴B在平均径向速度图上存在明显的中层气流辐合（MARC）,是即将出现地面强辐散风的标识。（3）强回波中心高度迅速降低是地面灾害性天气发生的标识,VIL密度比VIL本身更能反映风暴的强度,特别是当因雷达扫描策略的影响导致探测不到风暴顶或风暴底时。（4）在用冰雹探测算法（HDA）探测冰雹时,要注意修改可调参数．特别是0℃和-20℃环境温度的高度．这样才能大大降低冰雹误报率。     

超级单体引发的龙卷天气过程分析

利用营口市多普勒天气雷达资料,对2005年8月10日16时10-20分左右营口市东南部六个乡出现的龙卷天气过程进行了简要分析,该龙卷发生前的主要天气形势是:一个东移的东北低涡引导高空槽,沿高空等高线冷干气流与低空的暖湿气流产生对流不稳定层结,超低空南支急流与低空西南风急流以及高空西北风产生的较大垂直风切变,有利于龙卷天气的产生.产生该龙卷的对流系统是由渤海湾生成的片状层状云和积状云混合降水回波.自东向偏北方向移动,15:50以后低层反射率因子的强降水回波移入大连北部与营口南部临近区域,在层状云降水中含有一些零散的和有组织的对流降水回波,主体为一个近似团状的对流系统,而龙卷产生自该系统南端的一个超级单体.最初的中气旋形成于8月10日15:56,相应对流单体的反射率因子还没有呈现出超级单体的特征,随后中气旋迅速发展加强,在16:02-16:08反射率因子形态呈现出经典超级单体的特征:明显的低层入流缺口,入流缺口位于超级单体移动方向(偏东南方向)的右侧,低层的弱回波区和中高层的回波悬垂结构,最大反射率因子超过56 dRz.在龙卷产生前几分钟和龙卷进行过程中,中气旋保持较强,而后迅速减弱,低层入流缺口渐渐消失.在龙卷进行过程中,相应45 dBz超级单体的反射率因子区局限在6 km以下,此系统为低质心的对流系统,产生的天气是龙卷,伴随有大风短时强降水,与冰雹的高质心对流系统有明显区别.同时也初步探讨了引发此次龙卷的生成机制.     

2006年6月皖北龙卷多普勒雷达产品分析

分析了2006年6月29日发生在安徽泗县的龙卷多普勒雷达的中气旋和龙卷涡旋特征（TVS）等产品。龙卷发生前,卫星云图上有3个对流云团呈东北—西南向排列,每个云团的东南侧有弓状回波发展,3条弓状回波首尾相连,也呈东北-西南向排列,龙卷发生在最西南的弓状回波的顶部。龙卷发生前弓状回波在上游产生了短时强降水,2 h降水量达到60 mm以上。在弓状回波的前沿,雷达探测到一系列的中气旋,龙卷发生前30 min,最西南的弓状回波追上其前面的回波带,发生了2个回波带合并,回波合并前,回波带上有2个中气旋,回波合并后,探测到一个特大直径的中气旋（径向直径25.8 km）。在龙卷发生地的上游,有一条带状的灾害性大风区,实地位置测定结果,该带状大风区与一系列中气旋最大风速圈的南边缘移过的路径一致。分析认为中气旋最大风速圈的南边缘,中气旋的风向与弓状回波后的直线风方向相同,两者叠加造成灾害性大风。出现龙卷1 h 40 min之前（05：00）,在泗县上游淮北地区,雷达开始探测到中气旋产品,在12 min之前探测到TVS（龙卷涡旋特征）产品,这些雷达产品对大风灾害的临近预报无疑是非常有用的。     

新一代天气雷达与闪电定位仪资料在一次区域性暴雨过程中的分析应用

根据一次大范围暴雨天气过程的闪电定位资料和多普勒雷达资料,利用统计和对比分析的方法,发现了闪电发生频数、强度和雷达回波强度在时间序列上有较好的一致性;在雷达回波发展的不同阶段,闪电发生的位置与雷达强回波位置有时相同,有时偏离,有时甚至无闪电发生;雷达回波速度场分析表明,在低层存在不利于对流发展的环境风场特征时,降水回波在向测站移动的过程中趋于消散,闪电频数也随着减少;在降水回波速度辐合区,对应闪电活动频繁。     

Study of the total lightning activity in a hailstorm

A thunderstorm that developed over northeastern Spain on 16 June 2006 is analyzed. This severe thunderstorm produced hailstones as large as 40 mm and had a lifetime of 3 h and 30 min. Radar cross-sections show strong vertical development with cloud echo tops reaching an altitude of 13 km. The specific characteristics of the lightning activity of this storm were: (i) a large amount (81%) of negative cloud-to-ground (−CG) flashes with very low peak currents (< 10 kA in absolute value), (ii) a very large proportion of intra-cloud (IC) flashes with an IC/CG ratio reaching about 400, (iii) a large number of “short” IC flashes (with only 1-VHF source according to SAFIR detection), (iv) a large increase of the −CG flash rate and of the CG proportion near the end of the storm. The rate of −CG flashes with a low peak current were observed to evolve similarly to the rates of IC flashes. Most of them have been assumed to be IC flashes misclassified by the Spanish Lightning Detection Network (SLDN). They have been filtered as it is usually done for misclassified +CG flashes. After this filtering, CG flash rates remained very low (< 1 min^− 1) with +CG flashes sometimes dominant. All the particular lightning activity characteristics similar to those observed in the Severe Thunderstorm Electrification and Precipitation Study (STEPS) campaigns support the hypothesis that this thunderstorm could have had an inverted-polarity or complex charge structure. The maximum IC flash rate (67 min^− 1) peaked 24 min before the presence of reflectivity higher than 60 dBZ. The IC activity abruptly decreased during the period when reflectivity was dramatically increasing. The time of maximum reflectivity observed by radar was consistent with the times of reported hail at the ground.     

Contribution of realistic soil moisture initial conditions to boreal summer climate predictability

Seasonal climate forecasts mainly rely on the atmospheric sensitivity to its lower boundary conditions and on their own predictability. Besides sea surface temperature (SST), soil moisture (SM) may be an additional source of climate predictability particularly during boreal summer in the mid-latitudes. In this work, we investigate the role of SM initial conditions on near-surface climate predictability during ten boreal summer seasons using three complementary ensembles of AMIP-type simulations performed with the Arpège-Climat atmospheric general circulation model. First we have conducted an assessment of the SM predictability itself through a comparison of simple empirical SM models with Arpège-Climat. The statistical and dynamical models reveal similar SM prediction skill patterns but the Arpège-Climat reaches higher scores suggesting that it is at least suitable to explore the influence of SM initialization on atmospheric predictability. Then we evaluate the relationships between SM predictability and some near surface atmospheric predictability. While SM initialization obviously improves the predictability of land surface evaporation, it has no systematic influence on the precipitation and near surface temperature skills. Nevertheless, the summer hindcast skill is clearly improved during specific years and over certain regions (mainly north America and eastern Europe in the Arpège-Climat model), when and where the SM forcing is sufficiently widespread and strong. In this case, a significant impact is also found on the occurrence of heat waves and heavy rains, whose predictability at the seasonal timescale is a crucial challenge for years to come.