“9608”号台风登陆北上引发北方特大暴雨的中尺度对流系统研究

利用NCEP再分析资料和常规观测资料,对1996年8月3—5日发生在河北和山西的特大暴雨过程进行了分析和模拟研究,并对模拟结果用部分特殊观测资料进行了证实。天气形势的分析表明,“96.8”暴雨过程中登陆减弱的台风低压并未出现“75.8”暴雨中的台风与中纬度西风槽发生相互作用,入侵的弱冷空气是由华北高压南侧的偏东风引导至台风倒槽外围,是属于比较少见的登陆台风北上受高压阻挡停滞类型;台风倒槽内发生发展的2个中尺度对流云团是造成此次暴雨的直接影响系统,对流层低层的偏南风低空急流对中尺度对流云团的发展具有重要作用;暴雨期间,低空急流不仅强度大,而且伸展高度也相对较高;伴随强对流系统的主要入流和出流气流分别有2支,其中对流层高层的高空中尺度急流中心主要由强出流气流形成。分析还表明太行山对对流系统的阻挡可能是导致强降水长时间维持在石家庄附近的原因之一。     

2000年7月中旬河南连续性暴雨过程分析

利用2000年7月12－16日逐时雨量、逐时风场、探空及多普勒雷达回波资料，分析了2000年7月12－16日河南连续性暴雨的雷达回波特征及中尺度系统活动与暴雨的对应关系，揭示中尺度系统是造成暴雨的直接原因。     

北京层状云人工增雨数值模拟试验和机理研究

在中尺度WRF 模式的Morrison 双参数方案中引入了AgI 粒子与云相互作用的过程,在WRF 模式中实现了催化功能。利用加入了催化方案的中尺度模式对2008 年3 月20～21 日环北京地区一次层状云系降水过程进行模拟和催化试验。模拟自然降水与实测结果一致,分析微物理特征并在所得分析基础上进行催化试验。研究在不同催化剂量、高度和时刻进行试验对降水的影响。结果表明:以20 g 的碘化银进行催化作业,在催化后的前30min 之内,地面雨量轻微减小,最大累积减雨量为2010 t,30 min 后,净增雨量迅速增加,最大累积增雨量达到了3.4×105 t。催化开始阶段的减雨主要是由于播撒AgI 后,云水减少而雪晶增多,导致雨滴碰并云滴,云滴向雨滴自动转化过程的减少以及雪晶碰并雨滴过程的增多,然而空中增多的雪晶尚未下落到暖区融化成雨滴。而第二阶段的增雨则是空中增多的雪晶逐渐下落到暖区,雪晶融化成雨滴过程增多。AgI 的播撒率对降水量有明显影响,过量催化会使雪晶平均质量减少,下落速度锐减,从而雪融化成雨水减少,导致雨量减弱,不同催化高度和催化时间的催化结果表明在过冷水含量比较丰富而冰雪晶含量偏少的区域进行催化,增雨效果显著。     

武汉一次下击暴流天气的成因分析

对多普勒天气雷达、卫星以及高空地面等各种探测资料进行了综合分析。结果表明:2007年7月27日发生于武汉地区的局地强对流天气过程主要是在500 hPa副热带高压边缘反气旋性弯曲的西南气流中,受午后下垫面强烈加热、对流不稳定增强、局地中尺度辐合系统加强等共同作用下,产生的中尺度对流系统直接造成的。雷暴云中宏尺度(Macro-scale)、微尺度(Micro-scale)的下击暴流在近地面形成的破坏性直线风是导致武汉地区大风灾害的直接物理原因。     

平流计算方案对一次江淮梅雨暴雨模拟的影响

为对比MM5模式中原中央差平流方案和新型半拉格朗日插值计算方案CIP的模拟差别,针对2006年7月5~6日的江淮流域梅雨暴雨过程进行了模拟,证实了CIP方案对这次暴雨过程具有较好的模拟效果,它更加真实地再现了梅雨锋降水的中尺度结构,改进了模式的模拟结果,特别是对梅雨锋内对流运动结构的模拟,显示了CIP对刻画大梯度分布和强对流运动计算的良好性能。平流的高精度计算对提高这类系统的模拟精度非常重要。     

渤海西海岸带大暴雨中尺度云团空间结构分析

利用FY-2E和CloudSat卫星、多普勒雷达、T639L60物理量、雷电及实测降水等资料,分析了2010年7月19日、8月4日和21日渤海西海岸大暴雨天气的中尺度云团时空分布.结果表明,从红外云图、水汽云图与6h雨量叠加可明显看出有多个β、γ中尺度云团,并以50～60 km·h-1的速度沿副热带高压588 dagpm线西北侧的引导气流方向移动,强降水时段TBB温度值为-96～-115℃,涡旋状云系对应低层中尺度低涡,带状云系对应低层中尺度切变线.同步雷达反演显示,β中尺度对流云团强、弱交替变化周期为3～6h,天津地区减弱的雨团移到河北海岸带,在有利的低层风场辐合区产生了新的暴雨云团;该暴雨云团减弱后,对下游锦州上空的中尺度雨团发展是一个快速波动传输过程,且与多单体风暴的传播机理相似.闪电频数与强降水时段的峰值较一致,中小尺度雨团合并和低层辐合是造成短时强降水的原因之一.     

Numerical Modelling of the Effects of Vegetation and Environmental Conditions on the Lake Breeze

The influence of vegetation and environmental conditions on the lake breeze and associated boundary-layer turbulence structure has been studied using a two-dimensional nonhydrostatic, compressible mesoscale model coupled with the SiB2 land-surface scheme. The results show that the impacts of vegetation on the lake effects are dependent on the environmental conditions, such as soil wetness and background wind, as well as vegetation characteristics. Both soil wetness and background wind play important roles in modifying lake effects on boundary-layer turbulence and the lake breeze, while the effects of vegetation type are secondary compared to the other factors. Without background wind, and under the same soil wetness, the maximum horizontal windspeed of the lake breeze is insensitive to the type of vegetation. Soil wetness can greatly affect both the maximum horizontal windspeed and the maximum vertical velocities of the lake breeze. With background wind, the lake-breeze circulations, upward motion regions, and boundary-layer turbulence structure all change markedly. A weaker background wind can strengthen the lake breeze, while stronger background wind suppresses the lake breeze circulations. The distribution of sensible and latent heat fluxes is also very sensitive to the soil wetness and background wind. However, for the same soil wetness (0.25 and 0.4 were chosen), there is only a small difference in the distribution of sensible and latent heat fluxes between the bare soil and vegetated soil or between the types of vegetated soils.     

Convective Snowfalls Linked to the Interaction of a Boundary-Layer Front with a Mesoscale Cyclone Near Terra Nova Bay,Antarctica

During the XXII Italian expedition in Antarctica, in the summer of 2007, severe weather conditions associated with deep convective instability and heavy coastal precipitation occurred around the Terra Nova Bay area in the presence of an upper level trough and energetic katabatic winds flowing from the Ross Ice Shelf over the open sea. In this case study we document an example of boundary-layer frontal movement across the Ross Sea and mesocyclone development in conjunction with the frontal movement. A westward fast moving boundary-layer front, generated by the leftward turning katabatic airstream to the east of Ross Island, was observed propagating as a baroclinic wave disturbance in an easterly direction across the Ross Sea, merging later with a mesocyclone approaching Terra Nova Bay from offshore. The observed inertial trajectory and an estimation of the radius of curvature suggest that the vigorous katabatic airstream was sustained by the strengthening of a surface mesocyclonic circulation settled over the north-eastern Ross Ice Shelf, triggered by a sub-synoptic upper level trough passing over the area. We hypothesise that baroclinic instability in the low levels plays an important role in the development of a mesoscale vortex and for triggering convective precipitation.     

Numerical Study On Cloud Lines Over An Urban Street In Tokyo

`Kanpachi Street Cloud (KSC)' is an unusual small-scalecumulus cloud line visible during calm summer conditions over a major street in the Tokyo metropolitanarea. In order to understand the mechanism leading to the formation of this cloud line,numerical simulations have been performed using the Regional Atmospheric Modelling System.The general characteristics of the simulated KSC agree well with observations. On alarge-scale view, the KSC can be characterized as a cumulus cloud line generated at the convergenceline of two sea breezes on the western side of Tokyo Bay, while on the microscale view, it resemblesBenard-type thermal convection modified longitudinally by wind shear.The location of the convergence line leading to the KSCformation coincides withthe Kanpachi street location, although the street itself does notmake any direct effect on the KSC formation. Additional numerical experiments were performed toidentify causes of the KSC formation in accordance with urbanization, by changing anthropogenicheat impact, land-cover and grid resolution.They confirmed that the formation of the KSC requirestwo meteorological processes at the same time:(1) the convergence of two sea breezes – the localsouth-eastern sea breeze from Tokyo Bay andan extended southern sea breeze from thePacific Ocean, respectively; (2) Forcing due to an urban heat island. It is shown that urbanization couldchange the intensity and position of the KSC through enhancement of local upward motions and changesin the near-surface horizontal pressure gradient between urban and sub-urban areas. Further,fine horizontal grid resolution is needed to be able to resolve these local thermal convection issues.     

“96·8”特大暴雨和中尺度系统发展结构的非静力数值模拟

1996年 8月 3～ 5日 (“96· 8”) ,中国河南、山西、河北等省发生了一次特大暴雨过程 ,造成了严重洪涝灾害。文中的天气分析指出 ,稳定的大型鞍形场和北移台风 (登陆后减弱为低压 )与其东侧副热带高压的相互作用是“96· 8”特大暴雨发生的大、中尺度环流条件 ;而中尺度低压及其特有的动力热力结构与该暴雨过程直接相关。对该过程采用非静力中尺度数值模式 (MM5)进行了数值模拟研究。模拟结果分析发现 ,非静力 (MM5)的全物理过程模拟基本上可再现大尺度和中 -α尺度天气系统的发生、发展和演变。采用二重网格双向嵌套技术的细网格模拟结果揭示 ,低压的动力场和热力场之间具有一种强耦合机制 ,即发展的低压具有气旋性涡柱的暖心高湿结构 ,在涡柱低空是湿对流不稳定和负湿位涡结构 ;强垂直上升运动与高空强辐散和低空强辐合以及对流云团的发展互耦 ;与低压相伴的强南风急流不仅是低压和对流云团发展与维持的互伴互耦条件 ,而且也是“96· 8”特大暴雨的水汽源和热能输送带。降水模拟结果分析表明 ,尽管某些降水中心对粗网格偏小 ,对细网格偏大 ,但雨带和雨强分布与观测结果基本一致。