Orographical modification and large scale forcing of a cold front

Summary The development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge.The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave.The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.With 18 Figures     

Interaction of Orographic Disturbance with Front

1.IntroductionTheimportanceoforographiceffectsonfrontwasrecognizedintheearly20thcentury.Butforthecomplexityofthisproblem,theinvestigationoforographiceffectsonfrontfromdynamicalviewpointisnottakenuntilthe1980s.Bannon(1983)derivedanalyticalsolutionsforthequasi--geostrophicfrontforcedbyahorizontalwinddeformationfieldthatmovesoveratwo--dimensionalmountainridge.Thesolutionsshowthatasacoldfrontapproachestheridge,itweakens,relativetotheflat--bottomsolution,andthefrontstrengthensasitmovesdowntothelees…     

两次昆明准静止锋的对比分析

利用EAR5再分析气象数据,分析了两次昆明准静止锋天气过程的高低空环流形势、锋面结构、锋面位置、以及相关要素场等基本特征及其异同。结果表明：中高纬度地区阻塞形势的存在是昆明准静止锋形成的背景环流。当冷暖气团均强且势均力敌时,更有利于锋面的维持;冷空气较强时,锋面会向西移动,越过云贵高原地形的阻挡;反之,锋面则会向东移动。当锋面维持在高原地形东侧时,冷空气被地形阻挡时,锋面的移动缓慢,此时锋区最为明显。当锋面维持在地形东侧时,风场的最大水平切变、温度的最大水平梯度和相当位温的密集区,描述的锋面位置几乎相同。两次过程寒潮爆发时冷空气均只能维持到700hPa附近,且水平风场也均为600hPa以下的低空切变,说明昆明准静止锋为浅薄系统。昆明准静止锋与一般冷锋锋后不同,两次个例昆明准静止锋锋后低空为湿区,而锋前水汽是否充足则要看南支槽所处的位置。     

对称和非对称地形对冷锋锋生过程的影响

本文利用半地转模式研究了对称和非对称地形以及双地形对冷锋锋生过程的影响。计算结果显示，地形坡度愈大，越容易产生背风气旋，并且其强度也愈大；背风气旋的强度主要由背风坡坡度决定。山对锋面垂直速度的增幅作用随地形坡度增大而增大，地形坡度越大．出现多次极大上升速度的可能性也越大。冷锋过双地形时，其锋面强度有两次减弱和加强的过程，上升速度得到明显加强，并有可能在锋前产生多条中尺度上升运动带，在合适的水汽条件下，冷锋过双地形有可能产生多重中尺度雨带，其间隔与锋面本身的尺度相当。     

A Study on a Snowband Associated with a Coastal Front and Cold-Air Damming Event of 3-4 February 1998 along the Eastern Coast of the Korean Peninsula

A 24-h simulation with the Advanced Regional Prediction System (ARPS) nonhydrostatic model is performed for the heavy snowfall event of 3–4 February 1998 along the eastern coast of Korean Peninsula; the results are used to understand the snowfall process, including why the precipitation maxima formed along the Yeongdong coastal region rather than over the mountain slope and ridge top during. The numerical simulation with a 4-km horizontal grid spacing and 43 levels reproduces very well the narrow snowband located off the eastern Korean coast, away from, instead of over, the Yeongdong coastal mountain range. The general evolution of the snowband agrees quite well with radar observations, while the water-equivalent precipitation amount agrees reasonably well with radar precipitation estimate. The simulation results clearly show that the snow band developed due to the lifting by a coastal front that developed because of the damming of cold air against the eastern slope of the coastal mountain range. The damming was enhanced by the advection of cold air by a low-level mountain-parallel jet from the north, formed due to geostrophic adjustment as the on-shore upslope air was decelerated by the mountain blocking. As the onshore flow weakened later due to synoptic-scale flow pattern change, the cold front propagated off shore and the precipitation dissipated.     

京津冀地区与海风锋相互作用的对流系统的发展预判分析

运用WRF模式,对京津冀地形触发对流系统与海风锋相互作用的两个个例开展了数值模拟,通过对两个个例模拟的广义湿位涡异常和雷达回波分布的对比分析,对广义湿位涡异常预判该类天气过程中对流系统发展的潜力进行了检验。分析结果显示,广义湿位涡异常较雷达回波提前0.5至1 h示踪到山地背风坡和海风锋处的对流系统的发展。斜压与水汽梯度相互作用和非绝热加热对广义湿位涡异常提前示踪山地背风坡和海风锋处对流系统发展有重要的贡献。过山气流的显著扰动和午后相对于陆地大气较为冷湿的海风辐合上升是分别造成山地背风坡和海风锋出现明显的斜压与水汽梯度相互作用的主要原因。     

昆明静止锋下的云南强倒春寒天气分析

利用NCEP1°×1°的6小时再分析资料和常规观测资料,对2005年3月云南出现强倒春寒天气过程的环流特征和物理量场特征进行了分析,探讨了在无南支槽的配合下,由昆明静止锋和高空切变线引发的大范围降雪和降温的强倒春寒天气发生前后中低层环流的演变、热力学和动力学物理因子与倒春寒天气的关系及其形成条件,为今后预测这类天气提供参考依据。     

太行山地形影响下的极端短时强降水分析

2015年8月2日午夜和2011年8月9日前半夜,在两种不同天气系统背景下太行山东麓都出现了小时雨量超过50 mm的极端短时强降水天气,两次过程都是雷暴先在太行山区触发加强,经过下山2 h先后在丘陵站平山和山前平原站石家庄市区产生极端短时强降水。利用常规探测资料、地面加密观测资料、石家庄SA多普勒天气雷达资料,对不同天气系统背景下太行山特殊地形影响的极端短时强降水成因进行分析。结果表明:偏东气流被南北向的太行山地形强迫抬升,且与下山雷暴出流形成中尺度辐合线触发新的雷暴,雷达回波呈现后向传播特征和列车效应造成局地极端短时强降水。太行山地形通过增强辐合上升运动、增大垂直风切变使雷暴下山加强。不同天气系统强迫下,太行山特殊地形对雷暴发展作用不同。在偏西气流引导下,暖区极端短时强降水由阵风锋触发,具有突发性、降水时间短、伴随风力大的特点,下山雷暴出流加快且与山前偏东风的辐合加强,陆续在丘陵区和山前平原触发对流与下山雷暴合并加强造成极端短时强降水;而在东北气流引导下,回流冷锋和阵风锋共同触发的极端短时强降水具有持续时间较长、降雨总量较大、伴随风力较小的特点,太行山东坡对东北冷湿回流有阻挡积聚作用,东北偏北来的雷暴出流边界西端在迎风坡上强迫抬升使雷暴触发并加强,东北气流遇山后发生气旋性偏转使雷暴出流转向东南下山,与平原的偏东风辐合加强,造成丘陵区和山前平原的总降雨时间更长、降雨总量更大。     

云南8月气温与春季气温场的典型相关分析

利用典型相关理论,分析了云南８月低温与当年春季气温场之间的关系,得出了一些有意义的结果。春季影响云南的西风带南支槽、副热带高压和北方南下冷空气等天气系统的强弱（南支槽的强和弱用滇西气温的负距平和正距平表示,副热带高压的强和弱用滇东南气温的正距平和负距平表示,冷空气的强和弱用滇东气温的负距平和正距平表示）与８月份云南是否出现低温天气关系较大,８月份云南北部和西部出现低温天气与２月份冷空气和南支槽的强     

Formulation of a Semi-Geostrophic Model of Frontal Interaction with Isolated Orography

Summary  A semi-geostrophic model of frontal passage over topography was developed to examine the effects of the interaction of a well developed front with an isolated mountain, and the subsequent orographically induced flow evolution. The analytic representation of the primary wave and its frontal structure gives us the ability to control the environmental and geometrical constraints and perform an exploration of parameter space. A number of problems appearing because of this approach are discussed. The results shown relate the characteristics of the orographic perturbation to varying amplitude of the primary wave and varying frontal intensity, but suggest that the representation of flow blocking by the mountain,which this model lacks, is crucial for the representation of frontal intensification in the lee of orography. Received March 2, 1999/Revised May 7, 1999     

江西中西部地区一次暴雨过程的地形敏感性试验

利用常规观测、加密自动站资料和NCEP再分析资料,分析2005年5月江西萍乡地区春季一次暴雨过程发现,该过程仅萍乡南部地区出现暴雨,而中北部地区为小到中雨。考虑到萍乡中南部特殊地形,通过WRF中尺度模式模拟再现这次暴雨过程,并设计降低、增高和移除地形三组敏感性试验探究地形对降水的影响。结果表明：（1）萍乡地形的屏障作用在南部地区造成的风场辐合是南部产生暴雨的重要原因;（2）地形通过影响切变线附近风场辐合以及水汽汇集的位置和强度来改变暴雨的落区与强度;（3）较高的地形造成气流在山前堆积,造成明显的水平气压梯度,使局地气流与背景气流在山前辐合,有利于山前降水增强。     

贺兰山东麓极端暴雨的中尺度特征

利用近10年宁夏逐时自动气象站降水、银川CD雷达、FY-2、探空和ECMWF再分析0.125°×0.125°等高分辨率多源气象资料,在中尺度系统分型基础上,对比分析贺兰山东麓6次极端暴雨的中尺度特征。结果表明:(1)低空偏(东)南急流夜间增强并配合贺兰山地形,在东坡山前触发或增强了暴雨中小尺度系统,造成地形处降水增幅,极端暴雨都是伴有短时强降水的对流性暴雨,主要集中在东坡山前,中心在山洪沟口,夜雨特征显著。(2)环境场都满足对流性暴雨的3个基本条件:700 hPa(东)南急流将暖湿水汽输向暴雨区,低层高温高湿促进了大气不稳定与动力、热力、地形抬升触发机制;深对流过程850 hPa无明显急流,水汽主要来自孟加拉湾,水汽输送受限,但大气稳定度更低,更有利于对流性暴雨发生,混合对流过程850 hPa与700 hPa急流路径重合,水汽来自孟加拉湾、南海、黄海和渤海,水汽输送更充沛,更有利于持续性暴雨产生。(3)极端暴雨主要有暖区对流降水、锋面对流降水、锋区层状云降水3种性质;暖区对流主要在山区,地形抬升是触发机制,锋面对流的触发是低层暖湿气流沿着冷垫抬(爬)升,平原和山区皆有;对流系统的移动与低层风场一致,山区和平原分别沿山体和低空急流轴传播,通常移动与传播方向平行,山区低层为偏东风时,移动与传播近似垂直,列车效应明显。(4)线型对流系统过程冷空气弱,以暖区或(和)锋面对流性降水为主,对流系统在山前沿山体传播形成组织化程度高的带状线型回波,移动与传播有平行有垂直,受地形抬升作用,对流系统在山前稳定少动、发展强盛,降水历时短、范围小、雨强大、有间歇性,3~4 h的累计雨量占过程总量的85%左右,区域平均雨量远小于暴雨量级,地形性强对流暴雨特征凸显。(5)非线型对流系统过程冷空气强,以锋面对流性降水和锋区层状云降水为主,对流系统在山前和平原沿山体和急流轴传播和移动形成非线型回波,平原地区传播与移动平行,山区两者垂直,对流系统组织化程度不高、移速快、强度弱,降水历时长、范围大、雨强小,连续降水累计雨量大,区域平均雨量接近或达到暴雨量级,混合性降水特征明显。(6)降水强度R与CAPE增幅、回波强度Z、强回波持续时间、回波顶高、液态水含量呈正相关,与TBB呈负相关,相关性在深对流过程更清晰;Z≥40 dBZ时,Z-R满足关系式:R=3.67×10-8Z5.222+4.835。     

Impacts of topography and land degradation on the sea breeze over eastern Spain

Summary A three-dimensional non-hydrostatic atmospheric model RAMS, version3b, is used to examine the impact of complex topography on the sea breeze under heterogeneous and degradation land use characteristics. In the study, it is shown that topography plays an important role in the sea-breeze circulation by aligning the sea breeze front to the coastline and locating the convergence zones close to the mountain range. When the sea breeze is coupled with the upslope wind, the sea-breeze circulation is strengthened by the topography.Sensitivity analyses are carried out to determine the influence of vegetation and soil moisture, i.e., land surface modifications, to this thermally driven flow. Land degradation results in an enhanced sea-breeze circulation which is characterized by a stronger onshore flow, a stronger return current, a larger updraft velocity associated with the sea-breeze front and further inland penetration. Other important features are a deeper sea-breeze depth, a larger downdraft velocity behind the sea-breeze front, and a longer offshore extent. The results also show how land changes modify the sea breeze temporal evolution resulting in an earlier onset and later end. The study stresses the convenience of using three-dimensional models with detailed land surface information to model the sea breeze in complex terrain where land use is rapidly modified.Received February 25, 2002; accepted October 7, 2002 Published online April 10, 2003     

地形与双冷锋的锋生过程

本文利用半地转模式，讨论了双冷锋的锋生过程及其越过山脉的演变。结果显示，在变形场作用下，双冷锋在移行过程中会发生上层锋合并的现象。在锋生过程中，下层前一条冷锋加强远大于后一条，在上层则是后一条冷锋的加强远大于前一条。地形对双冷锋有重大影响，山的迎风坡对冷锋移行有阻挡作用，并削弱锋的强度，背风坡则加速锋的移行，并加强锋的强度。山对双冷锋的上层锋的合并有促进作用，并造成近地面锋区呈现鼻状突出；地形促使锋区垂直速度大大增大，并导致其产生跳跃式变化；地形对锋的影响限于下层锋区。     

A Study on Frontal Motion over Orography

肖庆农，伍荣生ＡＳｔｕｄｙｏｎＦｒｏｎｔａｌＭｏｔｉｏｎｏｖｅｒＯｒｏｇｒａｐｈｙ￥ＸｉａｏＱｉｎｇｎｏｎｓａｎｄＷｕＲｏｎｇｓｈｅｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＡｔｍｏｓｐｈｅｒｉｃＳｃｉｅｎｃｅｓ，ＮａｎｊｉｎｇＵｎｉｖｅｒｓｉｔｙ，Ｎａｍｉｎｇ...     

一次冷锋后飑线的大振幅重力波特性分析

本文用较密集的观测资料分析江淮地区一次冷锋后的飑线过程。分析表明飑线的生成和发展与中尺度冷涌活动有关，它具有大振幅重力波的特征。重力波在传播过程中于大别山东侧形成边缘锋，边缘锋的传播更类似于沿岸Kelvin波，大振幅重力波在边界层特征更明显。这类被地形拦截的大振幅重力波是江淮地区冷锋后飑线的一种可能激发机制。     

2005年12月山东半岛暴雪成因及多尺度信息特征

利用常规天气资料、地面加密观测资料、卫星云图资料和多普勒雷达产品,对2005年12月山东半岛特大雪灾作了综合分析。研究表明,这次的冷流降雪归因于前期气温和海温偏高、冷空气势力偏强引起的海陆温差大、半岛北面临海的丘陵地形以及海岸锋等因素,地面自动站、雷达、卫星等多种尺度的特征信息清楚地揭示了其结构和演变。利用改进的简化伴随模式,对12月6日的烟台单站多普勒雷达资料进行了三维风场反演,结果表明,海陆热力差异和山脉对于向岸风的摩擦辐合作用形成了中尺度海岸锋,它引起了低层浅对流的产生。高空补充弱冷空气和降雪的潜热释放进一步激发了垂直上升运动,使对流活动发展加强,引起强降雪。当高空弱冷空气引起的系统性辐合上升运动与低层海岸线附近的海岸锋引起的中尺度辐合上升运动叠加时,降雪特别强烈。     

Numerical simulation of January 28 cold air outbreak during GALE part II: The mesoscale circulation and marine boundary layer

A two-dimensional (2-D) mesoscale numerical model is applied to simulate the January 28 cold-air outbreak over the Gulf Stream region during the Intensive Observation Period-2 (IOP-2) of the 1986 Genesis of Atlantic Lows Experiment (GALE). The model utilizes a turbulence closure which involves the turbulent kinetic energy (TKE) and dissipation () equations and combines the level 2.5 formulations of Mellor and Yamada (1982) for better determination of the eddy Prandtl number.The modeled marine boundary layer (MBL) is in good agreement with the observations (Wayland and Raman, 1989) showing a low-level jet west of the Gulf Stream warm core and a constrained boundary layer due to the middle-level (2–4.5 km) stable layer. The MBL-induced single cloud and rain band first appears east of the Gulf Stream boundary, and then moves offshore at the speed of the circulation front. The front, however, moves slightly slower than the ambient flow. Removal of the tropopause does not influence the low-level circulation and the movement of the front. The speed of the front is slightly larger in the baroclinic downshear flow than in the barotropic flow. The results also indicate that the observed high cloud streets propagating downwind of the Gulf Stream may be related to upper-level baroclinic lee waves triggered by an elevated density mountain. The density mountain waves, however, become evanescent as the baroclinity (which gives a larger Scorer parameter) is removed.The modeled 2-D circulation systems are found to be sensitive to differing eddy Prandtl numbers, in contrast to the 1-D model results presented in Part I. Sensitivities become increasingly important as the clouds begin to interact with the MBL. A constant eddy Prandtl number of unity produces a more slantwise convection compared to that by the level 2.5 case. Cloud development is stronger in slantwise convection than in upright convection. The fastest development of clouds can be explained in terms of the conditional symmetric instability (CSI), which begins as the MBL baroclinity becomes sufficiently large.     

冬半年副热带西风南支槽结构和演变特征研究

南支槽是冬半年副热带南支西风气流在高原南侧孟加拉湾地区产生的半永久性低压槽, 本文从气候学角度探讨其结构和演变特征。结果表明: (1) 南支槽10月在孟加拉湾北部建立, 冬季 (11～2月) 加强, 春季 (3～5月) 活跃, 6月消失并转换为孟加拉湾槽; 10月南支槽建立表明北半球大气环流由夏季型转变成冬季型, 6月南支槽消失同时孟加拉湾槽建立是南亚夏季风爆发的重要标志之一。 (2) 南支槽在700 hPa表现明显, 其槽前干暖平流的输送有利于昆明准静止锋形成和维持, 槽后冷湿平流也与孟加拉湾冷涌关系密切。 (3) 冬季辐散环流下沉支抑制了南支槽前上升运动的发展, 这时低层辐合, 中层辐散, 南支槽前上升运动一般只伸展到对流层中层600 hPa左右。春季随着辐散环流减弱, 东亚急流入口区南侧辐散中心的出现使得垂直运动向上迅速伸展。 (4) 从气候平均看冬季水汽输送较弱, 上升运动浅薄, 无强对流活动, 南支槽前降水不明显, 雨区主要位于高原东南侧昆明准静止锋至华南一带。春季南支槽水汽输送增大, 同时副高外围暖湿水汽输送加强, 上升运动发展和对流增强, 南支槽造成的降水显著增加, 因此春季是南支槽最活跃的时期。     

Dynamic and thermal effects on surface airflow associated with southerly changes over the South Island,New Zealand

Summary The Southerly Change Experiment (SOUCHEX) was conducted to examine the influence of the New Zealand Southern Alps on the structure and evolution of cold fronts, locally called southerly changes, as they travel up the east coast. The extensive data obtained by the augmented surface weather station network is used to examine in detail the mesoscale wind field associated with the events observed during the experiment. A comparison of the wind fields observed during the different events illustrates the influence of local dynamic and thermal factors. In particular, lee trough-induced northeasterlies and thermally developed diurnal wind systems are seen to interact with the wind field created by the passage of the front over the Southern Alps.It is apparent that the wind field associated with southerly changes responds to a variety of factors as the cold fronts propagate northwards. For example, there is a tendency for the flow to turn onshore producing a southeast wind during daytime over the Canterbury Plains south of Banks Peninsula probably due to diabatic heating of the mountains and plains. This onshore flow is in direct opposition to pre-frontal foehn northwesterly flow which often continues in the mountain regions and aloft after the front has moved up the coast. The interaction of these air masses over Canterbury creates difficulties for local forecasting. Also, the nocturnal passage of a southerly change is often difficult to detect in surface anemograph traces because of the decoupling of the boundary layer air from that above, producing low level drainage flow over the Canterbury Plains. The overall effect is to create a complex mesoscale wind field resulting from interaction of cold fronts with regional orographic and thermal influences.With 8 Figures