Meso-scale surface wind changes associated with the passage of cold fronts along the eastern side of the Southern Alps,New Zealand

Summary The movement of cold fronts along the eastern side of the Southern Alps has been studied in detail during the Southerly Change Experiment (SOUCHEX). An enhanced network of surface wind monitoring stations was established in the Canterbury region of the South Island to allow detailed study of meso-scale wind fields during the passage of fronts. Fivesoutherly changes occurred during the experiment, one of which failed to produce a clear wind change over much of the area. The often erratic movement of these fronts along the eastern side of the mountains is illustrated by isochrone maps of the onset of the wind change. Other general characteristics of these events include their shallowness (1000–1500 m deep) and in most cases their abrupt onset. Detailed analysis of anemograph data collected during SOUCHEX indicates great variability in the surface wind field associated with passage of the fronts. Maps of the meso-scale wind field plotted for the first southerly change of the 14 January 1988 illustrate the complexity of the wind changes experienced over the eastern South Island in particular. The arrival of the cold fronts at individual sites provided features of air mass interaction which appear to relate to time of day, and regional and local site factors. Variations, in wind, temperature and relative humidity provide clear examples of both single and double air mass changes. However, nocturnal changes seem to be less distinct due to lower ambient temperatures and increased boundary layer stability.With 9 Figures     

Regional and local scale characteristics of foehn wind events over the South Island of New Zealand

Summary Regional and local scale windfield and air mass characteristics during two distinct synoptic foehn wind events over southern New Zealand are examined. The Southern Alps were observed to effectively block low level onshore gradient northwesterly airflow and to channel it through both Cook and Foveaux Straits. Blocking of the onshore synoptic northwesterly airstream also resulted in barrier jet formation along the western slopes of the Southern Alps. This feature of the regional windfield has not previously been documented and develops during favourable conditions to a height of between 1500 to 1800 m above sea level. In the immediate lee of the Southern Alps at Lake Tekapo, classic foehn conditions such as warm ambient air temperatures, low relative humidities and gusty winds were monitored throughout both foehn events examined. Differences in the local windfield were however observed, which reflect the importance of local topography on lee side windfield dynamics during foehn events. Spillover of precipitation to the lee of the mountains was monitored in the latter stages of each case study and appeared to be associated with the passage of the cold front over the Southern Alps. Observations made by this investigation have a number of applied and theoretical implications with respect to meso-scale modelling, orographic rainfall distribution and forecasting.With 12 Figures     

11月初北黄海一次海龙卷风暴的中尺度特征分析

利用多普勒天气雷达资料及反演风场和常规观测资料,对2014年11月2日发生在北黄海（山东半岛北部海上）一次罕见海龙卷风暴的中尺度特征进行了分析。结果表明：冷空气、暖湿海面热力边界、山东半岛北部近海岸西北风与偏西风的辐合线是海龙卷风暴发生的天气背景。海龙卷风暴发生时雷达回波PPI最大分贝反射率因子为60 dBZ,高度为2.0 km,最高风暴顶为4.5 km,最大垂直累积液态水含量VIL为21 kg·m-2。利用雷达反演风场进行中尺度特征分析,结果表明：在海龙卷风暴发生发展过程中,低层风辐合对应4.0 km高度上是风辐散,海上有较强的偏南暖湿气流输送到雷暴区。中尺度动力特征：最大正涡度和散度辐合在1.0 km以下,低层正涡度和散度辐合、高层散度辐散是雷暴发生初期动力特征;低层没有正涡度和散度辐合、高层为正涡度和散度辐合是雷暴开始发展的动力特征;低层和高层为大的正涡度和散度辐合是雷暴成熟阶段的动力特征。高空冷空气叠加上低空强的偏南气流,造成局地涡度加大和低层辐合加强,使低层暖湿气流倾斜上升。海龙卷与辐合区的冷空气和暖湿气流有关。     

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。     

一次长三角地区飑线过程的数值预报分析

2019年4月9日,长三角地区发生了一次罕见的长历时强飑线天气过程。在分析其天气形势背景和发展演变基础上,利用新一代华东区域数值模式对此次过程进行了预报分析,初步分析了其演变过程中的中尺度结构特征。结果表明,此次飑线发生在高空槽前、低层强烈辐合抬升天气背景下,强的垂直风切变、冷空气向南侵入与低层暖湿气流叠加建立了强的对流不稳定层结,是飑线发生发展和长时间维持的重要原因。数值模式成功模拟了飑线前部低层暖湿空气上升和后部中层干冷空气下沉这两支入流,以及飑线过境时边界层高度和大气可降水量迅速下降,地面中尺度冷池向东南方向的传播过程,冷池与对流风暴的移动速度基本一致,导致对流前部低层一直有风场的切变辐合抬升,有助于对流维持并发展。     

Analysis and numerical modelling of a frontal passage associated with thunderstorm development over the Po valley and the Adriatic sea

Summary A case study is presented of the meteorological situation of 26 July 1987, when a moderate lee cyclone formed south of the Alps associated with a cold front moving from the northwest. A convective line developed over the Po valley and the northern Adriatic sea, slightly in advance of the position of the cold front which, over that area, was associated with a north-easterly low-level flow. The mesoscale situation over the Po valley before and after the frontal passage was complex, with different flow configurations and weather phenomena affecting the eastern and western portions of the region.The analysis of observations has been complemented by the results of numerical experiments run with a limited area, high resolution model, initialized using interpolated ECMWF analyses. Sensitivity experiments show the important roles played by the orography and by the condensation-evaporation processes in determining the mesoscale field environment favourable for the development of the organized convection.With 26 Figures     

2006年3月末河南一次沙尘暴过程的天气成因分析

利用常规观测资料、地面自动站加密观测资料以及NCEP1°×1°格点再分析资料,对2006年3月27日发生在河南省的一次沙尘暴过程的天气成因进行了分析。结果表明,这是一次冷锋南下时产生的沙尘暴天气,高空影响系统是携带冷空气东南移并发展的小槽,此短波槽最终替代了原有的东亚大槽。前期降水少导致地表干燥,河南省北部、东部的黄泛平原沙土土层为沙尘暴的发生提供了丰富的沙源。冷锋影响时,强烈的温度平流作用使得风力加大,垂直运动加强。锋生作用使冷锋导致的天气现象更为剧烈。冷锋不仅是静力不稳定能量的触发系统,锋区内还存在对称不稳定,有利于垂直运动进一步加强,将沙尘输送至高空。地面中尺度滤波表明,中尺度低压的形成和发展不仅使得湍流混合加强,同时也加剧了不稳定能量的释放。沙尘在强风、强湍流混合以及强上升气流作用下最终形成沙尘暴。     

98.8松嫩流域一次东北冷涡暴雨的数值模拟初步分析

利用MM5中尺度非静力模式对1998年8月8~9日松嫩流域的东北冷涡切变型暴雨过程进行了数值模拟和对比试验。结果表明, MM5能够对此次过程进行较成功的模拟; 同时发现, 此次过程首先由高层强的正涡度平流触发, 低涡区强降水的产生是由于高低空急流的耦合。其中, 高空急流的作用强烈而短暂, 低空急流的作用较为持久, 强度稍弱。阻高前部下沉气流形成的中层偏东干冷气流是切变形成的关键, 它与偏南气流在对流层中层形成的风向切变是产生上升运动的主要强迫机制。减弱阻高使其底部偏东气流减弱及后部偏南气流在高层减弱、低层增强, 导致降水减弱, 并使系统位置偏东、偏南。西北路冷空气增强使低涡和切变北侧高层偏南气流增强、低层偏南气流减弱, 导致降水强度增强、尺度增大。     

Some Characteristics of the Surface Boundary Layer of a Strong Cold Air Process over Southern China

In southern China,cold air is a common weather process during the winter season;it can cause strong wind,sharp temperature decreases,and even the snow or freezing rain events.However,the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data,especially regarding turbulence.In this study,four-layer gradient meteorological observation data and one-layer,10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China.The results show that,with the passage of a cold air front,the wind speed exhibits low-frequency variations and that the wind systematically descends.During the strong wind period,the wind speed increases with height in the surface layer.Regular gust packets are superimposed on the basic strong wind flow.Before the passage of cold air,the wind gusts exhibit a coherent structure.The wind and turbulent momentum fluxes are small,although the gusty wind momentum flux is slightly larger than the turbulent momentum flux.However,during the invasion of cold air,both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed,and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period.After the cold air invasion,this structure almost disappears.     

广东博贺近海海面的一次冷空气过程强风特征分析

利用广东博贺离海岸6 km的一个海上100 m铁塔观测到的梯度资料,采用特征量分析方法,研究强冷空气过程海洋近地层强风的微观特征。结果表明：冷锋过境时,近地层的气象要素有明显的响应：温度骤降、风速加大、气压上升,风向有明显的转变;风向标准差和风速标准差在冷锋前、冷锋过境和冷锋后都有明显不同;冷锋过境大风湍流度为0.08,阵风系数为1.19;此次冷锋过境强风在时间上表现为周期0.5～1 h的低频阵性;在空间上,60 m附近存在近地层的最大风层。     

Boundary layer structure in two fronts passing a tower

Summary The atmospheric boundary layer in an ana-cold front and in a kata-cold front is investigated using measurements at a 200 m high tower near Karlsruhe, Germany. In the ana-cold front the wind speed decreased, whereas the passage of the kata-cold front was accompanied by strong gusts. The frontogenesis function of potential temperature was only weak. Turbulence increased at the fronts as expressed by the turbulent kinetic energy. However, the turbulent vertical momentum flux was relatively inefficient at the passage of the fronts. Also it is attempted to calculated flux profiles and friction in the cold air.With 15 Figures     

NUMERICAL STUDY ON THE LOCAL WIND STRUCTURES FORCED BY THE COMPLEX TERRAIN OF QINGDAO AREA

In this paper, some 2-D features of the down- and up-slope winds and sea-land-breeze generated over the com-plex terrain of the Qingdao area and the interaction between them are numerically analysed by use of a 2-Dnon-hydrostatic mesoscale model. The simulated results in the west-east vertical cross-section show that (1) when thelarge-scale wind is a southerly gentle one, the generated easterly down-slope wind is much stronger than with an oppo-site background wind, and the down-slope wind can trigger and intensify the land breeze corresponding to the easterncoast of Jiaozhou Bay; (2) a gentle westerly background wind will reduce the eastward sea breeze and up-slope windduring the daytime due to a cold advection, but shows a little effect on the mesoscale circulation formed in the nighttime.     

A study of orographic effects on mountain-generated precipitation systems under weak synoptic forcing

Summary ?Mountains profoundly impact precipitation systems in Taiwan, particularly in areas occupying roughly two-thirds of the island’s landmass. This study examines the terrain structures possibly affecting the formation of rainfall systems in northern Taiwan by analyzing radar data, surface rainfall data, and simulation results from MM5 (Fifth-Generation NCAR/Penn State Mesoscale Model) under a weak synoptic influence condition. More specifically, this study analyzes precipitation systems formed in three different days with different ambient wind directions (i.e., southwesterly, southerly and south-southeasterly flows) in a low Froude number regime in Mei-Yu (or Baiu) season. The southwesterly (southerly) predominant wind was blocked by CMR (central mountain range) over southwestern (southern) Taiwan. Consequently, the southwesterly (southerly) winds were diverted around southern Taiwan, traveled northward following the terrain contour of CMR and then converged in northeastern (northern) Taiwan to produce a NE-SW (N-S) orientated convergence area. As anabatic flow and onshore flow intensified in northern Taiwan and thus enhanced the existing convergence in the late morning and early afternoon, the precipitation system appeared over slope first and then moved down the slope following the predominant wind direction. Upwards motion persisted in this convergence region, and initiated a new precipitation system. Consequently, rainfall accumulation was orientated in a NE-SW (N-S) direction in northern Taiwan. On the windward side of CMR in central Taiwan, precipitation was first produced in the slope by anabatic flow and was generated in lower land because of the interaction between down slope and onshore flow in the late afternoon. When the flow was predominantly from the south-southeast, the convergence due to the splitting of the predominant over western Taiwan became weaken after onshore flow over west coast developed since the direction of onshore flow was against the splitting predominant flow. Precipitation only appeared in the sloping areas of northwestern and central Taiwan in the relatively dry environment resulting from the anabatic flow. Several sensitivity tests indicated that the lee-side convergence in a low Froude number regime superimposed by anabatic flow and onshore flow is important for producing rainfall in northern Taiwan. The prevailing wind direction determined the orientation of the rainfall accumulation in northern Taiwan. The high relative humidity is important for precipitation to form in lower elevations. Received February 9, 2001; Revised November 23, 2001     

Turbulent dispersion in coastal atmospheric boundary layers: An application of a Lagrangian model

A Lagrangian model is applied to simulate the dispersion of passive tracers (in particular, water vapour) in coastal atmospheric boundary layers under onshore wind conditions. When applied to convective boundary layers over uniform surfaces, the model gives results in agreement with those of similar studies. Numerical simulation of turbulent dispersion in coastal areas also reproduces the basic features known from experimental studies. Under onshore wind conditions, the humidity field is plume-shaped with the maximum vertical transport being over land downstream of the coast line. The model shows that the surface sensible heat flux over land, the static stability of the onshore air flow and the onshore wind speed are the most important factors determining the basic features of turbulent dispersion in coastal areas.     

On the air‐sea interaction in areas of thermal marine fronts in the Greenland Sea

Abstract

The effects of marine fronts on the local atmospheric surface layer and air‐sea interaction were studied. Several mesoscale fronts were crossed by a research vessel in the Greenland Sea. Air temperature, humidity and stability conditions, and the fluxes of momentum, as well as sensible and latent heat, were investigated. For relatively calm conditions, close air‐sea coupling was observed in the temperature whereas for stronger winds, the air temperature of the surface layer was not markedly modified by the front below. Changes in the moisture content in the frontal area were observed and, in one case, evaporation was observed on the warm water side and condensation on the cold water side of the front. Frontal differences in heating from the sea were assumed to affect the surface‐layer wind field.     

Water Circulation Off the Northeastern Coast of Sakhalin during the Passage of Three Types of Deep Cyclones over the Sea of Okhotsk

Based on the numerical simulation of water circulation in the Sea of Okhotsk in 1986 to 2015, the impact of deep cyclones on the circulation off the northeastern coast of Sakhalin is studied. The circulation in the Sea of Okhotsk is simulated with the COSMO-Ru-INMOM-CICE model configuration, where the COSMO-Ru and INMOM resolve explicitly the mesoscale atmosphere and ocean dynamics and the CICE resolves the ice cover evolution. The extreme atmospheric events associated with the intensive cyclone activity over the Sea of Okhotsk during the cold season are classified. It is found that high velocity is typical of the cyclones coming to the sea from Sakhalin, and wind speed on the periphery is higher for the cyclones coming to the Sea of Okhotsk from the south and southwest. The analysis of water circulation response off the northeastern coast of Sakhalin demonstrates that the meridional current velocity on the shelf increased by several times from the sea surface to the bottom for all types of cyclones. On the edge of the shelf, southern currents intensified in the surface and bottom layers during the passage of cyclones and at the intermediate depths during the passage of fronts. On the continental slope, southern currents intensified in the surface, intermediate, and bottom layers depending on the type of extreme events.

     

Orographically-forced cold fronts — Mean structure and motion

Shallow cold fronts with a north-west/south-east orientation sometimes experience blocking by the coastal mountain range of south-eastern Australia and greatly increase their propagation speed on the eastern (seaward) side of the range while they advance more slowly to the west of the range. The violent behaviour of some cold-frontal passages, or southerly busters, is found to be at least orographically initiated, but the phenomenon does not exhibit the characteristics of a coastally-trapped density current. The head of the front has the character of an evolving density current and its propagation is well predicted by density current theory over more than half of its lifetime. Nevertheless, it differs from steady laboratory-simulated examples in that a horizontal roll-vortex just behind the front is found to be accelerating relative to the rate of advection of cold air behind the front. This implies that its evolution will be governed by warm air entrainment, a fact which is confirmed by the observations. General agreement between extensive airborne observations over the ocean and measurements made at selected points along the coast indicates the usefulness of these basic studies for future improvement to forecasting in the near-coastal region.     

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.     

The exceptional Alpine south foehn event of 14–16 November 2002: a case study

Summary This study examines the exceptional Alpine south foehn event of 14–16 November 2002 using routine observations and high-resolution numerical simulations. Besides its long duration and an extremely high temperature level related to warm-air advection from the northern Sahara, this foehn event exhibited an unusual spatial structure of the low-level wind and temperature field. Whereas the foehn was largely restricted to the first half of 14 November in the western part of the Alps (Switzerland), it extended over the full period in the inner-Alpine valleys in the eastern Alps. The duration and intensity of the foehn also tended to decrease from the Alpine crest towards the northern rim of the Alps. Most surprisingly, continuous foehn even occurred on the windward side of the Alpine crest, namely in a basin located in the southeastern Alps. The distribution of the orographic precipitation associated with the foehn case was unusual as well. In Switzerland, intense precipitation was not restricted to the windward (southern) side of the Alps but extended to the northern side of the Alpine crest, particularly on 16 November. The results indicate that the spatio-temporal distribution of the foehn in the northern Alps was related to the fact that the western Alps were within a synoptic-scale transition zone between extremely warm air advected from the south and colder air lying over western Europe. The colder air was advected around the western Alps whereas extremely warm air descended from the Alpine crest farther east. Moreover, a small cyclone formed on 14 November north of the Alps and generated a shallow cold front propagating eastward along the northern Alps. Thus, the tendency towards foehn decreased from west to east and from the Alpine crest towards the north. The occurrence of foehn on the windward side of the Alpine crest was made possible by the extreme strength of the large-scale southerly flow, combined with the fact that the upstream precipitation field did not reach the southeastern edge of the Alps. Finally, the pronounced spillover of precipitation to the northern side in the Swiss part of the Alps appears to be related to the colder air present north of the crest. This prevented the formation of orographic gravity waves and downslope air motion, which usually leads to a rapid evaporation of the precipitation on the lee side of the Alpine crest.     

The Phoenix Evening Transition Flow Experiment (TRANSFLEX)

Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area.