气象专业英语2.pptx

Unit Two:Frontogenesis and frontal characteristics第二课第二课 锋生和锋的特征锋生和锋的特征 New words:Front 锋，锋面 frontolysis 锋消Frontogenesis(Frontogeneses)锋生Frontal 锋面的mid-latitude 中纬度 lower-latitude 低纬度High-latitude 高纬度Day-to-day 逐日的 daily 逐日的Mass 气团，质量Humidity 湿度Specific humidity 比湿Meteorologist 气象学家Integral 整数的，积分的，完整的 Integration 积分Interface 交界面，接口Interaction internet interannual(年际的）Numerical 数值的Model 模式Baroclinic 斜压的 barotropic 正压的Cyclone 气旋 anticyclone 反气旋cyclogenesis气旋生成气旋生成Depression 低压 isobaric 等压的 isothermal 等温的Iso+.等。Synoptic 天气的 PrincipleofSynopticMeteorology 天气学原理Convergence 辐合 divergence 辐散Apex 顶点，峰尖 peak 峰Zone 区域 regionCirrus 卷云 jet 急流 Low level jet 低空急流Geostrophic 地转的Convection 对流 convective 对流的Greenhouse 温室效应 albedo 反照率Evaporate 蒸发v.Evaporation 蒸发n.Occlusion 锢囚 squall 飑Oceanic 海洋的 lee 背风面Cyclogenesis 气旋生成 Trough 槽 ridge 脊Ana-front 上滑锋 kata-front下滑锋Slope 坡度 order 量级Multi-layered 多层的Multi-cell 多单体的Cirrostratus 卷层云 altostratus 高层云Nimbostratus 雨层云 stratocumulus 层积云Drizzle 毛毛雨 precipitation 降水Rainfall 降水量 saturate 使饱和Descend 下降 ascend 上升Medium-level 中层 high-levelLow-level 低层 mid-tropospheric 对流层中部的Airflow 气流Large-scale 大尺度的 meso-scale 中尺度的 momentum 动量Broad-scale 大范围的Potential 位势，潜在的，Potential heat 潜热 geopotential meter 位势米Rainband 雨带 rainbeltOrographic 地形的 topographic Down-wind 在下风方向Cumulonimbus 积雨云Downpour 倾盆大雨 thunder 雷Duration 持续时间 occlude 锢囚Trowal(trough of warm air aloft)高空暖舌Aloft 高的，上面的Poleward 向极地的 Phase 相位，阶段，方面TibetPlateaumoderateP1:The first real advance in our detailed understanding of mid-latitude weather variations was made with the discover that many of the day-to-day changes are associated with the formation and movement of boundaries,or fronts,between different air masses.Observations of the temperature,wind directions,humidity and other physical phenomena during unsettled periods showed that discontinuities often persist between impinging air masses of differing characteristics.The term“front”,for these surfaces of airmass conflict,was a logical one proposed during the First World War by a group of meteorologists working in Norway,and their ideas are still an integral part of most weather analysis and forecasting particularly in middle and high latitudes.1.FrontalwavesP2:It was observed that the typical geometry of the air mass interface,or front,resembles a wave form.Similar wave patterns are,in fact,found to occur on the interface between many different media,for example,waves on sea surface,ripples on beach sand,aeolian sand dunes,etc.Unlike these wave forms,however,the frontal waves in the atmosphere are commonly unstable:that is,they suddenly originate,increase in size,and then gradually dissipate.Numerical model calculations show that,in middle latitudes waves in a baroclinic atmosphere are unstable if their wavelength exceeds a few thousand kilometers.Frontal wave cyclones are typically 1500-3000 km in wavelength.Similar wave patterns are,in fact,found to occur on the interface between many different media,for example,waves on sea surface,ripples on beach sand,aeolian sand dunes,etc.It was observed that the typical geometry of the air-mass interface,or front,resembles a wave form.1500-3000 km in wavelength.Unlike these wave forms,however,the frontal waves in the atmosphere are commonly unstable:that is,they suddenly originate,increase in size,and then gradually dissipate.The initially attractive analogy between atmospheric wave systems and waves formed on interface of other media is,therefore,an insufficient-basis on which to develop explanations of frontal waves.In particular,the circulation of the upper troposphere plays a key role in providing appropriate conditions for their development and growth,as will be shown below.2.ThefrontalwavedepressionP3:A depression(also termed a low or cyclone)is an area of relatively low pressure,with a more or less circular isobaric pattern.It covers an area 100-3000 km in diameter and usually has a life-span of 4-7 days.Systems with these characteristics,which are prominent on daily weather maps are referred to as synoptic scale features.The depression,in mid-latitudes at least,is usually associated with a convergence of contrasting air masses.The interface between these air masses develops into a wave form with its apex located at the centre of the low-pressure area.The wave encloses a mass of warm air between modified cold air in front and fresh cold air in the rear.The formation of the wave also creates a distinction between the two sections of the original airmass discontinuity for,although each section still marks the boundary between cold and warm air,the weather characteristics found in the neighborhood of each section are very different.The two sections of the frontal surface are distinguished by the names warm front for the leading edge of the wave and cold front for that of the cold air to the rear.modifiedcoldairfreshcoldairwarmairP4:The depression usually achieves its maximum intensity 12-24 hours after the beginning of occlusion.FrontalcharacteristicsP5:The activity of a front in terms of weather depends upon the vertical motion in the air masses.If the air in the warm sector is rising relative to the frontal zone,the fronts are usually very active and are termed ana-fronts.Whereas sinking of the warm air relative to the cold air masses gives rise to less inactive kata-fronts.1.ThewarmfrontP6:The warm front represents the leading edge of the warm sector in the wave.The frontal zone here has a very gentle slope,of the order 1/2-1,so that the cloud systems associated with the upper portion of the front herald its approach some 12 hours or more before the arrival of the surface front.The ana-warm front,with rising warm air,has multi-layered cloud which steadily thickens and lowers towards the surface position of the front.The first clouds are thin,wispy cirrus,followed by sheets of cirrus and cirrostratus,and altostratus.高层云卷层云The sun is obscured as the altostratus layer thickens,and drizzle or rain begins to fall.The cloud often extends through most of the troposphere and with continuous precipitation occurring is generally designated as nimbostratus.Patches of stratus may also form in the cold air as rain falling through this air undergoes evaporation and quickly saturates it.P7:The descending warm air of the kata-warm front greatly restricts the development of medium-and high-level clouds.The frontal cloud is mainly stratocumulus,with a limited depth as a result of the subsidence inversions in both air masses.Precipitation is usually light rain or drizzle formed by coalescence since the freezing level tends to be above the inversion layer,particularly in summer.P8:In the passage of the warm front the wind veers,the temperature rises and the fall of pressure is checked.The rain becomes intermittent or ceases in the warm air and the thin stratocumulus cloud sheet may break up.P9:Forecasting the extent of rain belts associated with the warm front is complicated by the fact that most fronts are not ana-or kata-fronts throughout their length or even at all levels in the troposphere.For this reason,radar is being used increasingly to determine by direct means the precise extent of rain belts and to detect differences in rainfall intensity.P10:Such studies have shown that most of the production and distribution of precipitation is controlled by abroad airflow a few hundred kilometres across and several kilometres deep,which flows parallel to and ahead of the surface cold front.P11:Just ahead of the cold front the flow occurs as a low-level jet with winds up to 25-30m/s at about 1 km above the surface.The air,which is warm and moist,rises over the warm front and turns southeastward ahead of it as it merges with the midtropospheric flow.This flow has been termed a“conveyor belt”(for large-scale heat and momentum transfer in mid-latitudes).Broad-scale convective(potential)instability is generated by the over-running of this low-level flow by potentially colder,drier air in the middle troposphere.Instability is released mainly in small-scale convection cells that are organized into clusters,known as meso-scale precipitation areas(MPAs).These MPAs are further arranged in bands,50-100 km wide.Ahead of the warm front,the bands are broadly parallel to the airflow in the rising section of the conveyor belt,whereas in the warm sector they parallel the cold front and the low-level jet.In some cases,cells and clusters are further arranged in bands within the warm sector and ahead of the warm front.Precipitation from warm front rainbands often involves“seeding”by ice particles falling from the upper clouds layers.It has been estimated that 20%-35%of the precipitation originates in the“seeder”zone and the remainder in the lower clouds.Some of the cells and clusters are undoubtedly set up through orographic effects and these influences may extend well down-wind when the atmosphere is unstable.2.ThecoldfrontP12:The weather conditions observed at cold fronts are equally variable,depending upon the stability of the warm sector air and the vertical motion relative to the frontal zone.The classical cold-front model is of the ana-type,and the cloud is usually cumulonimbus（积雨云）.Over the British Isles air in the warm sector is rarely unstable,so that nimbostratus（雨层云）occurs more frequently at the cold front.With the kata-cold front the cloud is generally stratocumulus（层积云）and precipitation is light.With ana-cold fronts there are usually brief,heavy downpours sometimes accompanied by thunder.The steep slope of the cold front,roughly 2,means that the bad weather is of shorter duration than at the warm front.With the passage of the cold front the wind veers sharply,pressure begins to rise and temperature falls.The sky may clear very abruptly,even before the passage of the surface cold front in some cases,although with kata-cold fronts the changes are altogether more gradual.3.TheocclusionP13:Occlusions are classified as either cold or warm,the difference depending on the relative states of the cold air masses lying in front and to the rear of the warm sector.If the air is colder than the air following it then the occlusion is warm,but if the reverse is so,it is termed a cold occlusion.The air in advance of the depression is most likely to be coldest when depression occlude over Europe in winter and very cold of air is affecting the continent.a warm occlusiona cold occlusiona neutral occlusionP14:The line of the warm air wedge aloft is associated with a zone of layered cloud and often of precipitation.Hence its position is indicated separately on some weather maps and it is referred to by Canadian meteorologists as a trowal.The passage is an occluded front and trowal brings a change back to polar air-mass weather.P15:A different process occurs when there is interaction between a polar trough and the main polar front,giving rise to an instance occlusion.A warm conveyor belt on the polar front ascends an upper tropospheric jet forming a stratiform cloud band,while a low-level polar trough conveyor belt at right angles to it produces a convective cloud band and precipitation area poleward of the main polar front on the leading edge of the cold pool.P16:The occurrence of the frontolysis(frontal decay)is not necessarily linked with occlusion,although it represents the final phase of a fronts existence.Decay occurs when differences no longer exist between adjacent air masses.This may arise in four ways:through their mutual stagnation over a similar surface,as a result of both air masses moving on parallel tracks at the same speed,as a result of their movement in succession along the same track at the same speed,or by the system incorporating into itself air of the same temperature.