氣象專業(yè)英語2

1、Unit Two: Frontogenesis and frontal characteristics第二課第二課 鋒生和鋒的特征鋒生和鋒的特征 New words:Front 鋒，鋒面 frontolysis 鋒消Frontogenesis(Frontogeneses) 鋒生Frontal 鋒面的mid-latitude 中緯度 lower-latitude 低緯度High-latitude 高緯度Day-to-day 逐日的 daily 逐日的Mass 氣團(tuán)，質(zhì)量Humidity 濕度Specific humidity 比濕Meteorologist 氣象學(xué)家Integral 整數(shù)的，積分

2、的，完整的 Integration 積分Interface 交界面，接口Interaction internet interannual (年際的）Numerical 數(shù)值的Model 模式Baroclinic 斜壓的 barotropic 正壓的Cyclone 氣旋 anticyclone 反氣旋cyclogenesis氣旋生成氣旋生成Depression 低壓 isobaric 等壓的 isothermal 等溫的Iso+.等。 Synoptic 天氣的 Principle of Synoptic Meteorology 天氣學(xué)原理Convergence 輻合 divergence 輻散A

3、pex 頂點(diǎn)，峰尖 peak 峰Zone 區(qū)域 regionCirrus 卷云 jet 急流 Low level jet 低空急流Geostrophic 地轉(zhuǎn)的Convection 對(duì)流 convective 對(duì)流的Greenhouse 溫室效應(yīng) albedo 反照率Evaporate 蒸發(fā)v. Evaporation 蒸發(fā)n.Occlusion 錮囚 squall 颮Oceanic 海洋的 lee 背風(fēng)面Cyclogenesis 氣旋生成 Trough 槽 ridge 脊Ana-front 上滑鋒 kata-front下滑鋒Slope 坡度 order 量級(jí)Multi-layered 多層的

4、Multi-cell 多單體的Cirrostratus 卷層云 altostratus 高層云Nimbostratus 雨層云 stratocumulus 層積云Drizzle 毛毛雨 precipitation 降水Rainfall 降水量 saturate 使飽和Descend 下降 ascend 上升Medium-level 中層 high-levelLow-level 低層 mid-tropospheric 對(duì)流層中部的Airflow 氣流Large-scale 大尺度的 meso-scale 中尺度的 momentum 動(dòng)量Broad-scale 大范圍的Potential 位勢(shì)，潛

5、在的，Potential heat 潛熱 geopotential meter 位勢(shì)米R(shí)ainband 雨帶 rainbeltOrographic 地形的 topographic Down-wind 在下風(fēng)方向Cumulonimbus 積雨云Downpour 傾盆大雨 thunder 雷Duration 持續(xù)時(shí)間 occlude 錮囚Trowal (trough of warm air aloft) 高空暖舌Aloft 高的，上面的Poleward 向極地的 Phase 相位，階段，方面Tibet PlateaumoderateP1: The first real advance in our

6、 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 physic

7、al 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, a

8、nd their ideas are still an integral part of most weather analysis and forecasting particularly in middle and high latitudes. 1. Frontal wavesP2: 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 o

9、n 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 dissipa

10、te. 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 bet

11、ween 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 atmo

12、sphere 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 fron

13、tal 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. The frontal wave depressionP3: A depression (also termed a low or cyclone) is an area of relatively low pressure, with a

14、 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

15、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 formatio

16、n 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 fro

17、ntal 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.modified cold airfresh cold airwarm airP4: The depression usually achieves its maximum intensity 12-24 hours after the beginning of occlusion.Frontal characteri

18、sticsP5: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

19、gives rise to less inactive kata-fronts.1. The warm frontP6: 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

20、 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 altos

21、tratus . 高層云卷層云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 r

22、ain 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

23、 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 bec

24、omes 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 troposph

25、ere. 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 hundre

26、d 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 an

27、d 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 potentiall

28、y 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 para

29、llel 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 o

30、ften 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 inf

31、luences may extend well down-wind when the atmosphere is unstable.2.The cold front P12: 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

32、the ana-type, and the cloud is usually cumulonimbus（積雨云）. Over the British Isles air in the warm sector is rarely unstable, so that nimbostratus（雨層云） occurs more frequently at the cold front. With the kata-cold front the cloud is generally stratocumulus （層積云）and precipitation is light. With ana-cold

33、 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 temperatur

34、e 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. The occlusionP13: Occlusions are classified as either cold or warm, the difference depending on the relative states of

35、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