《气象科技英语》课件：专业英语5-大气环流 47页

Unit Five : The General circulation of the atmosphere 第五课 大气环流 New words: Vector: 向量 Zonal 纬向的 latitude 纬度 Meridional: 经向的 longitude: 经度 Subpolar: 副极地的 subgrid Jetstream 急流 Reversal 颠倒，反转 Westerly 西风 easterly 东风 Westerlies 西风带 easterlies 东风带 Retreat : 复原，恢复；撤退 withdraw Equator 赤道 ribbon 带状物， strip ;belt Coriolis 科里奥利力 Oceanographic 海洋学的 kinetic 运动的 大气环流：一般是指具有世界规模的、大范围的大气运行现象，既包括平均状态，也包括瞬时现象，其水平尺度在数千公里以上，垂直尺度在 10km 以上，时间尺度在数天以上。 大气大范围运动的状态。 某一大范围的地区（如欧亚地区、半球、全球），某一大气层次（如对流层、平流层、中层、整个大气圈）在一个长时期（如月、季、年、多年）的大气运动的平均状态或某一个时段（如一周、梅雨期间）的大气运动的变化过程都可以称为大气环流。 大气环流通常包含平均纬向环流、平均水平环流和平均经圈环流 3 部分。 ① 平均纬向环流 。指大气盛行的以极地为中心并绕其旋转的纬向气流，这是大气环流的最基本的状态，就对流层平均纬向环流而言，低纬度地区盛行东风，称为东风带（由于地球的旋转，北半球多为东北信风，南半球多为东南信风，故又称为信风带）；中高纬度地区盛行西风，称为西风带（其强度随高度增大，在对流层顶附近达到极大值，称为西风急流）；极地还有浅薄的弱东风，称为极地东风带。 ② 平均水平环流 。指在中高纬度的水平面上盛行的叠加在平均纬向环流上的波状气流（又称平均槽脊），通常北半球冬季为 3 个波，夏季为 4 个波，三波与四波之间的转换表征季节变化。 ③ 平均经圈环流 。指在南北 - 垂直方向的剖面上，由大气经向运动和垂直运动所构成的运动状态。通常，对流层的径圈环流存在 3 个圈：低纬度是正环流或直接环流（气流在赤道上升，高空向北，中低纬下沉，低空向南），又称为哈得来环流；中纬度是反环流或间接环流（中低纬气流下沉，低空向北，中高纬上升，高空向南），又称为费雷尔环流；极地是弱的正环流（极地下沉，低空向南，高纬上升，高空向北）。 控制大气环流状态的基本因子 ① 大气本身的特殊尺度（准水平） ② 太阳辐射随纬度分布的不均匀性（三圈环流） ③地球自转（准地转） ④地球表面的不均匀性（海陆分布、陆地起伏） ⑤地面摩擦（东西风维持） 赤道无风带是指赤道附近南、北纬 5° 之间的地带。这里太阳终年近乎直射，是地表年平均气温最高地带。由于温度的水平分布比较均匀，水平气压梯度很小，气流以辐合上升为主，风速微弱，故称为赤道无风带。它控制下的天气特点是气压低、湿度大、多云、多雷暴，是海上航行时要避开的区域。 a b fell P1: ① The general circulation comprises the movements of the atmosphere on a worldwide scale. ② Since it is usually studied by means of data averaged over several days , so that minor, local or day-to-day irregularities are smoothed out , any model of the general circulation must be generalized, and cannot include very many short-lived features of importance for local weather. ③ The general circulation is the overall pattern that must obviously affect local weather at sometime or another, directly or indirectly, and is in this sense the greatest single terrestrial cause of climate and weather. P2: ① Technically , the general circulation may be defined as the mean three-dimensional pattern of the meteorological elements, plus the “turbulence”, the oscillation or perturbations of the mean pattern , provided by changing, day-to-day synoptic weather patterns . ②Its basic features may be described in terms of global, seasonal vector mean winds as a function of height, or they may be derived by applying the geostrophic wind relation to means pressure-contour charts. P3: ① The three-dimensional aspects of the general circulation as actually observed must be particularly emphasized. ② For comparative purposes, it is convenient to separate the zonal (east-west) and meridional (north-south) components of the mean motion for the northern hemisphere. ③The mean meridional circulation is about a meter per second in the lower and middle latitudes throughout a substantial depth of the atmosphere: this is a much weaker circulation than the zonal one, but can nevertheless create or destroy momentum at the rate of 10 m per sec. P4: ① The simplest model that incorporates the main features of the observed mean meridional is given in the figures ( omitted ). ②The three kinds of cells are in the troposphere in each hemisphere: the Hadley cells in the tropics, the Ferrel cells in middle latitudes, and the weak subpolar cells beyond these. ③ Angular momentum is injected into the Ferrel cells as indicated by the arrows, and is later carried downward by small convective eddies. ④ Convection is most intense in low latitudes and thus for equilibrium to occur the Hadley cells must rotate faster than the Ferrel cells. 全球大气环流示意 P5: ① The model devised by Palmen takes into account the existence of jetstreams, which are the dominant features of the actual circulation . ②Observation shows that the Hadley cells, which are directly driven by heat , are the most important single elements of mean tropospheric circulation, but the Ferrel cells, driven by friction with the Hadley cells, probe to be more significant than Palmen envisaged. P6: ① An independent circulation is generated by heating and cooling in the stratosphere, down to about 10 mb. ②Stratospheric winds reveal remarkable reversals in direction. ③ A stratospheric monsoon occurs in the northern hemisphere: westerlies change to easterlies in April above 10 mb, the reversal proceeding downward and southward from the polar regions, reaching 100 mb in late May. ④Easterlies prevail above 100 mb from May to August. ⑤In late August and early September, these easterlies revert back to westerlies. P7: ① Over North America in April, stratospheric polar easterlies are separated by the middle-latitude westerlies from the tropical easterlies of the lower atmosphere, which move northward in the month . ②By July, easterlies prevail down to at least 15 km in low latitudes, to 20 km in middle latitudes and to 15-17 km in polar latitudes. ③By September, the polar and tropical easterlies begin retreating to their minima, in November and December, respectively. ④ In general, the picture at 10 mb is of a slow, steady transition from summer easterlies to winter westerlies, but during January and February 1958 this simple pattern broke down in a very complex manner. P8: ① General models of the upper atmospheric circulation have been produced by Keliogg and Schilling (1951), Murgatroyd (1957), and Batten (1961). ②According to Betten’s model, the major center of westerly winds is in the winter hemisphere, although these winds also cross the equator into the summer hemisphere. ③ Easterlies occur in spring in the lower ionosphere, building down from the mesosphere as westerlies develop aloft; in turn, the westerlies then build down as easterlies develop aloft. ④Small easterly centers occur in the lower mesosphere in late winter and spring. ⑤ The stratosphere winds above the Pacific equatorial region are extremely variable. P9: ① Important elements of the stratospheric circulation include the Berson westerlies and the Krakatoa easterlies. ②The former, first discovered at 50 to 60 mb over central Africa, but now known to occur anywhere up to 10 mb, form a continuous ribbon around the equator: the westerlies and easterlies alternate , one half-cycle being 12 to 15 months. ③ The Krakatoa easterlies occur at 25 mb: their existence was first inferred from the movement of volcanic dust after Krakaroa eruption. ④Radar wind observations now indicate that Krakatoa westerlies also occur. P10: ① The geographical importance of these stratospheric winds is that they make any simple, intuitive model of the atmospheric circulation untenable. ②In such a model, the rotation of the Earth from west to east is assumed to drag the lower part of the atmosphere with it, imparting a westerly motion to these layers. ③Thus slight variations in the momentum of the west-east rotating atmosphere would be interpreted at the Earth’s surface as indicating winds apparently coming from different directions. ④A local excess of momentum in the atmosphere, causing the latter to move more rapidly from west to east than the Earth’s surface, would be described as a west wind. ⑤A local deficit of momentum, causing the atmosphere to move less rapidly than the Earth’s surface, would give rise to an east wind. P11:①Recent observations indicate that the high atmosphere contains many circulation features that cannot be explained by a simple intuitive model. ② Thus, changes in the rotation of Sputnik 3 can be explained by the existence of a strong westerly wind, accompanying the Earth’s rotation, but well above any region of possible frictional drag with its surface. ③ Slow oscillations, representing a balance between inertia and Coriolis forces and static stability （静力） , have been measured by radarsonde theodolites （经纬仪） at Crawley. ④ They have period of 12 hours or so, and are due to disturbances with vertical and horizontal dimensions of 1 km and several 100km, respectively. ⑤ The complexity of data for the high atmosphere has made it necessary to split the circulations observed into mathematical components. P12:①The circulation models discussed so far have been on diffusion. ②In Brewer’s model, based on the diffusion of ozone and water vapor, air rises through the equatorial tropopause, which acts as a cold trap owing to its low temperature (around 80 ℃ ). ③The cold, dry air then moves horizontally, finally sinking in middle and high latitudes. ④According to Dobson’s model, ozone-enriched air arriving via Brewer’s meridional circulation is stored in the stratospheric polar-night jet, i.e., in the cold pool over the winter pole. ⑤From here, it gradually sinks into the lower stratosphere at temperate latitudes in late winter and spring. P13:① In Spar’s model, which is based on the diffusion of radioactive debris （放射性碎片） , the main exit for air from the stratosphere is through the gap in the tropopause, in which turbulent mixing takes place . ②More mixing takes place in the polar stratosphere (particularly in winter) than elsewhere, and much less mixing in the equatorial stratosphere than in the Brewer-Dobson model, which describes only one part of the whole circulation. ③ The highest parts of the Brerrer-Dobson circulation reach 80,000 feet; above the atmosphere was envisaged by Brewer and Dobson as stagnant region is moist, and meridional transfer is affected by small-scale turbulent diffusion. ④ The height of the transition from meridional-circulating to meridional-stagnant air varies both in time and in latitude. P14: ① In the Goldsmith-Brown model, rising air at the equator does not reach great heights, but turns poleward almost immediately above the tropopause. ② The meridional circulation is rapid just above the tropopause, the air taking slightly more than two months to reach temperate zones . ③ The upper flow is much slower, so that air remains in the ozone-producing layers for about a year. ④ From there, ozone-enriched air is fed slowly into the polar-night jet, where it becomes available for transfer into the middle-latitude lower stratosphere by the Dobson mechanism . P15: ① These circulation models for the high atmosphere are significant for weather at the Earth’s surface because they demonstrate that air is definitely exchanged between troposphere and stratosphere. ②Surface weather systems, especially if described in terms of air masses and fronts, cannot be regarded as closed systems, although the error in so regarding them may not be serious in daily weather analysis. ③A further complication for climatological work is that the atmospheric and oceanic circulations must be considered a complexly integrated single system, if a complete study is to be made of the general circulation. ④If the stratospheric and oceanographic complications are ignored, the general circulation can be regarded as a consequence of part of the solar radiation received by the Earth being transformed into kinetic energy.