Лекция: The general circulation

The observed patterns of wind and pressure prompt consideration of the mechanisms maintaining the general circulation of the atmosphere – the large – scale patterns of wind and pressure that persist throughout the year or recur seasonally. Reference has already been made to one of the primary driving forces, the imbalance of radiation between lower and higher latitudes, but it is also important to appreciate the significance of energy transfers in the atmosphere. Energy is continually undergoing changes of form. Unequal heating of the earth and its atmosphere by solar radiation generates potential energy, some of which is converted into kinetic energy by the rising of warm air and the sinking of cold air. Ultimately, the kinetic energy of atmospheric motion on all scales is dissipated by friction end small-scale turbulent eddies (i.e. internal viscosity). In order to maintain the general circula­tion, the rate of generation of kinetic energy must obviously balance its rate of dissipation. These rates are estimated to be about 2 W m-2, which amounts to only 1 per cent of the average global solar radi­ation absorbed at the surface and in the atmosphere. In other words, the atmosphere is a highly inefficient heat engine.

A second controlling factor is the angular momen­tum of the earth and its atmosphere. This is the tendency for the earth's atmosphere to move, with the earth, around the axis of rotation. Angular momentum is proportional to the rate of spin (that is the angular velocity) and the square of the distance of the air parcel from the axis of rotation. With a uniformly rotating earth and atmosphere, the total angular momentum must remain constant (in other words, there is a conservation of angular momentum). If, therefore, a large mass of air changes its position on the earth's surface such that its distance from the axis of rotation also changes, then its angular velocity must change in a manner so as to allow the angular momentum to remain constant. Naturally, absolute angular momentum is high at the equator and decreases with latitude to become zero at the poles (that is, the axis of rotation), so air moving

polewards tends to acquire progressively higher eastward velocities. For example, air travelling from 42 to 46° latitude and conserving its angular momentum would increase its speed relative to the earth's surface by 29 m s-1. This is the same principle that causes an ice skater to spin more violently when her arms are progres­sively drawn into the body. In practice, this increase of air-mass velocity is countered or masked by the other forces affecting air movement (particularly friction), but there is no doubt that many of the important features of the general atmospheric circu­lation result from this poleward transfer of angular momentum.

The necessity for a poleward momentum trans­port is readily appreciated in terms of the mainte­nance of the mid-latitude westerlies. These winds continually impart westerly (eastward) relative momentum to the earth by friction, and it has been estimated that they would cease altogether due to this frictional dissipation of energy in little over
a week if their momentum were not continu­ally replenished from elsewhere. In low latitudes, the extensive tropical easterlies are gaining westerly relative momentum by friction as a result of the earth rotating in a direction opposite to their flow, and this excess is transferred pole­wards with the maximum poleward transport occur­ring, significantly, in the vicinity of the mean subtropical jet stream at about 250 mb at 30°N and 30°S.


Упражнение 2.

Прочитайте следующие слова и определите их соответствия
в русском языке:

energy, form, potential, kinetic, turbulent, circulation, generation, momentum, absolute, equator, progressively, principle, practice, mask, transport, tropical, subtropical.


Упражнение 3.

Заполните таблицу, образовав недостающие части речи.

v. n. adj. adv.
    extend     conservation   relative   continually   progresssively

Упражнение 4.

В правой колонке найдите русские эквиваленты следующих английских словосочетаний из текста The general circulation:

1. driving force 2. unequal heating 3. angular velocity 4. jet stream 5. large-scale 6. same principle 7. violently 8. altogether 9. over a week 10. ultimately 11. therefore a. тот же самый принцип b. в течение недели c. все вместе d. следовательно e. струйное течение f. неравномерное нагревание g. в основном h. угловая скорость i. крупномасштабный j. управляющая сила k. сильно


Упражнение 5.

Составьте 5 вопросов к первому абзацу текста The general circulation.


Упражнение 6.

(Парная работа) Ответьте на вопросы в упражнении 6.


Упражнение 7.

Найдите подлежащее в каждом предложении второго абзаца текста The general circulation.


Упражнение 8.

Вставьте соответствующий предлог и подберите определение к каждому глаголу: about, down, off, into, up, in for.

1. break … 2. look … 3. pick … 4. leave … 5. go … 6. lead … 7. cut … 8. get … a. начать b. распространяться c. перестать e сокращать f. исследовать g. расщеплять h. получать i. интересоваться

Упражнение 9. Прочитайте текст и найдите ответы на следующие вопросы (Контрольное время 7 минут):

1. От чего зависит вертикальные изменения давления с высотой?

2. Какой тип центров действия атмосферы преобладает в субтропических, субполярных и экваториальных зонах?

3. Какие ветра преобладают в тропической и умеренной зоне?

4. Какой тип циркуляции называют ячейкой Уокера?


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