Thought Questions on the Geostrophic Wind and Real Winds Aloft at Midlatitudes

Transcription

1 Thought Questions on the Geostrophic Wind and Real Winds Aloft at Midlatitudes (1) The geostrophic wind is an idealized, imaginary wind that we define at each point in the atmosphere as the wind that blows just fast enough and in just the right direction so that the Coriolis force acting on it exactly opposes (balances) the pressure gradient force also acting it. (The winds in the atmosphere aloft on large scales, except near the equator, are usually pretty nearly equal to the geostrophic winds.) (a) Using the definition of the geostrophic wind, plus relevant properties of the Coriolis force and the pressure-gradient force, explain why the geostrophic wind at any particular point must necessarily blow in a direction parallel to isobar through that point. (The explanation is partly a geometric one--you should not start with a motionless or realistic wind, which doesn t ever have to be exactly geostrophic, and follow it s adjustment to something close to a geostrophic wind--that s a different question.) (b) Again using the definition of the geostrophic wind, plus the relevant properties of the Coriolis force and the pressure-gradient force, explain why the geostrophic wind speed must be greater when the isobars are closer together. (c) Given the foregoing results, together with the fact that the winds aloft on large scales (except near the equator) are usually pretty nearly equal to the geostrophic winds, where on earth, in the atmosphere aloft, would you expect to find the fastest winds? Why? Page 1

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3 (2) Figure 1 shows a pattern of height contours of a constant-pressure surface aloft at midlatitudes in the Northern Hemisphere. The region where these contours are relatively close together lies above the polar front in the Northern Hemisphere. (a) On which side of the polar front at this level, north or south, would you expect the height of the isobaric surface to be generally higher? Why? (b) Label the relatively low heights in troughs in this isobar pattern with an L and the relatively high heights in ridges with an H. (c) At the points indicated (black dots), draw arrows representing: (1) the direction of the pressure-gradient force (label it PG ); (2) the direction of the geostrophic wind (use a double arrow); and (3) the direction of the Coriolis force (label it Co ) acting on the geostrophic wind. The lengths of your two force arrows should reflect the magnitudes of each force relative to the other force. Based on your results, in what sense (clockwise or counterclockwise) do the geostrophic winds (and therefore actual winds) seem to blow around highs and around lows aloft in the Northern Hemisphere? Page 3

4 Figure 1 A simplified height pattern for a constantpressure surface aloft at midlatitudes in the Northern Hemisphere W N S E Page 4

5 (3) Figure 2 shows a pattern of height contours for a constant-pressure surface aloft at midlatitudes in the Southern Hemisphere. The region where these contours are relatively close together lies above the polar front in the Southern Hemisphere. (a) On which side of the polar front at this level, north or south, would you expect the heights of this isobaric surface to be generally higher? Why? (b) Label the relatively low heights in troughs in this isobar pattern with an L and the relatively high heights in ridges with an H. (c) At the points indicated (black dots), draw arrows representing: (1) the direction of the pressure-gradient force (label it PG ); (2) the direction of the geostrophic wind (use a double arrow); and (3) the direction of the Coriolis force (label it Co ) acting on the geostrophic wind. The lengths of your two force arrows should reflect the magnitudes of each force relative to the other force. Based on your results, in what sense (clockwise or counterclockwise) do the geostrophic winds (and therefore actual winds) seem to blow around highs and around lows aloft in the Southern Hemisphere? Page 5

6 Figure 2 A simplified height pattern for a constantpressure surface aloft at midlatitudes in the Southern Hemisphere W N S E Page 6