Quantcast Wind Estimates from Satellite Pictures

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Wind Estimates from Satellite Pictures

Clouds over the tropics, viewed from satellite altitude, reveal many features of the flow. The distribution of widespread cloud systems has definite relationships to major trough and ridge positions, and wind estimates for both upper and lower tropospheric levels can be obtained from analysis of cumulus and cirrus cloud formations as seen in singular satellite pictures or from cloud motion measurements taken from a series of pictures.

Two levels of the tropical atmosphere are usually plotted and analyzed: in the lower troposphere, it is the gradient level, and in the upper troposphere, it is the 300-or 200-mb level.

LOW-LEVEL STREAMLINES.—Gradient level (2,000 to 3,000 feet) winds are most often chosen for analysis, because the wind frequency is fairly equal, and since cloud bases average 2,000 feet, balloons are often lost above this level. Ship’s surface winds can also be used to supplement the winds of this field, and satellite pictures permit wind determination from low-level cloud fields.

· Research has shown that many cumulus cloud lines observed between 10°N and 10°S over the Atlantic and eastern Pacific oceans are oriented in approximately the same direction as the surface wind. However, because the orientation of cloud lines relates to wind shear, these lines may depart considerably from the surface wind direction. Therefore, a great deal of caution must be exercised when using cloud lines to define low-level wind flow.

There are no hard-and-fast rules that permit positive identification of the particular cloud lines that approximately parallel the surface wind. It has been observed that cloud lines that are very long, very narrow, and either wavy, zig-zag, or have knots (wide places along a line) are the type that most often parallel the surface wind. Figure 9-2-3 shows cloud lines that approximately parallel the wind flow. Surface wind reports are entered on the picture as white arrows.

Two drawbacks to using the gradient level are (1) the orographic influences and (2) in the trade wind belt, tropical disturbances are not readily apparent at this level; the wind fluctuations at 2,000 feet may be small and noninformative. Thus, the choice of level to be analyzed must be based on local and synoptic peculiarities of the area.

UPPER-LEVEL STREAMLINES.—All the rules applicable to low-level streamlines also apply to the streamlines of the upper troposphere. Upper-level streamline analyses are primarily used to locate areas of cyclonic and anticyclonic flow. Anticyclonic outflow areas are the most important

Figure 9-2-3.—Low-level clouds off the coast of Africa.

areas to look for, because they are your first clue to tropical cyclone development aloft. You will find, more often than not, that upper-level wind data (200-mb level, or 40,000 ft) diminishes to less than half that available for low-level analyses. In view of this, you will find qualitative streamline analysis more suitable for this level in most cases, and that satellite pictures play an, even more important part in the analysis.

Just as low-level winds are estimated from cloud patterns seen in satellite imagery, so too are upper-level winds. Two of the most useful cloud patterns used in determining upper-level flow are plumes from the tops of cumulonimbus (Cb) clouds and cirrus cloud shields associated with subtropical jet streams.

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