Upper-level Winds from Cumulonimbus Cloud Tops

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Upper-level Winds from Cumulonimbus Cloud Tops.— Vertical wind shear through an atmospheric layer containing clouds is one of the prime factors governing the shape of cloud formations. Most of the cloud lines and bands seen in satellite pictures parallel the thermal wind through the cloud layer (base to top). Under some circumstances, the direction of the shear is the same, or nearly the same, as the wind direction in the layer containing the cloud. This is the case if (1) the wind changes speed with height but not direction or (2) the wind changes direction with height, but the winds at the top of the Cb are much stronger than those near its base. When either of these conditions occurs, it is possible to estimate the wind direction directly from the orientation of the Cb plumes. These conditions are common in tropical regions, where the speed of the wind at the level of cumulonimbus anvils is several times greater than the mean WIND SPEED OF THE WIND THROUGH THE LOWER PORTION OF THE CB. In these cases, wind estimates based on orientation and dimensions of cirrus blow-off are quite accurate and closely approximate the winds near the cloud top. On the other hand, there are two conditions that can cause considerable error in wind direction estimates derived from Cb anvils: (1) light winds at the cirrus level and (2) a large difference between the direction of the shear through the convective layer and the direction of the wind at the top of the layer.

When the speed is light (less than 20 knots), the wind direction is variable causing the correla-tion between plume orientation and wind direction to be less reliable. If there is a large difference between the direction of the shear through the convective layer and the direction of the wind at the top of the layer, the plumes will parallel the shear but not the wind at the top of the layer. This condition is common in frontal areas where temperature advection is strong, but occurs less frequently in the tropics.

Upper-level Flow from Cirrus Cloud Shields.— It is common to observe huge masses of cirriform clouds extending poleward from the tropics. These clouds occur in advance of 200-mb troughs and represent a poleward transport of momentum and high-level moisture from the tropical region. A wind maximum is usually located on the poleward edge of these cloud formations. As with strong jet streams, the direction of the upper-level wind parallels the cloud edge. Because the air is moving away from the equator and accelerating, it is crossing the contour pattern toward lower pressure. For this reason, the general orientation of the cloud shield and the striations within it can differ as much as 15 degrees from the orientation of the upper-tropospheric contours. When this is taken into account, cirrus formations of this type provide good estimates of wind direction but less precise information as to wind speed. Figure 9-2-4 shows atypical cirrus cloud shield. The double shafted, white arrows (A to C) represent the 200-mb wind maximum suggested by the cirrus cloud edge. The black, single-shafted arrows represent the 200-mb flow based on individual cirrus elements that lie within the larger scale formation. The vertical cloud pattern (D) is associated with a cut-off low.

A cirriform cloud shield extends northeast from A in advance of an upper-level trough associated with the vortex. There are numerous small-scale lines of clouds (E to F) that are oriented approxi-mately perpendicular to the wind direction. These lines are believed to be caused by horizontal shear and should not be confused with cumulonimbus plumes which parallel the vertical shear. There are two basic methods of streamline analysis in use; the discontinuous or qualitative method and the streamline-isotach method. If reports are sparse, the discontinuous or qualitative method is generally used. The most complete analysis is made with the streamline-isotach method, and this is the more recommended procedure.

Discontinuous (Qualitative) Streamline Analysis

This method involves a single set of lines drawn tangential to the wind direction and spaced

Figure 9-2-4.—A typical cirrus cloud shield.

in proportion to the wind speed. This means that in

Figure 9-2-5.—Discontinuous or qualitive streamline analysis.

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