Location of Fetch In all cases, the first step toward a wave forecast is locating a fetch. A fetch is an area of the sea surface over which a wind with a constant direction and speed is blowing. Figure 6-4 shows some typical fetch areas. The ideal fetch over an open ocean is rectangular shaped, with winds that are constant in both speed and direction. As shown in figure 6-4, most fetch areas are bounded by coastlines, frontal zones or a change in isobars. In cases where the curvature of the isobars is large, it is a good practice to use more than one fetch area, as shown in figure 6-4(B). Although some semipermanent pressure systems have stationary fetch areas, and some storms may move in such a manner that the fetch is practically stationary, there are also many moving fetch areas. Figure 6-5

Figure 6-5.-Examples of moving fetches.

shows three cases where a fetch AB has moved to the position CD on the next map 6 hours later. The problem is determining what part of the moving fetch area to consider as the average fetch for the 6-hour period. In figure 6-5, Case 1, the fetch moves perpendicular to the wind field. The best approximation is fetch CB. Therefore, in a forecast involving this type of fetch, use only the part of the fetch that appears on two consecutive maps. The remaining fetch does contain waves, but they are lower than those in the overlap area.

In figure 6-5, Case 2, the fetch moves to leeward (in the same direction as the wind), Since waves are moving forward through the fetch area, the area to be used in this case is fetch CD.

Case 3, figure 6-5, depicts the fetch moving windward (against the wind). Since the waves move toward A, the region AC will have higher waves than the area BD. Experience has shown that in this case AB is the most accurate choice for a fetch,

The most obvious and accurate way to determine wind speed over a fetch is to average the reported values from ships. This method has the advantage of not requiring a connection for gradients or stability. But often there are only a few ship reports available, and ship reports are subject to error in observation, encoding, or transmission.

A second way to determine wind speed is to measure the geostrophic wind from the isobaric spacing and then correct it for curvature and stability. At first it would seem this would be less desirable due to the extra time necessary for corrections. However, barometric pressure is probably the most reliable of the parameters reported by ships, and a reasonably accurate isobaric analysis can be made from a minimum number of reports. For these reasons the corrected geostrophic wind is considered to be the best measure of wind speed over the fetch, except of course in cases where there is a dense network of ship reports where wind direction and speed are in good agreement.

The reason for correction to geostrophic wind is that the isobars must be straight for a correct measure of the wind. When the isobars curve, other forces enter into the computations. The wind increases or decreases depending on whether the system is cyclonic or anticyclonic in nature. The stability correction is a measure of turbulence in the layer above the water. Cold air over warmer water is unstable and highly turbulent, making the surface wind more nearly equal to the geostrophic wind. Conversely, warm air over colder water produces a stable air mass and results in the surface wind being much smaller than the geostrophic wind.

Three rules for an approximation of the curvature correction are as follows:

1. For moderately curved to straight isobars-no connection is applied.

2. For great anticyclonic curvature-add 10 percent to the geostrophic wind speed.

3. For great cyclonic curvature-subtract 10 percent ffom the geostrophic wind speed.

In the majority of cases the curvature correction can be neglected since isobars over a fetch area are relatively straight. The gradient wind can always be computed if more refined computations are desired.

In order to correct for air mass stability, the sea air temperature difference must be computed. This can be done from ship reports in or near the fetch area aided by climatic charts of average monthly sea surface temperatures when data is too scarce. The correction to be applied is given in table 6-2. The symbol Ts stands for the temperature of the sea surface, and Ta for the air temperature.

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