COLD OCCLUSIONS 

A cold occlusion is the occlusion that forms when a cold front LIFTS the warm front and the air mass preceding the front (fig. 4-5-2). The ver-tical and horizontal depiction of the cold occlu-sion is shown in figure 4-5-3. 

Cold occlusions are more frequent than warm occlusions. The lifting of the warm front as it is underrun by the cold front implies existence of an upper warm front to the rear of the cold occlusion; actually such a warm front aloft is

Figure 4-5-2.Vertica1 cross section of a cold type of occlusion.

rarely discernible and is seldom delineated on a surface chart.

Most fronts approaching the Pacific coast of North America from the west are cold occlusions. In winter these fronts usually encounter a shallow layer of surface air near the coastline (from about Oregon northward) that is colder than the leading edge of cold air to the rear of the occlusion. As the occluded front nears this wedge of cold air, the occlusion is forced aloft and soon is no longer discernible on a surface chart. The usual practice in these cases is to continue to designate the cold occlusion as though it were a surface front because of the shallowness of the layer over which it rides. As the occlusion crosses over the mountains, it eventually shows up again on a surface analysis. The passage of the cold type of occlusion over the coastal layer of colder air presents a difficult problem of analysis in that no surface wind shift ordinarily occurs at the exact time of passage. However, a line of stations reporting surface-pressure rises is the best criterion of its passage.

This should be verified by reference to plotted raob soundings where available. When a Pacific cold occlusion moves farther inland, it sometimes encounters colder air of appreciable depth over the Plateau or Western Plains areas; in this case, it should be redesignated as an upper cold front.

The occlusion lies in a low-pressure area; and in the latter stages, a separate low center may form at the tip of the occlusion, leaving another low-pressure cell near the end of the occlusion. The pressure tendency across the cold occluded front follows closely with those outlined for cold fronts; that is, they level off, or more often, rapid rises occur after the passage of the occluded front.

In the occlusions initial stages of develop-ment, the weather and cloud sequence ahead of the occlusion is quite similar to that associated with warm fronts; however, the cloud and weather sequence near the surface position of the front is similar to that associated with cold fronts. As the occlusion develops and the warm air is lifted to higher and higher altitudes, the warm front and prefrontal cloud systems disappear. The weather and cloud systems are similar to those of a cold front. View A of figure 4-5-3 shows the typical cloud and weather pattern associated with the cold occlusion. Most of the precipitation occurs just ahead of the occlusion. Clearing behind the

Figure 4-5-3.Cold front type of occlusion. (A) Vertical structure through points A and A; (B) horizontal structure.

 occlusion is usually rapid, especially if the occlu-sion is in the advanced stage. Otherwise, clear-ing may not occur until after the passage of the warm front aloft.

Upper Air Characteristics

If only one upper air sounding were taken so that it intersected either the cold or warm front, the sounding would appear as a typical warm or cold front sounding. However, if the sounding were taken so that it intersected both the cold and warm air, it would show two inversions.

The occlusion may appear on some upper air charts. It usually appears on the 850-mb chart, but rarely on the 700-mb chart. As the two air masses are brought closer together and as the occlusion process brings about gradual disap-pearance of the warm sector, the isotherm gradient associated with the surface front weakens. The degree of weakening depends on the horizontal temperature differences between the cold air to the rear of the cold front and that ahead of the warm front. The angle at which the isotherms cross the surface position of the occluded fronts becomes greater as the temperature contrast between the two cold air masses decreases. A typical illustration of the isotherms shows a packing of isotherms in the cold mass behind the cold front and less packing in the cool mass in advance of the warm front. A warm isotherm ridge precedes the occlusion aloft.

Learning Objective: Describe the forma-tion, structure, and characteristics of the warm occlusion.

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