Occlusion (meteorology)
In meteorology, occlusion is a process in a dynamic low-pressure area in which the warm sector is lifted from the ground by the union of two fronts .
In dynamic low pressure areas in middle latitudes , the Ferrel cell or westerly wind zone , two fronts form in the course of cyclogenesis , the warm front and the cold front . Both fronts rotate with the low pressure area, i.e. counterclockwise in the northern hemisphere and clockwise in the southern hemisphere .
Occlusion types
Cold front occlusion
With cold front occlusion, the air behind the cold front is colder than the air in front of the warm front. Both air masses are raised in front of the cold front, as they have a lower density due to the higher temperature. The warm front is "rolled over" on the ground weather map and forms a kink to the line that is formed by the occlusion and the cold front. In middle latitudes , cold front occlusions tend to occur in the summer months.
Warm front occlusion
Conversely with warm front occlusion: Here there is very cold air in front of the warm front, and only cool air behind the cold front. At the point of occlusion, this slides like the warm front onto the cold air. The front line of the warm front is therefore not changed on the map, the occlusion and warm front form a continuous line. The cold air in front of the front passage also cools the floor, which means that there is little convection behind the occlusion and only a few new swelling clouds are created . In middle latitudes, warm front occlusions tend to occur in the winter months.
Similarities
In both cases, a warm front (red line) is overtaken by a faster cold front (blue line). The point at which they combine to form an occlusion is called the occlusion point ; From this point on there is only one front, the occlusion front (purple line). There are weak circulating winds near the occlusion core. At the edge of the core, the wind can take on gale force . The wind speed only drops again at a greater distance .
The temperatures of both cold sectors (here: dark blue = cold; light blue = cool) differ only slightly, but both are colder than the warm sector. Nevertheless, the difference in density is usually sufficient, even with a small temperature difference, to allow one of the fronts to dominate the occlusion.
However, if the temperature difference is too small or does not exist, such a dominance can occasionally no longer be determined. One then no longer makes any conceptual distinction. This phenomenon can also be regarded as the third type of occlusion.
Image description
In the graphic, the terms mentioned so far are shown together with typical cloud types and precipitation events .
The first part shows the same ground weather map with an entire low pressure area. A warm front is represented here by semicircles on the front line, a cold front by triangles and an occlusion by triangles and semicircles next to each other. The orange, horizontal dividing lines mark certain areas of the low pressure area, which are shown in more detail in the following partial images. It should be noted that a low pressure area only has the shape shown in the middle latitudes and only in the northern hemisphere.
The second part shows a section through the low pressure area at line (b). Here the fronts are still separated, so this is the state before the occlusion occurred .
The temperature profile in the third part belongs to the state before the occlusion occurred and indicates its type.
The fourth partial image shows a section through the low pressure area at line (a). It represents the occlusion, i.e. the union of both fronts.
