101: Norwegian Polar Front Cyclone Model

               Each day, around the world, countless television stations, newspapers, weather agencies, and web sites issue weather maps depicting the current weather conditions or those forecast for the day. These might look something like this:

 

               This happens to be a rather historic storm from November 1998. What is likely most prominent on this map, as well as most other weather maps, are the large “H”s and “L”s and the bold colored lines. These symbols depict the weather based as described by a model developed in the 1920s by the Bergen School in Norway. This model, called the Norwegian Polar Front Cyclone Model, describes the process in which mid-latitude cyclones (MLC) begin, develop, and ultimately dissipate. The key characteristic of this theory is the dominance of fronts, especially the cold front. The fronts envisioned here were unlike any standard model of the day; the idea that strong temperature gradients could exist over long distances had typically been dismissed. Below is a weather map issued by the Signal Service of the U.S. Army back in September of 1872:

               First of all, the lack of observations over the western part of the country is immediately apparent. The black lines are isobars or isotherms (lines of constant temperature), depending on what they’re labeled as. On this map, a high pressure center is depicted over much of the Appalachian Mountains and a low center is identified just off the coast of Maine. Nowhere on this map are fronts, or even strong temperature gradients, despite the fact that it is likely that there was at least a weak cold front right off the east coast.



"Frontal Wave"

                The Norwegian model fixed many of these issues and was able to describe how the specific features came about. The MLC in their theory begins along what they called the “polar front”, which they envisioned as a zone of high temperature gradient that circles the earth in the mid-latitudes. These days, this would likely be shown as a stationary front located over places as far north as central Canada, Western Europe and Siberia. To some extent this front does show up, but only in sections. In the model, it is along this polar front that a weak low pressure center develops. This center will begin to cause the surrounding air to spiral cyclonically (counter-clockwise), thus pulling warm air from the south northward and pushing cold air from the north southward. At this point, the system is referred to as a “frontal wave”.



Fronts

               As this trend continues, the polar front will be twisted cyclonically. Since the cold air moving southward on the west side of the system is denser than the northward flow of warm air, it essentially plows into the warm air, forcing it to rise above the cold air. The edge of this cold air mass is the cold front, while the warm air moving northward will eventually have to slide up the cold air north of the system, forming the warm front.



Developing

               At this stage in development the fronts become well established and both circulate around the low center cyclonically. The overall appearance of the system is relatively unchanged, except that the cold front, which circulates faster than the warm front, is beginning to “catch up” to the warm front, causing the angle between them to decrease.



Mature

               In the final stage, the MLC is fully mature and is likely at its peak intensity. The cold front has caught up with the warm front and past the point where they intersect the front is now an occluded front, which combined look something like a zipper on a jacket. Since the occlusion began, the warm air from the south has no longer reached the low center. Eventually the lack of warm air will weaken the low center and cause its pressure to increase, or “fill”.

          

Cross-sections of the three front types helps show what the air masses are doing in three dimensions.

               The warm front is characterized a warm air mass that is sliding up the cooler air mass to the north, which is likely slowly retreating. It is the line on the ground where the two air masses meet that a surface warm front is depicted. Since the air gently slides up the cool air, clouds associated with warm fronts tend to be layer clouds with light precipitation. It is common for places on the surface in the cool air to receive precipitation from the clouds forming on the warm front far above the surface.

               The cold front is a zone of sharp contrast where the cold, dense air mass moving southward is pushing into the warm air mass. The warm air is forced to rapidly ascend the front of the cold air, resulting in a zone of cloudiness that is much thinner than around the warm front, but with much more vigorous precipitation or even thunder storms. Because the advancing cold air is pushing into the warm air, often the warmest area of a MLC will be right in front of the cold front.

 

 

               The occluded front is sort of a combination of the warm and cold fronts. Since the dense cold air catches up to the warm front, the warm front is essentially lifted up the face of the cold front. The result is that at the surface the occluded front represents the boundary between the cold air that was behind the cold front and the slightly less cold air that was ahead of the warm front. Now the only place where the warm air can be found is aloft, above where the three fronts meet.