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University of Alaska - Fairbanks: "Improving the understanding and prediction of the meteorology of the arctic front over Alaska"

Final Report

Although the Arctic front has received little attention in modern meteorological education, it is a significant feature. In certain locations, it is responsible for the majority of the wintertime precipitation. In addition, the Arctic front is intimately related to the problem of winter continental Arctic stratus and appears to affect the distribution of Arctic haze. The front is not simply a colder version of the Polar front, but has its own unique characteristics. For instance, the Arctic front in the mainland part of Alaska can rarely be detected as a surface discontinuity in either temperature or wind direction, as it is usually isolated from the ground by a well-developed ground inversion. Since a north-south transect across Alaska has at most 3 soundings over a 10 degree latitude interval, and since the front is defined by steep horizontal temperature gradients between 50 and 150 mb above the surface, simply locating the front is a major problem. Precipitation events associated with the Arctic front also differ from those along the Polar front. Polar front precipitation is generally the result of well-defined low pressure systems with a large frontal wave which usually winds up as a classic occluded front. When a Polar-front storm penetrates into interior Alaska, precipitation rates can be very high but such events are relatively rare. In contrast, the Arctic front is most often observed as a stationary or quasi-stationary feature. Precipitation is due either to general southward movement of the front or to short waves traveling along the front, producing rather low precipitation rates in relatively frequent events.

Investigations of the Arctic front under this Cooperative Project included assembling multi-year statistics on the importance of the front as a precipitation generator, locating the front, describing its relationship with continental stratus and Arctic haze, and looking at model forecasts of Arctic frontal passages. The work on detecting the Arctic front focused on assembling and plotting data from 40 winters. Another detection technique used data from the remote automated weather stations at elevations above the surface inversion.

Conclusions from the project were:

1) The Arctic front is not the major precipitation producer originally thought. Classic Arctic frontal passages produce anywhere from 0 to 56% of any given winter's snowfall in Fairbanks, and are responsible for rare snow events as far south as Anchorage.

2) The classic Arctic front (the boundary between continental Arctic (cA) and maritime Polar (mP) air is not the only way snowfall can be produced when Arctic air is near the surface. Fronts between two Arctic air masses can produce small amounts of precipitation, as can overrunning events which appear only as weak troughs at the surface.

3) Snow density is much less well correlated with temperature than had often been assumed.

4) The Arctic front may play a major role in the production of cA air by removing excess water very early in the cooling process. Paradoxically, this could actually slow the production of cA air, as the drier air should have a lower radiative cooling rate.

5) There are several possible ways of looking at the Arctic front, involving thicknesses and pseudoadiabats. At the present time, the usefulness of all but the simplest, the 1000-850 mb thickness, is limited by data and model output.