Frictional Convergence

Frictional convergence is speed convergence induced by the effects of friction over land. The roughness of the land causes the air to slow down as it crosses the land-water (or land-ice) interface. The upstream air catches up and converges on the air that is moving over the land. The head of Lake Superior is a land-water interface and thus experiences frictional convergence.

A lot of the snow that falls due to the enhanced lift is blown inland by the wind. Some of the extra snow can fall in Duluth itself depending upon the how far upstream the convergence extends and the strength of the winds. If the wind is from the east-southeast and more direct into the shore then heavier snow tends to be blown farther inland. If the wind is more east or slightly north of east, then heavier snow has a better chance of falling in communities adjacent to the shore.

Frictional convergence is only one of the mechanisms contributing to convergence at the head of the Lake Superior. When the other processes are added, you can get a broad area of convergence that covers both the water and the surrounding land.

Definition of Frictional Convergence in AMS Glossary of Meteorology