Wind regime

A prominent feature of the Greenland climate is its "katabatic" wind regime. In the right conditions such drainage winds can reach gale force. In its most generic sense, a katabatic wind (from the Greek word katabikos - to go down) refers to any downslope wind flowing from high elevation mountains, plateaus or hills down to valleys or plains. But more commonly, the term is reserved for winds that, despite the effects of adiabatic compression during descent, are colder than the air displaced at the bottom of the incline. A wind that is warmer than the air being displaced at the bottom of an incline is commonly referred to as a foehn, although there are many local names (e.g., Chinooks in the lee of the Rocky Mountains and the Santa Anna in the lee western side of the San Bernadino Mountains, California).

Dynamically, katabatic winds in Greenland are the same as those found in Antarctica. They relate to flows that are forced by radiational cooling of the lower atmosphere adjacent to the sloping terrain on the ice sheet. Cooling of the low-level environment means that the density of the air near the surface is greater than that situated some horizontal distance away from the sloping terrain. This establishes a buoyancy force, which promotes an acceleration in the downward slope direction (Parish and Cas-sano, 2003). While katabatic winds are thus directed with a downslope component, one must also consider the effects of the Coriolis force and frictional drag. For horizontal scales of motion of several kilometers and time scales of a few minutes, the Coriolis force can be ignored. For motions of hundreds of kilometers or time scales of hours, the Coriolis force is important, and will deflect the wind to the right of the motion. Friction acts to slow the winds. Because of the Coriolis force, Greenland's katabatic winds, when not greatly influenced by local topography (as would be the case in the upper regions of the ice sheet), tend to flow with a pronounced component across the fall line. However, winds near the coast are channeled by valleys and fiords. Intense katabatic winds are favored by particular synoptic conditions that enhance the large-scale pressure gradient. As discussed in Chapter 4, katabatic winds along the coast are also known to assist in the development of mesoscale polar lows.

Measurements at Swiss Camp during 1990-9 yield a maximum monthly mean wind speed of 9-11 m s-1 during November-January and a minimum of 5 m s-1 in July. The prevailing wind direction is from 120-130°, reflecting a katabatic regime (Steffen and Box, 2001). Winds show strong directional consistency over most of the ice sheet. Heinemann and Klein (2002) undertook a limited-area model analysis of all of Greenland for January 1990. Their results show a clear signal of katabatic winds in the mean wind field. The strongest katabatic winds are observed in western Greenland north of 70° N and in eastern Greenland north of 75° N. Katabatic storms are well known along the southeastern coast of Greenland and in the east coast valleys near

Angmassalik. By disrupting the surface-based temperature inversion, katabatic wind events can cause rapid changes in surface air temperature.

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