Geoscience Reference
In-Depth Information
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 very near the surface is greater than
that situated some horizontal distance away from the sloping terrain. This estab-
lishes a horizontal pressure gradient force. However, high density air moving hori-
zontally then encounters the lower density adjacent air. The result is a force with a
component directed down the slope (Parish and Cassano,
2003
). However, 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,
as is generally the case for Greenland, 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 1990-1999 yield a maximum monthly
mean wind speed of 9-11 m s
−1
during November-January with a minimum of
5 m s
−1
in July. The prevailing direction is from 120-130°, reflecting a katabatic
regime (Steffen and Box,
2001
). Winds show strong directional consistency over
most of the ice sheet. G. Heinemann and T. 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.
A special type of winds associated with the topography of Greenland is tip jets,
which are seen as short lived, shallow, and narrow high wind speed events that
extend from the southern tip of Greenland (Cape Farewell) to a few hundred kilo-
meters away from the coast. They occur multiple times each winter. The vicinity of
Cape Farewell is recognized as the windiest place on the ocean's surface (Sampe
and Xie,
2007
). There are two basic types: (1) westerly (sometimes knows as for-
ward) tip jets, so named because the winds have a westerly (from the west) com-
ponent with the jet extending eastward from the tip of Greenland (
Figure 8.3
); and
(2) northeasterly (sometimes known as reverse) tip jets, when winds are from the
northeast and the jet extends southwestward from the southern tip of Greenland
(Moore,
2003
; Vage et al.,
2009
). At times, wind speeds in the core of the jets
can reach 50 m s
−1
. Both types of tip jets represent interaction between the steep
