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down the lee side, converting potential into kinetic energy, giving rise to a tip jet.
If the Froude number is less than 1, air will flow around the barrier. It appears that
for westerly tip jets, flow around the barrier is most important (Vage et al., 2009 ).
With the low located off the southeast coast of Greenland, air on the west side of
Greenland is dammed, leading to locally high pressure along the west coast. The
air moves southward along the west coast. When air gets to the tip of Greenland,
the barrier is removed, and the piled-up (dammed) air accelerates down the pres-
sure gradient. It then adjusts to the mean pressure field, which is why the winds are
westerly.
Northeasterly tip jets tend to occur when a cyclone center is located south of
Cape Farewell (the southern tip of Greenland). In this case, the wind low at low
levels is directed into the east coast of Greenland. This damming creates a barrier
wind. However, the barrier flow is cross isobaric, so the wind accelerates as is trav-
els southwest, with the barrier to the right of the flow. As the strong barrier wind
approaches Cape Farewell and the topography decreases in elevation, the pressure
gradient force away from the barrier decreases and eventually disappears as the
wind passes the end of the island. The wind speed remains high, meaning a large
Coriolis force that turns the flow anticyclonically (Outten, Renfrew, and Petersen,
2009 ). The anticyclonic flow has an outward centrifugal pulling in the same direc-
tion as the synoptic-scale pressure gradient force, leading to further acceleration of
the wind. More recent work (Moore, 2012 ; Moore and Pickert, 2012 ) shows that
tip jets and barrier flow occur also in other regions along the Greenland coast with
steep topography, parts of the northern Bering Sea, and probably also in Norway,
Iceland, and Svalbard.
8.1.5
Precipitation, Accumulation and Sublimation
Some aspects of precipitation and accumulation over the ice sheet were outlined in
Chapter 6 . Accumulation basically represents the net effects of direct precipitation,
its redistribution on the surface via wind scour and drifting, and mass losses attrib-
uted to melt and evapo-sublimation. Accumulation is typically assessed via snow
pits or ice cores. Based on coastal station observations of precipitation, adjusted for
wind speed, and accumulation data from recent ice cores, the annual precipitation
averaged over the ice sheet is estimated to be 340 mm (Ohmura et al., 1999 ). On
average, only 40 percent of the total precipitation at the coastal stations is in solid
form. However, at Danmarkshavn, this figure rises to 83 percent. There are zones
of maximum precipitation exceeding 2,000 mm in the southeast coastal area and
600 mm in the northwest. Amounts in the north-central area are around 100 mm.
The southeastern maximum is strongly influenced by orographic uplift of southeast-
erly flow associated with traveling cyclones. The northwestern maximum is related
to flow off northern Baffin Bay and uplift. A trend surface analysis of accumulation
data (van der Veen, Bromwich, and Castho, 2001 ) indicates that 80 percent of the
spatial variance in average accumulation is a result of the large-scale atmospheric
circulation and its interaction with the ice sheet topography. From regression
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