Geoscience Reference
In-Depth Information
Figure 9.17
Schematic illustrations of
vortex developments in satellite imagery.
The sequences run from bottom to top.
Left. Comma cloud (c) developing in a
polar airstream. Centre. Instant occlu-
sion from the interaction of a polar
trough with a wave on the polar front.
Right. The classical frontal wave with
cold and warm conveyor belts (CCB,
WCB). C = enhanced convection; D
= decaying cloud band; cloud cover
stippled.
Source
: After Browning (1990) by per-
mission of the American Meteorological
Society.
when a primary depression is situated in the Davis Strait
and a breakaway wave forms south of Cape Farewell
(the southern tip of Greenland), moving away eastward.
Analogous developments take place in the Skagerrak-
Kattegat area when the occlusion is held up by the
Scandinavian mountains.
they are displaced equatorward in winter, when the
Atlantic frontal zone may extend into the Gulf of
Mexico. Here there is convergence of airmasses
of different stability between adjacent subtropical
high-pressure cells. Depressions developing here com-
monly move northeastward, sometimes following or
amalgamating with others of the northern part of the
polar front proper or of the Canadian Arctic front.
Frontal frequency remains high across the North
Atlantic, but it decreases eastward in the North Pacific,
perhaps owing to a less pronounced gradient of sea-
surface temperature. Frontal activity is most common in
the central North Pacific when the subtropical high is
split into two cells with converging airflows between
them.
Another section of the polar front, often referred
to as the
Mediterranean front
, is located over the
Mediterranean-Caspian Sea areas in winter. At inter-
vals, fresh Atlantic mP air, or cool cP air from southeast
Europe, converges with warmer airmasses of North
African origin over the Mediterranean Basin and
F ZONES OF WAVE DEVELOPMENT
AND FRONTOGENESIS
Fronts and associated depressions tend to develop in
well-defined areas. The major zones of frontal-wave
development are areas that are most frequently baro-
clinic as a result of airstream confluence (Figure 9.18).
This is the case off East Asia and eastern North
America, especially in winter, when there is a sharp
temperature gradient between the snow-covered land
and warm offshore currents. These zones are referred
to as the Pacific polar and Atlantic polar fronts, respec-
tively (Figure 9.19). Their position is quite variable, but
