Geoscience Reference
In-Depth Information
4.6
Polar Lows
4.6.1
Definition
Polar lows are intense maritime mesocyclones of typically 100-500 km in diam-
eter. They may intensify rapidly, and surface winds speeds can sometimes reach
hurricane force (Businger and Reed,
1989
). They tend to be short lived, generally
lasting only three to thirty-six hours. Polar Lows are the most intense category of
the family of mesoscale cyclonic vortices found poleward of the main polar front,
which are known generically as polar mesoscale cyclones. Polar Lows, which can
present significant hazards to shipping, are also known as Arctic Instability Lows,
comma clouds, and - in the Southern Hemisphere - Antarctic coastal vortices. In
the north polar regions, Polar Lows are particularly common in the Nordic Seas, the
Labrador Sea, the Bering Sea, the Gulf of Alaska, and the Sea of Japan. Hundreds
of mesoscale lows can develop in these regions on an annual basis (Renfrew,
2003
).
Satellite images of two Polar Lows appear in
Figures 4.14
and
4.15
. The charac-
teristics of Polar Lows have been examined in a number of studies. Papers by E.
Rasmussen (
1979
), Shapiro, L. Fedor, and T. Hampel (
1987a
), S. Businger and
R. Reed (
1989
), K. Emanuel and R. Rotunno (
1989
), S. Grønas and N. Kvamstø
(
1995
) and A. Carleton (
1996
) are worth reading. Rasmussen and J. Turner (
2002
)
provide a comprehensive assessment of Polar Lows while Renfrew (
2003
) provides
a useful shorter review, which we draw from here.
4.6.2
Development Processes
As outlined by Renfrew (
2003
), Polar Lows develop over open water. When moving
over land or the sea ice cover, they tend to rapidly dissipate. They can be thought
of as “hybrid” systems, typically having features both baroclinic and convective in
nature. A common feature of Polar Lows seen in satellite imagery is a spiral cloud
(comma cloud) signature. Some systems develop a clear eye at the center similar to
tropical cyclones (
Figure 4.14
). Generally, Polar Lows are warm-cored, and many
have well-defined fronts as seen in extratropical, synoptic-scale lows. Purely baro-
clinic or topographically forced systems tend to remain fairly weak. Rapid intensi-
fication of an intense Polar Low seems to require some element of convection. The
preferred areas for Polar Low development mentioned previously are those that are
commonly subject to cold polar outbreaks, where cold continental air is advected
over relatively warm open water - conditions favoring convection. Consequently,
Polar Lows are essentially cold-season phenomena.
Two related mechanisms have been proposed to explain Polar Lows. The first is
CISK (Conditional Instability of the Second Kind), which emphasizes the organi-
zation of cumulonimbus convection. An initial disturbance causes low level con-
vergence and ascent, which in a conditionally unstable atmosphere, results in latent
heat release through convection. The latent heat release leads to a drop in surface
pressure, favoring more cyclonic relative vorticity, more low-level convergence and,
