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 4.15). Generally, Polar Lows are warm cored, and many have well-defined fronts as seen in extratropical, synoptic-scale lows. Purely baroclinic or topographically forced systems tend to remain fairly weak. Rapid intensification of an intense Polar Low seems to require some element of convection. The preferred areas for Polar Low development mentioned above 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.

Figure 4.16 NOAA-12 image for January 19, 1998 of a Polar Low. Norway is on the right side of the image, with Iceland to the left side. Note the comma-shaped cloud band in this example (by permission of the NERC Satellite Receiving Station, University of Dundee, http//www.sat.dundee.ac.uk/).

Figure 4.16 NOAA-12 image for January 19, 1998 of a Polar Low. Norway is on the right side of the image, with Iceland to the left side. Note the comma-shaped cloud band in this example (by permission of the NERC Satellite Receiving Station, University of Dundee, http//www.sat.dundee.ac.uk/).

Two related mechanisms have been proposed to explain Polar Lows. The first is CISK (conditional instability of the second kind), which emphasizes the organization of cumulonimbus convection. An initial disturbance causes low-level convergence 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, through continuity, high-level divergence. The low-level convergence provides for a continued source of moisture and convective latent heat release - a feedback process that can lead to rapid development. It seems that cold polar outbreaks over open water in high latitudes can provide for the deep layer of conditional instability associated with the CISK process.

The second mechanism is known as WISHE (wind induced surface heat exchange). WISHE is a feedback between the circulation and surface sensible and latent heat fluxes from the sea surface, with a stronger circulation giving rise to larger surface fluxes of heat, which are then redistributed aloft by convection, in turn strengthening the circulation. The feedback arises in that the surface heat fluxes are proportional to the wind speed - turn on the winds, and the heat flux strengthens. The redistribution by convection results in a pressure drop, further strengthening the winds and hence the surface heat fluxes. WISHE can explain the growth of both tropical cyclones (hurricanes) and Polar Lows.

In WISHE, the emphasis is placed on the surface fluxes as the primary growth-limiting process - the convection only serves to redistribute the heat. This contrasts with CISK, which emphasizes that the circulations amplify through their interaction with the convection itself. However, in the real world the two mechanisms can be difficult to distinguish from each other. The common feature of both processes in Polar Low development, however, is that one needs an initial disturbance. The presence of some sort of baroclinic disturbance provides the key. Off East Greenland, mesocyclone development also appears to be forced by the effects of the ice sheet topography and low-level katabatic winds (Chapter 8). Klein and Heinemann (2002) show from simulations with a mesoscale model that channeling of the flow in the large valleys of East Greenland around Angmagssalik leads to convergence and cyclonic vorticity generation. This can be enhanced during strong katabatic events that supply cold air, augmenting low-level baroclinicity. These katabatic winds are themselves favored by the presence of a synoptic-scale low in the Greenland Sea. They propose that such synoptic support is important because the pure katabatic wind layer is only a few hundred meters deep. A positive feedback on the katabatic system can occur when a mesocyclone develops off the coast.

Numerical simulation of Polar Lows is still immature. Renfrew (2003) also points out that polar mesocyclones, including Polar Lows, may play an important role in the high-latitude climate system through strong coupling of the atmosphere and ocean via air-sea heat exchange.

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