On the Mechanism for the, Development of Polar Lows

Abstract

To evaluate the relative importance of different mechanisms responsible for the formation of polar lows, we have developed a three-layer, two-dimensional, quasi-geostrophic model, which includes both the effects of latent heating and baroclinity. Latent heating was parameterized for both stable precipitation associated with moist baroclinic processes as well as for convective precipitation associated with Conditional Instability of the Second Kind (CISK). Seven case studies and other observational studies are used to demonstrate that CISK or dry baroclinity does not individually provide the necessary forcing to allow the instability to grow to the observed wavelengths at the observed rates. It is found that moist baroclinic processes alone may explain the origin of Pacific polar lows, while moist baroclinity and CISK are essential in the genesis of Atlantic polar lows. Thus, there appears to be two types of polar lows. Sensitivity studies reveal the importance of low-level shear for the Atlantic disturbances. A further investigation with the analytic model used Lau's wintertime climatological data to find correlations between growth rates and preferred scales of disturbances and local effects such as latitude and climatology of the base state. The model predicts the preferred geographic regions where polar lows develop—in the Atlantic in the vicinity of Greenland/Iceland, and in the northern Pacific. We suggest that, because the former region is one of the resting places of Sander's “bombs,” polar lows in the Atlantic may result from the residual circulations or baroclinity of occluded systems. As these incipient disturbances move over warmer waters, conditionally unstable lapse rates develop in the lower troposphere and the CISK mechanism then becomes potentially important.