10.4 Vortex intensification of tropopause polar cyclones

Thursday, 11 June 2009: 11:20 AM
Pinnacle BC (Stoweflake Resort and Confernce Center)
Steven M. Cavallo, Univ. of Washington, Seattle, WA; and G. J. Hakim

Tropopause polar vortices (TPVs) are coherent structures based on the tropopause in the high latitudes that play an active role in extratropical weather. The Arctic is a particularly favorable region for these features due to its isolation from the strong shears associated with jet streams, which allows the vortices to last in some cases longer than one month. Despite their abundance and longevity, there remain a number of fundamental questions regarding the controls on their intensity and role in Arctic climate. In order to quantify the dynamics of intensity change, we adopt a budget approach based on the Ertel potential vorticity applied to numerical modeling experiments based on observed and idealized data.

Composite results for cyclonic TPVs over the Canadian Arctic show a tendency for intensification by cloud-top radiational cooling. This tendency is partially offset by latent heating, which contributes to weakening the average vortex. Radiation effects dominate latent heating in this location, such that cyclonic TPVs strengthen on average. We hypothesize that this result is due to the low temperatures, and lower water vapor saturation mixing ratios, of the Arctic troposphere, which limits latent heating and cloud optical thickness. In order to test this hypothesis and better understand the competition between radiation and latent heating, idealized numerical modeling experiments are performed for basic states derived from observed data. Results reveal the important role of both the dynamical and physical processes in the controlling the intensity of these vortices.

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