Deborah Hanley

Thesis Approved 1993

A Case Study of a Polar Low in the Laborador Sea

Polars lows are intense cyclonic systems that form poleward of major jet streams or frontal zones and are mainly convective in nature. Their small spatial scale (<500km) and time scale (<24 hours) are beyond the forecast capabilities of present operational numerical models. Recent climatological studies have shown that polar lows are a common feature of the Labrador Sea. Because of the potential for loss of life and vessel damage due to high winds and heavy precipitation associated with polar lows, it is important to improve the ability of numerical forecast models to forecast these systems.

The operational (100km) version of the Canadian Regional Finite Element Model was used to simulate a polar low that occurred in the Labrador Sea on January 11, 1989. The sensitivity of the model development to resolution was tested using 50km and 25km versions of this model. Enhanced sea-surface temperature analyses were used to test the sensitivity to surface fluxes of heat and moisture.

Increasing the model resolution resulted in a more intense polar low development. It was also found that the storm development depended strongly upon the sea-surface temperature analysis. This indicates the importance of fluxes of heat and moisture from the sea surface in the development of polar lows in this area.

An additional experiment was performed using the newest version of the RFE (1992) run at 25km resolution. This model incorporated four additional sigma layers, increasing the number in the boundary layer from 6 to 7 layers, and a much larger central high resolution domain. Because polar lows are shallow systems, increasing the vertical resolution resulted in better development. A conceptual model of the Labrador Sea polar low was created based on the output from this experiment.

The conceptual model showed that the polar low was generated in a region of lee troughing from the mountains of Baffin Island and Labrador and the incipient surface low was triggered by an upper-level short-wave. Intensification of the incipient low occurred as the result of the influence of diabatic heating. Comparisons were made to existing conceptual models of polar lows in other areas.

Debbie is currently studying for her Ph.D at the State University of New York at Albany. She can be reached by e-mail at DH7077%ALBNYATM.bitnet@UACSC2.ALBANY.EDU