9 June 2010 • Rare is the weather feature that can maintain its identity over a period of weeks. Steven Cavallo examined one such feature—tropopause polar cyclones—for his dissertation at the University of Washington. While a postdoctoral researcher at NCAR, Cavallo translated his doctoral work on these cyclones, dozens of which form each year, into several journal articles. The lows, usually located to the north of the polar jet stream a few kilometers above ground level, are little studied but a potentially significant factor in generating wintertime cyclones. Monthly Weather Review recently published the first of these papers, which were written by Cavallo and his Ph.D. adviser at UW, Gregory Hakim. Cavallo is now at the University of Oklahoma.
How important are tropopause polar lows to weather across North America?
They’re important because they are often precursors to winter storms. They don’t always have a surface cyclone with them, but when they move over a baroclinic zone with strong temperature and moisture contrasts—say, the east coast of the United States—they can play a role in the formation of surface lows. Their mean radius is about 450 kilometers (280 miles), but it can range anywhere from really tiny to over 1,000 km (620 mi).
What kind of weather occurs beneath a tropopause polar low?
In North America, it would be the type of weather associated with other upper-level lows. It’s often cold, with showers and possibly snow. The lows most frequently affect the United States in the winter because the jet stream can more easily pull the lows down from the Arctic. I’ve seen them move as far down as the Gulf Coast. Some of the major East Coast nor’easters have been associated with these features, including the Superstorm of 1993 and the Blizzard of 2006.
What have you learned about how the lows form and maintain themselves?
Diabatic mechanisms and radiative properties are important. Because the tropopause is lower in the middle of the vortex, it enhances the vertical water vapor gradient. That sends more long-wave radiation upward, which helps the lows intensify. That’s really the main thing happening. There’s also a little bit of latent heating, which can help in the Arctic. The latent heating rates are low because of cold temperatures, but you can still get clouds that further promote the radiative effect.
Is there any hope that identifying these lows could help provide advance notice of the potential for U.S. winter storms?
If you analyze potential temperature and potential vorticity, you can actually track some of these lows for weeks and months. There was one in the summer of 2006 in the Arctic that lasted for three months. Because of their smaller scales, these lows are hard to resolve and predict accurately in models. If they’re not resolved correctly in the models, then it can throw off the timing of storms. In the longer term, a small displacement can lead to big forecast errors downstream. Depicting these lows could definitely help improve predictions of winter weather.