It has been almost a year since early January 2010 when England was hit by snow storms – and now, just before Christmas 2010, the story seems to have a sequel. Airline passengers stranded in London were reminded that it is not only volcanoes that can impact our high-tech infrastructure, but also good old-fashioned "holiday" snow.
If it is any consolation, however, it was not only London that was impacted by cold and difficult weather conditions. The Atmospheric Infrared Sounder (AIRS) on board NASA’s Aqua satellite, by measuring atmospheric temperature profiles, revealed most of the Northern Hemisphere, in particular land areas north of 30° N, under very cold conditions (Figure 1).
Figure 1. Three-day average of surface air temperatures, retrieved by AIRS during the period December 17-19, 2010.
Figure 1 shows a three-day average of surface air temperatures as retrieved by AIRS for the period December 17-19, 2010. Extremely cold conditions persisted over the republic of Sakha, (Russian Federation), Greenland, and around the Alaska-Canadian border, which is perhaps not too much of a surprise to the locals. However, most of the normally moderate-climate European regions, including England, were well under the freezing point.
The AIRS-retrieved surface air temperatures in Figure 1 manifest three obvious troughs in the Northern Hemisphere's atmosphere, giving a hint that upper troposphere (300 mb, or at about 9 km altitude) Rossby waves may be the driving factor behind the bad weather observed hemispheres apart in both California and England. These particular Rossby waves are associated with kinks in the polar jet stream, which is located at roughly that altitude. AIRS has the ability to retrieve profiles of atmospheric temperatures up to the lower stratosphere (about 50 km altitude), and hence the 300-mb temperature retrieval may be a good proxy to reveal the polar jet stream and associated Rossby waves.
Figure 2 presents a three-day average of AIRS atmospheric temperatures, together with the wind vector field from NOAA’s Global Forecast System (GFS) – all at 300 mb – for the same time period as in Figure 1. The polar jet can be identified by following the 215 K temperature contours and the wind field, and it can be seen to take a southward dive around locations indicated with “L”. These dives are especially drastic off the coasts of California and England, which results in bringing cold Arctic air deep into the south, at the surface. The “L’’s indicate upper-level low-pressure troughs that can also be regarded as the troughs of the Rossby waves.
Thus, conditions in the upper troposphere command the weather at the surface, and because AIRS data carries valuable information about atmospheric state, forecasters use these data to improve their weather predictions.
Figure 2. Atmospheric temperature from AIRS, and wind vector field from GFC, all at 300-mb, for December 17-19, 2010