THE WINTER 2008-2009 MAJOR SUDDEN STRATOSPHERIC WARMING OBSERVED BY LIDAR AT THULE (76.5°N, 68.8°W), GREENLAND

Lidar measurements of atmospheric temperature profiles and aerosol backscatter ratio and depolarization have been carried out at Thule (76.5°N, 68.8°E), Greenland, in the period January - early March 2009. The Lidar, installed at Thule in 1990, is part of the Network for the Detection of Atmospheric Composition Change (NDACC). During winter 2008-2009, Lidar profiles were acquired on a regular basis with a maximum of 5-6 hours of measurements per day, except for a few periods characterized by poor weather conditions or instrumental problems. A total of 44 Lidar temperature profiles between 25 and 70 km were obtained during the measurement campaign. Radiosonde data obtained at the stations of Eureka (79.9°N, 85.9°W) and Alert (82.5°N, 62.3°W) were used to derive temperatures below 25 km. Lidar temperature profiles have permitted to show the evolution of the stratospheric thermal conditions. During the first part of the campaign, in mid-January 2009, the polar vortex was still present above Thule. A polar stratospheric cloud (PSC) of NAT particles was detected on January 17 and 18 between 17 and 19 km. The major sudden stratospheric warming (SSW) was observed during the second half of January. The warming affected the upper stratosphere (~ 40-45 km) first, and then propagated rapidly from the upper to the lower stratosphere. The temporal evolution of the stratospheric temperature was derived at fixed potential temperature levels between 500 and 1500 K. Lidar data show the first signs of the warming at the 1500 K level (~ 42 km) on 22 January, after a week of instrumental problems that prevented from carrying out measurements. After 2-3 days, the warming reached 1000 K (~ 34 km), 900 K (~ 32 km) and 800 K (~ 29 km), and after 5-6 days it reached 600 K (~ 23 km) and 500 K (~ 20 km). Comparison of Lidar data with CIRA model profiles indicates that during the SSW the measured temperature between 25 and 45 km altitude exceeded by 40-50 K the expected CIRA values, reaching a maximum of ~290 K at 40 km. The intensity peak of the SSW was observed between 22 and 24 January. The warming produced an abrupt and irreversible break of the polar vortex. Comparison of 2009 data with Lidar atmospheric temperature measurements obtained during several years between 1994 and 2007 indicates that the 2009 SSW was the strongest event ever observed by the Lidar at Thule.