Temperature of Polar Stratospheric Clouds formation in the Arctic and Antarctic
Résumé
Polar Stratospheric Clouds (PSCs) induce a remarkable reddenings of the sky at twilight. Such reddening has been observed regularly by the SAOZ UV-visible spectrometers operating in polar regions in Dumont d'Urville in Antarctica and Sodankyla in Finland. The PSC detection is based on a Color Index (CI) derived from the ratio of sunlight scattered at zenith at 550 to 350 nm. The cloud altitude is retrieved from the SZA (Sun Zenith Angle) of maximum CI, after calibration by comparison with PSCs altitude observed by the CALIPSO lidar in orbit since 2006, a ground-based lidar in Dumont d'Urville and backscatter sondes balloon flights in Sodankyla. The temperature threshold at which PSCs can form is then investigated using the ECMWF model at the cloud level. Shown will be the statistics based on more than 20 years of observations in Sodankyla in Finland and in Dumont d'Urville in Antarctica. The PSC threshold temperature is found to be lower by about 5K in the Antarctic than in the Arctic, which is consistent with the expected stronger de-nitrification and dehydration of the southern winter vortex compared to the northern.