Formation of ice supersaturation by mesoscale gravity waves

[摘要] We investigate the formation and evolution of an ice-supersaturated region(ISSR) that was detected by means of an operational radiosonde soundinglaunched from the meteorological station of Lindenberg on 21 March 2000,00:00 UTC. The supersaturated layer was situated below the local tropopause,between 320 and 408 hPa altitude. Our investigation uses satellite imageryfrom METEOSAT and the Advanced Very High Resolution Radiometer (AVHRR) andanalyses of the European Centre for Medium-Range Weather Forecasts (ECMWF).Mesoscale simulations reveal that the ISSR was formed by a temporary verticaluplift of upper tropospheric air parcels by 20 to 40 hPa in 1 to 2 h. Thisresulted in a significant local increase of the specific humidity by themoisture transport from below. The ascent was triggered by the superpositionof two internal gravity waves, a mountain wave induced by flow past theErzgebirge and Riesengebirge south of Lindenberg, and an inertial gravitywave excited by the anti-cyclonically curved jet stream over the BalticSea. The wave-induced ISSR was rather thick with a depth of about 2 km. Thewave-induced upward motion causing the supersaturation also triggered theformation of a cirrus cloud.METEOSAT imagery shows that the cirrus cloudgot optically thick within two hours. During this period another longerlasting thin but extended cirrus existed just beneath the tropopause.Thewave-induced ISSR disappeared after about half a day in accordance with thedecaying wave activity.

[学科分类] 大气科学