Abstract. The sedimentation of HNO 3 containing Polar Stratospheric Cloud (PSC) particles leads to a permanent removal of HNO 3 and thus to a denitrification of the stratosphere, an effect which plays an important role in stratospheric ozone depletion. The polar vortex in the Arctic winter 2009/2010 was very cold and stable between end of December and end of January. Strong denitrification between 475 to 525 K was observed in the Arctic in mid of January by the Odin Sub Millimetre Radiometer (Odin/SMR). This was the strongest denitrification that had been observed in the entire Odin/SMR measuring period (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010). Lidar measurements of PSCs were performed in the area of Kiruna, Northern Sweden with the IRF (Institutet för Rymdfysik) lidar and with the Esrange lidar in January 2010. The measurements show that PSCs were present over the area of Kiruna during the entire period of observations. The formation of PSCs during the Arctic winter 2009/2010 is investigated using a microphysical box model. Box model simulations are performed along air parcel trajectories calculated six days backward according to the PSC measurements with the ground-based lidar in the Kiruna area. From the temperature history of the backward trajectories and the box model simulations we find two PSC regions, one over Kiruna according to the measurements made in Kiruna and one north of Scandinavia which is much colder, reaching also temperatures below T ice . Using the box model simulations along backward Correspondence to: F. Khosrawi (farah@misu.su.se) trajectories together with the observations of Odin/SMR, Aura/MLS (Microwave Limb Sounder), CALIPSO (CloudAerosol Lidar and Infrared Pathfinder Satellite Observations) and the ground-based lidar we investigate how and by which type of PSC particles the denitrification that was observed during the Arctic winter 2009/2010 was caused. From our analysis we find that due to an unusually strong synoptic cooling event in mid January, ice particle formation on NAT may be a possible formation mechanism during that particular winter that may have caused the denitrification observed in mid January. In contrast, the denitrification that was observed in the beginning of January could have been caused by the sedimentation of NAT particles that formed on mountain wave ice clouds.

Abstract. During the MaCWAVE winter campaign in January 2003, layers of enhanced echo power known as PMWE (Polar Mesosphere Winter Echoes) were detected by the ESRAD 52 MHz radar on several occasions. The cause of these echoes is unclear and here we use observations by meteorological and sounding rockets and by lidar to test whether neutral turbulence or aerosol layers might be responsible. PMWE were detected within 30 min of meteorological rocket soundings (falling spheres) on 5 separate days. The observations from the meteorological rockets show that, in most cases, conditions likely to be associated with neutral atmospheric turbulence are not observed at the heights of the PMWE. Observations by instrumented sounding rockets confirm low levels of turbulence and indicate considerable small-scale structure in charge density profiles. Comparison of falling sphere and lidar data, on the other hand, show that any contribution of aerosol scatter to the lidar signal at PMWE heights is less than the detection threshold of about 10%.

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[1] We report on the shape of particles in a noctilucent cloud (NLC) as deduced from measurements by groundbased lidar. The experiment was performed on August 3, 2000 using the ALOMAR Rayleigh/Mie/Raman lidar, located at 69°N, 16°E. Over a period of 74 min, the instrument performed high quality measurements of the polarization state of 532 nm laser light backscattered from NLC particles. From the experiment we derive that the observed depolarization, averaged over the altitude range 84.2 to 85.5 km was d NLC = (1.7 ± 1.0) %. Considering the small ratio of particle size over wavelength this is an unexpectedly large depolarization. The layer of enhanced depolarization was centered 1 km above the maximum of the NLC layer. We compare the observed depolarization with that calculated for cylinder-shaped NLC particles. The observed depolarization can be explained by the presence of elongated particles with a length-over-diameter ratio larger than 2.5.