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The importance of polar stratospheric clouds (PSC) for polar ozonedepletion is well established. Lidar experiments are well suited toobserve and classify polar stratospheric clouds. On 5 January 2005 aPSC was observed simultaneously on the east and west sides of theScandinavian mountains by ground-based lidars. This cloud wascomposed of liquid particles with a mixture of solid particles inthe upper part of the cloud. Multi-colour measurements revealed thatthe liquid particles had a mode radius of r≈300 nm, adistribution width of σ≈1.04 and an altitudedependent number density of N≈2–20 cm−3. Simulationswith a microphysical box model show that the cloud had formed about20 h before observation. High HNO3 concentrations in the PSCof 40–50 weight percent were simulated in the altitude regionswhere the liquid particles were observed, while this concentrationwas reduced to about 10 weight percent in that part of the cloudwhere a mixture between solid and liquid particles was observed bythe lidar. The model simulations also revealed a very narrowparticle size distribution with values similar to the lidarobservations. Below and above the cloud almost no HNO3 uptake wassimulated. Although the PSC shows distinct wave signatures, nogravity wave activity was observed in the temperature profilesmeasured by the lidars and meteorological analyses support thisobservation. The observed cloud must have formed in a wave fieldabove Iceland about 20 h prior to the measurements and the cloudwave pattern was advected by the background wind to Scandinavia. Inthis wave field above Iceland temperatures potentially dropped belowthe ice formation temperature, so that ice clouds may have formedwhich can act as condensation nuclei for the nitric acid trihydrate(NAT) particles observed at the cloud top above Esrange

annales geophysicae

Simultaneous lidar observations of a polar stratospheric cloud on the east and west sides of the Scandinavian mountains and microphysical box model simulations