Polar stratospheric cloud observations by MIPAS on ENVISAT: detection method, validation and analysis of the northern hemisphere winter 2002/2003

摘要

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onENVISAT has made extensive measurements of polar stratospheric clouds (PSCs)in the northern hemisphere winter 2002/2003. A PSC detection method based on aratio of radiances (the cloud index) has been implemented for MIPAS and isvalidated in this study with respect to ground-based lidar and space borneoccultation measurements. A very good correspondence in PSC sighting andcloud altitude between MIPAS detections and those of other instruments isfound for cloud index values of less than four. Comparisons with data from theStratospheric Aerosol and Gas Experiment (SAGE) III are used to further showthat the sensitivity of the MIPAS detection method for this threshold valueof cloud index is approximately equivalent to an extinction limit of10-3km-1 at 1022nm, a wavelength used by solar occultationexperiments. The MIPAS cloud index data are subsequently used to examine,for the first time with any technique, the evolution of PSCs throughout theArctic polar vortex up to a latitude close to 90° north on a near-dailybasis.We find that the winter of 2002/2003 is characterised by three phases of verydifferent PSC activity. First, an unusual, extremely cold phase in the firstthree weeks of December resulted in high PSC occurrence rates. This wasfollowed by a second phase of only moderate PSC activity from 5-13 January,separated from the first phase by a minor warming event. Finally there was athird phase from February to the end of March where only sporadic and mostlyweak PSC events took place. The composition of PSCs during the winter periodhas also been examined, exploiting in particular an infra-red spectralsignature which is probably characteristic of NAT. The MIPAS observationsshow the presence of these particles on a number of occasions in Decemberbut very rarely in January. The PSC type differentiation from MIPASindicates that future comparisons of PSC observations with microphysical anddenitrification models might be revealing about aspects of solid particleexistence and location.