Investigation of noctilucent cloud properties and their connection with solar activity

Abstract

Noctilucent clouds (NLC) are optically thin layers of water ice particles present near the summer mesopause at high latitudes. Because of their extraordinary height of about 83km, they can become visible to the naked eye when the sun sinks between 6 and 16 degrees below the horizon, providing a dazzling display of bluish light. The observation of NLC conveys unique information concerning the different processes taking place in the upper mesosphere. Since NLC are extremely sensitive to changes in temperature and water vapor content, it is possible, by studying the variation of NLC properties on different timescales, to gain insights into the various atmospheric processes in action near the summer mesopause. Understanding which physical mechanisms are important for this region of the atmosphere is also crucial for quantifying the possible change in NLC properties which could be attributed to anthropogenic activity.The purpose of this thesis is to analyze NLC properties measured from the space-borne SCanning Imaging Absorption spectroMeter for Atmospheric ChartograpHY (SCIAMACHY) instrument. Limb observations of NLC scattered sunlight, in the spectral range 264-300 nm, are exploited for the retrieval of NLC occurrence, radiance, and altitude. The good spectral resolution of the instrument also permits the determination of the NLC particle size, provided assumptions are made on the particle shape and size distribution. The modeling of the particle scattering is based on the T-matrix approach. The NLC particle size can only be unambiguously retrieved in the northern hemisphere due to the particular geometry of observation. A climatology of NLC occurrence frequency, radiance, altitude, and particle size is presented for years 2002-2009 and for both hemispheres. The seasonal variations of NLC properties are in good agreement with independent data sets. It is also shown that despite the coarse vertical resolution of SCIAMACHY, the daily NLC altitudes correlate well with the supersaturation levels, especially in the northern hemisphere. SCIAMACHY's global coverage grants the possibility of observing the response of NLC activity linked to various atmospheric phenomena such as the solar proton events, planetary waves, and the zonal dependence of gravity wave forcing. The dependence of the NLC particle size on latitude, local time, and altitude is also investigated. It is shown that the NLC particle radii increase with latitude, decrease with increasing altitude and are slightly larger for a local time of 9~PM compared with 11~AM. A study of the sensitivity of the retrieved NLC particle size on the assumptions is also presented, showing a large variation of the NLC particle size depending on the PSD width and the particle shape. The SCIAMACHY retrieved NLC particle radii are also compared with different independent particle size data sets, revealing how much information can be gained by merging different data sets together.Finally, the effect of the 27-day solar cycle on NLC properties is investigated. An analysis based on the SCIAMACHY and the Solar Backscattering UltraViolet (SBUV) data sets shows that there is a clear anti-correlation of NLC occurrence rate and albedo with solar activity on short timescale, while the NLC daily altitude is positively correlated with solar activity. The response of the NLC properties to the 27-day solar forcing is generally larger at higher latitudes. The results of the analysis with the SBUV data set reveal that the sensitivity of the NLC albedo to the 27-day solar cycle is similar to that of the 11-year solar cycle, suggesting that similar mechanisms may be responsible for the variation of NLC properties. Inspection of the temperature and water vapor from the Microwave Limb Sounder (MLS) indicates that temperature, not water vapor, is responsible for the variation in NLC properties.