Short-term variations in middle atmospheric ozone induced by solar forcing

Abstract

Ozone is one of the most publicly discussed atmospheric trace gases since the discovery of the ozone hole over Antarctica in the mid 1980s. The purpose of this thesis is to investigate the solar influence on ozone of the middle atmosphere, with the focus being on variations on small time scales of less than a month. On the one hand the modulating nature of the 27-day solar rotation signal on stratospheric ozone using a new ozone profile data set from SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) is studied. In this context, common and new frequency analysis techniques help to unravel dominant signals. On the other hand data from the latest version 1.07 Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) tropical ozone (1.27 micrometer as well as the 9.6 micrometer retrieval) and temperature data are studied with respect to daytime variations in the upper mesosphere.The frequency analysis of the 27-day solar rotation signal in SCIAMACHY ozone data (< 20_ latitude, 20-60 km altitude, 2003-2008) was carried out with the help of commonly used tools, like the fast-Fourier transform (FFT) and cross-correlation (CC), but was also supplemented with the continuous wavelet transform (CWT), which was not used before in the analysis of satellite ozone data. The CC showed that the maximum correlation between the Mg II index (used in this thesis as solar proxy) and ozone is weaker during the maximum of solar cycle 23 (r = 0.38) than in the previous two solar cycles that have been investigated in earlier studies using different data sets. The magnitude of the ozone signal is highly time dependent and may vanish for several solar rotations even close to solar maximum conditions. The FFT analysis reveals, besides the 27-day signal, several frequencies close to 27-days. The ozone sensitivity (i.e. ozone change in % per 1 % change in 205 nm solar flux) is on average about 0.2 %/% above 30 km altitude and smaller by about a factor of two compared to earlier studies. For selected three month periods the sensitivity may rise beyond 0.6 %/% in better agreement with earlier studies. The analysis of the 27-day solar rotation signal was also carried out with stratospheric European Centre for Medium range Weather Forecast (ECMWF) temperature data from the operational analysis covering the same period as the SCIAMACHY data (2003-2008). Although direct radiation effects on temperature are weak in the upper stratosphere, temperature signals with statistically significant periods in the 25-35 day range similar to ozone could be found with the applied methods.In addition to the 27-day solar rotation signal, the investigation of the SABER tropical mesospheric ozone and temperature daytime variations was combined with a comparison to the output of the three-dimensional general circulation and chemistry Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA). The results show good agreement for ozone. The amplitude of daytime variations is in both cases approximately 60 % of the daytime mean. During equinox the daytime maximum ozone abundance is for both, the observations and the model, higher than during solstice, especially above 80 km. Furthermore, HAMMONIA output of daytime variation patterns of several other different trace gas species, e.g., water vapor and atomic oxygen, is discussed with respect to the daytime pattern in ozone. In contrast to ozone, temperature data show little daytime variations between 65 and 95 km and their amplitudes are on the order of less than 1.5 %. At last, SABER and HAMMONIA temperatures show significant differences above 80 km in their daytime pattern.