Validation and time series analysis of global stratospheric data sets
The depletion of ozone from the stratosphere and the impact of global warming have been identified as phenomena caused as a result of anthropogenic activity. Scientists are thus trying to find quicker and more reliable ways to analyse these environmental problems. Remote sensing of the atmosphere using satellites is perhaps the most cost effective method. One beneficial application is the acquisition of vertical profiles of atmospheric species on a global scale with good spatial and temporal resolutions. An example of an instrument that can achieve this is the Sub-Millimetre Radiometer (SMR) aboard the Odin satellite that launched in 2001, which retrieves vertical limb profiles of numerous trace gases, including stratospheric ozone. This thesis presents a validation summary for primarily the Odin/SMR ozone data products, v1.2, v2.0 and v2.1 (for the 501.8 GHz band in the microwave region). Comparisons are made to v4.61 data obtained from the Interferometer for Passive Atmospheric Soundings (MIPAS) instrument, on board the ENVISAT satellite. Further analysis compares Odin/SMR mixing ratios to balloon sonde data. It is suggested from the results found here that Odin/SMR v2.1 ozone data should be used for scientific purposes. Odin/ SMR is also compared to its sister instrument, the Optical Spectrograph Infra-Red Imaging System (Odin/OSIRIS). Results show an excellent agreement between coincident profiles (typically < 10%). The fact that the two fundamentally different measurement geometries agree so well, provides confidence in the robustness of both techniques. Data assimilation is also shown to be a viable application for validation of satellite measurements when very few coincident data are available between satellite measurements and balloon sondes.
The second topic covered in this thesis concerns time series analysis. One study involves modelling ozone’s evolution using linear regression. It is shown that by combining various atmospheric data sets that it is possible to estimate trends of stratospheric ozone, using an example of six satellite instruments covering 1979-2008. It is found that the upper stratosphere (35-45 km) at mid-latitudes shows the most promising signs of ozone recovery (1.7%/decade), although trend values are found not to be significant at a 95% confidence level. A similar method is applied to other shorter data sets of various other molecules such as; water vapour (H2O), chlorine monoxide (ClO), and hydrochloric acid (HCl), all three of which play important roles in the stratosphere and ultimately the recovery of stratospheric ozone. Trend analyses of ClO and HCl show that since the late 1990s abundances of these molecules have reduced in the upper stratosphere (35-45 km). The results found here confirm how effective the 1987 Montreal protocol and its amendments have been in reducing the total amount of stratospheric chlorine.