Mesospheric measurements using microwave spectroscopy

Abstract: The mesosphere is an altitude region of the atmosphere, covering altitudes between about 50 km to 100 km. Improved understanding of this area of the atmosphere offers possibilities of increasing the accuracy of weather forecast and climate models. Continuous measurements at these altitudes are difficult as neither balloon nor airplanes can reach such heights. However, by remotely measuring thermal emission emitted in the microwave region, properties such as the atmospheric temperature and the abundance of species such as water vapour and ozone, can be determined from instruments placed on the ground or on satellites. In this thesis microwave spectroscopy is used to measure carbon monoxide above the Onsala Space Observatory located south of Gothenburg, Sweden. The results of the measurements are compared to, and found in good agreement with, co-located data from satellite instruments. A new inversion technique is also demonstrated which integrates the temporal averaging of spectra directly into the retrieval. This shows that ground-based microwave instruments are well suited for mesospheric monitoring, which will become more important in the future as many of the satellite instruments used in the comparison are slated for retirement, with no immediate replacements planned. Microwave spectroscopy can also be used in studies related to polar mesospheric clouds. These are clouds that form in the uppermost part of the summer mesosphere and are extremely sensitive to temperature changes in the background atmosphere. In this thesis tomographic measurements from the sub-millimetre radiometer on-board the Odin satellite are used to retrieve 2-D images of the background temperature and water vapour around these clouds. Combining these results with measurements of ice content and ice particle sizes in the clouds from Odin's other instrument OSIRIS, allows us to investigate the relationship between the horizontal and vertical features in the clouds and their background atmosphere with a hitherto unsurpassed resolution.