Techniques for the validation of Odin and ENVISAT satellite data
Licentiate thesis, 2007
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 for Odin/SMR ozone data products, V1.2, V2.0 and V2.1 (for the 501.8 GHz band in the microwave region). Comparisons were made to V4.61 data from the Interferometer for Passive Atmospheric Soundings (MIPAS) instrument, on board the ENVISAT satellite. Further analysis was carried out by comparing Odin/SMR mixing ratios to balloon sonde data. By validating these three versions of Odin/SMR data, we can establish if any further adjustments need be made to the settings and calibrations of the instruments for the production of subsequent data sets. Such an investigation can be achieved either by matching coincident data using time and distance constraints directly or by using a data assimilation model
The latest and current version, V2.1, of Odin/SMR showed comparisons with MIPAS V4.61 and balloon sonde data to have the smallest systematic differences with respect to the previous two versions, especially below 25 km. In this lower stratospheric region, V2.1 was only slightly smaller than MIPAS by less than 0.25 ppmv, while comparisons to sonde measurements showed an agreement within +/-0.3 ppmv. The largest systematic differences for this version were seen in the tropics, where a bias of ~0.7 ppmv between 25-37 km was found.
An analysis was done in order to prove that data assimilation is a valid method for validation of satellite measurements when very few coincident data are available between satellite measurements and balloon sondes. Results showed that mean differences between the Isentropic Assimilation model For Stratospheric Ozone (IASCO) produced ozone profiles and co-located ozone sondes contained systematic differences similar to those obtained from co-located MIPAS and ozone sonde coincidences. Spatial and temporal constraints were used to obtain the MIPAS/sonde matches, where 12 hours and 800 km produced the best results for statistical analysis. The largest residual between the IASCO/sonde mean difference and MIPAS/sonde mean difference using these constraints was less than 0.25 ppmv between potential temperature altitudes of 425-975 K