In this study, the reprocessing of 18 years of mesospheric \(H_2O\) and temperature measurements from the Sub-Millimetre Radiometer (SMR) aboard the Odin satellite has been presented. Satellite observations of \(H_2O\) in the middle atmosphere have been performed since the 1970s by several instruments such as LIMS (Limb Infrared Monitor of the Stratosphere), SAMS (Stratospheric and Mesospheric Sounder), SABER (Sounding of the Atmosphere using Broadband Emission Radiometry), ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer), MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation), MLS (Microwave Limb Sounder), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography). Together with \(H_2O\), temperature is also a retrieval product obtained from the same spectra and it represents another useful tool to study the middle atmospheric dynamics.
Two different frequency modes (FMs) of SMR have been used in this study. FM13 refers to the frequency range 556.598-557.398 GHz and FM19 refers to frequency range of 556.550-557.350 GHz. To evaluate the quality of the new FM13 and FM19 data, the SMR v3.0 \(H_2O\) and temperature retrievals from these FMs with nearly coincident measurements from MIPAS, ACE-FTS and MLS are compared. The absolute and relative difference between coincident profiles are defined as:
Where \(x\) is the retrieved \(H_2O\) or temperature, \(z\) is the altitude and \(i\) is the profile number. Measurements done by satellite instruments suffer from large uncertainties so, their relative difference is calculated with respect to the mean of the two measurements, to avoid preference towards one instrument or the other.
The figures below show the summary of the results of all the comparisons done in this study. Previous data versions (v2.1) of SMR showed large biases to other instruments. After accounting for an underestimation of leakage of the image sidebands in the nominally single-sideband receivers, improved the new SMR version (v3.0). Globally averaged SMR v3.0 FM13 \(H_2O\) concentrations present relative differences within ±20 % between 45 and 80 km altitude. SMR is in very good agreement with ACE-FTS and MLS up to 70 km, with relative differences within 0 % and −5 %. Temperature absolute differences shows an improvement of about 5 K at all observed altitudes for the new v3.0 compared to v2.1.
Reference
Grieco, F., Pérot, K., Murtagh, D., Eriksson, P., Rydberg, B., Kiefer, M., Garcia-Comas, M., Lambert, A., and Walker, K. A.: Improvement of Odin/SMR water vapour and temperature measurements and validation of the obtained data sets, Atmos. Meas. Tech., 14, 5823–5857, https://doi.org/10.5194/amt-14-5823-2021, 2021.