Simulation study for the Stratospheric Inferred Winds (SIW) sub-millimeter limb sounder

Stratospheric Inferred Winds (SIW) is a Swedish mini sub-millimeter limb sounder selected for the 2 nd InnoSat platform launch planned near 2022. It is intended to fill the altitude gap between 30–70 km in atmospheric wind measurements and also aims at pursuing the limb observations of temperature and key atmospheric constituents between 10–90 km when current satellite missions are probably stopped. Line-of-sight winds are retrieved from the Doppler shift of the emission lines introduced by 5 the wind field. Observations will be performed with two antennas pointing toward the limb with perpendicular directions to reconstruct the 2-D horizontal wind vector. Each antenna has a vertical field of view of 5 km. The chosen spectral band near 655 GHz contains a dense group of strong O 3 lines suitable for exploiting the small wind information in stratospheric spectra. Using both sidebands of the heterodyne receiver, a large number of chemical species will be measured including O 3 -isopotologues, H 2 O, HDO, HCl, ClO, N 2 O, HNO 3 , NO, NO 2 , HCN, CH 3 CN and HO 2 . This paper presents the simulation study for assessing the measurement performances. The line-of-sight winds are retrieved between 30–90 km with the best sensitivity between 35–70 km where the precision (1-sigma) is 5–10 m s −1 for a single scan. Similar performances can be obtained during day and night conditions except in the lower mesosphere where the photo-dissociation of O 3 in day-time reduces the sensitivity by 50 % near 70 km. Profiles of O 3 , H 2 O and temperature are retrieved with a high precision up to 50 km ( −1 between 30–40 km can be induced by the air-broadening parameters uncertainties of O 3 lines. This highlights the need for a good knowledge of these parameters and to study methods to mitigate the retrieval bias.