Abstract
Abstract. Volume mixing ratio water vapour profiles have been retrieved from IASI (Infrared Atmospheric Sounding Interferometer) spectra using the MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water) processor. The retrievals are done for IASI observations that coincide with Vaisala RS92 radiosonde measurements performed in the framework of the GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN) in three different climate zones: the tropics (Manus Island, 2° S), mid-latitudes (Lindenberg, 52° N), and polar regions (Sodankylä, 67° N). The retrievals show good sensitivity with respect to the vertical H2O distribution between 1 km above ground and the upper troposphere. Typical DOFS (degrees of freedom for signal) values are about 5.6 for the tropics, 5.1 for summertime mid-latitudes, 3.8 for wintertime mid-latitudes, and 4.4 for summertime polar regions. The errors of the MUSICA IASI water vapour profiles have been theoretically estimated considering the contribution of many different uncertainty sources. For all three climate regions, unrecognized cirrus clouds and uncertainties in atmospheric temperature have been identified as the most important error sources and they can reach about 25 %. The MUSICA IASI water vapour profiles have been compared to 100 individual coincident GRUAN water vapour profiles. The systematic difference between the data is within 11 % below 12 km altitude; however, at higher altitudes the MUSICA IASI data show a dry bias with respect to the GRUAN data of up to 21 %. The scatter is largest close to the surface (30 %), but never exceeds 21 % above 1 km altitude. The comparison study documents that the MUSICA IASI retrieval processor provides H2O profiles that capture the large variations in H2O volume mixing ratio profiles well from 1 km above ground up to altitudes close to the tropopause. Above 5 km the observed scatter with respect to GRUAN data is in reasonable agreement with the combined MUSICA IASI and GRUAN random errors. The increased scatter at lower altitudes might be explained by surface emissivity uncertainties at the summertime continental sites of Lindenberg and Sodankylä, and the upper tropospheric dry bias might suggest deficits in correctly modelling the spectroscopic line shapes of water vapour.
Highlights
Atmospheric water plays a key role in the atmospheric energy balance and temperature distribution via radiative effects and latent heat transport
The retrievals are done for IASI observations that coincide with Vaisala RS92 radiosonde measurements performed in the framework of the GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN) in three different climate zones: the tropics (Manus Island, 2◦ S), mid-latitudes (Lindenberg, 52◦ N), and polar regions (Sodankylä, 67◦ N)
For the comparison study we proceed and examine bias and scatter, which means that we describe the variance in the MUSICA IASI data by the variance in the GRUAN data and the variance in the difference between MUSICA IASI and GRUAN
Summary
Atmospheric water plays a key role in the atmospheric energy balance and temperature distribution via radiative effects (clouds and vapour) and latent heat transport. Understanding the coupling between moisture transport, clouds, and atmospheric dynamics is seen as a major challenge for improving atmospheric models (Stevens and Bony, 2013) In this context the global monitoring of the water vapour distribution is important, whereby the large inhomogeneity in time and space (horizontally and vertically) is challenging. In this paper we perform a detailed theoretical error assessment and an empirical validation of the water vapour profiles as generated by the MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water Schneider et al, 2016) IASI retrieval processor.
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