Evaluation of MUSICA MetOp/IASI tropospheric water vapour profiles by theoretical error assessments and comparisons to GRUAN Vaisala RS92 measurements

摘要

Volume mixing ratio water vapour profiles have been retrieved from IASI (Infrared Atmospheric Sounding Interferometer) spectra by using the MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water) processor. The retrievals are made 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 HO distribution between 1–2 km above ground and the upper troposphere. Typical DOFS (degree of freedom for signal) values are about 5.5 for the tropics, 5.0 for summertime midlatitudes, 4.0 for wintertime mid-latitudes, and 4.5 for summertime polar regions. The errors of the IASI water vapour profiles have been theoretically estimated considering the contribution of a large number of uncertainty sources. For all three climate regions unrecognized cirrus clouds and uncertainties in atmospheric temperature have been identified as the most important error sources, whereby the total errors are estimated to be typically 25 %.The IASI water vapour profiles have been compared to 100 individual coinciding GRUAN water vapour profiles. The systematic difference between the IASI and GRUAN data is within 12 % at all altitudes. The scatter is largest close to the surface and close to the tropopause, but does never exceed 30 %. The study documents that the MUSICA MetOp/IASI retrieval processor provides HO profiles with good accuracy and captures the variations in HO volume mixing ratio profiles from 1–2 km above ground up to altitudes close to the tropopause with a precision that is in accordance to the theoretical error assessment.