Water cycle

OMEGA y SPICAM

Regarding OMEGA data, we will apply an inverse method to the NIR spectral data to retrieve cloud particle size and opacity overall the entire OMEGA dataset. In practice a model of the cloud properties is fitted to the observed spectra (along with the dust signature and the aerosol-free surface spectra estimated from observations acquired at other seasons). The feasibility of this method has been demonstrated in Madeleine et al. (2012a) where further details are given. They were able to map and characterize clouds on a selection of OMEGA observations. However, they were limited by the need for aerosol free surface spectra overall the planet. Our global mapping project will be made possible by the new surface spectral map by UPWARDS. This unique dataset will provide a new constraint on Climate models, for the first time at various local times.

For the SPICAM data, our main goal is to build an automatic tool to detect the presence of clouds using nadir observations in the UV spectral region, where they are much brighter than dust or the surface. The algorithm will be able to detect clouds above the dark regolith, but also above the brighter ice polar caps. It will benefit from the heritage of a first investigation performed on a subset of Mars Express SPICAM UV spectrometer as described in Mateshvili et al. (2009). This tool will then be applied to the full SPICAM nadir UV dataset to obtain climatology of the presence of clouds that will complement spatially and temporally that obtained from the analysis of OMEGA data.

While numerous studies have been performed to monitor water vapour and surface frost in the Martian atmosphere, some specific work is required to use them in a synergistic manner and in particular to make them usable to constrain climate models. For this purpose three specific studies will be performed:

• The various water vapour measurements available from Mars Global Surveyor and Mars Express will be gathered, compared and combined. Previous comparison of water vapour abundance results obtained from TES and the PFS, SPICAM and OMEGA instruments on Mars Express showed general agreement, but enough systematic differences to make precise comparison difficult. Within this project, we will conduct research on a selection of these enigmatic discrepancies (which could be, for instance, explained by an unexpected vertical distribution of water vapour) using the new synergistic algorithms that are able to combine spectral data from various instruments.
• Similarly, additional information on the vertical distribution of water vapour will be obtained using new Mars Express limb observations.
• In parallel, a climatology of seasonal surface frost as observed by the Mars Express OMEGA imaging spectrometer will be prepared in direct collaboration with the OMEGA team in order to allow an “apple to apple” comparison with the Global Climate Model.

Using the information obtained from the previous studies, a new generation model of the water cycle (including an improved representation of clouds, frost and exchange with the sub-surface) will be included in the LMD Global Climate Model and validated by the observations. The LMD Mars Global Climate Model includes a Mars water cycle which now account for the radiative effect of water ice clouds, a detailed cloud microphysics and for which a new parameterisation of exchange with the subsurface has been recently developed. It will be carefully validated and tuned using all observations from UPWARDS. In light of these comparisons and using the observations, new or improved parameterisations will be included in the model in order to better account for sub grid-scale cloudiness, the thermodynamics of surface frost, the sensitivity of cloud particles nucleation to temperature, and possibly all new physical processes that may be revealed by our studies.