Abstract
A Martian water-ice cloud climatology has been extracted from OMEGA data covering 7 Martian years (MY 26–32) on the dayside. We derived two products, the Reversed Ice Cloud Index (ICIR) and the Percentage of Cloudy Pixels (PCP), indicating the mean cloud thickness and nebulosity over a regular grid (1° longitude × 1° latitude × 1° Ls × 1 h Local Time). The ICIR has been shown to be a proxy of the water-ice column derived from the Mars Climate Database.The PCP confirms the existence and location of the main cloud structures mapped with the ICIR, but also gives a more accurate image of the cloud cover. We observed a more dense cloud coverage over Hellas Planitia, the Lunae Planum region and over large volcanoes (Tharsis volcanoes, Olympus and Elysium Montes) in the aphelion belt.For the first time, thanks to the fact that Mars Express is not in Sun-synchronous orbit, we can explore the clouds diurnal cycle at a given season by combining the seven years of observations. However, because of the eccentric orbit, the temporal coverage remains limited. Identified limitations of the dataset are its small size, the difficult distinction between ice clouds and frosts, and the impact of surface albedo on data uncertainty. We could nevertheless study the diurnal cloud life cycle by averaging the data over larger regions: from specific topographic features (covering a few degrees in longitude and latitude) up to large climatic bands (covering all longitudes). We found that in the tropics (25°S – 25°N) around northern summer solstice, the diurnal thermal tide modulates the abundance of clouds, which is reduced around noon (Local Time). At northern midlatitudes (35°N – 55°N), clouds corresponding to the edge of the north polar hood are observed mainly in the morning and around noon during northern winter (Ls = 260° – 30°). Over Chryse Planitia, low lying morning fogs dissipate earlier and earlier in the afternoon during northern winter. Over Argyre, clouds are present over all daytime during two periods, around Ls = 30° and 160°.
Highlights
Understanding the water cycle of current and past Martian climate and weather remains an objective of planetary climate research
We present the complete climatology of clouds as observed by the OMEGA (Observatoire pour la Mineralogie, l’Eau les Glaces et l’Activite) imaging spectrometer on board Mars Express
It is a better extractor of thin clouds at pixel scale, especially from pixels with values just above the threshold used for the calculation of the Percentage of Cloudy Pixels (PCP)
Summary
Understanding the water cycle of current and past Martian climate and weather remains an objective of planetary climate research. This study focuses on the life cycle of water-ice clouds, and, on their diurnal cycle It is based on an extensive use of data from the OMEGA imaging spectrometer onboard Mars Ex press, from which only a small subset has been used for cloud studies. Unlike Mars Express, put into orbit in December 2003, MGS, ODY and MRO were placed on heliosynchronous orbits As a consequence, they always cover the (non-polar) surface of Mars at the same Martian local time, approximately 2–3 h and 13–15 h local time (LT). A possible application is the comparison with the coming generation of high-resolution general circulation models in order to investigate small-scale processes Another potential application is the extraction and the validation of data from Exomars / Trace Gas Orbiter (TGO) instruments, in particular from the Atmospheric Chem istry Suite (ACS) and the Nadir and Occultation for MArs Discovery (NOMAD) spectrometers
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