It has been suggested that dust storms efficiently transport water vapor from the near‐surface to the middle atmosphere on Mars. Knowledge of the water vapor vertical profile during dust storms is important to understand water escape. During Martian Year 34, two dust storms occurred on Mars: a global dust storm (June to mid‐September 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162–260° and Ls = 298–345°) from the solar occultation measurements by Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). We show a significant increase of water vapor abundance in the middle atmosphere (40–100 km) during the global dust storm. The water enhancement rapidly occurs following the onset of the storm (Ls~190°) and has a peak at the most active period (Ls~200°). Water vapor reaches very high altitudes (up to 100 km) with a volume mixing ratio of ~50 ppm. The water vapor abundance in the middle atmosphere shows high values consistently at 60°S‐60°N at the growth phase of the dust storm (Ls = 195°–220°), and peaks at latitudes greater than 60°S at the decay phase (Ls = 220°–260°). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single pole‐to‐pole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322–327°), in particular at latitudes greater than 60°S.

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The NOMAD ("Nadir and Occultation for MArs Discovery") spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the comThis paper is dedicated to the memory of M. Allen, V. Formisano, and J. McConnell. position of Mars' atmosphere, with a particular focus on trace gases, clouds and dust. The detection sensitivity for trace gases is considerably improved compared to previous Mars missions, compliant with the science objectives of the TGO mission. This will allow for a major leap in our knowledge and understanding of the Martian atmospheric composition and the related physical and chemical processes. The instrument is a combination of three spectrometers, covering a spectral range from the UV to the mid-IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and explain the technical principles of the three spectrometers. We also discuss the expected performance of the instrument in terms of spatial and temporal coverage and detection sensitivity.

Abstract. This paper discusses the global analyses of stratospheric ozone (O 3 -VISAT). This corresponds to the entire period during which MIPAS was operating at its nominal resolution.Our analyses are evaluated against assimilated MIPAS data and independent HALOE (HALogen Occultation Experiment) and POAM-III (Polar Ozone and Aerosol Measurement) satellite data. A good agreement is generally found between the analyses and these datasets, in both cases within the estimated error bars of the observations. The benefit of data assimilation is also evaluated by comparing a BASCOE free model run with MIPAS observations. For O 3 , the gain from the assimilation is significant during ozone hole conditions, and in the lower stratosphere. Elsewhere, the assimilation does not provide significant improvement. For NO 2 , the gain from the assimilation is realized through most of the stratosphere. Using the BASCOE analyses, we estimate the differences between MIPAS data and independent data from HALOE and POAM-III, and find results close to those obtained by classical validation methods involving only direct measurement-to-measurement comparisons. Our results extend and reinforce previous MIPAS data validation Correspondence to: Q. Errera (quentin.errera@aeronomie.be) efforts by taking into account a much larger variety of atmospheric states and measurement conditions. This study discusses possible further developments of the BASCOE data assimilation system; these concern the horizontal resolution, a better filtering of NO 2 observations, and the photolysis calculation near the lid of the model. The ozone analyses are part of the PROMOTE project and are publicly available via the BASCOE website (www.bascoe. oma.be/promote).

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter measured a large increase in water vapor at altitudes in the range of 40-100 km during the 2018 global dust storm on Mars. Using a three-dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere, the temperature increases, and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70-100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high-altitude atomic hydrogen following the high-altitude water vapor increase by a few days. Plain Language Summary The ExoMars Trace Gas Orbiter (TGO) is currently in orbit aroundMars measuring the composition of the atmosphere. TGO was able to make observations before, during, and after a large planet-encircling dust storm that occurred from June to August 2018. The TGO measurements provide the first opportunity to observe how water vapor is distributed with height in the atmosphere during a global-scale dust event. It was found that there was a large increase in water vapor very high (40-100 km) in the atmosphere during the dust storm. Using a three-dimensional numerical model of the Mars atmosphere, we found that when dust from the storm is transported up to levels above~40 km, it warms the atmosphere due to solar absorption, and this in turn prevents ice clouds from forming at heights of 40-60 km and allows more water vapor to ascend to greater heights in the atmosphere. This is of interest in terms of the planetary evolution, as water molecules at greater heights are more readily dissociated by sunlight and lost from the atmosphere. This is an important factor for understanding the changes that have occurred since the period when surface features on Mars indicate that liquid water was present.

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