Assimilation of Temperatures and Column Dust Opacities Measured by ExoMars TGO‐ACS‐TIRVIM During the MY34 Global Dust Storm.

We assimilate atmospheric temperature profiles and column dust optical depth observations from the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite thermal infrared channel (TIRVIM) into the Mars Planetary Climate Model. The assimilation period is Mars Year 34 Ls = 182.3°–211.4°, covering the onset and peak of the 2018 global dust storm. We assimilated observations using the Local Ensemble Transform Kalman Filter with 36 ensemble members and adaptive inflation; our nominal configuration assimilated TIRVIM temperature profiles to update temperature and dust profiles, followed by dust column optical depths to update the total column dust abundance. The observation operator for temperature used the averaging kernels and prior profile from the TIRVIM retrievals. We verified our analyses against in‐sample TIRVIM observations and independent Mars Climate Sounder (MCS) temperature and dust density‐scaled opacity profiles. When dust observations were assimilated, the root‐mean‐square temperature error verified against MCS fell by 50% during the onset period of the storm, compared with assimilating temperature alone. At the peak of the storm the analysis reproduced the location and magnitude of the peak in the nighttime MCS dust distribution, along with the surface pressure diurnal cycle measured by Curiosity with a bias of less than 10 Pa. The analysis winds showed that, at the peak of the storm, the meridional circulation strengthened, a 125 m s−1 asymmetry developed in the midlatitude zonal jets, the diurnal tide weakened near the equator and strengthened to 10–15 K at midlatitudes, and the semi‐diurnal tide strengthened almost everywhere, particularly in the equatorial lower atmosphere. Plain Language Summary: ExoMars Trace Gas Orbiter has been in low orbit around Mars since early 2018. During 2018 a large amount of dust was lifted from Mars' surface into the atmosphere. This eventually obscured most of the planet, forming a global dust storm. These storms have a large effect on the temperature and weather in Mars' atmosphere. We used observations from the spacecraft's thermal infrared instrument. This measures how much light comes from the atmosphere and surface at wavelengths where heat is emitted, and can be used to work out the atmospheric temperature and dust content at different times of day. We combined these observations with a numerical model of Mars' atmosphere, considering uncertainties in the measured atmospheric properties and the modeled atmosphere. We compared our results with temperature and dust observations made by Mars Climate Sounder on board Mars Reconnaissance Orbiter. Including dust observations was better than just using temperature observations. While the dust storm was raging, north‐south wind speeds increased, the strong eastward wind in the northern hemisphere became much stronger than in the southern hemisphere, and differences between day and night became larger. Our results mean we can better understand aspects of Mars' atmosphere throughout the daily cycle. Key Points: We assimilate temperature and dust observations from ExoMars Trace Gas Orbiter‐Atmospheric Chemistry Suite‐TIRVIM during the MY34 global dust storm (GDS)The analysis verifies well against independent Mars Climate Sounder temperature and dust profiles, and Curiosity pressure measurementsAt the peak of the GDS the winds strengthen, the diurnal tide migrates poleward, and the semi‐diurnal tide strengthens [ABSTRACT FROM AUTHOR]