Diurnal Variations of Dust During the 2018 Global Dust Storm Observed by the Mars Climate Sounder.

We report observations by the Mars Climate Sounder showing strong diurnal variations in temperature and the vertical dust distribution during the 2018 (Mars Year 34) global dust event. The temperature field shows weak diurnal tidal activity at equatorial latitudes but a strong diurnal tide in middle to high latitudes with a maximum amplitude of 29 K in the lower atmosphere of the south polar region. The diurnal variability of dust is small in the equatorial region and increases toward higher latitudes. At middle and low latitudes, comparable dust amounts are found about 5–10 km higher in the atmosphere on the dayside than on the nightside. The dust reaches the highest altitudes in the late afternoon and is found at the lowest altitudes in the late night. In the southern high latitudes a persistent cold air mass with low dust content is identified on the nightside of the planet centered at 3–6 a.m. local time. The observed variations are well represented by model simulations with the Laboratoire de Météorologie Dynamique General Circulation Model. Comparisons between data and model results suggest that the diurnal variations in the dust are largely driven by the meridional circulation exhibiting diurnal tidal variations. The model results show that the compact air mass in the south polar region has a high potential vorticity, supporting its interpretation as a remnant of the southern polar vortex, which is forced toward the nightside of the planet due to the enhanced diurnal tide during the global dust event. Plain Language Summary: One of the most distinctive features of the Martian atmosphere are global dust storms, one of which occurred in 2018. We report on observations of the vertical structure of atmospheric temperature and dust by the Mars Climate Sounder onboard Mars Reconnaissance Orbiter. Strong differences between day and night are found in both temperature and dust vertical structure. The strongest temperature variations are observed in the south polar atmosphere, with temperature differences up to 58 °C/136 °F between day and night. Comparable dust amounts are found 5–10 km (3–6 miles) higher in the atmosphere on the dayside than on the nightside at central and equatorial latitudes. In the southern polar region a persistent cold body of air with low dust is identified on the nightside of the planet. The observations are compared with the results from a global atmospheric computer model. The observed temperature and dust distribution and their variations over the Martian day are well represented by the model. The diurnal variations in the dust are largely driven by diurnal changes in the large‐scale atmospheric circulation. The compact body of air in the south polar region is forced to the nightside by this altered circulation but does not dissipate for several months. Key Points: The Mars Climate Sounder observes strong diurnal variations in the vertical distribution of dust during the 2018 global dust stormDust diurnal variability at low and middle latitudes is largely related to the meridional circulation exhibiting diurnal tidal variationsDiurnal variations at south polar latitudes are related to a remnant of the polar vortex confined to the nightside of the planet [ABSTRACT FROM AUTHOR]