Southern hemisphere decrease from 68 km to 54 km (cf. figure 10) and the main 3 µm atmospheric absorption (i.e. the maximum of IBD for a given profile) decrease from 75 km to 58 km (cf. figure 9). Regardless that the haze top altitude remains at ∼85 – 90 km, and the latitude goes successively towards and from the equator. Nevertheless, we note a slight North/South asymmetry during the dust storm that is represented by the smaller number of high-altitude water ice clouds in the Northern hemisphere (see figure 11.c&d).

Ice clouds are detected at very high altitudes in the mesosphere during this period, up to ≥80 km at the beginning of the GDS. A few profiles show robust water detections at altitudes above 90 km, up to 100 km maximum at Ls ∼ 200◦(cf. figure 14.c). These high altitude water ice clouds are observed only at the beginning of the GDS. High altitude water ice clouds can also be seen during non-dust GDS years, when they occur during the storm season (Vincendon et al., 2011; Clancy et al., 2019). However, high cloud altitudes are nearly systematic in our observations during the peak phase of the GDS (cf. figure 10), while such altitudes are less typical during non-GDS years (Clancy et al., 2019). Moreover, peak cloud altitudes are slightly higher during the onset of the GDS than reported in non-GDS years. Indeed, models (Neary et al., 2019) showed that GDS are expected to slightly increase the average and maximum altitude of water ice clouds, which is consistent with our observations. The sharp increase in water ice cloud altitude observed during the GDS indeed confirms that this water ice cloud increase is directly connected to dust storm activity. The large-scale dust storm probably facilitates the formation of high altitude water ice clouds through increase in the altitude of both water vapor and condensation nuclei. This increase of water ice altitude during the GDS is indeed consistent with the recently reported increase of water vapor at higher altitudes (Fedorova et al., 2018; Neary et al., 2019).
We still observe a North/South latitudinal asymmetry during the GDS. In the Northern hemisphere, seven observations in the figure 11.c exhibit low haze top altitudes during the GDS. The haze top is between 26 km and 36 km, i.e. ∼50 km below the altitudes measured during the dust storm for most of the other profiles. Actually, these profiles were taken at high Northern latitude (60◦N – 66◦N) during the decay of the dust storm (Ls = 221◦−231◦) at longitudes of ∼25◦W (Acidalia Planitia) and ∼155◦E (Utopia Planitia), see table 2: this suggests that 60◦N represents the Northern extent of the dust storm activity in these regions from Ls ∼220◦, which is in agreement with MCS measurements (D. Kass et al., 2019). The scheme is different for the Southern hemisphere, with high haze top altitudes (> 80 km) observed during the GDS up to the polar regions (73◦S).
Ls
Latitude Longitude Local time Haze top altitude 219.0◦ 65◦N 113◦E 08h14 35.3 km 221.1◦ 66◦N 6◦W 08h38 27.5 km 224.3◦ 66◦N 38◦W 09h07 28.1 km 224.6◦ 66◦N 152◦W 09h09 29.2 km 224.7◦ 66◦N 150◦E 09h09 32.5 km 229.5◦ 62◦N 28◦W 09h16 26.2 km 231.1◦ 60◦N 159◦E 09h06 27.4 km [Table: Table 2.
Properties of profiles located outside the GDS beyond its Northern limit at
Ls
∼ 225◦. With a mean haze top altitude of 30 km, i.e. 50 km lower other measurements
during the same period, these observations can no longer be considered as part of the GDS.
These low haze top observations are located at latitudes greater than 60◦with some longitudinal
variability. This latitude thus correspond to the Northern maximum extent of the GDS at that
time, in agreement with MCS measurements (D. Kass et al., 2019).
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