Observation of aphelion cloud belt over Martian tropics, its evolution, and associated dust distribution from MCS data

The present study uses five Martian years of observations from Mars Climate Sounder onboard Mars Reconnaissance Orbiter for investigating the Aphelion Cloud Belt (ACB) over the tropics. Analysis of zonal mean water ice column opacity suggests that the spatial extension of the ACB is mainly confined over the tropics and mid-latitudes (-20 - 40 degrees N) during L-s similar to 45 -135 degrees (L-s = 0 degrees signifies northern spring equinox). The ACB is seen primarily in the nighttime only due to the truncation of the daytime profile observations at significantly higher altitudes (at similar to 30 km). Zonal mean ice extinction profiles show ACB's altitudinal range within similar to 10 - 40 km, and the existence of a thin cloud band in the absence of a thick ACB during aphelion season. Three phases of the ACB could be identified as the formation phase during L-s = 45 - 75 degrees (phase 1), the peak phase during L-s = 76 - 105 degrees (phase 2), and the decaying phase during L-s = 106 - 135 degrees (phase 3). Observation of the cloud latitude belt shows a northward movement starting from phase 2, prominent over regions nearby Lunae Planum and Xanthe Terra. During this phase, the top level of thick clouds within the ACB decreases to similar to 20 km in the southern hemisphere, while it increases a little over the northern hemisphere (NH). The decreasing tendency continues in phase 3 over the entire region -10 -10 degrees N, and the thick cloud base moves higher over the NH, though the vertical depth of it becomes narrower than phase 2. Temperature profiles do not show any noticeable influence on the northward evolution of the ACB. However, the study at a regional level indicates a possible association of upper tropospheric dustiness with the ACB's evolution. The mechanism is evident in the correlation analysis mostly at an altitude range of similar to 18 - 35 km. The migrating semidiurnal tide (SMD) as a proxy of dust or water ice forcing, and the calculated upper tropospheric dust radiative heating, shows an apparent northward movement of their peak amplitude within the three phases of the ACB. This match between the spatiotemporal variations of the SMD and the water ice was not observed previously. However, the correlating behavior seems to be prominent in the areas nearby Lunae Planum and Xanthe Terra and the upper-tropospheric region of the atmosphere. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study utilizes five Martian years of observations to investigate the Aphelion Cloud Belt (ACB) over the tropics, finding its concentration in the tropics and mid-latitudes, primarily observed at night. The altitudinal range of ACB is approximately 10-40 km, and a thin cloud band may exist during aphelion season instead of a thick ACB.