Abstract 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°N) during LS ∼ 45 – 135° (LS = 0° 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 ∼30 km). Zonal mean ice extinction profiles show ACB’s altitudinal range within ∼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 LS = 45 – 75° (phase 1), the peak phase during LS = 76 – 105° (phase 2), and the decaying phase during LS = 106 – 135° (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 ∼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°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 ∼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.

中文翻译：

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从 MCS 数据观测火星热带上空的远日点云带、演化和相关的尘埃分布

摘要 本研究利用火星勘测轨道飞行器上的火星气候探测器对热带地区远日点云带 (ACB) 进行的五年观测。对纬向平均水冰柱不透明度的分析表明，在 LS ∼ 45 – 135°（LS = 0° 表示北春分点）期间，ACB 的空间延伸主要局限于热带和中纬度地区（-20 – 40°N） ）。ACB 主要出现在夜间，只是由于在明显更高的高度（约 30 公里）上截断了白天的剖面观测。带状平均冰消光剖面显示 ACB 的海拔范围在 10 – 40 km 内，并且在远日点季节没有厚 ACB 的情况下存在薄云带。ACB 的三个阶段可以确定为 LS = 45 – 75°（阶段 1）期间的形成阶段，LS = 76 – 105°（第 2 阶段）期间的峰值相位，以及 LS = 106 – 135°（第 3 阶段）期间的衰减相位。云纬度带的观测显示从第 2 阶段开始向北移动，在 Lunae Planum 和 Xanthe Terra 附近的地区尤为突出。在这个阶段，ACB 内厚云的顶层在南半球下降到 20 公里，而在北半球（NH）略有增加。在整个-10 – 10°N区域，第3阶段继续下降，厚云底在NH上移得更高，但其垂直深度比第2阶段变窄。 温度剖面没有显示出任何明显的影响ACB 向北演化。然而，区域层面的研究表明，对流层上层尘埃可能与 ACB 的演变有关。该机制在相关分析中很明显，主要是在~18-35 公里的高度范围内。作为尘埃或水冰强迫的代表的迁移半日潮 (SMD)，以及计算出的对流层上层尘埃辐射加热，显示出其峰值振幅在 ACB 的三个阶段内明显向北移动。以前没有观察到 SMD 和水冰的时空变化之间的这种匹配。然而，相关行为似乎在 Lunae Planum 和 Xanthe Terra 附近的区域以及大气的对流层上层区域很突出。显示其峰值振幅在 ACB 的三相内明显向北移动。以前没有观察到 SMD 和水冰的时空变化之间的这种匹配。然而，相关行为似乎在 Lunae Planum 和 Xanthe Terra 附近的区域以及大气的对流层上层区域很突出。显示其峰值振幅在 ACB 的三相内明显向北移动。以前没有观察到 SMD 和水冰的时空变化之间的这种匹配。然而，相关行为似乎在 Lunae Planum 和 Xanthe Terra 附近的区域以及大气的对流层上层区域很突出。