Martian Atmospheric Characteristics Associated with Dust Storms and Clouds Realized Through Satellite Observations and MarsWRF

Abstract

Dust storms and clouds are the two major atmospheric phenomena frequently seen over Mars and have the most dynamic impact on its atmosphere. The dust can alter the temperature profile, thereby controlling the weather. Understanding the clouds will give an insight into the water cycle, and water is an essential resource for any living body. Satellite observations spanning across more than two decades are playing a pivotal role in understanding the meteorological processes associated with the dust and cloud activities over Mars. However, studies of the vertical distribution of dust and water ice and their dynamical, microphysical, and radiative interactions need more attention, which will help to understand their intrinsic variation. Another aspect is understanding and predicting the thermal behavior of the atmosphere adequately, considering the complex interplay of dust and water ice forcing. Therefore, this study has analyzed the clouds and dust lifting processes using several imaging and meteorological sensors onboard different Mars orbiters.
First, the analyses are carried out during a dust lifting sequence in early southern summer near the Lunae Planum region based on the observations from the Mars Color Camera (MCC) onboard the Mars Orbiter Mission (MOM). The Mars Daily Global Maps (MDGMs) of the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) showed that the dust storm was generated in the Acidalia-storm-track region, crossed the equator, and merged with a Hellas sequence over Noachis Terra. The same is confirmed by an increased column dust opacity derived from the observations of the Mars Climate Sounder (MCS) onboard MRO. The HATDM (High Altitude Tropical Dust Maximum) is formed but with a temporal delay of 2° LS (solar longitude) after the dust storm disappearance. The study implicates, variation of the planetary boundary layer (PBL) growth, visible heating rate, and the water ice mixing ratio during the dust storm scenario for this temporal delay in dust lifting.
The aforesaid mechanism is found to be valid for the Mars year (MY) 34 (2018) global dust storm. Dust lifting is observed to continue until LS ~207 – 208° in the MARCI images and MCS column dust opacity. However, the dust abundance in the middle atmosphere shows its peak around LS 211°. HATDM is evident after LS ~190°, i.e., a few LS after the large precursor Acidalia dust storm. Also, the diurnal tide is strongly amplified at high-altitudes away from the tropics, and the vertical extent of clouds is significantly reduced. These changes are consistent with the time scale of the evolving vertical distribution of dust observed by the MCS. Overall, these delays observed in the dust movement, clouds, and tidal distribution probably reflect a vertical mixing timescale of dust irrespective of the spatial scale of a storm.
Therefore, more detailed analyses of the vertical distribution of dust and associated atmospheric impacts is carried out for a regional and a global event over Acidalia Planitia using MCS observations and ‘Mars Weather Research and Forecasting (MarsWRF)’ model simulations. The dustiness at ~25–35 km altitude contributed significantly to the column opacity during the regional dust event (RDE), which peaks at ~40–50 km for the global one (GDE). This altitudinal difference is also visible in the inversion layer formed by the combined effect of decreasing surface temperature and the downward infrared radiation from the dust. This atmospheric warming by the dust radiative heating with the inversion layer below could be associated with the presence of heating and cooling layers centered around 30–50 and 20–35 km heights, which influence the variability of ice optical depth within them. The MarsWRF simulated downward wind, PBL height, and surface radiation flux showed lesser vertical mixing from the surface and also suggested dominating downward radiation from the suspended dust, which altogether influenced the formation of a heating/cooling layer and variability of water ice clouds. The Empirical Orthogonal Function analysis suggests that the seasonal cycle of the southern hemispheric dust storms, northern hemispheric active storm track, and the cap-edge storms possibly influenced the seasonality observed in the heating/cooling layer clouds.
In the clouds' case, the present study uses five MY MCS observations for investigating the Aphelion Cloud Belt (ACB) over the tropics. The observation suggests its appearance at altitudes ~10–40 km, within -20 – 40°N and during LS ~45 – 135°. The thick clouds within the ACB show a northward movement starting from the peak phase (LS ~76 – 105°), prominently over regions nearby Lunae Planum and Xanthe Terra. The temporal evolution of the ACB shows a decreasing cloud top over the southern hemisphere and little increase over the northern hemisphere (NH), though the vertical depth of it becomes narrower. A possible association of upper tropospheric dustiness with the ACB’s northward evolution is evident mostly at an altitude range of ~18–35 km. It is supported by the migrating semidiurnal tide (SMD) as a proxy of dust or water ice forcing and upper tropospheric dust radiative heating, which showed a northward movement of their peak amplitude with the temporal evolution of the ACB.
The investigation of tropical clouds extended to the Olympus and Arsia Mons region, which provides an exciting opportunity to explore the dynamical effects related to the orographic clouds. MCS observations suggest an appearance of thick, low altitude (at ~15–32 km) clouds during LS ~35 – 150°, which is strongly controlled by the air temperature, and an occurrence of high altitude (at ~30–50 km) haze clouds during LS ~225 – 315°, which is found to be more associated with the elevated dustiness and vertical advection of dust-laden mountain induced regional circulation, as suggested by MarsWRF simulated wind. The thick clouds followed by the haze clouds maintain the cloud water content at ~0.2 and ~0.05 pr. μm. in the 1st and 2nd half of the year. However, a higher cloud vertical depth is evident during the 2nd half of the year due to the appearance of high-altitude cloud haze. The Olympus Mons region shows a more favorable condition for the thick low-altitude clouds that are mostly influenced by the consistent and stable aphelion cloud cycle. In contrast, more prominent vertical advection over the Arsia Mons region during the perihelion season drives the noticeable occurrence of haze clouds and significant east-west directional asymmetry in the cloud abundance.
Lastly, a study of dust and water ice interrelated variation is carried out using the six years (MY 29 – 34) of MCS retrievals. The correlations between dust and water ice column opacity and profiles are found to ‘switch sign’ and ‘alter within 20–40 km altitudes’ between the low-dust and high-dust seasons. During the low-dust period, the positive correlations over the latitudes 40 – 80° S and 20° S – 40°N are mainly controlled by the water ice cycle in the south polar hood clouds and ACB besides the presence of atmospheric dust in ACB’s formation stage. During the high-dust period, at southern latitudes, significant dust lifting and the associated temperature changes are found to be the reason for the strong negative correlation. And, in tropical latitudes, the significant positive relationship at relatively high altitudes (~40 km) is seen, possibly due to the presence of thin or haze clouds. The global dust storm occurrence only modulates the correlation behavior during the high-dust period and above ~40 km altitude, indicating an enhanced vertical advection. The difference in correlation behavior between low and high-dust seasons could be explained from the variation in MarsWRF simulated PBL height. Moreover, the dust and water ice interaction pattern has a prominent seasonal and altitudinal variation, which is influenced by the water ice cycle, dust cycle, or the dustice microphysical relationship