Gravity waves in Mars’s atmosphere strongly affect the general circulation as well as middle atmospheric cloud formation, but the climatology and sources of gravity waves in the lower atmosphere ...remain poorly understood. At Earth, the statistical variance in satellite observations of thermal emission above the instrumental noise floor has been used to enable measurement of gravity wave activity at a global scale. Here is presented an analysis of variance in calibrated radiance at 15.4μm (635–665 cm−1) from off-nadir and nadir observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO); a major expansion in the observational data available for validating models of Martian gravity wave activity. These observations are sensitive to gravity waves at 20–30 km altitude with wavelength properties (λh=10–100 km, λz> 5 km) that make them likely to affect the dynamics of the middle and upper atmosphere. We find that: (1) strong, moderately intermittent gravity wave activity is scattered over the tropical volcanoes and throughout the middle to high latitudes of both hemispheres during fall and winter, (2) gravity wave activity noticeably departs from climatology during regional and global dust storms; and (3) strong, intermittent variance is observed at night in parts of the southern tropics during its fall/winter, but frequent CO2 ice clouds prevents unambiguous attribution to GW activity. The spatial distribution of wave activity is consistent with topographic sources being dominant, but contributions from boundary layer convection and other convective processes are possible.
•Lower atmospheric gravity wave activity surveyed from orbit over multiple Mars Years.•Gravity wave activity common over tropical volcanoes and winter westerly jets.•Boundary layer convection likely source of gravity wave activity elsewhere.•Gravity wave activity departs from climatology during large dust storms.•Gravity waves and thick mesospheric clouds hard to disambiguate.
Deep convection, as used in meteorology, refers to the rapid ascent of air parcels in the Earth's troposphere driven by the buoyancy generated by phase change in water. Deep convection undergirds ...some of the Earth's most important and violent weather phenomena and is responsible for many aspects of the observed distribution of energy, momentum, and constituents (particularly water) in the Earth's atmosphere. Deep convection driven by buoyancy generated by the radiative heating of atmospheric dust may be similarly important in the atmosphere of Mars but lacks a systematic description. Here we propose a comprehensive framework for this phenomenon of dusty deep convection (DDC) that is supported by energetic calculations and observations of the vertical dust distribution and exemplary dusty deep convective structures within local, regional, and global dust storm activity. In this framework, DDC is distinct from a spectrum of weaker dusty convective activity because DDC originates from pre-existing or concurrently forming mesoscale circulations that generate high surface dust fluxes, oppose large-scale horizontal advective-diffusive processes, and are thus able to maintain higher dust concentrations than typically simulated. DDC takes two distinctive forms. Mesoscale circulations that form near Mars's highest volcanoes in dust storms of all scales can transport dust to the base of the upper atmosphere in as little as two hours. In the second distinctive form, mesoscale circulations at low elevations within regional and global dust storm activity generate freely convecting streamers of dust that are sheared into the middle atmosphere over the diurnal cycle.
•We use infrared observations of polar carbon dioxide clouds to estimate the rate of snowfall during polar winter on Mars.•Snowfall in south polar winter accounts for 3–20% of the total rate of ...seasonal carbon dioxide ice accumulation.•Radiative cooling of the polar winter atmosphere on Mars is found to be sufficient to lead to CO2 precipitation.•The observed decay of polar CO2 clouds is consistent with calculated sedimentation rates if the particles are > 50μm in radius.
Wintertime observations of the martian polar regions by orbiting spacecraft have provided evidence for carbon dioxide clouds, which measurably alter the polar energy budget and the annual CO2 cycle. However, it has remained unclear whether snowfall contributes a substantial quantity to the accumulating seasonal ice caps. We develop models to constrain precipitation rates based on observations of south polar CO2 clouds by the Mars Climate Sounder (MCS), and show that snowfall contributes between 3% and 20% by mass to the seasonal deposits at latitudes 70–90°S. The lower bound on this estimate depends on a minimum effective cloud particle size of ∼50μm, derived by comparing the short lifetimes (less than a few hours) of some clouds with calculated sedimentation velocities. Separate constraints from infrared spectra measured by MCS suggest CO2 cloud particles in the size range 10–100μm. Snow particles are not likely to re-sublime before reaching the surface, because the lower atmosphere in this region remains near saturation with respect to CO2. Based on cooling rate calculations, snowfall originating below 4km altitude likely contributes a comparable or greater amount to the seasonal deposits than the rest of the atmosphere. Due to the positive feedback between cloud particle number density and radiative cooling, CO2 snow clouds should propagate until they become limited by the availability of condensation nuclei or CO2 gas. Over the south polar residual cap, where cloud activity is greatest, atmospheric radiative cooling rates are high enough to offset heat advected into the polar regions and maintain consistent snowfall. At latitudes of 60–80°S the lower atmosphere tends to be slightly sub-saturated and rapid cooling by mechanical lift driven by orography or convergent flow may be required to initiate a snowstorm, consistent with the more sporadic clouds observed by MCS in this region, and their correlation with topographic features. Snowfall and accumulation at the surface are found to be inevitable consequences of the polar energy budget, unless advection redistributes heat from lower latitudes in much greater quantities than expected.
Dusty convection, convective activity powered by radiative heating of dust, is a ubiquitous phenomenon in Mars's atmosphere but is especially deep (i.e., impactful on the middle atmosphere) and ...widespread during planet‐encircling dust events (PEDEs) that occur every few Mars Years (MYs). Yet the relative roles of dusty deep convection and global dynamics, such as the principal meridional overturning cell and the radiative tides, in dust storm development and the vertical transport of dust and water are still unclear. Here, observations from the Mars Climate Sounder on board Mars Reconnaissance Orbiter (MRO‐MCS) are used to study dusty deep convection and its impact on middle atmospheric water content during the MY 34 PEDE (commenced June 2018). Additional context is provided by MRO‐MCS observations of the MY 28 PEDE (commenced June 2007). This investigation establishes that a few, localized centers of dusty deep convection in the tropics formed in the initial phases of both PEDE simultaneously with a substantial increase in middle atmospheric water content. The growth phase of the MY 34 PEDE was defined by episodic outbreaks of deep convection along the Acidalia and Utopia storm tracks as opposed to less episodic, more longitudinally distributed convective activity during the MY 28 PEDE. The most intense convection during both PEDE was observed in southern/eastern Tharsis, where MRO‐MCS observed multiple instances of deep convective clouds transporting dust to altitudes of 70–90 km. These results suggest that Martian PEDE typically contain multiple convectively active mesoscale weather systems.
Plain Language Summary
Just as the heat released by condensing water vapor powers thunderstorms in Earth's atmosphere, very dusty air heated by the Sun in Mars's atmosphere can power dust clouds that tower many tens of kilometers. These dust towers are most common in Mars's rare and impressive planet‐encircling dust events, when dust is rapidly lifted from the surface and the planet's atmosphere fills with a thick haze of dust. But the role of dust towers is unknown. In one view, dust towers randomly form from dust lifted by stronger trade winds in the dust event. In another view, the dust towers organize into a few hurricane‐like storms that spread dust around the planet. Here we study the two most recent planet‐encircling dust events in 2007 and 2018. We find that dust towers first form at the same time as a rapid increase in water at high latitudes observed early in each event. In the 2018 storm, dust tower forming weather systems initially formed near the equator along low elevation pathways along which strong cold fronts in the northern hemisphere may have traveled. Dust towers east of Mars's high Tharsis volcanoes were especially strong.
Key Points
Deep convection during the MY 34 PEDE was more episodic than in the MY 28 PEDE
The tallest convective clouds during the MY 34 PEDE were more than 80 km high
Tropical hygropause altitude increased from 55 km to 65–70 km during the initial convective episode, peaking at 75–80 km during a later one
We present south polar winter infrared observations from the Mars Climate Sounder (MCS) and test three hypotheses concerning the origins of “cold spots”: regions of anomalously low infrared ...brightness temperatures, which could be due to enrichment in non‐condensable gases, low‐emissivity surface frost, or optically thick CO2 clouds. Clouds and surface frosts have been historically difficult to distinguish, but the unique limb sounding capability of MCS reveals extensive tropospheric CO2clouds over the cold spots. We find that both clouds and surface deposits play a significant role in lowering the infrared emissivity of the seasonal ice cap, and the granular surface deposits are likely emplaced by snowfall. Surface temperatures indicate the polar winter atmosphere is enriched by a factor ∼5–7 in non‐condensable gases relative to the annual average, consistent with earlier gamma ray spectrometer observations, but not enough to account for the low brightness temperatures. A large ∼500‐km diameter cloud with visible optical depth ∼0.1–1.0 persists throughout winter over the south polar residual cap (SPRC). At latitudes 70–80°S, clouds and low emission regions are smaller and shorter‐lived, probably corresponding to large‐grained “channel 1” clouds observed by the Mars Orbiter Laser Altimeter. Snowfall over the SPRC imparts the lowest emissivity in the south polar region, which paradoxically tends to reduce net accumulation of seasonal CO2 by backscattering infrared radiation. This could be compensated by the observed anomalously high summertime albedo of the SPRC, which may be related to small grains preserved in a rapidly formed snow deposit.
Key Points
The snowiest place in the south polar region is the south polar residual cap
A separate class of small, short‐lived CO2 clouds predominate 70‐80 S
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