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•We have produced a multiannual climatology of the dust distribution on Mars.•We grid retrievals of column dust optical depth from 3 heterogeneous instruments.•Biases among different ...instruments appear when carrying out detailed validation.•We show the interannual and interseasonal variability of dust over 8 martian years.•The years without global-scale storms display four phases in the dust distribution.
We have produced a multiannual climatology of airborne dust from martian year 24–31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages and uncertainties on a regularly spaced spatio-temporal grid, using an iterative procedure that is weighted in space, time, and retrieval quality. The lack of observations at certain times and locations introduces missing grid points in the maps, which therefore may result in irregularly gridded (i.e. incomplete) fields. In order to evaluate the strengths and weaknesses of the resulting gridded maps, we compare with independent observations of CDOD by PanCam cameras and Mini-TES spectrometers aboard the Mars Exploration Rovers “Spirit” and “Opportunity”, by the Surface Stereo Imager aboard the Phoenix lander, and by the Compact Reconnaissance Imaging Spectrometer for Mars aboard MRO. We have statistically analyzed the irregularly gridded maps to provide an overview of the dust climatology on Mars over eight years, specifically in relation to its interseasonal and interannual variability, in addition to provide a basis for instrument intercomparison. Finally, we have produced regularly gridded maps of CDOD by spatially interpolating the irregularly gridded maps using a kriging method. These complete maps are used as dust scenarios in the Mars Climate Database (MCD) version 5, and are useful in many modeling applications. The two datasets for the eight available martian years are publicly available and distributed with open access on the MCD website.
Hydrogen chloride was discovered in the atmosphere of Mars for the first time during the global dust storm in Mars year (MY) 34 (July 2018) using the Atmospheric Chemistry Suite mid-infrared channel ...(ACS MIR) on the ExoMars Trace Gas Orbiter. The simultaneity of variations in dust and HCl, and a correlation between water vapour and HCl, led to the proposal of a novel surface-atmosphere coupling analogous to terrestrial HCl production in the troposphere from salt aerosols. After seasonal dust activity restarted in MY 35 (August 2020), we have been monitoring HCl activity to determine whether such a coupling was validated. Here we present a new technique for analysing the absorption features of trace gases close to the ACS MIR noise level and report that HCl mixing ratios are observed to rapidly increase in both hemispheres coincidentally with the onset of the MY 35 perihelion dust season. We present the temporal evolution of the vertical distribution of HCl (0.1–6 ppbv) and of dust activity in both hemispheres. We also report two observations of >2 ppbv HCl below 10 km in the northern hemisphere during the aphelion period.
Airborne dust is the main driver of Martian atmospheric temperature, and accurately accounting for its radiative effect in Global Climate Models (GCMs) is essential. This requires the modeling of the ...dust distribution and radiative properties, and when trying to simulate the true climate variability, the use of the observed dust column opacity to guide the model. A recurrent problem has been the inability of Mars GCMs to predict realistic temperatures while using both the observed dust radiative properties and column opacity. One would have to drive the model with a tuned opacity to reach an agreement with the observations, thereby losing its self‐consistency. In this paper, we show that using the most recently derived dust radiative properties in the LMD (Laboratoire de Météorologie Dynamique) GCM solves this problem, which was mainly due to the underestimation of the dust single scattering albedo in the solar domain. However, an overall warm temperature bias remains above the 1 hPa pressure level. We therefore refine the model by implementing a “semi‐interactive” dust transport scheme which is coupled to the radiative transfer calculations. This scheme allows a better representation of the dust layer depth in the model and thereby removes the remaining warm bias. The LMD/GCM is now able to predict accurate temperatures without any tuning of the dust opacity used to guide the model. Remaining discrepancies are discussed, and seem to be primarily due to the neglect of the radiative effect of water‐ice clouds, and secondarily to persisting uncertainties in the dust spatial distribution.
Key Points
Most recent dust optical properties significantly improve Mars GCM predictions
A new strategy for transporting dust is coupled to the radiative transfer model
Simulating changes in dust layer depth is key to predict realistic temperatures
Polar vortices on Mars provide case‐studies to aid understanding of geophysical vortex dynamics and may help to resolve long‐standing issues regarding polar vortices on Earth. Due to the recent ...development of the first publicly available Martian reanalysis dataset (MACDA), for the first time we are able to characterise thoroughly the structure and evolution of the Martian polar vortices, and hence perform a systematic comparison with the polar vortices on Earth. The winter atmospheric circulations of the two planets are compared, with a specific focus on the structure and evolution of the polar vortices. The Martian residual meridional overturning circulation is found to be very similar to the stratospheric residual circulation on Earth during winter. While on Earth this residual circulation is very different from the Eulerian circulation, on Mars it is found to be very similar. Unlike on Earth, it is found that the Martian polar vortices are annular, and that the Northern Hemisphere vortex is far stronger than its southern counterpart. While winter hemisphere differences in vortex strength are also reported on Earth, the contrast is not as large. Distinctions between the two planets are also apparent in terms of the climatological vertical structure of the vortices, in that the Martian polar vortices are observed to decrease in size at higher altitudes, whereas on Earth the opposite is observed. Finally, it is found that the Martian vortices are less variable through the winter than on Earth, especially in terms of the vortex geometry. During one particular major regional dust storm on Mars (Martian year 26), an equatorward displacement of the vortex is observed, sharing some qualitative characteristics of sudden stratospheric warmings on Earth.
By measuring the regular oscillations of the density of CO2 in the upper atmosphere (between 120 and 190 km), the mass spectrometer MAVEN/NGIMS (Atmosphere and Volatile EvolutioN/Neutral Gas Ion Mass ...Spectrometer) reveals the local impact of gravity waves. This yields precious information on the activity of gravity waves and the atmospheric conditions in which they propagate and break. The intensity of gravity waves measured by MAVEN in the upper atmosphere has been shown to be dictated by saturation processes in isothermal conditions. As a result, gravity waves activity is correlated to the evolution of the inverse of the background temperature. Previous data gathered at lower altitudes (∼95–∼150 km) during aerobraking by the accelerometers on board MGS (Mars Global Surveyor), ODY (Mars Odyssey) and MRO (Mars Reconnaissance Orbiter) are analyzed in the light of those recent findings with MAVEN. The anti-correlation between GW-induced density perturbations and background temperature is plausibly found in the ODY data acquired in the polar regions, but not in the MGS and MRO data. MRO data in polar regions exhibit a correlation between the density perturbations and the Brunt-Väisälä frequency (or, equivalently, static stability), obtained from Global Climate Modeling compiled in the Mars Climate Database. At lower altitude levels (between 100 and 120 km), although wave saturation might still be dominant, isothermal conditions are no longer verified. In this case, theory predicts that the intensity of gravity waves is no more correlated to background temperature, but to static stability. At other latitudes in the three aerobraking datasets, the GW-induced relative density perturbations are correlated with neither inverse temperature nor static stability; in this particular case, this means that the observed activity of gravity waves is not only controlled by saturation, but also by the effects of gravity-wave sources and wind filtering through critical levels. This result highlights the exceptional nature of MAVEN/NGIMS observations which combine both isothermal and saturated conditions contrary to aerobraking measurements.
•Gravity wave activity causes density perturbations in the Martian thermosphere.•MAVEN found a correlation between GW activity and inverse background temperature.•Lower-altitude aerobraking data do not show this correlation, except for Mars Odyssey.•Aerobraking data and GCMs suggest instead wave activity correlated with Static stability.•When no such correlation, a mix of saturation, critical levels and sources is suspected.
The climate on Earth is generally determined by the amount and distribution of incoming solar radiation, which must be balanced in equilibrium by the emission of thermal radiation from the surface ...and atmosphere. The precise routes by which incoming energy is transferred from the surface and within the atmosphere and back out to space, however, are important features that characterize the current climate. This has been analyzed in the past by several groups over the years, based on combinations of numerical model simulations and direct observations of the Earth's climate system. The results are often presented in schematic form to show the main routes for the transfer of energy into, out of and within the climate system. Although relatively simple in concept, such diagrams convey a great deal of information about the climate system in a compact form. Such an approach has not so far been widely adopted in any systematic way for other planets of the Solar System, let alone beyond, although quite detailed climate models of several planets are now available, constrained by many new observations and measurements. Here we present an analysis of the global transfers of energy within the climate systems of a range of planets within the Solar System, including Mars, Titan, Venus and Jupiter, as modelled by relatively comprehensive radiative transfer and (in some cases) numerical circulation models. These results are presented in schematic form for comparison with the classical global energy budget analyses for the Earth, highlighting important similarities and differences. We also take the first steps towards extending this approach to other Solar System and extrasolar planets, including Mars, Venus, Titan, Jupiter and the ‘hot Jupiter’ exoplanet HD 189733b, presenting a synthesis of both previously published and new calculations for all of these planets.
We present and discuss here the average fields of the Venus atmosphere derived from the nighttime observations in the 1960–2350 cm−1 spectral range by the VIRTIS‐M instrument on board the Venus ...Express satellite. These fields include: (a) the air temperatures in the 1–100 mbar pressure range (~85–65 km above the surface), (b) the altitude of the clouds top, and (c) the average CO mixing ratio. A new retrieval code based on the Bayesian formalism has been developed and validated on simulated observations, to statistically assess the retrieval capabilities of the scheme once applied to the VIRTIS data. The same code has then been used to process the entire VIRTIS‐M data set. Resulting individual retrievals have been binned on the basis of local time and latitude, to create average fields. Air temperature fields confirm the general trends previously reported in Grassi et al. (2010), using a simplified retrieval scheme and a more limited data set. At the lowest altitudes probed by VIRTIS (~65 km), air temperatures are strongly asymmetric around midnight, with a pronounced minima at 3LT, 70°S. Moving to higher levels, the air temperatures first become more uniform in local time (~75 km), then display a colder region on the evening side at the upper boundary of VIRTIS sensitivity range (~80 km). As already shown by Ignatiev et al. (2008) for the dayside, the cloud effective altitude increases monotonically from the south pole to the equator. However, the variations observed in night data are consistent with an overall variation of just 1 km, much smaller than the 4 km reported for the dayside. The cloud altitudes appear slightly higher on the evening side. Both observations are consistent with a less vigorous meridional circulation on the nightside of the planet. Carbon monoxide is not strongly constrained by the VIRTIS‐M data. However, average fields present a clear maximum of 80 ppm around 60°S, well above the retrieval uncertainty. Once the intrinsic low sensitivity of VIRTIS data in the region of cold collar is kept in mind, this datum is consistent with a CO enrichment toward the poles driven by meridional circulation.
Key Points
Venus air temperatures above clouds are driven by atmosphere dynamics
Cloud altitude increases toward the equator also on the nightside
CO at 65‐70 km increases from 40S to 60S
► Thermal structure of Venus in the pressure range 100–4
mbar in the night. ► Cold collar feature is located at 60–70° on both hemispheres. ► Thermal structure supports a North–South symmetry. ► ...Sun-synchronous waves are present at 100, 32, 12, and 4
mbar and confirmed by the LMD Venus GCM model.
We present the spatial distribution of air temperature on Venus’ night side, as observed by the high spectral resolution channel of VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), or VIRTIS-H, on board the ESA mission Venus Express. The present work extends the investigation of the average thermal fields in the northern hemisphere of Venus, by including the VIRTIS-H data. We show results in the pressure range of 100–4
mbar, which corresponds to the altitude range of 65–80
km. With these new retrievals, we are able to compare the thermal structure of the Venus’ mesosphere in both hemispheres.
The major thermal features reported in previous investigations, i.e. the cold collar at about 65–70°S latitude, 100
mbar pressure level, and the asymmetry between the evening and morning sides, are confirmed here. By comparing the temperatures retrieved by the VIRTIS spectrometer in the North and South we find that similarities exist between the two hemispheres. Solar thermal tides are clearly visible in the average temperature fields. To interpret the thermal tide signals (otherwise impossible without day site observations), we apply model simulations using the Venus global circulation model Venus GCM (Lebonnois, S., Hourdin, F., Forget, F., Eymet, V., Fournier, R. 2010b. International Venus Conference, Aussois, 20–26 June 2010) of the Laboratoire de Météorologie Dynamique (LMD). We suggest that the signal detected at about 60–70° latitude and pressure of 100
mbar is a diurnal component, while those located at equatorial latitudes are semi-diurnal. Other tide-related features are clearly identified in the upper levels of the atmosphere.
Using a ground‐to‐exosphere general circulation model for Mars we have simulated the variability of the dayside temperatures at the exobase during eight Martian years (MY, from MY24 to MY31, ...approximately from 1998 to 2013), taking into account the observed day‐to‐day solar and dust load variability. We show that the simulated temperatures are in good agreement with the exospheric temperatures derived from Precise Orbit Determination of Mars Global Surveyor. We then study the effects of the solar variability and of two planetary‐encircling dust storms on the simulated temperatures. The seasonal effect produced by the large eccentricity of the Martian orbit translates in an aphelion‐to‐perihelion temperature contrast in every simulated year. However, the magnitude of this seasonal temperature variation is strongly affected by the solar conditions, ranging from 50 K for years corresponding to solar minimum conditions to almost 140 K during the last solar maximum. The 27 day solar rotation cycle is observed on the simulated temperatures at the exobase, with average amplitude of the temperature oscillation of 2.6 K but with a significant interannual variability. These two results highlight the importance of taking into account the solar variability when simulating the Martian upper atmosphere and likely have important implications concerning the atmospheric escape rate. We also show that the global dust storms in MY25 and MY28 have a significant effect on the simulated temperatures. In general, they increase the exospheric temperatures over the low latitude and midlatitude regions and decrease them in the polar regions.
Key Points
Eight Martian years simulated with global model including day‐to‐day variable solar flux and dust
Important effect of solar cycle and solar rotation on simulated exobase temperatures
Significant effects of planetary‐encircling dust storms on simulated exobase temperatures
The impact of gravity waves (GW) on diurnal tides and the global circulation in the middle/upper atmosphere of Mars is investigated using a general circulation model (GCM). We have implemented a ...stochastic parameterization of non‐orographic GW into the Laboratoire de Météorologie Dynamique (LMD) Mars GCM (LMD‐MGCM) following an innovative approach. The source is assumed to be located above typical convective cells (
∼250 Pa), and the effect of GW on the circulation and predicted thermal structure above 1 Pa (
∼50 km) is analyzed. We focus on the comparison between model simulations and observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter during Martian Year 29. MCS data provide the only systematic measurements of the Martian mesosphere up to 80 km to date. The primary effect of GW is to damp the thermal tides by reducing the diurnal oscillation of the meridional and zonal winds. The GW drag reaches magnitudes of the order of 1 m/s/sol above 10
−2 Pa in the northern hemisphere winter solstice and produces major changes in the zonal wind field (from tens to hundreds of m/s), while the impact on the temperature field is relatively moderate (10–20 K). It suggests that GW‐induced alteration of the meridional flow is the main responsible for the simulated temperature variation. The results also show that with the GW scheme included, the maximum day‐night temperature difference due to the diurnal tide is around 10 K, and the peak of the tide is shifted toward lower altitudes, in better agreement with MCS observations.
Key Points
A stochastic non‐orographic gravity wave (GW) scheme is implemented into the LMD‐MGCM
Non‐orographic GW generated above typical convective layers control diurnal tides
The implemented GW scheme improves the accuracy of the LMD‐MGCM between 1 and 0.01 Pa in comparison with MCS
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