Regional, diurnal and seasonal variations of surface

temperature are particularly large on Mars. This is mostly due to the Martian surface remaining close to radiative equilibrium. Contrary to most terrestrial locations, contributions of sensible heat flux (i.e. conduction/convection exchanges between atmosphere and surface) to the surface energy budget [hereinafter SEB] are negligible on Mars owing to lowatmospheric density and heat capacity (e.g. Figure 2 in Savijärvi and Kauhanen, 2008). This radiative control of surface temperature is a key characteristic of the Martian environment and has crucial consequences on the the Martian geology, meteorology, exobiology, etc.



In order to identify the impact of this Martian peculiarity to near-surface regional-to-local atmospheric circulations,

we employ our recently-built Martian limited-area meteorological model (Spiga and Forget, 2009). We use horizontal resolutions adapted to the dynamical phenomena we aim to resolve: from several tens of kilometers to compute regional winds (mesoscale simulations) to several tens of meters to compute atmospheric boundary-layer winds (microscale or turbulent-resolving simulations, also called Large-Eddy Simulations, LES)

Forget, F.

Hinson, D. P.

Lewis, S. R.

Madeleine, J.-B.

Millour, E.

Montabone, L.

Spiga, A.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsMartian meso/micro-scale winds and surface energybudgetConference or Workshop ItemHow to cite:Spiga, A.; Forget, F.; Madeleine, J.-B.; Montabone, L.; Millour, E.; Lewis, S. R. and Hinson, D. P. (2011).Martian meso/micro-scale winds and surface energy budget. In: Fourth International Workshop: Mars AtmosphereModelling and Observations, 8-11 Feb 2011, Paris, France.For guidance on citations see FAQs.c© 2011 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:http://www-mars.lmd.jussieu.fr/paris2011/abstracts/spiga1 paris2011Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukMARTIAN MESO-/MICRO-SCALE WINDS & SURFACE ENERGY BUDGET.A. Spiga, F. Forget, J.B.Madeleine, L.Montabone, E.Millour, Laboratoire deMétéorologie Dynamique, UniversitéPierre et Marie Curie, France, S. R. Lewis, Department of Physics and Astronomy, The Open University, UnitedKingdom, D. P. Hinson, Carl Sagan Center, SETI Institute, Mountain View, USA.Introduction and general discussionRegional, diurnal and seasonal variations of surfacetemperature are particularly large on Mars. This ismostly due to the Martian surface remaining close toradiative equilibrium. Contrary to most terrestrial lo-cations, contributions of sensible heat flux (i.e. con-duction/convection exchanges between atmosphere andsurface) to the surface energy budget [hereinafter SEB]are negligible on Mars owing to low atmospheric densityand heat capacity (e.g. Figure 2 in Savijärvi and Kauha-nen, 2008). This radiative control of surface temperatureis a key characteristic of the Martian environment andhas crucial consequences on the the Martian geology,meteorology, exobiology, etc.In order to identify the impact of this Martian pecu-liarity to near-surface regional-to-local atmospheric cir-culations, we employ our recently-built Martian limited-area meteorological model (Spiga and Forget, 2009).We use horizontal resolutions adapted to the dynami-cal phenomena we aim to resolve: from several tens ofkilometers to compute regional winds (mesoscale sim-ulations) to several tens of meters to compute atmo-spheric boundary-layer winds (microscale or turbulent-resolving simulations, also called Large-Eddy Simula-tions, LES).Our discussions herein focus on the contributionto surface energy budget of sensible heat flux, whichcouples atmospheric temperature, surface temperatureand near-surface winds. We show that, while day-time boundary layer convection indicates the promi-nence of radiative forcing over sensible flux over flatterrains (Spiga et al., 2010, and first section of this ab-stract), this is no longer true over slopes, as e.g. night-time katabatic winds can cause sensible heat flux tobecome comparable to radiative flux (Spiga et al., 2011,and second section of this abstract). The contribution ofsensible heat flux to the surface energy budget has beenoverlooked thus far: the Martian surface and atmosphereare more coupled than hitherto thought.SEB drives wind: exotic boundary layer convectionIn idealized conditions simulating an infinite flat plain,turbulent motions responsible for boundary layer mixingin afternoon convective conditions are resolved throughLarge-Eddy Simulations with grid spacing 50 m (Spigaet al., 2010). Mixing layer depths in various MartianFigure 1: Mixing layer depths in various Martian regions have beendetermined through Mars Express radio-occultations. In low latitudes,the Martian convective boundary layer appears to extend at higher al-titudes over high plateaus than in lower plains despite similar surfacetemperatures, a behaviour which is reproduced by Large-Eddy Sim-ulations including radiative transfer. Top plots Topographical map ofthe Amazonis/Tharsis region and vertical profiles of potential temper-ature obtained through radio-occultations on board Mars Express or-biter. Bottom plots Horizontal/vertical section of vertical velocity (ar-bitrary units) showing 50 m LES predictions of mixing layer heights.Martian Meso-/Micro-Scale Winds & Surface Energy Budgetregions have been determined through Mars Expressradio-occultations (Hinson et al., 2008), hence can becompared to model predictions to help to identify the un-derlying physical processes governing their behaviour.In low latitudes, both in observations and in simulations,the Martian convective boundary layer appears to extendat higher altitudes over high plateaus (e.g. Tharsis) thanin lower plains (e.g. Amazonis) despite similar surfacetemperatures. Through Large-Eddy Simulations (Figure1), it is possible to relate such behaviour to the dominantradiative forcing of the Martian boundary layer througha so-called “pressure effect" (equations and detailed ex-planations can be found in Spiga et al., 2010, note alsothat a similar effect is observed in very dusty boundarylayer in the Saharian desert). Mars appears in strikingcontrast with terrestrial dry conditions where sensibleheat flux dominates (Spiga, 2011). New scaling lawsmust be built for the Martian example to account for theturbulent heat flux not being maximum near the surfacebut a few hundreds meters above it.Figure 2: The evolution of daytime coinvective boundary layerheight for a set of LES results in which various dust opacity or back-ground wind were assumed. Simulations have been carried out allthings being equal, in a “reference" Meridiani Terra site. Note thatdust opacity is supposed to be uniform in the whole domain (well-mixed hypothesis).Figure 2 shows that background wind and dust opac-ity can influence the mixing layer depths too, but more assecond-order contributions. It is not possible to accountfor the nearly 3 km boundary layer depth difference be-tween the Tharsis and Amazonis cases (cf. Figure 1)with even the most extreme combination of those twofactors.LES modeling also showed that maximum frictionvelocities in the boundary layer (caused by dust devils)follow the same tendency as the boundary layer depth:larger velocities are observed in regions where surfaceis higher and pressure is lower (Spiga and Lewis, 2010,and Lewis et al., this issue, Figure 2). This could explaine.g. why dust devils are found over Arsia Mons wheredensity is very low (Reiss et al., 2009), hence liftingof dust from the surface much more difficult than inlower-latitude Martian terrains. We found that enhancedfriction velocity over the volcano would be large enoughto compensate the decrease in density to yield a similarwind stress as in lower-altitude terrains.SEB is driven by winds: slope thermal departuresTo describe circulations in the vicinity of Martian to-pographical obstacles, mesoscale simulations are em-ployed. We found that surface temperature can be im-pacted by mesoscale atmospheric circulations associatedto the presence of topography. According to nighttimeTES measurements in various non-dusty seasons, theMartian surface is up to 20 K warmer on slopes than onsurrounding plains in the Olympus Mons/Lycus Sulciregion.Thus far, it has been thought that such behaviourwas due to contrasts in soil thermophysical properties,especially thermal inertia (see e.g. Mellon et al., 2000,for a comprehensive description of this parameter and itsretrieval from orbit through infrared spectrometry). Weshow through mesoscale modeling that those warm sur-face departures in the night, correlated with slope steep-ness, could just as well be accounted for by mesoscale at-mospheric winds only, notably powerful katabatic windswhich form over Martian sloping terrains all night longin various seasons. Observed nighttime warmings aroundOlympus Mons are indeed qualitatively and quantita-tively reproduced in dedicated mesoscale simulationswith uniform surface thermophysical properties (Figure3). Katabatic winds are found to have a twofold impacton the Martian local atmosphere and surface: 1. theirvertical component yields adiabatic compression of air,thereby atmospheric heating that could overwhelm in-frared radiative cooling; 2. their along-slope horizontalcomponent enhances sensible heat flux, thereby allow-ing for atmospheric heating described in point 1 to warmthe surface, as sensible heat flux becomes comparableto radiative contributions [see Figure 4].Despite the low density of the Martian atmosphere,sensible heat flux cannot be systematically neglectedin surface energy budget: over Martian slopes in thenight, it can become comparable to longwave radiativeloss. Hence surface temperature can be far from ra-diative equilibrium over numerous sloping terrains onMars, where the atmosphere adopts a more earth-likebehaviour. Part of the surface temperature signal con-veys information about slope winds.One consequence is that warm signatures of surfacetemperature over slopes, observed through infrared spec-trometry, cannot be systematically associated to con-trasts of intrinsic soil thermal inertia. Apparent thermalinertia maps retrieved thus far possibly contain wind-induced structures. This also makes surface temperaturemaps in uneven terrains of great potential value to vali-date katabatic wind speeds predicted through mesoscaleMartian Meso-/Micro-Scale Winds & Surface Energy BudgetFigure 3: Mesoscale modeling show that 15-20 K nighttime sur-face warmings over the slopes of Olympus Mons observed by thermalinfrared spectrometry, hitherto believed to be related to thermal iner-tia contrasts, are caused by powerful katabatic circulations. Top plotMesoscale model predictions of horizontal wind and surface tempera-ture over the Olympus Mons volcano in northern fall nighttime condi-tions (predictions for surface temperature are in close agreement withobservations of thermal infrared spectrometry despite an assumed uni-form thermal inertia in the simulations). Bottom plot Apparent ther-mal inertia retrievals based on surface temperature measurements ob-tained through infrared spectrometry (Putzig and Mellon, 2007).modeling, provided systematic intrinsic thermal inertiacontrasts can be ruled out through geological analysis.We showed that katabatic winds can have a strongthermal impact, in addition to their well-known aeo-lian influence. The impact of mesoscale winds on sur-face temperature (hence possibly on retrievals of appar-ent thermal inertia) is particularly prominent over steepslopes in low thermal inertia terrains. Olympus Monsis only an example amongst others: observations andmesoscale modeling indicate that e.g. Terra Meridianicratered terrains are prone to the same phenomenon.Figure 4: Despite the low density of the Martian atmosphere, sen-sible heat flux cannot be systematically neglected in surface energybudget: over Martian slopes in the night, it can become comparable tolongwave radiative loss. Hence surface temperature can be far fromradiative equilibrium over numerous sloping terrains on Mars. TopDownward longwave radiative flux (W m−2) and bottom downwardsensible heat flux (W m−2) at the surface predicted in the OlympusMons/Lycus Sulci area by the mesoscale simulation. Topography iscontoured. Sensible heat exchanges warm the surface in white areas(bottom plot).Our conclusions offer the possibility of improvedfuture thermal inertia retrievals, unambiguously repre-senting intrinsic characteristics of Martian soil, whichwould complement e.g. the recent efforts by Putzigand Mellon (2007) to take into account surface hetero-geneities. The effect is even general enough to applyunder daytime conditions, thereby providing a possibleexplanation for observed afternoon surface cooling, andto ice-covered terrains, thereby providing new insightson how winds could have shaped the present surface ofMars [Figure 5].REFERENCESFigure 5: The effect is general enough to apply to ice-covered ter-rains, thereby providing new insights on how winds could have shapedthe present surface of Mars. Downward sensible heat flux (W m−2)and superimposed horizontal wind vectors 10 m above local sur-face (m s−1) predicted in the northern polar region by a mesoscalesimulation with 10 km horizontal resolution and polar stereographicprojection. Season is northern spring, universal time 09:00. Topogra-phy is contoured. Vectors each 3 grid points. Sensible heat exchangesact to warm the surface in yellow/red areas of this figure.ConclusionThis work exemplifies how studying the Martian meso-and micro-scale circulations through observations ormodeling: 1. gives insights into key characteristics ofthe Martian environment, through phenomena usuallyleft unresolved by general circulation models; 2. resultsin constraints not only on the meteorology but also onthe geology of Mars; 3. represents a rich source forcomparative planetology approachReferencesHinson, D. P., Pätzold, M., Tellmann, S., Häusler, B.,and Tyler, G. L. (2008). The depth of the convectiveboundary layer on Mars. Icarus, 198:57–66.Mellon, M. T., Jakosky, B. M., Kieffer, H. H., and Chris-tensen, P. R. (2000). High resolution thermal iner-tia mapping from the Mars Global Surveyor ThermalEmission Spectrometer. Icarus, 148:437–455.Putzig, N. E. and Mellon, M. T. (2007). Apparent ther-mal inertia and the surface heterogeneity of Mars.Icarus, 191:68–94.Reiss, D., Lüsebrink, D., Hiesinger, H., Kelling, T.,Wurm, G., and Teiser, J. (2009). High Altitude DustDevils on Arsia Mons, Mars: Testing the Green-house and Thermophoresis Hypothesis of Dust Lift-ing. In Lunar and Planetary Institute Science ConferenceAbstracts, volume 40 of Lunar and Planetary Inst. Tech-nical Report, pages 1961–+.Savijärvi, H. and Kauhanen, J. (2008). Surface andboundary-layer modelling for the Mars ExplorationRover sites. Quarterly Journal of the Royal Meteorologi-cal Society, 134:635–641.Spiga, A. (2011). Elements of comparison between mar-tian and terrestrial mesoscale meteorological phenom-ena: Katabatic winds and boundary layer convection.Planetary and Space Science, in press.Spiga, A. and Forget, F. (2009). A new model to simulatethe Martian mesoscale and microscale atmosphericcirculation: Validation and first results. Journal ofGeophysical Research (Planets), 114(E13):2009–+.Spiga, A., Forget, F., Lewis, S. R., and Hinson,D. P. (2010). Structure and dynamics of the con-vective boundary layer on mars as inferred fromlarge-eddy simulations and remote-sensing measure-ments. Quarterly Journal of the Royal Meteorological So-ciety, 136:414–428.Spiga, A., Forget, F., Madeleine, J., Montabone, L.,Lewis, S. R., and Millour, E. (2011). The impact ofmartian mesoscale winds on surface temperature andon the determination of thermal inertia. submitted toIcarus.Spiga, A. and Lewis, S. (2010). Mesoscale and mi-croscale wind variability of relevance for dust lifting.Mars, 5:146–158.

Martian meso/micro-scale winds and surface energy budget

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Martian meso/micro-scale winds and surface energy budget

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