Lewis, S.R.

Montabone, L.

Read, P.L.

English

Open Research Online (Open)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsResults on dust storms and stationary waves in threeMars years of data assimilationConference or Workshop ItemHow to cite:Montabone, L.; Lewis, S.R. and Read, P.L. (2006). Results on dust storms and stationary waves in threeMars years of data assimilation. In: Second Workshop on Mars atmosphere modelling and observations, 27 Feb - 3Mar 2006, Granada, Spain.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://adsabs.harvard.edu/abs/2006mamo.conf..231MCopyright 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.ukRESULTS ON DUST STORMS AND STATIONARY WAVES IN THREE MARSYEARS OF DATA ASSIMILATION.L. Montabone, Dept. of Physics › AOPP, University of Oxford, Oxford, UK › now at Laboratoire de M·et·eorologie Dy›namique du CNRS, Paris, France (montabone@lmd.jussieu.fr), S. R. Lewis, Dept. of Physics & Astronomy, CEPSAR,The Open University, UK, P. L. Read, Dept. of Physics › AOPP, University of Oxford, Oxford, UK.IntroductionData assimilation is a technique which provides a com-plete, balanced, four-dimensional best-fit to observa-tions for all the atmospheric variables, including thosefor which no direct measurements are available, such aswind and surface pressure. It represents, therefore, aunique opportunity to study both the inter-annual vari-ability of the dust cycle and the behaviour of waves inthe atmosphere of Mars. The access to information onsurface wind stress, for instance, allows us to correlatedust storms and meteorology, and the global spatial andtemporal coverage gives enough resolution to extract theproperties of either stationary, transient or thermal tidewaves (Lewis and Barker, 2005), which are difficult tostudy with measurements from a single sun-synchronoussatellite.We assimilated almost three complete Martian yearsof atmospheric temperature profiles and total dust opac-ities below about 40 km altitude into a general circula-tion model of the atmosphere, the Oxford version of theAOPP/LMD Mars general circulation model (MGCM)(Forget et al., 1999). The assimilation scheme whichwe used is based on the sequential Analysis Correctionscheme of Lorenc et al. (1991), adapted and tuned forMartian data assimilation (Lewis et al., 1997, 2006).The data were provided by the Thermal Emission Spec-trometer (TES) in nadir mode during the Mars GlobalSurveyor spacecraft (MGS) mapping phase which beganin April 1999 and is still ongoing (though not routinelywith the TES instrument).In this paper we give some examples of the resultswhich we obtained with this technique. These resultsare related to two topics: 1) the study of the inter-annualvariability of dust storms and the conditions which cantrigger the onset of a planet-encircling dust storm such asthat of year 2001,and 2) the stationary waves in the lowerMartian atmosphere. The stationary wave analysis hasalso been used for the purpose of validation of the resultsof the assimilation while comparing to an independentand simultaneous source of observations, such as thetemperature profiles retrieved from the radio occultationmeasurements made with the ultra-stable radio oscillatoron board MGS (Hinson et al., 1999, 2001), and acquiredduring approximately the same period of time as theassimilated TES profiles.Inter›annual variability of dust stormsFigure 1: Evolution of dust storms in MY 24–26: distributionof the dust total optical depth normalized to 700 PaThe overall variability of dust loading in the atmo-sphere over the three observed dust seasons appears to bequite large, although there are features with repeatableseasonal and spatial characteristics in all three Martianyears (see Fig. 1 in Lewis et al., this issue). In particular,the picture highlights the extreme nature of the stormseason in Martian Year (MY) 25 with respect to theother two, which corresponds to the planet-encirclingdust storm of year 2001. The dust season in MY 25shows both an increase in dust opacity of about threetimes more than the other seasons and an earlier andmore rapid growth (see Fig. 1). The overall opacitystarts to increase monotonically at Ls ∼ 185◦ at a rate ofabout 0.14 per sol until Ls ∼ 195◦ (on average, over alatitude band 40◦N–60◦S. See also Fig. 2, upper panel),reaches its maximum at Ls ∼ 214◦, then decreases moreslowly over a range of more than 100 degrees of areo-Dust and waves in data assimilation: Montabone et al.centric longitude. This global dust storm originated asa regional storm between Hellas and Isidis Planitia andrapidly grew to planetary scale thanks to the eastward mi-gration and the consequent contribution of dust from theTharsis region and the plains south of the Tharsis ridge.This could be due to a positive feedback within the dustcloud which serves to strengthen the near-surface windstress, which in turn enhances the lifting of more dust(Newman et al., 2002). In MY 24 two regional duststorms developed in the Chryse region (at Ls ∼ 210◦ andLs ∼ 222◦) and in the Amazonis region (Ls ∼ 226◦),and faded out rapidly. MY 26 also exhibited a regionaldust storm developing between Hellas and Isidis Plani-tia, although later than the previous year, but this did notgrow sufficiently to produce a global storm, and fadedout after only 12 sols. Nevertheless, a secondary peakof dust loading showed in this year around Ls = 340◦,corresponding to regional storms which contributed to-wards local warming of the lower atmosphere aroundthe time of the landing of ESA’s Beagle 2 and NASA’sMars Exploration Rovers.Figure 2: The upper panel represents the average of the dustoptical depth at 700 Pa over the latitude band 40◦N–60◦S,smoothed in time by using a 1-sol running mean. The middlepanel is the average of the near-surface wind magnitude (windspeed at 4.6 m) over the same latitude band, and the lowerpanel is the average of the near-surface wind stress.The onset and early development of dust storms areconnected to the intensity of surface winds, which canbe local or a manifestation of an enhanced global cir-culation. The variability of dust storm occurrence isdetermined mostly by the interannual variability of thelocal meteorology, enhanced or reduced by a numberof positive or negative feedbacks which can originatefrom the the global circulation. Established dust storms,however, can have a big impact on the large scale vari-ability. The possibility of dust devils (Renno´ et al.,1998) triggering the onset of local dust storms shouldalso be considered, since the amount of dust lifted bythese atmospheric vortices can be considerable. Theirstrength, and therefore their ability to lift more dust, isdetermined by surface to air temperature difference andthermodynamic efficiency.In order to characterize and try to explain the interan-nual variability of the onset of dust storms, we thereforeanalysed in the assimilation the variables which can besomehow related to the process of dust lifting, namelythe near-surface wind stress (which depends on the windspeed), the surface to air temperature difference and thethermodynamic efficiency (Newman et al., 2002).The interannual variability of near-surface winds andnear-surface wind stress (at about 5m altitude) correlatesvery well in time with the variability of dust opticaldepth, as Fig. 2 shows. This picture plots the average ofnear-surface wind speed and wind stress over the latitudeband 40◦N–60◦S, smoothed by a 1-sol running mean,and compares them to the zonal mean of dust opticaldepth at 700 Pa averaged over the same latitude band.The comparison highlights that,on average, the dramaticincrease in optical depth at Ls ∼ 185◦ in MY 25 matchesperfectly with the steep increase both in the magnitudeof surface wind which, over 5 degrees of areocentriclongitude (about 8 sols), becomes about 35% larger thanin the other two years at the same time of year, and in thesurface wind stress. Such an increase of surface windmagnitude with respect, for instance, to the previous yearis at first concentrated around the northern and easternslopes of Hellas basin (high slope winds), then extendsto the Tharsis ridge, the Syria Planum and Solis Planum,south of Tharsis. According to the model of dust liftingdescribed in Newman et al. (2002), this in turn shouldincrease the amount of dust which is lifted in the air,although a preliminary study without transporting dustin the model was not able to reproduce the explosiveincrease of observed dust optical depth in MY 25 withrespect to the increase in the previous year. Future workwill address this issue in detail.The effects of the planet-encircling dust storm areevident not only in the change of the dynamics of theatmosphere but also in the increase of the temperaturebelow 60 km altitude, as shown in the upper panels inFig. 3 for the period Ls = 195◦–225◦. The temperatureincrease during the global dust storm (right panels) is aslarge as ∼ 30 K at a height of 20 km above the equator.This effect is due to the increased absorption of visibleradiation by the suspended dust, which in turn has theconsequence of cooling the surface by more than 20 Kin some locations during the day. The combination ofthese two effects yields a negative feedback on the de-velopment of dust devils during dust storms, reducingthe surface to air temperature difference, as describedby Newman et al. (2002) and shown in Fig. 4, whichis not compensated by any anomalous increase of ther-Dust and waves in data assimilation: Montabone et al.Figure 3: The plots show the temperature and mean merid-ional circulation averaged over 30 degrees of areocentric lon-gitude (Ls = 195◦ − 225◦) in two different years.modynamic efficiency (see Fig. 5). The anticorrelationbetween these two variables suggests that the role playedby convective vortices in lifting dust before and duringthe development of the global storm was marginal withrespect to the lifting of dust by near-surface wind stress,which seems to be the prevalent mechanism for duststorm initiation.Figure 4: Difference between MY 25 and MY 24 of the sur-face to air temperature difference at noon at 90◦E for four solsbefore and during the development of the dust storm aroundHellas which leads to the global storm in MY 25.Stationary wavesData assimilation with a general circulation model al-lows us to gain access not only to variables not directlyassimilated, but also to full global coverage, both inspace and time. The latter feature is essential for study-ing waves, transient and tidal in particular.Figure 5: Same as in Fig. 4 for the thermodynamic efficiencyAn important issue, nonetheless, is represented bythe validation of the obtained results, ideally performedagainst an independent and simultaneous source of ob-servations. We present here some results on waves whichcan be directly compared with observations. These arestationary waves which have also been measured bymeans of radio occultation soundings performed withthe ultra-stable radio oscillator on board MGS.A series of four direct comparisons between station-ary waves obtained with assimilation and those obtainedwith radio occultation (Hinson et al., 2001, 2004) havebeen performed, namely for Ls = 150◦–160◦ at lati-tude 67◦S in MY 24 and for Ls = 52◦–58◦ at latitude62◦N, Ls = 76◦–82◦ at latitude 65◦N and Ls = 99◦–109◦ at latitude 75◦N in MY 25. The comparison ismade by using the stationary component of the tem-perature field, i.e. the average of temperature over theindicated period from which the zonal average has beensubtracted. It is also possible to compare directly thewave-1 and wave-2 components of the field, obtainedthrough a fourth-order decomposition at each pressurelevel p, T ′(λ, p) =∑4s=1Cs(p) cos[sλ − γs(p)]], whereCs and γs are the amplitude and phase, respectively, atzonal wavenumber s, and λ is the longitude. Figure 6shows an example of these quantities.For the three last periods investigated, data assimi-lation shows a very good agreement with observations,although the MGCM does not resolve sharp vertical vari-ations of temperature, as expected, such as those that ap-pear in the first kilometers above the surface. For the firstexamined period, namely for the southern polar winter,however, the comparison highlights differences betweenthe model and the observations which are known to bepresent during polar night. These differences might inpart be related to problems of calibration for the TES ob-servations, but are also certainly related to an incompleterepresentation of CO2 processes (convection and forma-tion of CO2 clouds) during the Martian polar nights inthe model. Work is in progress to improve these modelparameterizations.The latter result highlights another advantage of dataREFERENCESFigure 6: Temperature field, temperature deviation field,wave-1 and wave-2 components at latitude 62◦N and Ls =55◦ in MY 25assimilation, which can be used to test the model perfor-mance and to discover possible problems in a systematicway.ConclusionsIn this paper we focused on some scientific results whichwere obtained by applying the data assimilation tech-nique on Mars. These results on some aspects of theinter-annual variability of the dust storms and on thestationary waves show the capabilities of this technique,which is able to give access to global, 4-D informationon all the atmospheric variables, even if the assimilatedmeasurements are limited to a set of asynchronous tem-perature profiles and total dust opacities.Further details about the topics discussed here canbe found in Montabone et al. (2005a) and Montaboneet al. (2005b)ReferencesForget, F., F. Hourdin, R. Fournier, C. Hourdin, O. Ta-lagrand, M. Collins, S. R. Lewis, P. L. Read, J.-P.Hout, 1999. Improved general circulation models ofthe martian atmosphere from the surface to above 80km. J. Geophys. Res. 104, 24,155–24,176.Hinson, D. P., Simpson, R. A., Twicken, J. D., Tyler,G. L., 1999. Initial results from radio occultation mea-surements with mars global surveyor. J. Geophys. Res.104 (E11), 26,997–27,012.Hinson, D. P., Tyler, G. L., Hollingsworth, J. L., Wil-son, R. J., 2001. Radio occultation measurements offorced atmospheric waves on mars. J. Geophys. Res.106 (E1), 1463–1480.Hinson, D. P.,M. D. Smith, B. J. Conrath, 2004. Compar-ison of atmospheric temperatures obtained through in-frared sounding and radio occultation by Mars GlobalSurveyor. J. Geophys. Res. 109 (E12002).Lewis, S. R., M. Collins and P. L. Read, 1997. Dataassimilation with a Martian atmospheric GCM: Anexample using thermal data. Adv. Space Res. 19(8),1267–1270.Lewis, S. R., Barker, P., 2005. Atmospheric tides in aMars general circulation model with assimilation ofMars Global Surveyor data. Adv. Space Res. 36(11),2162–2168.Lewis, S. R., P. L. Read, B. J. Conrath, J. C. Pearland M. D. Smith, 2006. Assimilation of ThermalEmission Spectrometer atmospheric data during theMars Global Surveyor aerobraking period. Submittedto Icarus.Lorenc, A. C., Bell, R. S., Macpherson, B., 1991. Themeteorological office analysis correction data assim-ilation scheme. Quart. J. R. Meteor. Soc. 117, 59–89.Montabone, L., S. R. Lewis, P. L. Read, 2005a. Interan-nual variability of Martian dust storms in assimilationof several years of Mars Global Surveyor observa-tions. Adv. Space Res. 36(11), 2146–2155.Montabone, L., S. R. Lewis, P. L. Read, and D. P. Hinson,2005b. Validation of Martian meteorological data as-similation for MGS/TES using radio occultation mea-surements. Icarus. Submitted.Newman, C. E., S. R. Lewis, P. L. Read and F. Forget,2002. Modeling the Martian dust cycle, 1. Represen-tations of dust transport processes. J. Geophys. Res.107(E12), 5123, doi:10.1029/2002JE001910.Newman, C. E. and P. L. Read and S. R. Lewis, 2004.Investigating atmospheric predictability on Mars us-ing breeding vectors in a gen eral circulation model.Q. J. R. Meteorol. Soc. , 130(603), 2971–2989.Renno´, N. O., M. L. Burkett and M. P. Larkin, 1998.A simple thermodynamical theory for dust devils. J.Atmos. Sci., 55, 3244–3252.

Results on dust storms and stationary waves in three Mars years of data assimilation

http://oro.open.ac.uk/4807/1/Montabone_Granada2006.pdf

Results on dust storms and stationary waves in three Mars years of data assimilation

Abstract

Similar works

Full text

Available Versions

Open Research Online (Open)