Workshop on Planetary Atmospheres (2007) 9077.pdf LARGE-SCALE EXTRATROPICAL CYCLOGENESIS AND FRONTAL WAVES: EFFECTS ON MARS DUST. J.L. Hollingsworth1, M.A. Kahre1, R.M. Haberle1, 1Space Science and Astrobiology Division, Planetary Systems Branch, NASA Ames Research Center, Moffett Field, CA 94035, ([email protected]). Introduction: Mars reveals similar, yet also vastly scimitar-shaped dust fronts in the northern extratropi- different, atmospheric circulation patterns compared to cal and subtropical environment during late autumn and those found on Earth [1]. Both planets exhibit ther- early spring [8,9]. mally indirect (i.e., eddy-driven) Ferrel circulation cells Results: The time and zonally-averaged tempera- inmiddleandhighlatitudes. Duringlateautumnthrough tureandzonalwindfieldfromourbaselinehigh-resolution early spring, Mars’ extratropics indicateintense equator- (i.e., 2.0 × 3.0◦ longitude-latitude) simulation is shown to-pole temperature contrasts (i.e., mean “baroclinic- in Fig. 1. The mean zonal temperatures appear rather ity”). From data collected during the Viking era and re- symmetric about the equator. Upon closer inspection, it cent observationsfromtheMarsGlobal Surveyor(MGS) can be seen that in the northern extratropics the north- mission, such strong temperature contrasts supports in- south temperature contrasts at this season, particularly tense eastward-traveling weather systems (i.e., transient near the surface, are significantly stronger than in the synoptic-period waves) [2,3,4] associated with the dy- southern hemisphere. This asymmetry in mean zonal namical process of baroclinic instability. The travel- fields is also apparent in the mean zonal wind (Fig. 1, ing disturbances and their poleward transports of heat bottom) where the northern hemisphere’s westerly po- and momentum, profoundly influence the global atmo- lar vortex is roughly twice as strong than in the south, spheric energy budget. In addition, the transient baro- with peak wind speeds of O(120 m s−1). That this clinic waves impact other atmospheric scalars (e.g., tem- season can support vigorous synoptic-period transient perature, horizontal winds, dust mixing ratio, etc). weather systems in the extratropics and subtropics, can Recently, global circulationmodels havingvery high be seen in Fig. 2. This figure shows a variety of near- resolution have been implemented to investigate Mars’ surface meteorological fields (i.e., within 1 km of the baroclinic waves, both in a simplified,mechanistic frame- surface) at three different times associated with the large- work [5] and in a more realistic setting [6]. Our goal scale cyclogenesis and subsynoptic-scale frontal waves. is to ascertain the atmospheric environmental conditions The color-shaded field corresponds to surface pressure under which near-surface and/or upper-level fronts (i.e., anomalies. A series of extratropical cyclones and anti- narrow zones with enhanced mass density, momentum cyclones that develop, intensify and decay can be clearly and thermal contrasts within individual transient baro- noted. Tracking of the anti-cyclone just west of the clinic waves) form in Mars’ high latitude baroclinic prime meridian in the top-most panel indicates a east- zone, and whether the dynamical development, inten- west (zonal) phase speed, c(x), of O(20 m s−1). The sification and decay of such frontal waves can be as- weather systems are often most intense just in the lee of sessed using modern diagnostics. Further, we wish to the Tharsis highlands, in the Acidalia/Chryse regions. investigatetheinteractionsofthesefrontal-wavesystems This region has been recognized to be an active storm with Mars’ highly-variable surface relief (i.e., on large- zone within the western hemisphere [10]. scales), and the interactions with other atmospheric cir- It can be seen in Fig. 2 that the low-level horizon- culation components. tal wind exhibits a distinct line of convergence which Model: Using very highhorizontalresolutionglobal is associated with two (one middle and the other high- circulation models that are driven by highly simplified latitude) low-pressure anomalies that merge in time. The physical parameterizations (i.e., an SGCM approach), appearance of this line of flow convergence shows sig- we have been modeling over the last several years [5] natures associated with frontogenetic processes: that cyclogenesis and frontal waves in the atmosphere of is, shear and stretching deformations, and flow contrac- Mars in order to mechanistically ascertain dynamical tions/dilatations which significantly alter atmospheric processes associated with baroclinic disturbances. In scalars (e.g., temperature, dust mixing ratio, etc). Also this investigation, we build upon that mechanistic ef- depicted in the figure are contours of instantaneous sur- fort by utilizing a state-of-the-art, full-physics general face stress (indicated in white, with regions that exceed τ ∗ circulation model (GCM) for Mars. The particular ver- the threshold value of 0 = 22.5 mPa indicated in red). sion adapted implements a full radiatively-active dust It can be seen from the figure that a broad region in cycle for a range of dust particle sizes, wherein dust lift- the Tharsis highlands exists where instantaneous stress ing is parameterized in terms of dust-devil and surface- values exceed the threshold value. Large stresses are stress schemes which are self-consistent with the atmo- often associated with strong anti-cyclonic circulations spheric environmental conditions as simulated by the (i.e., a near-surface ridge) just upstream (to the west of) model [7]. This model includes the lifting, transport the developing frontal wave further to the east. Fre- and sedimentation of radiatively-active dust. Our ap- quently, the strongest gradient in surface stresses occurs proach is motivated by recent MGS imaging from the just upstream of the near-surface front. Mars Orbiter Camera (MOC) which indicate large-scale, Summary: The impositionMars’ strong baroclinic- spiralling, “comma”-shaped dust cloud structures and ity supports intense eastward traveling weather systems, Workshop on Planetary Atmospheres (2007) 9077.pdf ◦ Fig. 2: Longitude-latitude sections at Ls = 350 of the instantaneous surface pressure anomalies (percent de- viations from a global mean value of 6.11 mbar); the instantaneous time deviations of the low-level vector horizontal wind (m s−1); and, the instantaneous surface t t t t Fig. 1: The time and zonally averaged (a) tempera- stress magnitudes (mPa) at (a) = 0; (b) = 0 +6 hr; − t t ture (K) and (b) zonal wind (m s 1) during late north- and (c) = 0 +12 hr. The surface stress contour inter- ◦ ern winter (Ls = 350 ) from an annual, high-resolution val is 5 mPa and regions exceeding the threshold value radiatively-active dust gcm simulation which imposes of 22.5 mPa are highlightedin red. both dust-deviland wind-stress dust lifting. The contour intervalis10K and20m s−1 in (a) and (b), respectively. J. Atmos. Sci., 37, 2002; Barnes J.R. (1981) J. Atmos. Sci., 38, 225. [3] Barnes J.R. (2003) Mars Atmosphere Modelling and Observations workshop, Granada, Spain particularly in northern late winter/early spring that on (abstract); Banfield D. et al. (2004) Icarus, 170,365. [4] large scales have accompanying sub-synoptic scale ram- Hinson D.P.and Wilson,R.J. (2002) Geophys. Res. Lett., ifications on the atmospheric environment through cy- 29, 10.1029/2001GL014103.; Hinson D.P. (2006) J. clonic and anti-cyclonic winds, deformations and con- Geophys. Res., 111, 10.1029/2005JE002612. [5]Hollin- tractions/dilatations in temperatures,and sharp perturba- gsworth J.L. (2003) Mars Atmosphere Modelling and tions amongst atmospheric tracers. Compared to models Observations workshop, Granada, Spain (abstract); [6] of cyclo-, fronto-genesis on Earth, Mars’ extratropical Wilson R.J. (2006) AGU Fall Meeting, San Francisco, weather systems appear short-lived. Our high-resolution CA (abstract); [7] HaberleR.M. et al.(1999) J. Geophys. simulations utilizing the NASA Ames Mars general cir- Res., 104, 8957; Kahre M.A. et al. (2006) J. Geophys. culation model with a consistent, interactive dust cy- Res., 111, doi:10.1029/2005JE002588. [8] James P.B. cle indicate that cyclogenetic and frontal-wave circula- and CantorB.A. (2001) Icarus, 154,131. [9]WangH.et tions can significantly alter dust transport and structuring al.(2003) Geophys. Res. Lett., 30, 10.1029/2002GL0168- (bothhorizontallyand vertically)withinthe atmosphere. 28.; WangH.et al.(2005) J. Geophys. Res., 110,doi:10.- Modeling the circulation at high spatial resolution is nec- 1029/2005JE002423. [10] Hollingsworth J.L. et al. essary in order to illuminateprocesses important to local (1996) Nature,380, 413; Hollingsworth J.L.et al.(1997) and regional dust activity. Adv. Space Res., 19, 1237. References: [1] Zurek R.W. et al. (1992) in Mars, Acknowledgements: This research has been sup- editedby Kieffer H.H. et al., 835. [2]Barnes J.R. (1980) ported by the NASA Planetary Atmospheres Program..