Weather in the Northern Hemisphere of Mars: Dust Storms and Baroclinic Eddies

1. Weather in the Northern Hemisphere of Mars: Dust Storms and Baroclinic Eddies David Hinson1 and Helen Wang2 1SETI Institute / Stanford University 2Smithsonian Astrophysical Observatory SETI Institute Mountain View, CA 3 December 2008

2. Annual Dust Cycle • Measurements by MGS Thermal Emission Spectrometer (TES) [e.g., M.D. Smith, Icarus, 2004] • Annual dust cycle in years without major global dust storm • Dust opacity peaks in midautumn and midwinter • Weather systems (baroclinic eddies) at high northern latitudes are believed to initiate these storms [Wang et al., GRL, 2003; Wang et al., JGR, 2005; Wang, Icarus, 2007] • Seasonal cycles of dust opacity and eddy activity are correlated [Wang et al., JGR, 2005; Hinson, JGR, 2006; Wang, Icarus, 2007] • Events in MY 24-26 studied using MGS observations (MOC, TES, RS) Hinson and Wang

3. Objectives • First close look at atmospheric dynamics in northern autumn of MY 27 • Investigate in more detail the relationship between baroclinic eddies and dust storms in the northern hemisphere • Use data obtained by MGS in its final martian year of operation • MOC images (top) • Atmospheric sounding by radio occultations (bottom) • Global coverage with IR sounder (such as MGS TES) was not available at this time Hinson and Wang

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11. Key Results from MDGMs • We are assembling a catalog of dust storms (chronology and morphology) to correlate with other observations • Regional dust storms occurred in all three major basins of the northern hemisphere (Acidalia, Arcadia, and Utopia) in MY 27 • Distribution is not uniform, with far more events in Acidalia (7) than in Arcadia (2) or Utopia (1) • During Ls = 221°-226° (sols 31-38), dynamic regional dust storms occur repeatedly in Acidalia, consisting of a frontal/flushing sequence with a distinct 2-day periodicity Hinson and Wang

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15. Samples of geopotential height at 610 Pa (with trend removed) • Longitude of successive measurements moves steadily westward (360° per sol) • Apply least-squares spectral analysis to identify eddy “modes” [Chapman et al., Proc. R. Soc., 1974; Salby, JAS, 1982; Lait and Stanford, JAS, 1988; Hinson, JGR, 2006] Hinson and Wang

16. Spectrum of space-time variations in geopotential height at 610 Pa • Expressed in terms of frequency f (cycles per sol) observed from polar orbit at fixed local time • Measurement longitude varies systematically with time of observation (as with all polar orbiters) • Does not yield unique solution for zonal wave number s and frequency  observed from fixed location on surface: f =  + s/T • Ambiguity can be resolved through comparisons with previous measurements by the Viking Landers [Barnes, JAS, 1980, 1981; Collins et al., Icarus, 1996] and MGS TES [Banfield et al., Icarus, 2004; Wang et al., JGR, 2005] Hinson and Wang

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19. Conclusions (part 1) • Baroclinic eddies evolve through a sequence of transitions among modes with different periods and zonal wave numbers • This is an important basic property of martian weather (Collins et al., Icarus, 1996; Hinson, JGR, 2006) • These baroclinic mode transitions strongly influence the timing of regional dust storms in the northern hemisphere • Large-amplitude wave-3 mode results in very strong meridional winds, favorable conditions for dust storms Hinson and Wang

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21. Time evolution of eastward-traveling, wave-3, baroclinic eddy • Instantaneous waveform at 8 time steps over 2 sols • Amplitude varies significantly with longitude, presumably due to interaction with topography [e.g., Hollingsworth et al., Nature, 1996] • Waveform is not symmetric about zero due to modulation by wave-2 stationary wave Hinson and Wang

22. Meridional winds implied by geostrophic balance • v = (2  sin)-1∂F/∂x • Traveling wave only • Amplitude varies significantly with longitude • Winds are strongest around Alba Mons (25 m/s), considerably weaker in Utopia (15 m/s) Hinson and Wang

23. Meridional winds implied by geostrophic balance • Both traveling wave and stationary waves • Circulation around Alba Mons is asymmetric • In Arcadia peak northward winds (30 m/s) are about twice as large as the peak southward winds (15 m/s) • Reverse is true in Acidalia, where the meridional winds have a strong southward bias Hinson and Wang

24. Conclusions (part 2) • Asymmetry of eddy wind field around Alba Mons favors regional storms in Acidalia, inhibits them in Arcadia and Utopia (consistent with observations) • Stationary waves (s=2) play an important role in this behavior • Strong, asymmetric winds associated with wave-3 baroclinic eddies influence both the timing and location of regional dust storms in the large topographic basins of the northern hemisphere Hinson and Wang

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27. Probability that wind magnitude exceeds 15 m s-1 • Main peak is associated with wave-3 baroclinic eddies (Ls=215°-240°) • Frontal/flushing storm events generally occur as winds intensify, then cease as winds diminish • Is surface reservoir of dust temporarily exhausted at key locations? Hinson and Wang

28. Future Directions • Revisit observations from previous years in light of these results; investigate interannual variability of northern winter weather • What mechanisms control the sequence and timing of baroclinic mode transitions? • What processes produce the modulated structure of the wave-3 eddy modes? • What causes dust storms to stop? • Evaluate performance of General Circulation Models (GCMs) against these new observations; use validated simulations to address key questions • Major goal is improved understanding of regional dust storms Hinson and Wang