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

Hinson and Wang2 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 studied using MGS observations (MOC, TES, RS)

3 December 2008Hinson and Wang3  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 Objectives

3 December 2008Hinson and Wang4

3 December 2008Hinson and Wang5

3 December 2008Hinson and Wang6

3 December 2008Hinson and Wang7

3 December 2008Hinson and Wang8

3 December 2008Hinson and Wang9

3 December 2008Hinson and Wang10

3 December 2008Hinson and Wang11 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 L s = 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

3 December 2008Hinson and Wang12

3 December 2008Hinson and Wang13

3 December 2008Hinson and Wang14

3 December 2008Hinson and Wang15 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]

3 December 2008Hinson and Wang16 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]

3 December 2008Hinson and Wang17

3 December 2008Hinson and Wang18

3 December 2008Hinson and Wang19 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

3 December 2008Hinson and Wang20

3 December 2008Hinson and Wang21 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

3 December 2008Hinson and Wang22 Meridional winds implied by geostrophic balance v = (2  sin  ) -1 ∂  /∂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)

3 December 2008Hinson and Wang23 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

3 December 2008Hinson and Wang24 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

3 December 2008Hinson and Wang25

3 December 2008Hinson and Wang26

3 December 2008Hinson and Wang27 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?

3 December 2008Hinson and Wang28 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