The Role of Thermal Tides in the Martian Dust Cycle R. John Wilson NOAA/Geophysical Fluid Dynamics Laboratory Princeton, New Jersey
. Modeling studies have generally focused on simulating major dust storms in the SH summer solstice season due to the expectation that dust is most efficiently lifted and distributed by the Hadley circulation. . However, the observational record indicates that the dust cycle in most years is dominated by pre- and post-solstice regional dust lifting. In some years major dust storms occurred well before the solstice (Ls=270°), suggesting that the Hadley circulation does not necessarily play the dominant role in dust storm initiation and development. It is likely that thermal tides play a more prominent role. •
Winter Westerlies + baroclinic waves Thermal Tides Subtropical Jet Retreating polar cap boundary Polar CO, caps are shaded Units:  10-3 Nm-2
Mass Transport Streamfunction Strong, low-level circulation into the summer hemisphere Units: 108 kg/s Strong seasonal variation associated with the migration of the subsolar heating latitude off the equator Streamfunction:
circulation intensity Diurnal –mean, near-surface winds are not substantially altered; especially the zonal mean component
Global Scale Response to Diurnally-Varying Solar Forcing Migrating (Sun-synchronous)  Tides Winds are strongly divergent/convergent strong upward motion in the afternoon. S² ～ Dust Heating
W 380 Pa surface MY24 Simulation Diurnal Mean   ώ 104Pa/s Rising motion in the summer hemisphere
Hadley Diurnal Mean -14 x 10-4 Pa/s  at  Ls= 270° 1500 LT Tide Contribution -40 x 10-4 Pa/s  at  Ls= 235° Note Change in Color Scale
V Diurnal Tide Amplitude: Ls= 190° Maximum amplitudes at ~ ±30° Semidiurnal Amplitude:   V Ls = 238° contour interval 5 ms-1 ressure
U ressur Diurnal Tide Amplitude contour interval 3 ms-¹ Phase (shading  0-24 LT) V Meridional wind is equatorward in late afternoon Maximum amplitudes at ~ ±30° ressure
Tide component of wind dominates the diurnal mean Slope effects less influential away from the surface:  tide winds assume the characteristics of the sun-synchronous tide; especially for increased dust loading. Slope effects can increase the diurnal range of tide winds Counterclockwise rotation in the SH
Hodographs: Near surface x and y wind components as functions of local time Color indicates Local Time: Clockwise rotation in NH Counterclockwise in SH Black axes indicates semimajor and semiminor axis of fitted ellipse (least squares) Blue line indicates local slope direction Red line indicates diurnal mean wind
Observed 1.6 m Simulated 40 m
Lon= 256 E Tide wind + nighttime downslope wind
Contours of zonal mean U: Hadley circulation  (5 m/s intervals) 0-20 m/s Fixed Dust simulation T= 0.5 Diurnal U Amplitude Diurnal V amplitude Semidiurnal V amplitude 0-20 m/s Contours of zonal mean field: Hadley circulation
Winter Westerlies + baroclinic waves Thermal Tides Subtropical Jet Retreating polar cap boundary Polar CO, caps are shaded Units:   10-3 Nm-2 MOC Image Dust front at 24° S and 250° E MGCM Simulation: 1400 LT Meridional Winds dominated by Tides 10 ms-¹ intervals V
• The significant influence of tides on Mars is in notable contrast with the terrestrial atmosphere. Low level wind variability on Mars is dominated by tides. Of course, slope wind effects (nonmigrating tides) are a major influence as well. • The seasonal variation in diurnal tide winds appears to be correlated with the pre- and post- solstice regional storm activity. • A negative feedback mechanism that can account for dust storm decay is still missing. The intensity of winds associated with the tides and the Hadley circulation are positively correlated with dust opacity. The availability of finite mobile surface dust deposits is an obvious possibility for limiting dust lifting in a particular region. •Vertical transport of dust out of the boundary layer is evidently dominated by migrating and nonmigrating tides  in MGCM simulations. The Hadley circulation is still the prime circulation element for global scale transport. • Simulations with 2°x2° spatial resolution are not able to represent small-scale convective plumes that may be important for vertical transport of dust into the free atmosphere.
Deep, meridionally broad heating projects very efficiently onto the main semidiurnal mode. O 3 contributes to zonally uniform response Diurnal tide is weaker, more localized to continental regions . Covey et al.,  JAS 68, 2011
4 Year record at Viking Lander 1 Amp= P tid High degree of regularity in the L s =0-180 period
Semidiurnal Tide:   S₂(p) 1 % Amplitude Consistent Tide response over 4 Mars years:  MY12-15 Ls= 100
Wave 2 Interference Pattern . S 1 & DK1 modes dominant . Simultaneous Phase Advance at two lander sites for diurnal tide as DK1 increases –As observed Wave 4 Interference Pattern . S, &  SDK2 modes dominant . Simultaneous Phase Delay for Semidiurnal tide as SDK2 increases As observed
Fixed Dust simulation: Local Tide Migrating component Migrating tide phase is relatively invariant Relatively little variation in migrating tide amplitude over season (~40%)---much larger longitudinal variation. Westward migrating solar radiation modulated by topographic influences cos(Ωt + λ)   cos2λ cos(Ωt + 3λ) + cos(Ωt - λ)
DK1 Similarly,   cos(2Ωt + 2λ) cos4λ    yields    cos(2Ωt - 2λ)   SDK2 cos(Ωt - 2λ) DK2 Similarly,   cos(Ωt + λ) cos3λ   yields Migrating tides are scattered into nonmigrating tides; induced upslope/downslope winds play a significant role
Fixed dust simulation --- Simulated SD at VL1 --- S2 mode only --- S2 + SDK2
Reanalysis Control Run and Fixed Dust run  (black)
MCS
LMD MCD5
Figure 30. Longitude/pressure sections of equatorial cloud opacity (top row) and temperature (bottom row). Temperature contoured in intervals of 5 K in all plots. Cloud opacity Δτ is shaded in units of 10 -3 km-1
Mars Climate Sounder   MCS Zonally-averaged Temperature 0-80 km Tavg= (T3pm + T3am)/2 Diurnal Average* Tdiff= (T3pm - T3am)/2 Sun-Synchronous Tide* aka Migrating Tide Nonmigrating Tide Forcing Topographically Locked Nighttime Water Ice Clouds
and Zonal Wave Components Arrows show pairs of possible wave modes Prominent DK1 (m=2) & DK2 (m=3) & DK4(m=4)
. Well-defined forcing period: Atmospheric response determined by the horizontal and vertical structure of the forcing: For Mars, sensible and radiative exchange with the surface and absorption of insolation by airborne dust are dominant forcing mechanisms. • Well-developed Linear Tide Theory provides a basis for relating temperature structure and forcing. • Examples of diurnal variability in the Martian atmosphere • Temperature Structure . Diurnal variation in boundary layer winds:  dependence on slope and dust Surface pressure variation, focusing on the dependence of the migrating • semidiurnal tide on aerosol opacity .
F(λ, t) ~   Σ F s, σ cos[sλ + σt]; F(λ, tLT) ~   Σ F s,σ cos[ (s-σ)λ + σtLT] = Σ F s,σ cos[σtLT]
X-track and along-track observations  yield up to 6 local times Allows fitting of diurnal and semidiurnal harmonics of the sun-synchronous tide Diurnal Tide Amplitude Semidiurnal Tide Amplitude Semidiurnal Tide:  5-8 K amplitude in tropics  !!
1977a Storm 1977b Storm Viking IRTM T 15 ( 0.5 hPa  or ~ 25 km ) Observed Latitude x Local Time Simulated (MGCM) binned in local time and zonally averaged
Dust distribution is shaped by the Hadley circulation Global distribution
S 1 stronger in 1977a than 1977b, and at a higher latitude:  consistent with the influence of zonal mean westerlies at Ls=225 in 1977a. S, stronger for 1977b– Significantly higher dust opacity in the 2’d storm. Latitude UK Reanalysis:   Equatorial Semidiurnal P SFC Amplitude and Dust Opacity S 2 (P sfc)
F = (α + βτ) {cos(δ) R-²} α = 0.48;     β = 1.32 Seasonal variation in equatorial (symmetric) insolation based on orbital radius R and declination δ α  is due to boundary layer heating τ ~ 0.5
No Clouds Radiatively Active Water Ice Clouds
Seasdust.free TES.Seasdust MCS.seasdust MCS.seasdust.icecloud
The evidence for coupling between tropical clouds and the thermal tide first seen in MGS Radio Science observations has been reinforced with the much more extensive and comprehensive data returned from MCS. •The presence of strong elevated nighttime temperature inversions in the Tharsis region is a robust feature of the equatorial atmosphere during the L s = 0-140° season, with little difference seen between the two Mars years examined (MY 29 and 30). •The tropical structure appears to evolve over the spring and summer seasons in response to the waxing and waning of tropical cloud opacity. MGCM simulations suggest that radiative forcing by water ice clouds plays a significant role in establishing the observed structure. •The zonal temperature anomalies described here are dominated by tide modes that include eastward propagating, diurnal period Kelvin waves and shorter vertical wavelength westward propagating tide modes. MCS temperature and cloud observations will provide valuable guidance and constraints for future model development. •Modeling of 32 micron (~surface) brightness temperature with radiatively active clouds should yield estimates column cloud opacity. MCS retrievals do not provide this observation and retrievals are limited by optically-thick clouds. •The vertical extent of clouds should be strongly influenced by cloud microphysics