Atmospheric Dynamics on Solid-Surface Bodies

1. Atmospheric Dynamics on Solid-Surface Bodies Mark I. Richardson

2. What counts? • Continuum fluid • Mean free path << scale height • Particle size << mean free path • Bound atmosphere • Mean thermal velocity << escape velocity • Sharp bottom boundary • Planet has a surface

3. Considered here • Venus • Earth • Mars • Titan • Close, but no cigar: • Triton, Pluto

4. Determining characteristics • Surface-atmosphere interface: • Heat and momentum exchange • Very different radiative properties • Overall energy balance • Net solar energy in = net thermal out • Atmospheric motions modify thermal structure and are in turn modified by thermal structure

5. Balanced flow and secondary circulations • In thin (O(10-100 km)) atmospheres, large (O(100-1000 km)) motions are hydrostatic • Hydrostatic plus Co+Ce = thermal wind (T(y) -> u(z), T(y,z) -> u(y,z)) • Will always be some mixing of air across strong P gradients • Secondary circulations result as the atmosphere is strongly driven to hydrostatic • e.g. “Hadley” cell (note Hadley cell NOT fundamentally CONVECTION!)

6. Latitudinal Distribution of Heating • Net heating at equator, cooling at pole • BUT this is a consequence of atmospheric motions, NOT the original driver of them • Column-wise radiative eqm is a valid solution • Column-wise radiative eqm atmosphere corresponds to enormous available potential energy • Surface drag (mechanical or thermal convection)

7. What does it all boil down to? • Radiative forcing: • > column rad eqm • Latitudinal T, U, and Ps gradients: • > eddy and wave transports of mass, momentum, and/or heat are possible • T, U, and Ps linked: • > to retain balance, mean meridional circulations are induced

8. Fundamental Global Questions • What determines radiative heating? • What wave and eddy motions are important for transporting heat, momentum, and mass? • What mean meridional circulations result? • For the range of observable atmospheres and their variability, can we predict what mix of motions will occur?

9. “Branching” “pure” studies • Nature of convection near the surface • Waves (tides, Rossby waves, bouyancy “gravity” waves) • Eddy dynamics (flow instabilities, diurnal topographic flows)

10. “Branching” “dynamical feedback” studies • Dynamical feedbacks involving the generation of radiatively important clouds • Lifting and transport of radiatively active haze • Dynamical feedbacks involving latent heating due to trace or major atmospheric gas

11. “Implication” studies • Atmospheric modification of the surface: • Winds (dunes) • Precipitation (channels, lakes, ice caps) • Thermal structure and trace species mixing: • Chemistry • Dynamical feedback on climate history • Variation of surface environment over geological time

12. Methodology • Measurements of the circulation (direct, tracer track, thermal, etc.) • Zonal mean circulation • Eddy / wave components • Measurements of the forcing • Net energy deposition (OLR, absorbing layers, etc.) • Predictive modeling (not a competition, need both or you’re fooling yourself) • Conceptual or “toy” models (inc. axisymmetric) • Numerical modeling (fully three-dimensional)

14. Venus • We don’t know what controls the circulation • Zonal winds tracked from cloud measurements: • 0-40/50deg roughly const. zonal vel. • 40/50deg-pole roughly const. ang. vel. • Superrotating by more than factor of 50

16. Paradigm • A “Hadley” cell seems unavoidable • Zonal wind not ang. mom. conserving at cloud top - some torques needed • Waves / eddies modify the upper-branch • Shear instabilities? • Kelvin/Rossby waves? • Zonal velocity is “smeared” equatorward, instead of very strong polar jets

17. Paradigm • Does frictional Hadley cell explain superrotation? • Tidal torques? (“push” on cloud level with reaction force on surface) • Why isn’t momentum simply frictionally lost back to surface? (stability due to cloud deck?)

18. Observational constraints • Can assess eddy fluxes from u’v’ net correlations - but need day and night (VIRTIS will build this up with near IR images) • Need wind measurements at other levels

19. Models • Resurgence of Venus GCM’s • Venus has huge thermal mass - very slow system - “worst case scenario” for GCM modeling - increase in cpu power and “cheap” parallel computers are key • Pseudo idealized GCM’s • Models don’t use realistic radiative timescales

22. Good and bad from GCM’s • Consistent with the GRW mechanism for superrotation • A lot of variation in magnitude of circulation between models with identical forcing (not in nature of circ) • Not forced with realistic timescales

23. More issues… • Banding structure in clouds • Polar “hurricane” (modified Hadley downwelling?) • Time variability

26. Venus bottom line • We are still grappling with the basic mechanisms of the general circulation • The relative magnitude of major circulation components are unknown • Clouds and immense atmosphere make observations difficult • Venus is the most challenging terrestrial atm for GCM’s - timescales and apparent sensitivity of exquisite balances to details of numerical discretization

28. Titan - baby Venus? • In some ways easier to observe • Thermal sounding from Voyager, Cassini • Thinner atmosphere than Venus, but much colder, strong seasonality • Solsticial version of GRW mechanism? • Early GCM modeling says ‘yes’ • No current GCM can maintain meridional temperature structure and hence get much slower zonal winds than inferred from thermal obs???

29. Problems, needs • Regular mapping data needed (Titan orbiter) • Can’t get wave information from cloud tracking - need regular thermal mapping and/or regular sounding of zonal and meridional winds • Concomitant haze measurements • Probably along way off…

30. What extra do we have • Dune orientations • Major problem: tropical westerlies • How can these be representative of the mean flow and be consistent with momentum exchange? • Methane clouds • Do predominant formation latitudes indicate upwelling (or geology?) • Haze distribution • Tracer on upper level circulation

34. Methane cycle • Geology indicate “wetter” and “drier” latitudes, inc. lakes and channels • Global transport • Vigor of precipitation • Cloud dynamics modeling • How well do we predict precipitation vigor on Earth? • Patterns of convective structure

36. Mars • Fast system with wide variety of forcing (seasonality and dustiness) • What mechanisms control expansion of Hadley cell, change in wave modes • Large topography • Influence on circulation • Partial resolution - modeling and data • still not completely known, e.g. cf. Earth monsoons

39. Pressure cycle • Seasonal cycle of bulk atmosphere is understood in mechanistic sense • Unknown why strange cap optical properties are needed (hood clouds?) • Only two lander stations - how much large-scale dynamical influence?

41. Dust • Greatly modifies heating rates • Questions about: • How the global mean circulation is modified • How storms intensify from local to global • How do storms turn off • How homogeneous are storm systems • What determines interannual variability (stochastic, surface dust sources)

48. Challenge of observing in storms • Most difficulty locations to observe with IR sounding • Next step, microwave+IR • Imaging provides morphology of shape • Next step, some means of mapping lifting Storms aren’t whole story - what maintains the background haze (local storms, dust devils…)

49. Water cycle • Surface source asymmetry (but don’t really know why) • Atmospheric transport, and moderated by clouds to some degree (sensitivity to microphysics) • How much interaction with the subsurface (regolith adsorbate, ice)

50. Next steps for water • Vertical distribution of vapor with same mapping structure as temperature, with cloud (MCS) • Near-surface water vapor (REMS) • Also need to understand boundary layer better…