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 youre fooling yourself) –Conceptual or toy models (inc. axisymmetric) –Numerical modeling (fully three-dimensional)

14. Venus We dont 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 isnt momentum simply frictionally lost back to surface? (stability due to cloud deck?)

18. Observational constraints Can assess eddy fluxes from uv 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 GCMs –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 GCMs –Models dont use realistic radiative timescales

22. Good and bad from GCMs 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 GCMs - 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) –Cant 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 arent whole story - what maintains the background haze (local storms, dust devils…)

49. Water cycle Surface source asymmetry (but dont 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…

53. Mars applications Paleoclimate –Orbital change –Ancient, thicker atmospheres

54. Conclusion Continuum from Venus to Earth in terms of confidence in understanding –But basic systems still being investigated in all Fundamentally different from many point-and-shoot experiments in planetary science: –Monitoring, simultaneously, with several instruments needed