Atmospheric Dynamics on Solid-Surface Bodies

Mark I. Richardson

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

Considered here

• Venus

• Earth

• Mars

• Titan

• Close, but no cigar:– Triton, Pluto

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

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!)

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)

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

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?

“Branching” “pure” studies

• Nature of convection near the surface

• Waves (tides, Rossby waves, bouyancy “gravity” waves)

• Eddy dynamics (flow instabilities, diurnal topographic flows)

“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

“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

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)

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

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

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?)

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

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

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

More issues…

• Banding structure in clouds

• Polar “hurricane” (modified Hadley downwelling?)

• Time variability

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

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???

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…

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

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

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

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?

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)

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…)

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)

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…

Mars applications

• Paleoclimate– Orbital change– Ancient, thicker atmospheres

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