Saturn Weather Forecast: Hazy, Windy, Chance Of Storms

Saturn Weather Forecast:Hazy, Windy, Chance of StormsTony Del GenioCassini Imaging TeamCHARMteleconteleconCHARM4/29/084/29/08

Motivations for studying the dynamics ofSaturn’s atmosphere Comparative planetology- Classification of Saturn among the planets- Saturn-Jupiter similarities and differences- Does our knowledge of Earth inform our study ofSaturn? Global issues- Why is there banded cloud structure?- What maintains the alternating eastward andwestward jets? Regional topics of interest- Evolution of equatorial jet- South polar vortexCHARM telecon4/29/08

Weather exists primarilyto move heat from placesof excess to places ofdeficitSlowly rotating planets(Venus, Titan): polewardTerrestrial planets (Earth,Mars): poleward upwardJovian planets (Jupiter,Saturn, Neptune): upward Less stableCHARM telecon4/29/08 More stableUpward heat flux on Saturnreflects need for internalheat to get out

Earth: Anobserver’s dreamSaturn: Anobserver’s nightmare Partly clear, partly cloudy;can see top to bottom Overcast; can’t see below theclouds where action is Small – global observingeasy Big – need to stay far away tosee it all No rings to get in the way! Lots of rings obscure the view Strongly forced –phenomena of interesthappen often Weakly forced – relevantprocesses occur once in a while People who launch weatherballoons for “ground truth” Saturnians haven’t yet releasedtheir weather data to NASACHARM telecon4/29/08

Water clouds buried beneath ammonia and ammoniumhydrosulfide clouds, deeper on Saturn than on JupiterTropospheric haze thicker on Saturn than on JupiterJUPITERCHARM telecon4/29/08SATURN

Result is thatimage contrastsare much greateron Jupiter thanon Saturn(e.g., Voyager: 10xas much wind data onJupiter vs. Saturn)To compensate:- Image in near IRwhere contrast isgreater- Image repeatedly tobuild up statistics(created by Björn Jónsson)CHARM telecon4/29/08

But haze isthicker inSouthern(summer)hemisphere –much easierto trackwinds inNorthernHemisphereright nowCHARM telecon4/29/08

Rapid (10 hr 39 min) rotation produceszonally oriented cloud bands andalternating eastward/westward jets of 225CHARMteleconfor decades, except 800 mphmph,stable4/29/08equatorial jet – what maintains the jets?

Jupiter: zones (bright) anti-cyclonic shear,belts (dark) cyclonic shearSaturn: More fine structure; if anything,dark eastward jetsbeltzoneSaturnJupiterCHARM telecon4/29/08

Theories for maintenance ofeastward and westward jets“Weather layer” hypotheses Coriolis force on poleward-drifting air due to cells ofrising-sinking motion (like Earth tropics) (Hess andPanofsky, 1951) Baroclinic instability (like Earth high-low pressurecenters) due to latitudinal heating and temperaturedifferences (Williams, 2003; Lian and Showman,2007) Pumping by thunderstorms feeds energy into jets(Ingersoll et al., 2000; Li et al., 2006; Showman, 2007) Cloud-level manifestation of deep rotatingconvective cylinders (Busse, 1976; Heimpel andAurnou, 2007)CHARM telecon4/29/08

Early observers: Bright “zones” and dark“belts” on jovian planets reminiscent of Earth’sown banded cloud structurestorm tracksdesertsrain forestsCHARM telecon4/29/08

Gave rise to traditional view that zones and belts areanalogous the Earth’s tropical Hadley cellRising in warm anticyclonic regions (“zones”)condenses bright cloudSinking in cool cyclonicregions (“belts”)evaporates cloudCoriolis force on air driftingfrom zones to beltsdeflects air to right (in N.H.)and sustains eastward jetsSmoking gun: Does air rise in zones and sink in belts?CHARM telecon4/29/08

Another possibility: Tilted eddies (disturbances in themean flow) carry higher-than-average momentum air intojet, accelerating it, and lower-than-average momentum airout of the jet, which also accelerates itPumping by thunderstorms, or baroclinic instabilityFaster air movestoward jetSlower air moves awayfrom jetEastward jetSmoking gun: “Chevron” cloud morphology – but dowind measurements bear this out?CHARM telecon4/29/08

Strategy: Focus on daysidelarge apoapsis orbits 2007Sun 2004/5Multiple-frame image mosaics everySaturn rotation in several filterssensitive to different altitudesCHARM telecon4/29/08

ID same feature in images on successive orbits usingautomated algorithm, measure departures of wind speed ineast-west (u’) and north-south (v’) directions:Are they correlated? If so, only at certain latitudes?Flux toward northwhere windstrengthenstoward north and viceversaImplies that kinetic energy of eddiesis being converted to kinetic energyof mean flow in the jet everywhereCHARM telecon4/29/08

Convective storm detection: Image in continuum filter (weakabsorption, sees deep) and methane band filter (strongabsorption, sees only to high altitude) simultaneouslySmall, rapidly evolving features bright in both filters areconvection rising motionLow –level cloudConvective cloudThin high cloud“Dragon storm”: Waterthunderstorm 100 miles deep– lightning not detected bycameras, but appearance ondisk correlates with SaturnCHARM teleconelectrostaticdischarges4/29/08SED #/hr

Other examples of Saturn convective stormsDo they occur randomly or in preferred locations?CHARM telecon4/29/08

Convective features mostly in cyclonic shear (“belts”),rarely in anti-cyclonic shear (“zones”) –so air rises in belts, not zones!cyclonicanti-cyclonicSaturnJupiterNumber of featuresBut some storms in E and W jets on Saturn, unlike JupiterCHARM telecon4/29/08

No sign of preferred drift direction orconcentration of large momentum fluxes nearconvective storms – so probably not thedirect source of momentum for the jetsCHARM telecon4/29/08

Scorecard on jet maintenance mechanisms:Coriolis force on poleward-drifting air associated with cellsof rising-sinking motion (like Earth tropics) – inconsistentwith convection in belts rather than zonesBaroclinic instability (like Earth high-low pressure centers)due to latitudinal temperature differences – consistent witheddy momentum flux into jet from both sides and risingmotion in beltsPumping by thunderstorms feeds energy into jets – noevidence for special behavior near storms, inconsistentwith eddy momentum flux on both sides of jetsCloud-level manifestation of deep rotating convectivecylinders – unknown how it would manifest at cloud levelother than jet structure itselfCHARM telecon4/29/08

Old picture of jovian planet meteorology: Likerising/sinking branches of Earth’s Hadley cellCHARM telecon4/29/08

Old picture of jovian planet meteorology: Likerising/sinking branches of Earth’s Hadley cell New picture: Like rising/sinking branches ofEarth’s Ferrel cell driven by baroclinic instabilityCHARM telecon4/29/08

Latitudinal cross-section of Earth’s general circulation(Hartmann, 2007) – valid for Jupiter and Saturn too?Visible cloudlevel ( 1-2 barEastwardjetAnti-cyclonicWater condensationlevel ( 10 bar)CHARM telecon4/29/08CyclonicBaroclinic eddiesConvection

Changes in Saturn’s equatorial zone over time1994VOYAGER 1981Higher cloud tops after’90’s storm eruptionsCASSINI 2004CHARM telecon4/29/08

Equatorial wind speed changes over time: Real or artifact?VoyagerHubbleCassinimethane bandCassinicontinuumCHARM telecon4/29/08CIRS(Flasar etal., 2005)thermalwindHubble reports much slower winds thanVoyagerCassini methane band matches Hubble,continuum winds much strongerSo mostly an artifact of higher cloud topand wind decreasing with height?Convection can cause some weakeningof wind (Sayanagi and Showman, 2007)

Saturn’s south polar vortex (Dyudina et al. 2008)Low cloudtops in center High cloud tops cast shadows SpiralbandsCHARM telecon4/29/08Is it a hurricane?

SaturnEarth “Warm core”in center “eye”(maybe) Winds increasetoward center upto “eyewall”But No ocean underneath to provide energy source Fixed spatially, maybe long-lived, rather than propagating andtransient so probably notCHARM telecon4/29/08

Conclusions General circulation of the jovian planets may indeedbe an analogy to Earth but to Earth’s midlatitudesrather than its tropics Wind-albedo relationship on Saturn yet to beunderstood Dramatic equatorial changes in clouds and perhapswinds on Saturn needs to be understood Seasonalshading by rings destabilizing upper levels?Propagating waves? South polar vortex hurricane-like in its organizationbut not in its sourceCHARM telecon4/29/08

Apr 29, 2008 · Slowly rotating planets (Venus, Titan): poleward . Terrestrial planets (Earth, Mars): poleward upward . Jovian planets (Jupiter, Saturn, Neptune): upward . Upward heat flux on Saturn reflects