# Presentation: 506962

## Saturn Weather Forecast: 

**4/29/08**

**Saturn Weather Forecast: **
**Hazy, Windy, Chance of  Storms**

**CHARM telecon**
**4/29/08**

**Tony Del Genio**
**Cassini Imaging Team**

## Motivations for studying the dynamics of Saturn’s atmosphere

**4/29/08**

**Motivations for studying the dynamics of Saturn’s atmosphere**

**Comparative planetology**
**	****- Classification of Saturn among the planets**
**	****- Saturn-Jupiter similarities and differences**
**	****- Does our knowledge of Earth inform our study of**
**	****	****Saturn?**
**Global issues**
**	****- Why is there banded cloud structure?**
**	****- What maintains the alternating eastward and ****	****westward jets?**
**Regional topics of interest**
**	****- Evolution of equatorial jet**
**	****- South polar vortex**

## ←                       →

**4/29/08**

**←                       →**
**Less stable                             More stable**

**Weather exists primarily**
**to move heat from places**
**of excess to places of**
**deficit**
**Slowly rotating planets**
**(Venus, Titan): poleward**
**Terrestrial planets (Earth,**
**Mars): poleward + upward**
**Jovian planets (Jupiter,**
**Saturn, Neptune): upward**
**Upward heat flux on Saturn reflects need for internal heat to get out**

## Earth: Anobserver’s dream

**4/29/08**

**Earth: An****observer’s dream**

**Partly clear, partly cloudy; can see top to bottom**
**Small – global observing easy**
**No rings to get in the way!**
**Strongly forced – phenomena of interest happen often**
**People who launch weather balloons for “ground truth”**

**Overcast; can’t see below the clouds where action is**
**Big – need to stay far away to see it all**
**Lots of rings obscure the view**
**Weakly forced – relevant processes occur once in a while**
**Saturnians haven’t yet released their weather data to NASA**

**Saturn: An**
**observer’s nightmare**

## Water clouds buried beneath ammonia and ammonium hydrosulfide clouds, deeper on Saturn than on JupiterTropospheric haze thicker on Saturn than on Jupiter

**4/29/08**

**Water clouds buried beneath ammonia and ammonium hydrosulfide clouds, deeper on Saturn than on Jupiter****Tropospheric haze thicker on Saturn than on Jupiter**

**JUPITER**

**SATURN**

## Result is that

**4/29/08**

- (created by Björn Jónsson)

**Result is that**
**image contrasts**
**are much greater**
**on Jupiter than **
**on Saturn**
**(e.g., Voyager: 10x**
**as much wind data on**
**Jupiter vs. Saturn)**
**To compensate:**
**- Image in near IR  where contrast  is greater**
** ****Image repeatedly to build up statistics**

## But haze is thicker in Southern (summer) hemisphere – much easier to track winds in Northern Hemisphere right now

**4/29/08**

**But haze is thicker in Southern (summer) hemisphere – much easier to track winds in Northern Hemisphere right now**

## Rapid (10 hr 39 min) rotation produces zonally oriented cloud bands and alternating eastward/westward jets of ~225 mph, stable for decades, except ~ 800 mph equatorial jet – what maintains the jets?

**4/29/08**

**Rapid (10 hr 39 min) rotation produces zonally oriented cloud bands and alternating eastward/westward jets of ~225 mph, stable for decades, except ~ 800 mph equatorial jet – what maintains the jets?**

## Jupiter: zones (bright) → anti-cyclonic shear, 

**4/29/08**

**Jupiter: zones (bright) ****→ anti-cyclonic shear, **
**	****belts (dark) → cyclonic shear**
**Saturn: More fine structure; if anything, **
**	****dark → eastward jets**

**Jupiter**

**Saturn**

**belt**

**zone**

## Theories for maintenance ofeastward and westward jets

**4/29/08**

**Theories for maintenance of****eastward and westward jets**

**“****Weather layer” hypotheses**
**Coriolis force on poleward-drifting air due to cells of rising-sinking motion (like Earth tropics) (Hess and Panofsky, 1951)**
**Baroclinic instability (like Earth high-low pressure centers) due to latitudinal heating and temperature differences (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 rotating convective cylinders (Busse, 1976; Heimpel and Aurnou, 2007)**

## Early observers: Bright “zones” and dark “belts” on jovian planets reminiscent of Earth’s own banded cloud structure

**4/29/08**

**Early observers: Bright “zones” and dark “belts” on jovian planets reminiscent of Earth’s own banded cloud structure**

**rain forests**

**deserts**

**storm tracks**

## Rising in warm anti-cyclonic regions (“zones”) condenses bright cloud

**4/29/08**

**Rising in warm anti-cyclonic regions (“zones”) condenses bright cloud**
**Sinking in cool cyclonic regions (“belts”) evaporates cloud**
**Coriolis force on air drifting from zones to belts deflects air to right (in N.H.) and sustains eastward jets**

**Gave rise to traditional view that zones and belts are**
**analogous the Earth’s tropical Hadley cell**

**Smoking gun: Does air rise in zones and sink in belts?**

## Another possibility: Tilted eddies (disturbances in the mean flow) carry higher-than-average momentum air into jet, accelerating it, and lower-than-average momentum air out of the jet, which also accelerates it

**4/29/08**

**Faster air moves**
** ****toward jet**

**Slower air moves away**
** ****from jet**

**Another possibility: Tilted eddies (disturbances in the mean flow) carry higher-than-average momentum air into jet, accelerating it, and lower-than-average momentum air out of the jet, which also accelerates it**
**Pumping by thunderstorms, or baroclinic instability**

**Eastward jet**

**Smoking gun: “Chevron” cloud morphology – but do**
**wind measurements bear this out?**

## Strategy: Focus on dayside

**4/29/08**

**Strategy: Focus on dayside**
**large apoapsis orbits **

**← ****2007**

**Sun ****→**

**← ****2004/5**

**Multiple-frame image mosaics every**
**Saturn rotation in several filters**
**sensitive to different altitudes**

## ID same feature in images on successive orbits using automated algorithm, measure departures of wind speed in east-west (u’) and north-south (v’) directions:Are they correlated?  If so, only at certain latitudes?

**4/29/08**

**ID same feature in images on successive orbits using automated algorithm, measure departures of wind speed in east-west (u’) and north-south (v’) directions:****Are they correlated?  If so, only at certain latitudes?**

**Flux  toward north**
**where wind**
**strengthens**
**toward north**

**...****and vice-**
**versa**

**Implies that kinetic energy of eddies**
**is being converted to kinetic energy**
**of mean flow in the jet everywhere**

## “Dragon storm”: Water thunderstorm ~100 miles deep – lightning not detected by cameras, but appearance on disk correlates with Saturn electrostatic discharges

**4/29/08**

**“****Dragon storm”: Water thunderstorm ~100 miles deep – lightning not detected by cameras, but appearance on disk correlates with Saturn electrostatic discharges**

**SED #/hr**

**Convective storm detection: Image in continuum filter (weak**
**absorption, sees deep) and methane band filter (strong**
**absorption, sees only to high altitude) simultaneously**
**Small, rapidly evolving features bright in both filters are**
**convection ****→ rising motion**** **

**Low –level cloud**

**Thin high cloud**

**Convective cloud**

## Other examples of Saturn convective storms

**4/29/08**

**Other examples of Saturn convective storms**

**Do they occur randomly or in preferred locations?**

## Convective features mostly in cyclonic shear (“belts”), rarely in anti-cyclonic shear (“zones”) – so air rises in belts, not zones! 

**4/29/08**

**Convective features mostly in cyclonic shear (“belts”), rarely in anti-cyclonic shear (“zones”) – ****so air rises in belts, not zones! **

**Number of features**

**cyclonic**

**anti-cyclonic**

**Saturn**

**Jupiter**

**But some storms in E and W jets on Saturn, unlike Jupiter**

## No sign of preferred drift direction or

**4/29/08**

**No sign of preferred drift direction or**
**concentration of large momentum fluxes near convective storms – so probably not the direct source of momentum for the jets**

## Scorecard on jet maintenance mechanisms:

**4/29/08**

**Coriolis force on poleward-drifting air associated with cells of rising-sinking motion (like Earth tropics) – ****inconsistent with convection in belts rather than zones**
**Baroclinic instability (like Earth high-low pressure centers) due to latitudinal temperature differences – ****consistent with eddy momentum flux into jet from both sides and rising motion in belts**
**Pumping by thunderstorms feeds energy into jets – ****no evidence for special behavior near storms, inconsistent with eddy momentum flux on both sides of jets**
**Cloud-level manifestation of deep rotating convective cylinders – ****unknown how it would manifest at cloud level other than jet structure itself**

**Scorecard on jet maintenance mechanisms:**

## Old picture of jovian planet meteorology: Like rising/sinking branches of Earth’s Hadley cell 

**4/29/08**

**Old picture of jovian planet meteorology: Like rising/sinking branches of Earth’s Hadley cell**** **

## ↑

**4/29/08**

**Old picture of jovian planet meteorology: Like rising/sinking branches of Earth’s Hadley cell**** **

**New picture: Like rising/sinking branches of **
**Earth’s Ferrel cell driven by baroclinic instability**

**↑**

**↓**

## Latitudinal cross-section of Earth’s general circulation (Hartmann, 2007) – valid for Jupiter and Saturn too?

**4/29/08**

**Latitudinal cross-section of Earth’s general circulation (Hartmann, 2007) – valid for Jupiter and Saturn too?**

**Anti-cyclonic**

**Cyclonic**

**Convection**

**Visible cloud level (~1-2 bar**

**Eastward**
**jet**

**Water condensation**
**level (~10 bar)**

**Baroclinic eddies**

## Changes in Saturn’s equatorial zone over time

**4/29/08**

**VOYAGER 1981**

**CASSINI 2004**

**Changes in Saturn’s equatorial zone over time**

**1994**

**Higher cloud tops after**
**’****90’s storm eruptions**

## Equatorial wind speed changes over time: Real or artifact?

**4/29/08**

**Equatorial wind speed changes over time: Real or artifact?**

**Voyager**
**Hubble**
**Cassini**
** ****methane band**
**Cassini**
** ****continuum**** **

**CIRS**
**(Flasar et**
**al., 2005)**
**thermal**
**wind**

**Hubble reports much slower winds than**
**   ****Voyager**
**Cassini methane band matches Hubble,**
**   ****continuum winds much stronger**
**So mostly an artifact of higher cloud top**
**   ****and wind decreasing with height?**
**Convection can cause some weakening**
**   ****of wind (Sayanagi and Showman, 2007)**

## Saturn’s south polar vortex (Dyudina et al. 2008)

**4/29/08**

**Saturn’s south polar vortex (Dyudina et al. 2008)**

**Is it a hurricane?**

**High cloud tops cast shadows**

**Low cloud**
**tops in center ******

******Spiral**
**bands**

##  “Warm core”

**4/29/08**

****** “****Warm core”**
**in center “eye”**
**(maybe)**

****** ****Winds increase**
**toward center up**
**to “eyewall”**

**But...**
** ****No ocean underneath to provide energy source**
** ****Fixed spatially, maybe long-lived, rather than propagating and**
**	****transient**
**	****	****	****... ****so probably not**

**Saturn**

**Earth**

## Conclusions

**4/29/08**

**Conclusions**

**General circulation of the jovian planets may indeed be an analogy to Earth...but to Earth’s midlatitudes rather than its tropics**
**Wind-albedo relationship on Saturn yet to be understood**
**Dramatic equatorial changes in clouds and perhaps winds on Saturn needs to be understood...Seasonal shading by rings destabilizing upper levels?  Propagating waves?**
**South polar vortex hurricane-like in its organization but not in its source**