6 Dynamics of Jupiter's Atmosphere

Andrew P. Ingersoll California Institute of Technology

Timothy E. Dowling University of Louisville

P eter J. Gierasch Cornell University

GlennS. Orton J et P ropulsion Laboratory, California Institute of Technology

Peter L. Read Oxford. University

Agustin Sanchez-Lavega Universidad del P ais Vasco, Spain

Adam P. Showman University of A rizona

Amy A. Simon-Miller NASA Goddard. Space Flight Center

Ashwin R . V asavada J et Propulsion Laboratory, California Institute of Technology

6.1 INTRODUCTION occurs on both planets. On Earth, electrical charge separa­ tion is associated with falling ice and rain. On Jupiter, t he Giant planet atmospheres provided many of the surprises separation mechanism is still to be determined. and remarkable discoveries of planetary exploration during The winds of Jupiter are only 1/ 3 as strong as t hose t he past few decades. Studying Jupiter's atmosphere and of aturn and Neptune, and yet the other giant planets comparing it with Earth's gives us critical insight and a have less sunlight and less internal heat than Jupiter. Earth broad understanding of how atmospheres work that could probably has the weakest winds of any planet, although its not be obtained by studying Earth alone. absorbed solar power per unit area is largest. All the gi­ ant planets are banded. Even Uranus, whose rotation axis Jupiter has half a dozen eastward jet streams in each is tipped 98° relative to its orbit axis. exhibits banded hemisphere. On average, Earth has only one in each hemi­ cloud patterns and east- west (zonal) jets. All have long-lived sphere. Jupiter has weather patterns ("storms'') that last storms, although Jupiter's Great Red Spot (GRS), which fo r centuries. Earth has stationary weather patterns fixed may be hundreds of years old, seems to be the oldest. to the topography, but the average lifetime of a traveling storm is rvl week. Jupiter has no topography, i.e. , no con­ tinents or oceans; its atmosphere merges smoothly with the 6.1.1 Data Sets planet·s fluid interior. Absorbed sunlight (power per unit area) at Jupiter is only 3.3% that at Earth, yet Jupiter's Early astronomers, using small telescopes with their eyes winds are 3-4 times stronger. The ratio of Jupiter·s internal as detectors. recorded the changing appearance of Jupiter·s power to absorbed solar power is 0.7. On Earth the ratio atmosphere. Their descriptive terms - belts and zones, 4 is 2 X lQ- . Jupiter's hydrologic cycle is fundamentally dif­ brown spots and red spots, plumes, barges, festoons, and ferent from Earth's because it has no ocean, but lightning streamers - are still used. Other terms - describing vorticity, 106 Ingersoll et al. vertical motion. eddy fluxes. temperature gradient . cloud 6.1.2 Scope of t h e Chapter and Role of Models height . and wind shear - have been added. bringing the This chapter reviews the observations and theory of tudy of Jupiter"s atmospheric dynamics to a level similar Jupiter's atmospheric dynamics. Sections 6.2 and 6.3 cover to that of Earth during the pioneering days of terrestrial the banded structures and discrete features. respectively. meteorology everal decades ago. Section 6.4 covers vertical structure and temperatures. ec­ Jupiter bas what is perhaps the most photogenic at­ tion 6.5 discus es lightning and models of moist convection. mosphere in the solar system. :\1ost of the visible contrast Section 6.6 reviews numerical models of the bands and zonal arises from clouds in the 0.7- to 1.5-bar range (see Chap­ jets, and ection 6. 7 review numerical models of the dis­ ter 5). The clouds come in different colors, and usually have crete features. Finally. Section 6. provides a discussion of textme on scales as small as a few tens of kilometers, which outstanding questions and how they might be answered. The is comparable to thee-folding thickness (scale height) of the chapter is aimed at a general planetary science audience. al­ atmosphere. At this resolution. cloud tracking over a few though some familiarity with atmospheric dynamics is help­ 1 ful for the modeling sections. hours yields wind estimates with errors of a few m s- . In contrast. the winds around the GRS and many of the zonal AI3 in the terrestrial atmospheric sciences, validated nu­ 1 merical model are the key to understanding. :\Iodels of jets exceed 100m s- . Winds are measured relative to Sys­ tem lll. a uniform rotation rate with period 9 h 55 m 29.71 s. Jupiter's atmosphere tend to be less complex than models of which is defined by radio emissions that are presumably tied Earth's atmosphere. They nevertheles contain much of the to the magnetic field and thus to the planet"s interior. nonlinear physics associated with large-scale stratified flows in rotating systems. Ideally. the complexity of the models Traditional Earth-based telescopic resolution is 3000 matches that of the observations. so that hypothese can km, which is enough to image the major atmospheric fea­ be tested cleanly. Some pure fl uid dynamics models, e.g., tures. Pioneers 10 and 11 improved on Earth-based res­ of two-dimensional flows without viscosity, find their best olution, but Voyagers 1 and 2 provided a breakthrough. applications on Jupiter and the other giant planets. Exam­ For cloud tracking. the most important data were the ··ap­ ples include the Kida vortex model. the models of inverse proach·· movies that were recorded during the three months cascades and beta-tUl·bulence. and the statistical mechan­ prior to each of the two encounters in :t\Iarch and July of ical models of two-dimen ional coherent tructures. The e 1979. The spacecraft obtained a view of each feature every models are discu ed in ections 6.6 and 6. 7. "'10 hours as the resolution improved from 500 km to 60 Peek (195 ) is the definitive book for early ob erva­ km. Occasional Yiews of selected features continued down tions of Jupiter· atmosphere. Gehrels (1976) is a collec­ to a resolution (pi.xel ize) of "'5 km. The Voyager infrared tion of chapters by various authors following the Pioneer pectrometer (IRIS) viewed the entire planet at a resolu­ encounters. Rogers (1995) is the modern equivalent of Peek. tion of several thousand kilometers and obtained spectra of There are many review articles (Ingersoll1976b, tone 1976. all the major dynamical features. Galileo obtained les data Williams 19 5. Beebe et al. 19 9. Ingersoll 1990, l\1arcu than Voyager. but the imaging resolution, usually 25 km. 1993, Gierasch and Conrath 1993. Dowling 1995a, Inger oil and the wa,·elengtb coverage were better. In particular. the et al. 1995). As an ensemble. the articles record the variance near-infrared response of the Galileo camera allowed imag­ of expert opinion. As a time series. they record the progress ing in the absorption bands of methane, from which one that has been made and bring clarity to t he remaining unan­ separates cloud at different altitudes. Cassini combined the swered questions. high data rate of Voyager with the broad spectral coverage For a point on the surface of an oblate planet. there are of Galileo. yielding a best resolution of 60 km (the Cassini two definitions of latitude. Planetographic (PG) latitude is data were still being analyzed at the time of thi writing). the elevation angle (relative to the equatorial plane) of the vector along the local vertical. and planetocentric (PC) lati­ Ground-based telescopes and the Hubble pace Tele­ tude is the elevation angle (relative to the equatorial plane) scope (HST) provide a continuous record of Jupiter's cloud of the vector from the planet's center. PG latitude is greater features at everal-month intervals. These data document than PC latitude except at the equator and poles where they the major e\·ents and also the extreme steadiness of the are equal. For Jupiter the maximum difference (4.16°) is at atmosphere. Ground-based telescope provide the highest 46.6° PG latitude. Unless otherwise specified, we use P G pectral resolution. Several trace gases. which provide im­ latitudes in this chapter. portant diagnostics of vertical motion. were discovered from the ground. Earth-based radio observations probe t he deep atmo phere. The HST was essentia l during the collisions of Comet Shoemaker- Levy 9 with Jupiter in 1994. Besides 6.2 BANDED STRUCTURE recording the waves and debris from the collisions, the HST 6.2.1 Belts and Zones defined the prior dynamical state of the atmo phere. Jupiter's visible atmosphere is dominated by banded struc­ The Galileo probe provided profiles of wind. tempera­ tures (Figure 6.1). Traditionally. the white bands are called ture. composition, clouds, and radiation as functions of pre - zones and the dark bands are called belts. The zonal jets sure down to the 22-bar level. but only at one point on the (eastward and westward currents in the atmosphere) are planet. Except at the Galileo probe site. these quantitie are strongest on the boundaries between the belts and zone uncertain below the 1-bar level. The base of the water cloud (Figure 6.2). The zones are anticyclonic, which means they is thought to lie at the 6- or 7-bar level, "'75 km below the have an eastward jet on the poleward side and a westward jet clouds that produce the visible contrast. on the equatorward side (in the reference frame of the planet, Atmospheric Dynamics 107

Figure 6 .1. See Plate 2. Whole disk views of J upiter. T he left image is from Voyager 2 in J une 1979. T he right image is from Cassini in ·ovember 2000. an anticyclone rotates clockwise in the northern hemisphere cle the entire planet (Beebe et al. 19 9. Simon-Miller et al. and counterclockwise in the southern hemisphere). The belts 2001b). Amateur and professional observers haYe recorded are cyclonic, which means they rotate the oppo ite way. In many such disturbances (e.g .. Sanchez-Lavega et al. 1991. an inertial frame. the rotation period varies with latitude in Sanchez-Lavega and Gomez 1996. Rogers 1995). Although a range ±5 min on either side of the System III period. The the belt/ zone boundaries align closely with the zonal jets. major belts and orne inertial rotation periods are labeled they do change in latitudinal extent and can recede or ex­ in Figure 6.3 (Peek 195 . Stone 1976). Individual features tend beyond the cores of the jets (Beebe et al. 19 9, Rogers like the GRS tend to have the same sign of vorticity (sense 1995. Simon et al. 199 ). of rotation) as the band in which they sit. Imbedded in the zones are the major anticyclonic ovals Jupiter is not bright orange or red in color, but more like the GRS at 22.5°S, the White Ovals at 33°S, and smaller of a muted brown (Peek 195 , Simon-l\filler et al. 2001a). ovals at 41°8 , 34° , 40°1\, and 45°N PG latitudes. T hese The colors of the belts and zones vary with time. The origin ovals usually extend into the neighboring belt on the equa­ of the colors and how they respond to the winds are un­ torward side, and sometimes block it off. Then the belt be­ certain. The major cloud constituents - ammonia. H2S, and comes a series of clo ed cyclonic cells. each one spanning water - are colorless, but elemental sulfur. phosphoru . and the region between two anticyclonic ovals. Activity is great­ organic compounds could combine in trace amounts to form est on the eastern end of each cyclonic cell, giving it the the muted colors. appearance of a turbulent wake extending off to the west of The zones appear more uniform than the belts. partic­ the anticyclonic oval. The best example is the South Equa­ ularly in the northern hemisphere. In the zone the small­ torial Belt (SEB). whose active part extends westward. just scale texture has low contrast. The large-scale features in the north of the GRS. Both the SEB and the ·orth Equato­ zones are generally steadier in time than those in the belts. rial Belt (KEB) are sites of intense convective activity - The clouds in the zones generally extend to higher altitudes lightning storms with high. thick clouds that double in area than those in the belts; the corresponding pressure differ­ in less than half a day (Gierasch et al. 2000). ence i a few hundred mbar. The gaseou ammonia abun­ Jupiter's Equatorial Zone (EZ) lies between the east­ dance i higher in the zones. and the upper tropo pheric tem­ ward jets at PG latitudes ±7°. The vorticity is anticyclonic peratures are lower (Comath and Gierasch 19 6. Gierasch (clockwise north of the equator and counterclockwise south et al. 19 6, Simon-1\Iiller et al. 2001b). The darker belts have of the equator), but the EZ is different from other zones. deeper clouds overall and more variation in cloud height. 1\Iethane band images that sound the upper troposphere re­ There are holes in the visible cloud deck (5-~ hot spots. veal an elevated haze that is thicker than that at neigh­ Figure 6.4) that allow radiation to escape from the warmer boring latitudes. Visible band images reveal a bland cloud layer below (Terrile and Westphal 1977, Ortiz et al. 199 ); deck whose northern boundary is punctuated by a dozen 5- thi radiation is most intense in a narrow wavelength region I-LID hot spots and plumes (Ortiz eta~. 199 ). The latter are around 5 !liD where there are no gaseous absorption lines to high. thick clouds that trail off 10 000 km to the southwest. impede it. The belts are the sites of initially mall convec­ The plume head are located just west of the hot spots and tive events that sometimes grow to great height and encir- sometimes exhibit convective activity (Hunt et al. 19 1). 108 Ingersoll et al.

Period JUPITER's BELTS AND PERIODS CF ROTATION

80 SPR sire s1 TB STB S£8 ""'

60

X 40

...... 0 +40" 0'1 Latitude q) -o ...... 20 q) Fig ure 6 .3 . Jupiter's belts and zones and periods of rotation. -o ;;) The figure is from Stone (1976). who used data summarized by Peek (1958). Those data were derived from decades of Earth­ 0 _J based telescopic observations. The belts are NEB = North Equa­ 2 0 0 torial Belt. 1 TB = North Temperate Belt, N TB = 1 orth 1 orth ·c Temperate Belt, etc., and similarly in the south. The zones are c: q) EZ = Equatorial Zone, NTrZ = North Tropical Zone, NTZ = 0 0 North Temperate Zone, N2 TZ = North North Temperate Zone, q; c: -20 etc., and similarly in the south. Periods are measured by track­ 0 ing features larger than ~ 3000 krn over time intervals of days or a: weeks. Short periods represent flow to the east relative to Sys­ tem III. which is the 9 h 55 m 29.71 s period defined from radio - 40 frequency observations.

-60

- 80

- 100 -50 0 50 100 150 Eastward Velocity (m/ sec)

Fig ure 6.2 . Zonal winds vs. latitude in 1979 and 2000. The dashed line is from Voyager (Limaye 1986). and the solid line is from Cassini (Porco et al. 2003).

The Galileo probe entered on the southern edge of a hot spot at PG latitude 6.5°N (Orton et al. 1998). Neither plumes nor hot spots look like vortices; nevertheless non­ zonal motions have been associated with them (Vasavada et al. 1998). Between 10- 13 hot spot/plume pairs have been present since the Voyager era; however Pioneer images and historical records indicate that there may have been fewer in the past. The train of features translates to the east with 1 a velocity of ~100m s- . When this translation is removed Figure 6.4. '"'hole disk image at a wavelength of 5 1-illl (Ortiz from time-series images of Jupiter's equator, the growth, in­ et al. 1998). The brightest areas, termed 5-!-lll hot spots, are boles in the visible cloud deck that reveal the warmer, deeper layers teractions. and decay of individual features over months to below. ~la.ximurn brightness temperatures sometimes exceed 273 years become apparent (Ortiz et al. 199 ). Cassini movies, K. The Galileo probe entered on the south edge of a hot spot at Galileo probe results, and numerical simulations suggest 6.5° latitude. that the features are probably a nonlinear wave traveling 1 westward on a fast ("-'160 m s- ) eastward jet (Showman and Dowling 2000). tions extend to ±80° at least (Garcia-Melendo and Sanchez­ The banded appearance at low latitudes gradually gives Lavega 2001. Porco et al. 2003). Methane-band images dis­ way at mid latitudes to a mottled appearance at high lat­ play prominent polar caps of elevated and thicker haze, pos­ itudes, which are dominated by closely spaced anticyclonic sibly maintained by auroral processes. with wave-like bound­ ovals and cyclonic features (Figure 6.5). Despite this mot­ aries (Rages et al. 1999, Sanchez-Lavega et al. 1998a. Chap­ tled appearance. movies show that organized zonal rno- ter 5). Recent observations at ultraviolet wavelengths, which Atmospheric Dynamics 109 are ensitive to stratospheric aerosols. reveal vortices and Simon 199 . Garcia-t-.lelendo et al. 2001. Porco et al. 2003). other feature clearly distinct from those of the visible cloud Between 1979 and 1995 the eastward jet at 23° ~ slowed from Jeck and po ibly associated with the auroral footprint (Vin­ 1 0 m s-1 to 140 m s- 1 and then remained constant. The cent et al. 2000, Porco et al. 2003). westward jet at 30° ' and the jets between 40°1'\ and 55° · 1 also show significant (1D-20 m s- ) changes and small shifts in latitude. 6.2.2 Changes in Appearance Although Jupiter·s banded appearance is quite stable, 6.2.4 Two Hypotheses about the Banded changes are visible in the Voyager and Cassini images ac­ Structure quired in 1979 and 2000, respectively (Figure 6.1). The equa­ torial plumes were less well defined with respect to their Jupiter·s large-scale winds are in approximate geostrophic surroundings in 2000 than they were in 1979, although they balance: therefore anticyclones are high-pressure centers were present in roughly the same numbers. There was a re­ and cyclones are low-pressure centers. \Varm-core features ,·ersal in the north-to-south color gradient aero the EZ as (warmer than their urroundings at the same pressure level) well (Simon-~Iill er et al. 2001b). become more anticyclonic with altitude because pressure de­ The 'EB was more active around the time of the creases with altitude more slowly when the air is warm than Cassini flyby. Dark material extended further to the north when it is cold. By the same token, cold-core features be­ than in the Voyager era. t>.Iany active sites were visible, and come more cyclonic with altitude. Thus in the Earth"s atmo­ po ible brown barges (elongated cyclonic dark ovals not sphere. a warm-core feature like a hurricane changes from visible in Figure 6.1) were reported for the first time since strongly cyclonic at low altitude to weakly anticyclonic at the Voyager era (neither HST nor Galileo saw brown barges high altitude. And in the Earth's ocean, warm-core features in the 1990 to mid-2000 time period). The orth Temper­ may be weakly cyclonic or anticyclonic at depth. but they ate Belt (NTB, from 23° · to 31°N). showed more contrast become strongly anticyclonic at the ocean surface. These with respect to the surrounding zones than in the Voyager are examples of a quantitative relation between wind shear era. ~one of these changes is particularly unusual. The belts and horizontal temperature gradient called the thermal wind and zones often change color or width. Good historical ac­ equation (e.g .. Pedlosky 19 7). counts of similar events are found in Peek (195 ) and Rogers For Jupiter, the traditional view (Hess and Panofsky (1995). Detailed studies of recent disturbances in the SEB 1951 , Ingersoll and Guzzi 1969) is that the wi nds are weak in and ~TB can be fow1d in 8anchez-Lavega and Gomez (1996) the deep atmo phere as in the deep oceans: in other word . and Sanchez..Lavega et al. (1991), respectively. the winds that we ee are shallow. This implies that the The GRS decreased in longitudinal extent and became zones and anticyclonic oval are warm-core features - the much row1der in appearance during the 21 years between the air between the deep ··level of no motion·· and the surface Voyager and Cassini epochs. The three largest white ovals on which the winds are measured is warmer than the sur­ (not visible in the Cassini image) also decreased in size and roundings. Since warm air tends to rise and cold air tends eventually merged into a single vortex. The mall ovals at to sink. it i natural to assume that the air in the zones is -tl 0 S have not changed in appearance or number. Despite slowly rising and the air in the belts is slowly sinking. And the sHght differences in the ovals and belt/zone appearance, since clouds tend to form on updrafts. this view seems to be the overall appearance of the planet and its major features consistent with the observation that the visible cloud deck i in both frames of Figure 6.1 is remarkably unchanged. higher in the zones {and in the anticyclonic ovals) and lower in the belts. This view also seems to be consistent with the observation that the 5-1-lm hot spots. which are holes in the 6.2.3 Changes in Zonal Velocity visible cloud deck. are concentrated in the belts (Terrile and T he velocities of Jupiter·s zonal jets have been inferred from Westphal 1977). the translation of cloud features for hundreds of years (Peek An alternate view {Busse 1976) is that the wind are 195 . Smith and Hunt 1976). Uncertainties arise from differ­ just as strong in the deep atmosphere as they are in the ent instruments and wavelengths, inaccurate image naviga­ visible cloud deck. If the fl uid is barotropic. meaning that tion, changes in the morphology of tracked cloud features. temperature is constant at constant pressure. the zonal jets confusion of measurements by non-zonal circulation , and would be the surface manifestation of differentially rotating imperfect coupHng of tracked feature to the underlying cyHnders concentric with the planet's rotation axis (Poincare zonal flow (e.g .. Beebe et al. 1996). Kevertheless Voyager, 1910). The fluid would then move in columns. according Galileo, HST, and Cassini images have produced a 21-yr to the so-called Taylor- Proudman theorem (e.g., Pedlo ky record of high-quality velocity measurements capable of re­ 1987). On the other hand, if the fluid is baroclinic, mean­ vealing any decadal-scale variations greater than about 10 ing that temperature varies at constant pressure. the wind m s-1 (Figure 6.2). The number and magnitude of Jupiter·s would not obey the Taylor- Proudman theorem and the fluid jets have remained virtually unchanged, in spite of the pres­ would not move in columns. Distinguishing between these ence of turbulence, convection, uncertainty in altitude. and two extremes, shallow vs. deep, requires knowledge of winds major changes in the brightness and width of the bands. and temperatures in the deep atmosphere. T he measured winds probably refer to levels in the 0.7- to 1.0-bar range (Banfield et al. 199 ). 6.2.5 Evidence of Upwelling and Downwe lling Some minor variations in jet shape and speed have been reproduced by everal analyses, however, including the re­ Large-scale vertical velocities are estimated to be rvl0- 3 m 1 sults hown in Figure 6.2 (Limaye 19 6. Vasavada et al. 199 . s- , which is too small to be measured directly. Departures 110 Ingersoll et al.

Figu re 6.5. Polar views of Jupiter. Images from different longitudes were map projected to show, from a vie·.vpoint directly over the pole all t he features in sunlight at the same time. Latitude varies linearly with radial distance in the image, from 0° in the corners to 90° in the center. (Left) South pole in 1979 from Voyager. (Right) l\orth pole in 2000 from Cassini. from chemical and thermal equilibrium provide indirect ev­ i.e .. the winds get weaker with altitude. Figure 6.6 compares idence of vertical velocity when the equilibrium state is a the thermal wind shear au;az, computed from the measured function of altitude. We consider four examples. The first temperature gradient aT /By, with the mean zonal wind u involves the fraction of H2 molecules in the two possible measured by cloud tracking, where y and z are the north­ spin states, ortho and para. The equilibrium para fraction ward and upward coordinates, respectively. This decay of decreases with depth due to the increase in temperature, the zonal winds with altitude takes place over two or three so a para fraction below the equilibrium value is a sign of scale heights. Cloud optical depths and ammonia abundance upward motion. Second, a stably stratified atmosphere is are displayed in Figure 6.7, and a ground-based 5-~m image one in which the potential temperature (or equivalently, the is shown in Figure 6.4 (Orton et al. 1996, 1998). Regions entropy) increases with height; therefore rising air tends to of low 5-~m optical depth appear bright because they al­ have low potential temperature and sinking air tends to have low thermal radiation from below to escape. The belts are high potential temperature. It follows that when there are regions of low optical depth and low ammonia abundance. no other heat sources, low and high temperatures mean up­ The inference is that the air in t he belts is sinking, at least welling and downwelling. respectively. Third, ammonia con­ \vithin the upper troposphere (from 0.1 to 0.5 bars). Under denses and precipitates in the upper troposphere. so high t his interpretation, the mean meridional motions (longitu­ ammonia abundance is generally a sign of upwelling. Fourth, dinally averaged motions in the vertical and north-south clouds form on updrafts, so increased cloud optical t hickness directions) agree with the traditional view of zones as sites is generally a sign of upwelling. of upwelling and belts as sites of downwelling. The temperatures of the upper troposphere (warm The Voyager IRIS spectra allow simultaneous determi­ belts, cold zones) are opposite to those postulated for the nation of the ortho-para ratio. the temperature, the ammo­ lower troposphere according to the t raditional view based nia concentration, and cloud optical depths at two differ­ on a level of no motion below t he visible cloud deck. Yet in ent wavelengths (5 and 45 ~m) . all with spatial resolution both cases one infers rising motion in the zones and sink­ of a few thousand km over most of the planet. The tem­ ing motion in the belts. The difference is that in the upper perature and para fraction refer to pressure levels of a few troposphere there are no obvious heat sources that would hundred mbar; t he 45- ~m cloud optical depth and the am­ make the belts warmer - one has to invoke dov.'llwelling. In monia concentration refer to levels between 1 bar and space; the lower troposphere one can invoke latent heat to keep and the 5- ~m optical depth refers to levels between a few the zones warmer (Ingersoll and Guzzi 1969, Barcilon and bars and space (Conrath and Gierasch 1986). An orderly Gierasch 1970). pattern related to the zonal mean jets emerges from these measurements (Gierasch et al. 19 6). Upper tropospheric The inferred circulation in the upper troposphere has temperatures are higher over the belts than over the zones, hot air sinl.:ing and cold air rising. This is a thermally in­ implying that the zones lose their anticyclonic vorticity and direct circulation. which stores potential energy and must the belts lose their cyclonic vorticity as altit ude increases, be mechanically driven. Gierasch et al. (19 6) and Conrath Atmospheric Dynamics 111

45

,..... li 30 100 2 'i.. ;:: i( .. IS so co 'iii ~ 1.5 ~ ..•" •. C! .. 0 0 .! .3 "" 0 i'. ::> :l.. -so ..!;.

-100 --- .. (oloucll}

40 20 0 - 20 - 60 PUNETOGRAPHIC LATITUDE

Figure 6.6. Upper tropospheric (~270 mbar) thermal wind shears compared with cloud-tracked wind velocities. The figure is from Gierasch et al. (1986). who computed the thermal wind shears from Voyager IRlS data. The cloud-tracked winds are from Limaye (1986) and refer to the ~0.7 bar level. .. et al. (1990) argue that the mean zonal flow at cloud-top level provides the energy. That flow is subject to dissipation, which they parameterize as Rayleigh drag and Ne,vtonian radiative damping. The dissipation causes the zonal winds to decay with altitude. The upwelling and downwe!Ung above 's 0" ..0 the clouds are part of a mean meridional overturning t hat 0 balances the dissipative effects with Coriolis acceleration e.. and vertical advection of potential temperature. Pirraglia (1989) and Orsolini and Leovy (1993a. 1993b) show that shear instability produces large-scale eddies that give the required decay of jets within t he upper troposphere. The in­ stabilities thus may be t he physical process underlying the F igure 6.7. Estimates of zonal mean ammonia concentration, 1 drag coefficient parameterization in t he interpretation by 5- J.liTI cloud optical depth ( 2050 em- ) • and 45- J.liTI cloud optical 1 Gierasch et al. (1986). depth (225 cm- ) from Voyager lRlS spectra. Absolute values of these retrieved quantities are model dependent. but the relative West et al. (1 992) and :\Ioreno and Sedano (1997) values from latitude to latitude are reliable. Ammonia and 45-J.UTI haYe calculated the residual mean meridional circulation cloud refer to levels between about 1 bar and space, and 5-J.UTI (in the altitude-latitude plane) taking into account the cloud refers to levels between a few bars and space. All three belt-zone temperature differences as well as the absorbing quantities correlate well with continuum brightness in the visible aerosols that are found especially over the polar regions. (Gierasch et al. 19 6). Such aerosols increase t he solar heating rates, and result in a hemisphere-wide circulation from 1 to 100 mbar. The belt­ zone downwellings and upwellings were found to persist only 6.3 DISCRETE FEATURES up to t he vicinity of the tropopause at ~100 mbar. The hydrogen para fraction shows a large-scale gradient 6.3.1 Great Red Spot from a minimum near the equator to higher values near the The GRS is probably t he largest and oldest vortex in the poles. which is consistent with upwelling near the equator atmospheres of the planets. Its oval shape appears in draw­ and sinking near t he poles, but it does not show a systematic in!!S from 1831 but it was tentatively first observed by J .P. correlation with belts and zones the way the clouds and c:ssini and others from 1665 to 1713 (Rogers 1995). Mea­ ammonia do (Gierasch et al. 1986). However these ortho­ surements in 1880 showed that it had an east- west length of para data from the IRIS spectra refer to a higher level in 39 000 km and a north- south width of 12 500 km. Its east­ the upper troposphere (a few hundred mbar) than do the west length has decreased since then to its present 17 000 km cloud optical depths and the ammonia concentration. and (Beebe and Youngblood 1979, Rogers 1995. Simon-:\Iiller et thus may be diagnostic of a different dynamical regime. al. 2002). The GRS is an anticyclonic vortex (high pressure center) extending from 17°8 to 27.5°8 PG latitude. In 1979 it had a maximum velocity of 120 m s- 1 along a peripheral 5 1 collar and maximum relative vorticity ~6 x 10- s- . which is about 1/ 3 the local planetary vorticity (vorticity due to the planet's rotation). As shown in Figure 6.8. its central 112 Ingersoll et al.

-15 ,.. :l =..., - -2!1

120 110 longitude

Figure 6.8. GRS in three filters with measured velocities. The images are from Galileo and the velocities are from Voyager (Dowling and Ingersoll 19 ). The flow is a counterclockwise high-speed jet \\ith a quiet region inside. The upper left panel is a continuum filter image at 756 nm. The upper right panel i a violet image at 410 nm. The lower right panel is a methane band image at 89 nm. High clouds appear bright at 9 nm. Two regions with high clouds are visible in the image - the interior of the GRS. which is relatively quiet, and the mall region in the upper left corner, which is sbort-Ji,·ed and acth·e. Lightning often appears in these active regions. which are probably ites of moist convection. part are quiescent (1Iitchell et al. 19 1, Dowling and In­ do streamlines in the circulating current around the GRS gersoll 19 9, Sada et al. 1996, Vasavada et al. 199 ). Recent (Simon-1Iiller et al. 2002). measurements from Galileo images indicate an increase in maximum tangential velocities to 190m s- 1 (Simon-Miller During the period 1 0- 2002. the GRS moved westward 1 et al. 2002). relative to System III with an average speed of 3 m s- . The speed varies slowly on a long (multi-year) time cale. Voyager. Galileo, and recent Cassini temperature mea­ Superpo ed on thi motion, the GRS osciUates in longitude surement how that the GRS has a cold core at upper with a period of 90 days and peak-to-peak amplitude of --..1 ° tropospheric levels (Flasar et al. 19 1, Orton et al. 1996. (Solberg 1969. Trigo-Rodriguez et al. 2000). Thi motion Simon-11iller et al. 2002) with a peripheral ring of high 5-J.!m is perturbed when the GRS interacts with features drift­ emi ion (Terrile and Beebe 1979). Figure 6.9 shows a tem­ ing relati,·e to it in nearby latitudes. The GRS engulfs the perature map obtained by the Galileo PPR instrument. The smaller anticyclones of ize "'2000 km and po ition PG lat­ cold temperatures over the GRS indicate that the anti­ itude "'20°S that approach from the east with a speed of 1 cyclonic \'Orticity decays with height. reaching zero at P 50 m s- (Smith et al. 1979a. :\lac Low and Inger oil 19 6). "' 50 mbar (Fiasar et al. 19 1). The para fraction shows On other occa ions. before they reach the GRS these mali a mini mum within the GRS. which is consistent with an up­ vortices are deflected outhward into the eastward current at welling, zone-like. anticyclonic behavior (Sada et al. 1996, PG latitude 27°S by a dark curved feature (Peek 1958, Smith Simon-Miller et al. 2002). et al. 1979b). the so-called South Tropical Zone Disturbance (STrZD), which forms sporadically. Several encounters be­ Photometry from the UV to the near-IR indicates that tween the GRS and the STrZD have been documented in the GRS has one main cloud deck at 0. 7 bar that is over­ detail (Smith et al. 1979b, Sanchez-Lavega and Rodrigo lain by a dense tropospheric blue-absorbing haze at about 19 5. Rogers 1995). In 1997 the GRS interacted with a 200 mbar and an uppermo t thin trato pheric haze extend­ 14-year old. 000 km anticyclone at 21.5°S (the White Trop­ ing to P"' 10 mbar (Banfield et al. 199 ). There are signif­ ical Oval), ab orbing part of its material and expelling the icant internal variations from point to point ( imon-~Iiller rest (Sancbez-Lavega et al. 199 b). On the equatorward side et al. 2002). The GRS is dark at violet and blue wavelengths. the GRS sometimes generates a stable plume-like feature at giving it a brick-red color. The chemical agent responsible 6°S that compre es the white material in the SEB (Sanchez­ for thi color is unknown. The combined data bow that Lavega and Rodrigo 19 5). These interactions produce tran­ the GRS cloud deck slopes upward from outh to north. as sient accelerations and decelerations in the GRS motion. Atmospheric Dynamic 113

Figure 6.10. Galileo image of white ovals DE and BC shortly before their merger in 199 (Vasavada et al. 199 ). The ovals DE (left) and BC (right) are at 30°8 planetocentric latitude. They are anticyclones (counterclockwise in the southern hemisphere), and there is a cyclonic region between them. The eastward current at 32°8 flows south of DE and creates the white cloud on the west side of the cyclonic region. It then flows north. clockwise around the cyclonic region, and finally south of BC and out of the figure to the east. The white oval to t he south did not participate in the merger.

Figure 6.9. Galileo PPR images of the GRS. The instrument records t hermal emission from the gas in the upper troposphere, also form when an anticyclonic zone breaks up. as the STZ where the GRS is some 10 K colder than its surroundings. Since did in 1939-40. Ovals disappear by merging and by getting there are no radiative processes to account for these cooler tem­ stretched out in the large-scale shear flow. Observations and peratures. they are most likely due to upwelling of air with lower dynamical simulations suggest that within each mjd-latitude potential temperature. zone oval ingest or merge with others. suggesting that they would grow in size until one or a few dominate (Mac Low and Ingersoll 1986, Dowling and Ingersoll 1989). However, 6.3.2 White O vals and Other Anticyclones historical observations reveal that the semi-major axes of the The GRS is the largest anticyclonic oval. but it is not unique. largest white ovals and the GRS decrease over time (Simon­ :\lo t of the others are white. but some are red. \Vhite ovals :\Iiller et al. 2002). are most conspicuou near PG latitudes 33° and 41°S but The anticyclonic rotation of t he largest white ovals is al o occur near 17° -. 34° :--1 and 40°1\. The major diameter well defined by their interior cloud texture. Tangential ve­ ranges from rvlQOO km to over 5000 km. The ones at high locity increases approxjmately li nearly with radial distance latitudes are smaller and rounder than those at low latitudes out to the visual boundary (Mitchell et al. 19 1. Vasavada (:\lac Low and Ingersoll19 6. ~Iora l es-Juberfas et al. 2002a). et al. 199 ). Like the GRS. the white ovals are cold at up­ The ratio of meridional to zonal extent approaches unity for per troposphere levels. even after mergers (Sanchez-Lavega t he mallest ovals. et al. 1999. 2001). Their anticyclonic vorticity. t he presence The three large white ovals at 33°S (termed BC. DE. of colder upper-level temperatures. the observed increased and FA) formed when an anticyclonic, planet-encircling altitude of overlying haze . t heir bright, white coloration zone. the STZ, broke into three part in 1939- 40 (Peek and dark halo all suggest moderate upwemng within white 195 , Beebe et al. 19 9. Rogers 1995). The ovals were similar ovals (Conrath et al. 19 1. Banfield et al. 199 ). The GRS is in appearance and size (minor and major a.xes about 5000 distinguished from white ovals by its annular velocity struc­ and 10 000 km) but exhibited varied longitudinal drift rates ture (surrounding an interior with little organized motion) (possibly correlated ""ith latitude). spacing, and interactions and its coloration. which may indicate its greater ability to with neighboring cyclonic features and t he GRS. In the late dredge and/or confine trace species. Little red spots have 1990s. t he eastward drift rate of oval BC lowed, causing the occasionally been seen in the TrZ, which is the nort hern other ovals and intervening cyclonic feature to pile up (com­ counterpart of the STrZ where t he GRS resides (Beebe and press) on the we tward side of BC (Simon et al. 199 ). In Hockey 19 6). These small anticyclones have the arne char­ early 199 . ground-based telescope documented the merger acteristic UV ab orber that is present in t he GRS but is not of ovals BC and DE into a larger oval and possibly a small. present in the belts. cyclonic vortex (Sanchez-Lavega et al. 1999). Figure 6.10 shows BC and DE just before their merger. with a vastly re­ duced cyclonic region queezed in between them. Two year 6.3.3 Cyclonic Features later the new oval merged with FA ( anchez-Lavega et al. 2001) to form a single oval named BA. The cyclonic regions tend to be more spread out in the Ovals form in several way . Small ovals ( <1000 km) zonal direction than the anticyclonic ovals. They have a may form in updrafts (e.g.. thunderstorm clu ters) who e more chaotic. fi lamentary texture and tend to evolve more preading motion produces anticyclonic vorticity. Ovals may rapidly, though orne survive for a few years. The cyclonic 114 Ingersoll et al. region contain a variety of organized morphologies that can be grouped in the following main categories (Smith et al. 1979a, 1979b. Mitchell et al. 1979. l\Iorales-Juberlas et al. 2002b): (1) filamentary turbulence related to the highest­ speed jets in the SEB (west of the GRS). ~EB, and ~TB; (2) organized folded filamentary regions (size 15000 km. fil­ ament width "'600 km): (3) elongated areas with contours clo ed by a ribbon-like feature; ( 4) discrete brown elongated ovals called "barges"' (zonal extent "'5000 km). Hatzes et al. 05 {1981) measured the peripheral circulation of a barge and Q) :g. its shape o cillations. Like the cyclonic belts. the closed cy­ clonic features are warmer than their surrounding at upper ~0 .3 L...-- tropospheric levels, consistent with downwelling (Conrath j et al. 19 1). At orne latitudes the anticyclones ··invade·· the belt on their equatorward side and break it into a series of closed cyclonic cells. The cyclones alternate in longitude with the anticyclones. but they are offset from each other in latitude. This alternating pattern resembles a classic Karman vortex street (Youssef and Marcus 2003). In the laboratory and in nature. such configurations form in wakes behind blunt bod­ ies and are stable to small perturbations. On Jupiter there is - 6~2 0 an asymmetry between the anticyclones and cyclones: The 0 du/dy (e-05/sec) r(u',v') former are more compact: the latter are more elongated and have a more chaotic textme. For example, the 12 compact Figure 6.ll. Zonal velocity gradient dii.f dy (left) and the corre­ anticyclonic white ovals at 41°8 alternate in longitude with lation coefficient r(u', v') from Voyager 1 (center) and Voyager 2 chaotic cyclonic patches that are a few degrees closer to the (right), from Ingersoll et al. (1981). Here u' and v' are the east­ equator than the anticyclonic ovals (Figme 6.5, left). Such ward and northward velocity components after subtracting the an asymmetry i not present in a classic vortex treet but mean winds. The imilarity of the three curves as a function of latitude indicates that the correlation between the components is could arise in a rotating planetary atmo phere, perhaps be­ significant. The fact that the correlation tends to ha\"e the same cause the anticyclones are vertically thicker, which follows sign as dilj dy indicates that the eddy momentum tran port is from the thermal wind equation, or perhaps because the cy­ into the jets and tending to accelerate them. clonic belts are the sites of moist convection.

6.3.4 Eddy Momentum Flux ratio is "'0.1, assuming the energy transfer i taking place The word "eddy.. refers to all the non-zonal features - the in a layer 2.5 bars thick, e.g.. from the 0.5-bar level to the residuals after subtracting off the zonal mean (average with 3.0-bar level (Ingersoll et al. 19 1). The ratios are a mea­ re pect to longitude). Eddy winds u' and v' are the residual sure of power in the mechanical energy cycle compared to eastward and northward velocity component after subtract­ that in the thermal energy cycle. and seem to imply that the ing off the means il and ii for that particular latitude band. jovian heat engine is much more efficient than the Earth's. The covariance pu'v' is the northward eddy flux of eastward Up-gradient tran fer do not violate physical laws as long as momentum and is an important diagnostic of the flow. The the eddies have a ource of energy that is separate from the eddy heat flux pCpv'T' has never been measmed. and the shear flow. Buoyancy-driven convection is an obvious energy values of ii are smaller than the measurement error. source that operates on both Earth and Jupiter. Beebe et al. (19 0) and Ingersoll et al. (19 1) used a Sromovsky et al. {19 2) challenged this estimate of the data set containing over 14 000 individual velocity vectors eddy momentum flux. They correctly pointed out that bi­ to determine pu'v' for 120 latitude bands. each 1° wide. ases could arise in measuring the 14 000 velocity vectors. A from 60°8 to 60°N. They found that the sign of the eddy human operator had to choose a cloud feature and find it momentum flux depends on the sign of dil/ dy, where y is in a second image taken at a different time. This target-of­ the northward coordinate. At latitudes where dil/ dy is posi­ opportunity approach does not sample the planet uniformly. tive the eddy momentum flux tends to be positive. and vice A spmious signal could arise, for example, if there were more versa (Beebe et al. 19 0. Ingersoll et al. 19 1). Figure 6.11 features on the SE and N\V sides of a large vortex and fewer shows dil/ dy and the correlation coefficients r·(u'. v') from on the SW and NE sides. Clearly the procedure needs to be \loyagers 1 and 2. all as function of latitude. The data automated and the measurement of pu'v' needs to be re­ refer to cloud-top levels. 0. 7 to 1.0 bars. The fact that the done. three curves how in-phase variation indicates that the eddy momentum flux is into the jets. which is opposite to what one would expect from turbulent diffusion. This up-gradient momentum transfer occms in the terrestrial jet streams as well. but the ratio of energy transfer into the jets to the power radiated by the planet is only "'0.001. On Jupiter the Atmospheric Dynamics 115

PIOileer 11 (1t1bound) -- Pioneer 10 - - P1oneer 11 (outbound) -

-40 -20 0 20 60 80 latitude (deg)

Figure 6.12. Brightness temperature (right-hand ordinate) and intensity (left-hand ordinate) as a function of latitude for three different values of the emission angle cosine (Ingersoll et al. 1976) at wavelength bands centered at 20 and 45 IJ.Ill. The data are from Pwneer 10. which viewed the low latitudes, and Pioneer 11. which reached higher latitudes than a ny other spacecraft. Significant features of the curves include:" (1) the agreement between Pioneers 10 and 11 , (2) the lack of pronounced equator-to-pole contrasts, and (3) t he higher brightness temperatures in belts (B) compared to zones (Z).

6.4 TEMPERATURES AND VERTICAL the o cillation but that forcing by small-scale gravity waves STRUCTURE provides a better fit to the ob ervations (cf. Li and Read 2000). 6.4.1 Global Te mperature Variations Orton et al. (1994) also noted a large cooling at the 250-mbar level from 19 5 to 1990 in a region between ap­ As shown in Figure 6.12, J upiter has no appreciable equator­ proximately 15° r and 27° J (planetocentric), i.e., between to-pole temperature gradient (Ingersoll et al. 1976. Pirraglia the northern boundary of the EB and the northern bound­ 19 4). Except for variations on the cale of the belts and ary of the ·orth Temperate Belt (. TB). They estimated zones, the emitted infrared radiation is independent of lati­ that if wind were steady at the cloud-top level near 60Q- tude. This means that energy is being transported poleward. 700 mbar then a large cooling trend at the 250-mbar level either in the atmo phere or in the interior. to make up for recorded between 19 5 and 1990 implied, through the ther­ the extra sunlight absorbed at the equator. Ingersoll (1976a) mal wind relationship, that the zonal 'vind decreased by at and Ingersoll and Porco (197 ) argued that Jupiter's inter­ least 3 m s-1 per terrestrial year. nal heat flux is diverted poleward by lightly lower polar temperatures at the top of the convection zone. Deep con­ ,·ection acts as a thermostat that maintains the equator and 6.4.2 Thermal Waves poles at essentially the same temperature. The fluid interior short-circuits the atmosphere. they argued. leavin g it with The profiles of the Voyager radio occultation experiment no role in the global energy budget. Earth ·s oceans cannot (Linda! et al. 1981) show wave-Like featmes (Figm e 6. 13), do this because they are heated from above and are therefore a lthough Linda! et al. suggested that they could be the result dynamically less active than the atmosphere. of local particulate layers that absorb sunlight. The features Jupiter has seasons despite its low 3° obliquity. Orton have vertical length scales of rvl.5 pressure scale heights et al. (1994) found high-latitude temperature maxima two and amplitudes of 5-25 K. The horizontal structure is un­ year after solstice at the 250-mbar level. The data cover known, as is the wave period. Vertical waves are evident one jovian year. from 1979 to 1993. This phase lag is con i - in the Galileo probe measurements of Jupiter's temperature tent with the computed radiative time con tant, which has structure (Seiff et al. 199 ). Stellar occultation result show­ a minimum of 4 x 107 s at the tropopau e (Flasar 19 9). ing temperature oscillations in the upper stratosphere rein­ A prominent non-seasonal variation occurs in the Equa­ force the wave interpretation of the Galileo probe results. torial Zone (EZ). whose 250-mbar temperature o ciliates Longitudinally varying thermal features that do not cor­ with a 4-year period and appears to be opposite in phase relate with visible features have been ob erYed in the up­ with the 20-mbar temperature (Orton et al. 1991. Chap­ per tropo phere (Magalhaes et al. 19 9. Deming et al. 19 9. ter 7). Leovy et al. (1991) termed this the quasi-quadrennial 1997, F i her 1994. Orton et al. 1994, Harrington et al. 1996). oscillation (QQO) of Jupiter, and related it to upward­ The amplitude is largest over the NEB and SEB. but is also propagating, equatorially trapped waves in analogy with the evident in belts farther from the equator. The waves are es­ quasi-biennial oscillation (QBO) of Earth's tropical atmo­ sentially stationary relative to System Ill. independent of sphere. Using a numerical model. Friedson (1999) showed cloud-tracked winds at the same latitude. Power spectra of that large-scale equatorial waves are ineffective in driving these oscillations show that longitudinal wavenumbers less 116 Ingersoll et al.

Jupiter

3 Dust-free model -;::-10 5 Voyager I .D"' h egress (1.N) .s h Voyager I ~30 ::l Iris rJl rJl (near Voyager I ~ I ingress latitude) 0.. 100 ~ 10 Voyager I 113 ingress (12.S) .0 300 ~ ::J 1/) 1/) ...Q) 1000 100 110 120 170 a.. 15

Figure 6.13. Temperatures in the upper troposphere and strato­ sphere (Lindal et al. 19 1). The Voyager 1 ingress and egress curves are from the radio occultation experiment and are for spe­ cific points on the planet. They show large-amplitude wave-like 20 features. The Voyager 1 IRJS curve is an inversion of radiance data and covers a much wider area than the occultation profiles. The dust-free model assumes radiative equilibrium above the tern­ perature minimum and does not take into account po ible dust particles that might absorb sunlight and heat the atmosphere.

25+---~---r--~--~----r---~~ than 15 predominate (Deming et al. 1997, Orton et al. 199 , 60 80 100 120 140 160 180 200 Fisher et al. 2001). These features are widely assumed to Wind Speed, m/s be vertically propagating Rossby waves (e.g., Deming et al. 1997. Friedson 1999, Li and Read 2000). Fundamentals of Figure 6.14. Eastward wind vs. altitude measured by the the phenomenon. such as how t hey are forced and whether Doppler wind experiment on the Galileo probe (Atkinson et al. 1998). The three curves show the range of acceptable solutions. they are exactly fixed to System III are not known. The 100m s-1 speed at the 0.7-bar level agrees with the cloud­ tracked wind speed at this latitude (6.5° :-<).

6.4.3 Vertical Structure - Winds Conversely, a layer that is neutrally stratified (dry adiabatic, The Galileo probe measured the zonal wind profile from the i.e., potential temperature constant with altitude) is more 0.5-bar pressure level down to the 22-bar level (Atkinson likely to be barotropic. In other words. a stably stratified et al. 199 ). The measurement was supposed to settle the layer acts to decouple the winds above from the wind question of whether the winds are shallow or deep (Section below. 6.2.4). The general expectation was that the winds would The wind profile measured by the probe i at least either decrease to zero at the base of the water cloud or consistent with t his picture: The wind varied with depth would be constant with depth. In fact the winds increased (baroclinic behavior) inside the clouds in the 1- to 4-bar with depth from 1 to 4 bars and then remained constant range where moist convection is expected to produce poten­ (Figure 6.14). Clearly the winds are not confined to the al­ tial temperature gradients, and the wind remained constant titude above the water cloud base at 6- to 7-bars. In that \vith depth (barotropic behavior) below the clouds where sense, the winds are ·'deep: ' but the interpretation is com­ dry convection i expected to eliminate the potential tem­ plicated by the local meteorology of the probe entry site. perature gradients. A problem with this picture is that the Winds are related to temperatures t hrough the t hermal measured temperatures followed a dry adiabat more closely wind equation. A barotropic fluid has constant temperature than a moist adiabat in the 1- to 4-bar region, but that may on constant-pressure surfaces. and the wind are constant be a special property of 5-J.im hot spots. with depth. If the fluid is not barotropic it is referred to as baroclinic. and the winds vary with depth. A single temper­ ature profile. like the one derived from the Galileo probe. 6.4.4 Vertical Structure - T e mperature cannot distinguish between a barotropic and a baroclinic state. But if the flow is baroclinic. there must be gradients of The Voyager radio occultation results (Lindal et al. 19 1) potential temperature (gradients of specific entropy). There­ reveal a statically stable atmosphere above 300 mbar and a fore a layer that is stably stratified. with potential tempera­ dry adiabat near 1 bar (Figure 6.13). The bulk of Jupiter· ture increasing with altitude, is more likely to be baroclinic. interior is expected to be convective. and the simplest model Atmospheric Dynamics 117 is one where the atmo phere follows a dry adiabat from the interior up to the base of the water cloud and a moist adiabat A ---Galileo ················solar within the cloud. The latter is indistinguishable from a dry 1 ------2 x solar adiabat near 1 bar where latent heat effects are negligible. - · - ·-·- · 3 x solar Thi picture seems to work in the Earth" tropics. where the atmosphere over the ocean i close to moi t adiabatic. Figure 6.15. from howman and Inger oil (199 ). how a comparison between moist and dry adiabats for three cases, in which the deep abundance of wa ter is 1. 2. and 10 3 times solar (elemental abundance ratios equal to those on the Sun). Ammonia and H2S are as umed to be solar, and · their effects on the latent heat release and molecular mass 100 200 300 are included. Virtual temperature Tv i related to buoyancy T v [K] and i defined as Tmo/m. where mo i the molecular mass l of dry air and m i molecular mass of the mixture - dry 8 c I a ir plus condensable vapor. As pressure decreases, Tv in­ 1 1 creases relative to t he dry adiabat. both because latent heat is released and because the heavier condensate falls out. The effect of water can easily exceed 10 K. and the as ociated I ·tatic stability (virtual temperature gradient minus the adi­ /. abatic gradient) is large. The effect of ammonia and H2 "I:• ~ are only several times 0.1 K, largely because these gases are 10 : ~ less abundant and al o because their latent heats are smaller I ; (Atreya 19 6). ;.Jumerical simulations that explicitly model the interaction between convection and condensation in the -20 -10 0 10 20 -20 -10 0 10 20 1- to 10-bar layer give the same result - a statically stable T-Tref [K] layer at a few bars that is overlain and underlain by neutrally T v-T v(ref) [K] stable Layers (. akajima et al. 2000). F igure 6.1 5 . Temperatures computed for an atmosphere with The Galileo probe found a temperature profile that was 1. 2. and 3 times solar abundances of H20. The abundances of close to dry adiabatic at all levels below 1 bar (Seiff et al. NH3 and H2S are solar in each case. The dashed li nes how the 199 ). using the probe data. i11agalhaes et al. (2002) derive temperature along the three moist adjabats, all of which pass a small static stability that v-aries between 0 and 0.2 K km - l through the point 169 K at 1 bar as measured from the Voyager in the 1- to 22-bar region. The measurement uncertainty i radio occultation (Linda! et al. 19 1). The solid line is the profile 1 rv0. 1 K km- . Inferences based on a gravity wave interpre­ measured by the Galileo probe (Seiff et al. 199 ). Buoyancy is tation of the probe·s ,-ertical motion (Allison and Atkin­ measured by virtual temperature T,·. which includes the effects son 2001) are generally consistent with this result. For com­ of both physical temperature and molecular mass of the conden­ pari on, if Jupiter· atmo phere were isothermal t he static sate. (A) Tv vs. log P. (B) The same. but with the dry adiabat subtracted. (C) Temperature T vs. log P with the dry adiabat stability would be rv2 K km _,, and if t he water abundance subtracted (Showman and Ingersoll 199 ). were 1- 3 times olar the static stability. defined by the dif­ ference in Tv between moist and dry adiabat (Figure 6.15). 1 would be rv1 K km- . To infer the tatic tability away from the probe ite. near the clouds have been generally successful in explaining one relies on indirect methods. which do not always agree. the basic features of Jupiter· jets and vortices (e.g.. Dowling Several types of waves that require a stable layer to· prop­ and Inger oil 19 9. Cho and Polvani 1996. chterberg and agate have been observed. ~Jesoscale waves with rv300 km Inger oll 19 9. 1994, W illiams 1996, tllarcus and Lee 199 . horizontal wavelength are seen in Voyager and Galileo im­ Showman and Dowling 2000. Cho et al. 2001). The defor­ ages. Bosak and Ingersoll (2002) uggest they are an exam­ mation radius Ld. which is the distance beyond which two ple of shear instability in a layer of mall tatic stability. vortices do not interact. is estimated to be ""'2000 km within Flasar and Gierasch (19 6) suggest they are ducted gravity a factor of about two. Ld is related to the static stability of waves in a ub-cloud layer of large tatic stability. Such a the atmosphere, and the 2000 km value is roughly consistent "tatically stable layer would help to explai n the existence of with the stabilities expected from t he moist-convection and the equatorial plumes (Allison 1990). And narrow, expand­ wave con iderations listed above. However, the large-scale ing rings observed after the collision of Comet Shoemaker­ dynan1ics models are not yet detailed enough for a defini­ Levy 9 with Jupiter (Hammel et al. 1995) have been inter­ tive comparison. preted as the trato pheric tails of gravity waves ducted by a Showman and Ingersoll (199 ) point out that the de­ stable layer below the clouds (Inger oil and Kanamori 1995). crease of probe-derived wind speed with altitude in the It is possible that the L9 \Yave can be explained without 1- to 4-bar pressure range implies substantial gradients of the tropospheric table layer (\Valterscheid et al. 2000). If temperature with latitude, and that these gradient change the Ingersoll and L

6.5 M OIST CONVECTION AND LIGHTNING -40 6.5.1 Lightning Distribution Voyager Galileo, and Cassini detected lightning in long­ expo ure images of Jupiter·s nightside (Borucki and ~ag­ -60 alhaes 1992, Little et al. 1999. Gierasch et al. 2000. Porco et al. 2003). The lightning strikes were concentrated in clus­ -100 ters. suggesting that everal discrete storms produced multi­ ple strikes during each of t he exposures. Twenty-six un ique storms were documented in t he two Galileo data sets. The lo­ Figure 6.16. Latitude of lightning torms (horizontal lines) ob­ cations of lightning clusters have been correlated with t he lo­ served in Galileo nightside images compared to the zonal velocity cation of small. bright clouds on dayside images. Although profile (Limaye 19 6). The bars on the right show the number of data are carce. these t hunderstorm clusters appear to be lightning storms per unit area in latitude bins 5° wide. Mo t of the observations are from a broad survey that covered more than associated with high levels of humidity (Roo -Serote et al. half the planet in late 1997 (Little et al. 1999). The observations 2000) and clouds at deep levels where water would be ex­ near 15°8 are from an intensive study of the SEB in mid-1999 pected to condense (Banfield et al. 199 ). (Gierasch et al. 2000). Lightning storms predominate in the cy­ As shown in Figure 6.16. lightning-bearing storms ap­ clonic bands, where the velocity is decreasing (increasing) with pear to be concentrated within narrow latitudinal bands latitude in the northern (southern) hemisphere. that are related to Jupiter·s zonal jet st ructure. In fact, ev­ ery storm occurs within a region of cyclonic hear or t he that moist convection involving water is occurring. Veloc­ neighboring westward jet. and 10 of the 11 regions of cy­ ity vectors show divergence within the high cloud OYer one clonic shear (belts) equatorward of :::::60° latitude are known of the torm centers, consistent with termination of an up­ to contain lightning torms. Belts near 4rX and 52°8 pro­ draft. Near-infrared observations of the NEB (Roos-Serote duced significantly more lightning strikes per area than other et al. 2000) showed high concentrations of water vapor in belts. Finally. Galileo·s probe detected radio emissions that the vicinity of one of these high. thick cloud . In thls case can be explained by a lightning-like source about 12° from there was no nightside imaging, so it was not possible to the probe site, which was at 6.54°. PG latitude (llinnert confirm that t his was a lightning torm. et al. 199 ). Both the lightning and t he small intense, rapidly diverging storms are observed almost exclusively in the belt . It is possible that this is an observational effect - that 6.5.2 Convective Heat Flux and Struct ure of the the uniform hlgh cloud of the zones are covering convec­ Lightning Clouds tive activity below. but t hi po sibility is unlikely for two In May 1999, Galileo took time-lapse images of the SEB on reasons. F irst, the mall int ense storms penetrate to higher t he dayside followed by lightning searches on the nightside levels than the uniform cloud in the zones. and therefore two hours later. Two lightning storms were found (Gierasch should be visible if they were present there. Second. the et al. 2000). Figure 6.17 displays a false color image revealing photons from the lightning seem able to reach the surface optically thick clouds at high elevation within a few hundred from great depths through optically thick clouds. They too kilometers of a deep cloud. located where the nightside im­ should be visible in the zones if they were present. ages bowed lightning flashes. Radiative modeling (Banfield Gierasch et al. (2000) estimate that the lightning torms et al. 199 ) of methane band and continuum images places are carrying mo t of the planet" internal heat flux. They the high cloud at a pressure of a few hundred mbar and base their estimate on (1) the temperature difference (~5 K) the deep cloud at a pressure exceeding 3 bars. where wa­ between t he at mo phere at the top of the convective clouds ter is the only possible condensate. T he authors conclude and the adiabat from the deep interior, (2) the vertical mass Atmospheric Dynamics 119

Jovian lightning occurs in storms whose sizes range from 200 km to over 1000 km a nd whose separation distance is "'104 km (Little et al. 1999). A 1-min exposure captures 10- 20 flashes. which therefore overlap in the image. Overlap is not a problem if one is calculating the average optical power of the storm. but it prevents one from estimating the prop­ erties of individual flashes. Fortunately, the Galileo camera captured three lightning storms in a "scanned .. frame - a 59. s exposure that was deliberately smeared across the disk so that each storm left a trail of bright dots where the individual flashes occmred. The brightest flash in the scanned frame was 1.6 x 1010 J (Little et al. 1999). This is three times brighter than the largest terrestrial superbolts (Borucki et al. 1982). Smaller flashes are more numerous. but most of the storms' optical energy is carried in t he largest flashes. The detection threshold for the Galileo and Voyager cameras is about 2 x 108 J. which is larger than the aver­ age terrestrial fl ash. Thus it is not possible to compare the global flash rates (number of flashes per unit area per unit -10 t ime). However the average optical power per unit area is 7 2 about the same for Earth and Jupiter, 3- 4 x 10- \V m- • -12 even though the convective heat fluxes differ by more than an order of magnitude ("'80 W m-2 for Earth vs. "'6 W 2 -14 m - for Jupiter) and the hydrologic cycles are fundamen­ tally different. 1 -16 The spectral energy density (W nm- ) measmed by Galileo was greatest in the red filter. next greatest in vio­ let. and least in green. The Cassini H a filter (centered on a strong line of atomic hydrogen at 656 nm) had the highest Figure 6.17. See Plate 3. Lightning storms (Gierasch et al. 2000) spectral energy density of all. While these results are con­ in the southern hemisphere. The top panel is a superposition of sistent with a mixture of line and continuum emission in a a continuum wavelength (756 nm) in the red plane, a medium hydrogen- helium atmosphere (Borucki et al. 1996), it is dif­ methane band (727 nm) in the green plane. and a strong methane ficult to infer physical properties of the lightning (discharge band (889 nm) in the blue plane. The location of lightning is rate. temperature, or pressure) from these data alone. shown by the blue overlay on to a continuum image in the mid­ dle panel. l\ote the close proximity of red (deep) features and bright white (high) features to the flash locations. The bottom 6.5.4 D e pth of Light ning panel shows velocity vectors derived from three time-steps in the continuum. The flags point downwind, and the largest flag cor­ Since t he photons are diffusing up through the intervening 1 responds to a speed of 70 m s- . The large-scale fl ow structure clouds. the depth of the lightning is roughly proportional to is eastward near the top of the frame (the north edge) and west­ t he width of the bright spot in the image. Width is defined as ward near the bottom. In the southern hemisphere this represents the half-width at half-maximum (HWH:\1), the radius of the cyclonic shear. Approximate latitude and longitude are indicated circle where the intensity is one-half the value at the center on the bottom panel. This region is ~30° west of the Great Red Spot. of the spot. Scattering models put t he ratio depth/ HWHM in the range 1- 2 (Borucki and Williams 1986. Little et al. 1999. Dyudina et al. 2002). The difficulty is finding light­ t ransport. which they get from t he rate of horizontal diver­ ning flashes that are well resolved (pixel size ::;25 km), not gence. and (3) the number of convective storms per unit overlapped. and not saturated. surface area. The latter estimate comes from earlier Galileo Borucki and Williams (1986) report that the average observations t hat surveyed most of the planet's surface for HWHM for lightning observed in the Voyager images is lightning (Little et al. 1999). 55 ± 15 km. The HWH:\1s for six Galileo flashes are 7. 69. 37, 72, 42. and 50 km (Little et al. 1999, Dyudina et al. 2002). This puts the average depth in the range 6Q-120 km. 6.5.3 Energy of L ig ht ning F lash es depending on the parameters of t he scattering model. The The measurable quantities are optical energy per flash and largest flashes could be even deeper. average optical power per unit area. Flash rate and color are With these large depths the lightning could be below measurable in principle. The optical range is here defined by the freezing level or even below the base of t he water cloud. t he transmission of the Galileo clear filter. which goes from unless the water abundance is much higher than impli ed by 3 5 nm to 935 nm (Little et al. 1999). One assumes that solar values of t he 0 / H ratio. The radiative properties of the t he photons are emitted uniformly in all directions. This clouds introduce a large (factor of 2) uncertainty. ot only gives a lower bound on the energy because clouds above the are the radiative properties uncertain, but the shape of the fl ash site may scatter the photons back down where they are clouds are uncertain and are apparently not plane-parallel. absorbed. Optical depth is greatest over the lightning and falls off with 120 Ingersoll et a1 . horizontal distance (Dyudina et al. 2002}. There is a small scale of motion). and f = 20-sin(¢) with 0. the planet·s possibility that some of the flashes are doubles. ).levertheless. angular velocity and 1> the latitude (e.g .. Pedlosky 19 7). the conclusion is that the lightning flashes are deep - that Attributed to Rossby, L d is the horizontal distance beyond they must be occurring within or below the jovian water which two vortices do not st rongly interact. Alternatively. cloud (Little et al. 1999}. it is the maximum size of features for which the fluid is barotropic and vertical stretching of vortex tubes is negli­ gible. The deformation radius is relevant where f # 0, i.e .. 6.5.5 Models of Moist Convection away from the equator. If there is a stable layer associated with moist convection within the water cloud (Achterberg Conrath and Gierasch (19 4) discussed the relative buoy­ and Ingersoll 19 9. Ingersoll and Kanamori 1995}, then Ld ancy effects of latent heat release, hydrogen ortho-para con­ may be written cllf l where c is t he speed of gravity waves version, and molecular weight differentiation on the outer that are ducted in the layer. lts value is estimated to be planets and found that all t hree are in principle capable of "'2000 km in Jupiter"s troposphere at mid latitudes. with causing density perturbations on the order of 1%. Smith and both the uncertainty and the natural variability probablv a Gierasch (1995) showed that ort ho-para effects are less im­ factor of 2 in each direction. The value of Ld could be m~ch portant for Jupiter than t hey are for Uranus and 1'\eptune. smaller if the low values of N measured by the Galileo probe Detailed modeling of moist buoyancy effects on J upiter, \vith are typical of the planet as a whole. the environment (in which the plume is imbedded) fixed 1 2 The second length scale is L B (Uif3) 1 . where U by initial conditions. yielded updraft velocities as high as = tens of m s-1 (Stoker 1986. Lunine and Bunten 1987). Self­ is the magnitude of the horizontal velocity, f3 = df I dy = consistent convective adjustment experiments (Delgenio and 20- cos( 1>) I a is the planetary vorticity gradient, and a is t he planetary radius. Attributed to Rhines, it is the scale l\fcGrattan 1990) gave layered profiles in t he vertical and a above which the speed of a barotropic Rossby wave is greater subsaturated. stably stratified mean state. than t he wind speed. Alternatively, it i the critical width Convective adjustment predict mean profiles but not of the zonal jets below which they might be unstable. The detailed flow fields. which are necessary eventually to explain barotropic stability criterion says that the flow is stable pro­ charge separation and lightning. Yair et al. (1995, 199 ) use an axisymmetric numerical flow model to study examples vided Q y = (3 -uyy > 0 at all latitudes, where the sub cripts of moist convection. Hueso and Sanchez-Lavega (2001) and denote differentiation with respect to y . Here Qy is the ab­ solute vorticity gradient, the sum of the planetary vorticity Hueso et al. (2002) developed a three-dimensional numerical model of moist convective storms that include vertical wind gradient {3 and the relative vorticity gradient -Uyy. Voyager shears. Again an environmental stratification and specific data imply that Uyy varies between ±2{3 and therefore that the criterion is violated (Ingersoll et al. 1981, Limaye 19 6). initial conditions are imposed. These authors obtain flows Reproducing this observation is a major challenge for the consistent with precipitation and lightning when sufficient water vapor is in troduced (OIH 2: solar) and low stability models. One possibility is t hat the variation of wind with is assumed. altitude. which is ignored in barotropic model . is affecting The fact that Ughtning storms and moist convection the stability of the flow. seem to occur in the cyclonic belts needs an explanation. Rhines (1975) demonstrated that zonal jets emerge from particularly since t he air in the belts is sinking. at least in decaying t urbulence on a {3-plane - a planar coordinate sys­ the upper troposphere. On Earth moist convection is as­ tem that preserves the important effects of the planet"s cur­ sociated with low-level convergence and rising motion. One vature and rotation. \1\"illiams (1978) first applied these ideas possibility is t hat the air in t he belts is rising in the lower tro­ to Jupiter. These ,8-turbulence models have some common posphere. with horizontal divergence at intermediate levels features. First, they describe motion in a thin layer, either (Ingersoll et al. 2000). Such divergent flow might be driven on a ,8-plane or on t he surface of a sphere; motions in t he by the eddy flux pu'v', which accelerates t he jets on eit her planet"s interior are neglected. Second. they rely on small­ side of the belt. Balancing the eddy acceleration of an east­ scale forcing. The classic inverse cascade models (Vallis and ward (westward) jet requires transport of low (high) an­ }.1altrud 1993. Huang and Robinson 1998. Marcus et al. gular momentum air from higher (lower) latitudes. Since 2000. Sukoriansky et al. 2002) have positive and negative the eastward jets are on the equatorward sides of the belts sources of vorticity at small scales. The baroclinic models and the poleward sides of zones. the net result is horizontal (Panetta 1993) have an unstable temperature gradient that divergence in t he belts and horizontal convergence in the produces eddies at the Ld scale. Other models (Williams zones. The updraft in the lower troposphere beneath the 197 , Cho and Polvani 1996) start with an initial eddy field belts brings water vapor up from the interior and leads to that e,·olves without dis ipation to a et of zonal jets. There moist convection. is a strong ani otropy between the zonal and meridional di­ rections; zonal jets develop in all the models. But in all cases the resulting jets have Uyy < ,B; they are too wide and too weak to violate the barotropic stability criterion and there­ 6.6 MODELS OF THE ZONAL JETS fore do not fully agree with the Jupiter data. 6.6.1 Banding Controlled in the Weather Layer Another mystery is why Jupiter has weaker winds than any other giant planet despite its greater radiative en­ Two length scales have been invoked to e:>..'J)lain the widths ergy fluxes - absorbed and emitted pmver per unit area. of t he zonal jets. The first is the deformation radius Ld = For example, Neptune·s winds are .-v3 times stronger t han NH/lfl . where is the Brunt- Viiisala frequency (the buoy­ Jupiter's. but the radiative fluxes at Neptune are .-v20 times ancy frequency). H is the pressure scale height (::::;vertical weaker. One po sibility (Inger oll 1990, Ingersoll et al. 1995} Atmospheric Dynamic 121

•.., that the radiative fluxes determine the level of atmo­ This condition corre pond to the case of marginal stability spheric turbulence. which dissipates the energy of the large­ ''-ith respect to a criterion that traces back to Kelvin and i -.cale winds. If the turbulence levels decrease by a large now known as Arnol'd's second stability criterion (although amount as the radiath·e f!tLxes decrease. a nd the energy it is notably ab ent from most meteorology textbooks}. It al­ source that drive the \dnds decrease by a smaller amount. lows a shear flow to be stable e,·en though the flow does not the lru·ge- cale winds would increase. By this token, the high­ satisfy other. better-known stability conditions. Here again, ::.peed jet of . eptune are coasting in an atmo phere where the aby al circulation plays an essential role in stabilizing dissipation i low. Jupiter· atmo phere i more turbulent. the flow in the upper layer . which limits the speed of the large-scale winds. Interestingly. By as uming that the rv450 m s-• speed (Hammel et al. Earth has the weakest large-scale winds and the strongest 1995) of the dark ring seen propagating outward from each radiative heating of any atmosphere-covered planet in the of the Comet Shoemaker- Levy 9 impact sites is the grav­ solar sy tern. ity wave speed in Jupiter's atmosphere (not a firmly estab­ lished fact}. Dowling (1995b} singled out the corresponding member in the family of deep circulations mentioned above 6.6.2 D eep Winds and Stability of the J ets to predict that Jupiter· westward jets change little with .Jupiter· trong. narrow jet are unstable if one a umes depth. but that it eastward jets increase in trength by that the wind ru·e confined to a thln horizontal layer. For 5Q-100o/c with depth. Thi prediction for the eastward jet in tance. Dowling and Ingersoll (19 9} showed that Jupiter's closely matches the sub equent results of the Galileo probe cloud-top winds suffer barotropic instability and cvoh·e away Doppler "-ind experiment. with the caveat that the probe' from what is observed when initialized in a shallow-water latitude ofT')[ is too close to the equator for the strong Cori­ model with no deep circulation. However. Ingersoll and olis effect assumed by this quasigeostrophic (mid-latitude) Cuong (19 1} found that the upper-layer wind profile is sta­ theory. ble if it re t hydro tatically on a co-moving deep lower layer who e density i greater than that of the upper layer. T his is 6.6.3 Banding Controlled in the Interior an extension of the idea that the observed zonal jet are the surface manifestation of differentially rotating cylinder con­ Several groups have considered the possibility that .Jupiter 's centric with the planet's rotation a.xis (Busse 1976}. Ingersoll jet streams are rooted deep in the interior where the planet's and Pollard (19 2} showed that the rotating cylinders could internal heat source drives com·ection. and where there i no be stable even though they violate the barotropic stability confinement of motions in ide a thin spherical shell. Bu e criterion becau e that criterion applies only to motions in (1976} investigated uch convection. and showed that it Call thin hell . They developed a criterion that applie to mo­ generate alternating jets at the top of the convecting phere. tion inside a compressible fluid sphere. where the effective Condie and Rhines (1994} studied a laboratory analog con­ J is negative and i 2 or 3 times larger than the traditional sisting of a rotating bowl of warm "·ater that is uniformly 3. .Jupiter's obsen·ed winds are closer to marginal stability cooled at the free urface. The cooling generates con\'ec­ according to thi criterion. However. the rotating cylinders tion cells that give ri e to azimuthal jet when they en­ penetrate into the interior where the electrical conductivity counter the free urface. ).1anneville and Olson (1996} find i;, high and the magnetic field may interfere with the flow symmetric bands and zonal jets in a rotating convecting (Kirk and tevenson 19 7}. A complete theory would take fluid sphere. Sun et al. (1993} cruTied out numerical sim­ these hydromagnetic effects into account. ulations of a rapidly rotating. deep fluid shell and achieved Orsolini and LeO\-y (1993a. 1993b} examined the linear a broad eastward flow at the equator with alternating jet in tability problem in cases where there is a deep circulation. at higher latitude . Howe\'er, the runplitude of their zonal to see whether the motions in the overlying atmosphere can flow i an order of magnitude le than the amplitude of the be tabilized. They found that jets that decay with height non-axisymmetric flow: in other words. the jet are barely are more table that tho e that do not. Thi agrees with work discernable through the large noise of the convection. Zhang by Pirraglia (19 9} and i consistent with the jet decay with and Schubert (2000} de,·eloped a model that combines con­ height inferred for Jupiter via the thermal-wind analysis. wction in the deep interior with an overlying stable atmo- The e two studies demonstrate the potential importance of phere. They find that strong motions can concentrate in the deep flow for the stability of the jet . but they do not the atmosphere as a re ult of convection that is dri\'en t her­ discuss how the deep flow is maintained. mally in the deep interior. a phenomenon they term "tele­ Dowling and Ingersoll (19 . 19 9} deduced the nature convection ... More coupled atmo phere-interior model like of the deep circulation by observing changes in ab c lute vor­ thls one are needed. Eventually the atmospheric component ticity a parcel move around the GRS and white oval BC. should include effects of moist convection. and the interior Changes in ab olute vorticity are due to vortex tube tretch­ component should include the effects of electrical conduc­ ing. which ru·ises when the parcels cro the pre ure ridge tivity and magnetic fields. as ociated with the flow underneath. The analy i produced a family of possible deep circulation with as the un­ L~ 6.6.4 Modes o f Inte rnal H eat Transfe r known scaling factor. For each case. the deep flow is not in solid rotation; it seems to have a jet-like structure some­ Ingersoll and Porco (197 ) pointed out that if Jupiter has a what like that ob en·ed in the upper layer. Dowling (1993} convective interior. only very small lateral temperature gra­ showed that thi family of abyssal circulation corresponds dient hould be expected at the outer edge of the convective to the pecial case Ld = £ 13 . where {3 i the full gradient region. If the emi ion to space is from t he top of the con­ of potential vorticity including the vertical tretching term. vective interior. it hould be uniform with latitude. This is 122 Inger oll et al. the accepted explanation for the ob ervation {Figure 6.12) than at other latitudes. Earth. Uranus, and t\eptune exhibit that emission to space on Jupiter is essentially independent equatorial subrotation. Although the equator of Jupiter is a of latitude (Inger oil et al. 1976. Pirraglia 19 4). But nei­ local minimum of zonal velocity due to the zonal jet max­ ther the zonal mean insolation nor the emission to space ima at ±7° PG latitude (Figure 6.2). it is a local maximum is determined very accurately by observations, and dynam­ of absolute angular momentum. Since rings of fluid circling ically important temperature gradients at. say. the 10-bar the planet at constant latitude tend to conserve their an­ level cannot be ruled out. Temperature contrasts of only a gular momentum as they move, any mixing between rings few degrees would be important for the dynamics. and could will reduce the angular momentum at the equator. A the­ possibly expla in t he mean eastward bias in globally averaged orem due to Hide {1970) states that a circulation t hat is surface wind {Gierasch 1999). symmetric about the axis of rotation (one with no eddies) A new wrinkle was introduced when Guillot et al. {1994) cannot ustain a local maximum of angular momentum. The pointed out that a radiative zone might exist in Jupiter inference i that the equatorial maximum on Jupiter is main­ {and Saturn) near the depth where the temperature reaches tained by eddy fluxes. e.g.. pressure variations in longitude. about 2000 K. This occurs where the pressure is a few tens In a paper on the superrotation of \'enus. Gierasch {1975) of kilobars and the depth is a few percent of the radius. noted that friction tends to mix angular momentum down At this temperature the peak of the Planck function has the gradient of angular velocity. toward a state of solid body shifted to near 5 1-!ITI. where jovian material has relatively rotation. For Jupiter. this down-gradient mixing is toward low opacity. Since 1994, Guillot has discovered new sources the equator, at least up to the latitude of t he zonal jets of opacity. and this radiative zone may not exist on Jupiter. at ±7°. Since friction in a planetary atmosphere invoh·es If it does exist. the Ingersoll and Porco reasoning should be turbulent eddies. the conclusion i that Jupiter·s equatorial reexamined. Gierasch {1999) experimented with radiative­ superrotation is maintained by eddy fliLxes. \Vhether the convective models that include a radiative zone and no lat­ mixing is from higher latitude or from lower altitudes is eral heat transport. These models s how that even a modest unknown. radiative zone would break the tight constraint on latitudi­ 1lost inverse-cascade models (Section 6.6.1) use a (3- nal temperature gradients that is imposed by a fu lly convec­ plane geometry, and so are unable to address the question tive interior. of equatorial superrotation. Cho and Polvani (1996) consider A major question about the outer planets is the depth decaying barotropic turbulence in a thin layer on a full ro­ of the dynamical region that produces the \'isible jets and tating sphere. Zonal jets develop at mid latitudes. but the spots. \Vithout a solid surface and without a stability tran­ equator subrotates as on Uranus and )ieptune. Yano et al. sition there is no externally impo ed boundary to form a {2002) con ider decaying barotropic turbulence in a deep base. leading to debate about ··deep cylinder"' flow configura­ fluid sphere. where the (3 effect has the opposite sign from tions that extend through the planet as opposed to "shallow that in a shallow surface layer (Ingersoll and Pollard 19 2). weather layer.. configurations (Busse 1976. Allison 2000). If a In this case a superrotating flow develops at the equator as radiative zone exists. it could be the location of the .. windy on Jupiter and Saturn. This is a suggestive result. but it is jovian thermocline"' discussed by Alii on {2000). Allison·s not a proof that the fluid phere model is correct. since Venus thermocline is a stable layer that is conjectured to close off superrotates and Earth doe not. and both atmospheres are the weather layer circulation and separate it from the deep thin compared to the radiu of the planet. interior. In general, the basic state stratification is a fun­ damental paramet~r affecting dynamics and heat t ransport and it is of high priority to determine whether a radiati,·e zone exists on Jupiter and Saturn. 6. 7 MODELS OF DISCRETE FEATURES 6.7.1 Stable Vortices 6.6.5 Banding Controlled by Tides The simplest model that produces stable vortices is an invis­ The third class of hypotheses concerning the control of cid. two-dimensional. non-divergent flow with shear (Moore Jupiter·s jet streams involves the intriguing possibility that and affman 1971, Kida 19 1). The fluid has uniform vor­ the winds are shaped and accelerated by satellite tides. Ioan­ ticity inside an isolated patch and a different uniform vor­ nou and Lindzen {1994) showed that if the interior of Jupiter ticity out ide. A steady. stable configuration occurs when is even modestly stable to convection, tides that are domi­ the anomalous patch has elliptical shape. with the long axis nated by higher-order Hough mode can couple to it. These oriented parallel to the flow at infinity (east- west on the tend to produce banding with alternating accelerations on giant planets). T he aspect ratio (ratio of long axis to short 1 1 the order of 1 em s- d- , which i significant. The authors axis) depends on the ratio of the vorticity inside to that out­ find that the dominant tides come from Io. Titan. Ariel, side the patch. Finite amplitude perturbations lead to stable and Triton. respectfully. for Jupiter. Saturn. Uranus, and oscillations in the aspect ratio and orientation. eptune. This idea adds to the motivation to search for ob­ Poh·ani et al. (1990) bowed that this ··Kida vortex·· servational evidence of the tidal response at Jupiter"s cloud model does a good job of matching the ob ervations of vor­ level. tices on the giant planets. For the GRS and white ovals of Jupiter, where we have measurements of the vorticity inside and outside. the model accurately accounts for the average 6.6.6 Equatorial Superrotation aspect ratio (Ingersoll et al. 1995). For the Great Dark Spot Jupiter. Saturn. Venus. and the Sun exhibit equatorial su­ (GDS) of. eptune. which oscillates in aspect ratio and orien­ perrotation - the atmosphere rotate faster near the equator tation, the model accurately accounts for the relative phase Atmospheric Dynamics 123 and relative ampli tude of the two oscillation . The model steady shapes of jo,·ian vortices. The model does not account does not explain the amplitude it elf, which i a free pa­ for the o cillations. since the equilibrium state is steady. rameter of the theory. nor does it account for the ob erved Turkington et al. (2001} propose using the equHibrium bedding of fi laments. statistical t heory for inverse modeling of the small-scale vor­ It is remarkable that the Kida vortex model works as ticity distribution. They start with realistic zonal jets and well as it doe . It has no vertical structure. no gradient in t he the underlying zona] flow as defined by Dowling (1995b}. ambient vorticity ((3 effect). no forcing. and no dissipation. The theory give one GR.S. one white oval. and realistic zonal Introducing these effects adds to the complexity of the mod­ jets - but only if the initial vorticity distribution i skewed els and greatly increases the number of free parameters. In toward anticyclonic values. Thrkington et al. cite the recent fact, stable vortices exist in models with a wide variety of as­ Galileo results (Gierasch et al. 2000, Ingersoll et al. 2000} sumptions about the vertical thermal structure. the flow un­ t hat support the occurrence of intense small-scale anticy­ derneath. and the energy source (Inger oil and Cuong 19 1, clonic forcing. Williams and Yamagata 19 4. l\Iarcus 19 . W illiams and Wil on 19 . Dowling and Ingersoll 19 9. Williams 1996. 2002. LeBeau and Dowling 199 . ~! arcus et al. 2000. Cho 6.7.3 E quatorial H ot Spots and the Galileo P robe et al. 2001. You sef and t- Iarcus 2003). table vortice form In 1995. the Galileo probe took in situ measurements of also in laboratory experiments ( ezlin 19 6. Read 19 6}. compo ition. wind . temperature. and clouds from about 0.4 The GR.S oscillate in longitude with ..,...1 ° ampli tude bar to 22 bars. a 150-km range of altitude. These mea­ (peak-to-peak} and 90-day period (Solberg 1969, Trigo­ surements have raised questions about conditions below the Rodriguez et al. 2000}. Other jovian pots oscillate in longi­ clouds. Before the probe·s entry. many experts t hought t hat tude as well (Peek 195 ). -eptune·s second dark spot discov­ the atmosphere would be well mixed below the condensation ered in Voyager 2 images oscillated in longitude with ±45° levels. That is. the ammonia mixing ratio would level off be­ amplit ude and 36-day period (Ingersoll et al. 1995}. The low 0.7 bars, and the H2S and water mixing ratio would Kida model has oscillations in orientation and aspect ratio level off below 2 bars a nd 6 bars. respectively. Instead t he only. Achterberg and Ingersoll (199-1) developed a model in ammonia mixing ratio leveled off at 10 bars. H2S leveled off which t he longit ude o dilations arise when the top and bot­ at 16 bar . and water was still increasing with depth at 20 tom halves of the vortex orbit around a common vertical bars (Xiemann et al. 199 . Folkner et al. 199 , ~I ahaffy et al. axis. They obtained peak-to-peak amplitudes up to 15°. but 2000). These observations require a dynamical explanation. they were not able to reproduce t he large-amplitude o cil­ The probe entered one of Jupiter·s 5-J.!m hot spots. lation of the Neptune pot because the two halve of the where a hole in the visible cloud 5000 km wide allows ,·ortex tended to eparate and drift off separately. 5-J.!III radiation to escape. To explain the hole in the cloud These inviscid theories shed no light on what maintains and the depre ed volatile abundances. several authors sug­ the vortices or t heir o dilations against dissipation. Obser­ gested that hot spots contain downdrafts that advect dry vations of mergers suggest that the large vortices feed on the air from t he upper troposphere down to rv20 bars or deeper. smaller ones. The inverse cascade of energy from small scales The simplest version of this idea has dense air that descends to large scale apparently maintains t he vortices as well as because it is convectively unstable (Atreya et al. 1999. how­ the zonal jet . One then must ask where do t he small vor­ man and Ingersoll 199 , Baker and Schubert 199 ). A diffi­ tice get their energy? The possibilities include: in tability culty is that any tatic stability from 1- 10 bars (as seems to associated with latitudinal temperature gradients and the exist; see Section 6.4.4) would act to halt the descent. More­ corresponding vertical ·hear. horizontal shear instability of over. downdrafts produced in numerical simulations (Baker the zonal jets. and moist convection. Finding the answer is and Schubert 199 ) are two orders of magnitude too small. an active area of current research. The convective downdraft hypothesis also has problem with the wind shear. which tends to pull the hot spot apart in 1 or 2 days. and does not explain the layered distribution of volatiles. 6.7.2 Statistical Mechanics Models The econd idea hypothesizes that hot spots are the .-\n entirely different approach. one that bypasses the tem­ downwelling branch of an equatorially trapped wave (Fried­ poral development of the flow . is to solve fo r the equilib­ son and Orton 1999. Showman and Dowling 2000). Allison rium state that maximizes a global integral that is analo­ (1990) and Ortiz et al. (199 ) documented wavelike behavior gous to the entropy in statistical mechanics ( ommeria et for the plumes and hot spots. and Showman and Dowling al. 1991, :Miller et al. 1992, t-Iichel and Robert 1994. Steg­ (2000} performed numerical simulations t hat support t his ner and Zeitlin 1996. Thrkington et al. 2001. Bouchet and idea. Air parcels that enter t he hot spot from t he we t (at 1 Sommeria 2002). The parcels are allowed to mix as if they tens of m s- ) are deflected downward; the parcels return to were chemically distinct molecules. such that each parcel their original altitudes a few days later as t hey exit the hot conserve it initial value of potential vorticity (PV). This spot to the east. Thi dowmvelling wave model explains the con ervative mixing is appropriate for an invi cid, adiabatic layered tructure in the ammonia. H2S. and water. Show­ fluid. but it is not clear how it would \YOrk in a real at­ man and Dowling's (2000} irnulations also suggest that the mo phere. where t he PV values may change during mixing. increase in \Vi nds with depth observed by the Galileo probe Also. the initial PV distribution is arbitrary and i not deriv­ results from local dynamics at the south edge of hot spots able from the maximum-entropy principle. The proponents and may not be a large- cale property of Jupiter·s equato­ of this approach point out that the equilibrium tates agree rial atmo phere. Although some issues remain. the model with numerical simulation (e.g.. ~!arcus 19 ) and with the successfully explains the ob erved dryness as a local me- 124 Ingersoll et al. teorological effect. and it is consistent with the idea that spheres. and Jupiter· atmo phere i no exception. The data Jupiter·s deep water abundance is at least olar. pro\·ide constraints on the models. ~Iod els of key process like moist convection in a bottomle s atmosphere need to be developed. The GCJ\ls for Jupiter are less well constrained than GCr-.ls for Earth or !\Iars. but that makes them all the 6.8 UNANSWERED QUESTIONS AND more interesting. ?deteorologists and oceanographer recog­ POSSIBLE SOLUTIONS nize Jupiter' value as a fluid dynamics laboratory. The goal orne of the big questions are: \\"hy are the giant planets is to capture the truth in a mall range of parameter pace. banded? What controls the speed and width of the zonal \Vhen the combined con traints of observation and theory jets? \Vhy are the jets stable? Where do the jet get their rule out most of the hypotheses and the models all resem­ energy? Do t he winds extend into the fluid interior? Why ble each other. then we can truly claim to understand the are the large vortices so stable? \\'hat are the clouds made dynamics of Jupiter·s atmosphere. of. and why are they colored? \\'hat is the composition of the deep atmosphere? \\'hat is the water abundance? How Acknowledgements. We thank the many agencies important is moi t com·ection? \\nere does lightning occur. that have funded planetary science research. and we thank and what is its relation to global feature ? J\Iany of these the many people who have contributed to our knowledge of questions concern the deep atmo phere - it compo ition. Jupiter's atmosphere. thermal tructure. and dynamics. Here we de cribe how ob­ ervations and modeling can provide the answer . Gravity measurements can determine whether the deep atmo phere, 1000s of km down. is in solid-body rotation REFERENCES or has jet streams comparable in speed to the surface jets. The centrifugal forces associated with the deep jets cause Achterberg, R. K. and A. 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