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2.1 Expectations based on Voyager results

Virtually all presently available information about Titan's atmospheric dynamics and meteorology derives from the Voyager 1 reconnaissance, with essential contributions from the infrared interferometry spectrometry (IRIS) and radio occultation (RSS) investigations (Hanel et al. 1981; Tyler et al. 1981).

Titan's vertical temperature-pressure structure, retrieved from Voyager RSS data (Lindal et al. 1983), is represented by the profile shown in Fig. 1. The occultation data extend all the way to the surface, where the temperature is approximately 97 K (with a systematic uncertainty of about ± 7K). Methane clouds may form at altitudes above a few kilometers, depending upon the relative humidity near the surface (Toon et al. 1988). The decrease of temperature with altitude below 40 km implies the absorption of sunlight at the surface and a weak tropospheric greenhouse. The radio occultation measurements indicate a nearly adiabatic lapse rate below 3 - 4 km, consistent with the inferred turbulent surface layer, but a statically stable profile above this height. Hinson & Tyler (1983) have found evidence for vertically propagating gravity waves at altitudes between 25 and 90 km, based on their analysis of radio occultation scintillations.

Figure 1: Titan meteorology. The vertical thermal structure retrieved from Voyager radio science data (Lindal et al. 1983) is represented by the solid-line temperature profile T(P) in the left panel. The equator-to-pole temperature contrast, as extrapolated from Voyager IRIS data (Flasar et al. 1981), is represented by the dashed-line profile referred to the inset scale. The dashed curve in the right-hand panel is the corresponding mid-latitude wind profile inferred from the vertical integration of the thermal wind equation, assuming vanishing velocity at the surface. The nominal descent time scale of the Huygens Probe is indicated in the middle of the figure.

The indirect inference of 100 m/s zonal cyclostrophic winds on Titan is based primarily upon the latitudinal contrast of temperature at infrared sounding levels observed by the Voyager 1 IRIS experiment (Flasar et al. 1981; Flasar & Conrath 1990). As interpreted by the thermal wind equation under the assumption of hydrostatic, gradient-balanced flow, the vertical variation of the Coriolis plus centripetal acceleration of the zonal velocity u is related to the latitudinal temperature gradient by


where = is the vertical log-pressure coordinate with the surface pressure, the latitude, a the planetary radius, the planetary rotation velocity, the mean molecular weight (mass per mole) of the atmospheric gas, and R the gas constant. With the further plausible assumption of relatively weak winds near the surface (cf. Allison 1992), the thermal wind equation may be solved for the zonal velocity at all levels for which there are vertically continuous observations of the horizontal temperature gradient. The second term in the thermal wind equation (1) can be neglected, except very close to the surface, because of the slow Titan angular rotation. The resulting circulation is in cyclostrophic balance with . However, the direction of the zonal wind (prograde or retrograde) cannot be uniquely determined from (1) under this condition.

The vertical integration of the thermal wind equation for the practical inference of zonal motions requires the independent specification of the velocity at one or more levels where the thermal gradient can be accurately estimated. For sufficiently strong surface drag, the zonal velocity at the bottom of the atmosphere may be assumed to be nearly zero. On this basis, and assuming an equator-to-pole temperature contrast given by the dashed line in the inset (left panel) of Fig. 1, Flasar et al. (1981) estimated a zonal wind as depicted schematically in Fig. 1 (dashed line in right panel), approaching 100 m/s in the upper stratosphere at a latitude of = 45.

Although the Titan thermal spectrum is partly convolved with variable aerosol opacity, the 1304/cm region, which comes mainly from the 0.5 mbar pressure level (230 km), is presumed to be relatively free of these effects. Measured brightness variations across the disk at this channel could thus be interpreted as a real difference in kinetic temperature between low and high latitudes of roughly 16 K (Flasar et al. 1981). More recently, Flasar & Conrath (1990) retrieved stratospheric temperatures from a combination of the measured radiances at 1304/cm and the P and Q branches of methane at 1260-1292/cm, essentially confirming the results of the earlier brightness temperature analysis. The data also indicated a hemispheric asymmetry in Titan's latitudinal temperature distribution at the time of the Voyager encounters. Coustenis (1991) has performed an independent retrieval of the latitudinal temperature structure as inferred from the 1304/cm channel, with similar results. IRIS measurements for a second thermal sounding channel at 200/cm, corresponding to emission in the vicinity of 100 mb (40 km), suggest that latitudinal thermal contrasts at this level may be less than about 1 K. Measurements for a third thermal channel at 530/cm, corresponding to emission near Titan's surface, were interpreted by Flasar et al. (1981) as a latitudinal temperature difference of about 2 K between the equator and 60 latitude. This interpretation has since been qualified, however, by the study of Toon et al. (1988), who pointed out that stratospheric aerosols may also contribute significantly to the brightness temperature at this wavenumber. The apparent latitudinal contrast may thus be best interpreted as a crude upper limit to actual variations in the kinetic temperature. A more comprehensive review of the evidence supporting our contemporary model of Titan's zonal winds has been compiled by Flasar et al. (1995).

As with Venus, the inferred cyclostrophic flow regime on Titan is not yet understood, representing a fundamental unresolved problem in the theory of atmospheric dynamics. The magnitudes of the meridional and vertical winds are also quite speculative. Assuming the poleward advection of heat is balanced by radiative cooling, Flasar et al. (1981) estimated a mean meridional motion of cm/s in the lower troposphere. Invoking continuity, the associated mean vertical motion may be estimated as cm/s, where km is the pressure scale height. It is possible that much stronger vertical motions exist in locally turbulent regions of the atmosphere, such as a methane thunderhead, but probably only over a relatively small fraction of the total area. The associated depth of the Ekman planetary boundary layer (PBL in Fig. 1), where the surface winds are rotated and strongly sheared with altitude until they match the thermal wind aloft, may be estimated as km (Allison 1992). Because the PBL mediates the transfer of angular momentum from the surface to the atmosphere, it is an important target for observational characterization. With strong surface drag and a global meridional thermal contrast no larger than assumed by Flasar et al. (1981), the vertically integrated thermal wind equation implies a geostrophic regime () extending up to about 5 km altitude.

In view of the various gaps and uncertainties in the thermal structure data, we cannot claim to know very much about the vertical and horizontal structure of Titan's zonal wind. Assuming that at least the 1304/cm measurements from the Voyager IRIS experiment have been more or less correctly interpreted, however, it is difficult to escape the conclusion that the thermal wind balance will involve zonal motions of the order of = 70 m/s at stratospheric levels.

The direct observational confirmation of the inferred zonal cyclostrophic motion of Titan's atmosphere by DWE will represent a fundamental contribution to the study of planetary meteorology. If its conjectured analogy to Venus is established, it would imply the probable ubiquity of atmospheric superrotation as a robust feature of slowly rotating, differentially heated planets.

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