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.
