LETTERS
PUBLISHED ONLINE: 15 JANUARY 2012 | DOI: 10.1038/NGEO1374

Two boundary layers in Titan’s lower troposphereinferred from a climate model
Benjamin Charnay * and Sébastien Lebonnois
Saturn’s moon Titan has a dense atmosphere, but its thermal
structure is poorly known. Conflicting information has beengathered on the nature, extent and evolution of Titan’splanetary boundary layer—the layer of the atmosphere that is
influenced by the surface—from radio-occultation observations
by the Voyager 1 spacecraft1 and the Cassini orbiter2 ,measurements by the Huygens probe3–5 and by dune-spacing
analyses6 . Specifically, initial analyses of the Huygens datasuggested a boundary layer of 300 m depth with no diurnal
evolution4 , incompatible with alternative estimates of 2–
3 km (refs 1,2,6). Here we use a three-dimensional generalcirculation model7 , albeit not explicitly simulating the methane
cycle, to analyse the dynamics leading to the thermal profile ofTitan’s lowermost atmosphere. In our simulations, a convective
boundary layer develops in the course of the day, rising to an
altitude of 800 m. In addition, a seasonal boundary of 2 km
depth is produced by the reversal of the Hadley cell at theequinox, with a dramatic impact on atmospheric circulation.
We interpret fog that had been discovered at Titan’s southpole earlier8 as boundary layer clouds. We conclude that
Titan’s troposphere is well structured, featuring two boundary
layers that control wind patterns, dune spacing and cloudformation at low altitudes.
The planetary boundary layer (PBL) is the atmospheric layer
located between the surface (land or sea), where friction slows down
air motion, and the free atmosphere, where it becomes negligible9 .
The PBL is not restricted to Earth and should exist on any planetor moon with an atmosphere. The main differences between PBLs
originate from their diurnal or seasonal evolution. On Earth, theheight of the PBL (generally between 500 m and 3 km) is essentiallycontrolled by solar heating and therefore by the diurnal cycle. A
convective boundary layer appears during the morning when the
ground is heated9 , and the associated rise of warm air leads to theformation of cumuli that disappear in the evening. Because of its
distance from the Sun and its thick and opaque atmosphere, Titan
has a much weaker diurnal cycle than Earth (the insolation on
Titan’s surface is about 0.1% of that on Earth). The impact of thisdifference on the PBL has remained largely unexplored.
The thermal structure of Titan’s lower troposphere (Titan’s
troposphere rises to 40 km) is difficult to measure. The firstobservations were obtained during the Voyager 1 flyby with two
radio-occultation temperature profiles (at dawn and dusk). Bothprofiles gave a thermal lapse rate close to the dry adiabatic value(−1.31 K km−1 ) below 3.5 km, followed by a slope change to−0.9 K km−1 above this altitude1 . It was concluded that the PBL
should correspond to this 3.5 km layer. A more precise temperature
profile was obtained by the Huygens probe during its descent3 . The
probe landed on Titan on 14 January 2005 at 9:47 am local true solar
time. The landing site was located at 10.3◦ ±0.4◦ S,167.7◦ ±0.5◦ E insouthern midsummer (solar longitude Ls = 300.5◦ ). It was possible

to derive the potential-temperature profile and the atmosphericcomposition there4,10 . The potential temperature decreased from93.65 K at the ground to 93.48 K in the first 10 m and thenremained approximately constant in the next 300 m. Above 300 m,
the potential-temperature gradient becomes positive (Fig. 1a). An
analytical calculation on the basis of these measurements led tothe conclusion that this layer, interpreted as a convective layer,
would not evolve by more than a few centimetres during Titan’s
day in this season4 . In the thermal profile, a lapse-rate change
at around 2 km altitude, together with a discontinuity at 800 m,
has also been noticed, complicating the understanding of the PBL(ref. 5). Recent measurements by Cassini radio occultations2 show
adiabatic profiles up to 2 km for latitudes of around 30◦ S and amore stable profile at 53◦ S. Another piece of information on Titan’s
PBL can be derived from observations of its giant linear dunes inthe equatorial band (Titan has the largest dune fields in the Solar
System)11 . The formation of giant dunes is a complex phenomenon
depending on both surface winds and the PBL. The latter controls
the height of the dunes and the spacing by promoting the growthof dunes with wavelength of the order of magnitude of the PBL
height12 . Thus, observation of dune spacing of around 3 km suggests
the presence of a PBL 2–3 km thick6 .
To clarify the situation concerning Titan’s PBL and its evolution
during the diurnal cycle, we used a three-dimensional generalcirculation model (GCM; Methods). As the radiative time constantin Titan’s troposphere is larger than 2.5 Titan years13 , direct solar
heating of the troposphere is unable to generate a diurnal or evenseasonal cycle. However, solar heating of the surface can induce
diurnal variations in the lower troposphere through the sensible
heat flux. The temperature lapse-rate profile simulated by the GCM
for the Huygens site at the time of landing is in agreement withthe measured profile and features three particular levels, denoted
A, B and C (Fig. 1b).
Level A corresponds to the top of a convective boundary
layer (also called the mixed layer) developed at 10 am by groundheating. For the Huygens landing site, the solar flux at the top of
the atmosphere reaches a maximum of 15 W m−2 with a diurnal
average value of 4.8 W m−2 , 10% of which reaches the surface.
The modelled ground temperature varies by 0.4 K over one Titan
day (Supplementary Fig. S1), so less than Cassini Composite
Infrared Spectrometer observations14 (1–1.5 K) . The sensible flux at
ground level varies from −0.05 W m−2 in the night to +1.1 W m−2
at midday (Supplementary Fig. S1), with an average value of+0.23 W m−2 . This represents approximately 5% of the solar flux
at the top of the atmosphere and around 60% of the solar flux
absorbed by the surface. The modelled wind is about 0.3 m s−1 at
30 m. In the convective layer, the temperature profile follows theadiabatic lapse rate. According to our simulations, dry convection
starts after 7 am local true solar time in the southern hemisphere(Fig. 2). Then, the convective boundary layer rises, reaching aLaboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Box 99, 75252 Paris Cedex 05, France. *e-mail: benjamin.charnay@lmd.jussieu.fr.
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NATURE GEOSCIENCE DOI: 10.1038/NGEO1374
a

LETTERS

3.5

1,000

4
3

3.0

800

2

2.0

C

1.5
1.0

0
93.2

93.4

93.6

93.8 94.0 94.2 94.4
Potential temperature (K)

94.6

¬1

400

¬2
¬3
¬4

94.8

95.0

0

0

5

10
15
Solar local time (h)20¬5Figure 2 | Simulated diurnal evolution of dry convection in the first
kilometre during one typical Titan day at the Huygens site. Temperaturetendencies (in 10−7 K s−1 ) due to dry convection, illustrating the verticalextension of the convective layer during one typical Titan day at the
Huygens probe landing site and season. The steps in the figure are causedby the vertical discretization of our model.

4.5
4.0
3.5

Altitude (km)

0

200

A

5.0

3.0
2.5
2.0

1
600

B

0.5

b

Altitude (m)Altitude (km)2.5

maximal altitude of approximately 800 m at 3 pm, and disappears
after 5 pm. After sunset, the air cools close to the ground, producing
a stable 100 m layer (observed in the Cassini radio-occultation
dawn profiles2 ). Owing to low turbulent heat diffusion and strong
atmospheric stability, the structure at 800 m (called the residual
layer, level B) remains observable throughout the day, with thetemperature lapse rate slightly below the adiabatic value. A similar
layer has been observed at 900 m in one of the Cassini radiooccultation profiles2 .
Above 1 km and up to 2 km (level C), the thermal profile ofthe troposphere is again close to the adiabatic lapse rate. This
involves a physical process different from the diurnal boundary
layer (limited to the first 800 m) mixing the air at this height. We
explain this phenomenon by the seasonal cycle15,16 producing aseasonal boundary layer. For most of the time, Titan’s circulationis controlled by a pole-to-pole Hadley cell. The circulation of thiscell reverses at the equinox, but this transition is not instantaneous:the intertropical convergence zone (ITCZ, where the air rises)
runs through all latitudes from one pole to the other, with theformation of two Hadley cells during the transition17 . Thus, twicea Titan year, the ITCZ crosses the equatorial band and warm air
rises to the free atmosphere at 2 km. The potential temperature in
the free atmosphere shows little seasonal or latitudinal variationand follows a linear profile. The 2 km height is then fixed by theconstant lapse rate in the free atmosphere (around −0.8 K km−1 )and the ground temperature (Fig. 3). The layer remains as a residual
layer after the passage of the ITCZ. For example, the layer atthe Huygens site was produced ten terrestrial years before thelanding. Moreover, sporadic convection is maintained, owing to
global circulation. In our simulation, the slope change at 2 km
has a dramatic consequence for meridional circulation. It acts
as a cap layer, blocking most of the vertical rising (weak on
Titan). Therefore most of the meridional circulation is confinedto the first 2 km, leading to heating and convection in this layer
at all latitudes covered by this circulation. The seasonal cycle
enforces a higher stability in winter (Fig. 3) or at high latitude,
except for the summer solstice, as observed2 . This trapped Hadleycirculation controls winds in the lower troposphere. Zonal winds
in the first 2.5 km of the Huygens profile18 correspond to summer
monsoon winds (Supplementary Fig. S2). The free atmospherebegins above this level: the temperature, controlled by direct
solar heating, is maximal at the equator, causing an increase of
zonal winds with altitude. In conclusion, although Titan’s Hadleycirculation reaches the tropopause and above, its strength is mainly
confined to the first 2 km. The thermal structure is thus basedon two boundary layers: a diurnal layer produced by groundheating and a seasonal layer produced by the ITCZ displacement
at the equinox, weaker the rest of the year, but maintained by
the global circulation.
Averaged over more than one day, only a weak PBL signature remains at level B. The slope change (level C) becomes thelargest significant feature in the thermal structure (Fig. 3). In
this respect, it is equivalent to a permanent boundary layer ofaround 2 km for all in the equatorial band (30◦ S–30◦ N). Interfacial
waves can appear on the top of this layer, controlling giant dune
size12 . The height of 2 km is consistent with the observed dune
spacing of 3 km (ref. 6). Furthermore, at these latitudes, thislayer is particularly marked at the equinox, in agreement witha scenario proposed for the preferential formation of the dunes
during this period19 .
As our model does not take into account clouds and themethane cycle, all results correspond to a dry case. Episodic deep
convection can happen20,21 when the atmosphere is saturated.
In such a case, its lapse rate is reajusted to the moist adiabtic
lapse rate (around −0.6 K km−1 ; refs 5,17). Such events mightleave footprints in the thermal profile, but they remain rare.
Furthermore deep convection is expected to be mainly restricted
to the middle troposphere5 . Over an area of liquid methane (for

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107

C

1.5
1.0
0.5

B
A

0
¬2.0 ¬1.8 ¬1.6 ¬1.4 ¬1.2 ¬1.0 ¬0.8 ¬0.6 ¬0.4 ¬0.2
Lapse rate (K km¬1)

0

Figure 1 | Thermal structure of the first kilometres at the Huygens landing
site, measured by the Huygens Atmospheric Structure Instrument andsimulated by the GCM. a, Potential temperature observed during the
Huygens probe descent (blue crosses; the temperature uncertainty is±0.25 K) and simulated with our GCM (black line). b, Temperature lapse
rate observed during the Huygens probe descent (blue) and simulated with
our GCM (black). The red line indicates the adiabatic lapse rate. Note thatthe model faithfully predicts the major features in the thermal structure(levels A, B and C). The characteristic peaks A and B are related to thediurnal evolution of the PBL, whereas the slope change C is related to theseasonal evolution of the PBL.

© 2012 Macmillan Publishers Limited. All rights reserved.

NATURE GEOSCIENCE DOI: 10.1038/NGEO1374

LETTERS

Huygens for the radiative transfer. Our model produces superrotation in thestratosphere with a winter jet of 120 m s−1 , slightly below the observed values25,28,30 .
Tropopause temperature is close to 70 K and the zonal wind profile from thesurface to 140 km closely follows the profile retrieved from the Huygens probe,
including a minimum in the 60–80 km region. Simulations were carried out
with a 32 × 48 longitude–latitude horizontal resolution and 73 vertical levels(35 in the troposphere). The first level is at 30 m. Physical constants used forthe surface were albedo = 0.15, thermal inertia = 400 J m−2 K−1 s−1/2 , emissivity= 0.95 and rugosity = 0.005 m. We verified that the development of the PBL
occurs for any realistic values of these constants. Further details are given in
Supplementary Information.

6

Altitude (km)540° S (equinox)
50° S (northern summer solstice)
0° S (northern summer solstice)
50° S (late southern summer)
Lapse rate of free atmosphere

3
2
1
0
91

Received 5 August 2011; accepted 16 December 2011;
published online 15 January 2012
92

93
94
95
Potential temperature (K)9697References

For this study, we used a three-dimensional GCM (ref. 7) derived from the first
Titan GCM developed by our team25 , but benefiting from the most up-to-date
dynamical core26 . It covers altitudes from the surface to 500 km. It includes the
McKay radiative code27 with diurnal cycle, fully coupled aerosol microphysics28 ,a Mellor–Yamada boundary-layer scheme29 and gravitational tides4 but nomethane cycle. We simply used a methane profile close to that observed by

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Figure 3 | Simulated averaged potential-temperature profiles giving theseasonal evolution of the thermal structure for the first kilometres.
Averaged potential-temperature profile over ten Titan days for different
latitudes (red at the equator, blue at 50◦ S) and at different seasons: solidline at the ITCZ crossing (equinox for the equator and late southernsummer for 50◦ S) and dashed line in the coldest period (northern summer
solstice for both). The green line corresponds to the lapse rate of the freeatmosphere (above 2 km).

example the polar lakes), methane evaporation and the associatedlatent flux will strongly modify the development of the diurnal
PBL. However, for lands with limited moisture at the ground (thisincludes areas surrounding lakes), the diurnal PBL should remainclose to the dry case.
Clouds are observed in Titan’s middle or higher troposphere.
Most of them are produced by large-scale ascending motion of
methane to altitudes where the air is saturated22–24 . Clouds have also
been detected at 750 m, below the usual altitude of troposphericclouds (higher than 10 km). They have been interpreted as fog
formed above the ground in high-humidity conditions8 , but have
not yet been explained, diurnal temperature variations being too
limited to generate fog by cooling. In our simulations, the topof the diurnal PBL reaches 700–800 m for all summer latitudes(5◦ S–80◦ S, except at the ITCZ, where it rises to 2 km), precisely
the altitude of the fog. Therefore, we propose this fog corresponds
to boundary-layer clouds, produced by the same mechanism as
fair-weather cumuli on Earth. During summer in polar regions(where lakes and pools of methane are present), humidity close to
the ground may reach high values. In the morning, dry convection
is triggered and methane-rich air close to the ground rises to the
top of the PBL. For an air parcel close to saturation near the
ground that rises to 750 m, the relative humidity can increase by
up to 8%, which is sufficient to trigger condensation. These clouds
could appear during the morning at high latitudes in summer, butalso after precipitation in other regions20,21 , in particular at the
ITCZ, where they could form up to 2 km. Mesoscale simulations,
including cloud-formation schemes, will be required to fully
validate our conclusions.
Our simulations provide a general framework that explains a
number of different meteorological observations and features on
Titan. They reveal that, despite the limited solar insolation, thediurnal cycle on Titan is an active and observable phenomenon. In
this matter, Titan therefore seems closer to an Earth-like world thanpreviously anticipated4 .

Methods© 2012 Macmillan Publishers Limited. All rights reserved.

NATURE GEOSCIENCE DOI: 10.1038/NGEO1374
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LETTERS
Acknowledgements
We are grateful to F. Forget and R. Wordworth for advice and corrections to the paper.
We thank N. Rochetin for discussions of the terrestrial climate. This study was supported
by an Agence Nationale de la Recherche grant.

Author contributions
S.L. and B.C. developed this version of the model. B.C. ran and analysed simulations.
B.C. and S.L. wrote the manuscript.

Additional information
The authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissions
information is available online at http://www.nature.com/reprints. Correspondence andrequests for materials should be addressed to B.C.

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