Anomalous solar heating dependenceof Venus’s cloud-level convection
T. Higuchi (Univ. Tokyo), T. Imamura (JAXA),
Y. Maejima (MRI, JMA), M. Takagi (Kyoto Sangyo Univ.),
N. Sugimoto (Keio Univ.), K. Ikeda (JAMSTEC)

0

Clouds on Venus• Venus is covered by clouds of
H2SO4-H2O which float in thealtitude region from 48 to 70 km.
Ultraviolet image of Venus takenfrom Pioneer Venus
80

Altitude ( km )7060
50Clouds40• The clouds play a major role via
reflection of sunlight (albedo of0.78) and absorption ofupwelling thermal radiation.

3020100100 200 300 400 500 600 700 800

Temperature ( K )Vertical temperature distribution of the Venus
atmosphere1

Vertical structure
Photochemically-sustained
aerosol layer
Dynamically-sustainedcondensational clouds

Upwelling thermal radiation

Pioneer Venus LCPS
(Knollenberg and Hunten, 1980)

Static stability measured
by entry probes

Latitudinal variation
Static stability profiles obtained by radio occultation (Tellmann et al. 2009)
Low latitude

Mid latitude

High latitude

CLOUD• The depth of convection layer increases with latitude.
 More vigorous convection at higher latitudes ?

3

Latitudinal variation
Amplitude of small-scale temperaturefluctuation deduced by radio occultation

Variance of radio scintillation power
observed during radio occultation

Woo et al. (1980)
Tellmann et al. (2012)• Amplitude of small-scale temperature/density fluctuation tends to belarger at higher latitudes.  More vigorous wave excitation at higher
latitudes ?

4

2-D convection model (Baker, Schubert & Jones, 1998; 2000)
Solar flux

Diffusion heat flux
60% subsolar heating

80% subsolar heatingBaker et al. introduced upward eddy diffusion
heat flux at the upper and lower boundaries
to drive convection. The value of the eddy
diffusion heat flux at the boundaries is equalto (but opposite in sign from) the solar heat
flux at that altitude.
The results show more vigorous convection
for stronger solar fluxes.

100% solar heating5

Origin of the latitudinal dependences ofconvection depth and gravity wave activity• Baker et al. argues occurrence of more vigorous
convection at lower latitudes.
 Gravity wave generation by other mechanisms ? (forexample, fluctuation of high-latitude jets, shear instability,topographyically-generated waves) How about the latitudinal variation of convection
depth ?
• Shoud convection be really stronger at lower latitudes ?
(This study)
6Energy input to the cloud layer
Shortwave heating

Longwave heating and cooling

Heating

Cooling

Cloud

Cloud

Heating7

Statement in Baker et al. (2000)“ Because solar heating is strongest at the top of the domain, one
may initially conclude that absorption of solar radiation provides astabilizing influence in Venus’s atmosphere. However, the Venus
profile of solar heating actually provides a destabilizing influence on
the atmosphere. Since the atmosphere behaves diffusively below
60-km altitude, negative potential temperature gradients are
established to transfer the absorbed solar radiation. Larger valuesof solar heating result in steeper (more negative) potential
temperature gradients, and convectively unstable regions areproduced. The situation is somewhat analogous to a uniformlyinternally heated system in which convection occurs even thoughheating is uniformly distributed. “ However, this seems to be true only when the net heat
flux is zero at the bottom boundary. This is not the case in
the real atmosphere.

Latitude dependence of the zonally-averaged radiative
energy flux
Solar flux

Radiative flux (W m-2)

EarthThermal flux
Solar flux

Venus
Thermal flux

- Excess solar flux in the low
latitude- Excess thermal flux in thehigh latitude

Moroz et al. (1985)
latitudeEnergy transport by atmospheric circulation
Net radiative heating rate(Crisp 1989)• Mean heating/cooling of the cloud-levelatmosphere by atmospheric dynamics is
expected to occur mostly through– adiabatic heating/cooling associatedwith large-scale descent/ascent instably-stratified region (Crisp 1989;Imamura 1997)– diffusive flux by convection in
neutral stability region• We focus on the convection layer andignore contributions of large-scale
dynamics

unit: K/day
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Two-dimensional convection model• 2-D numerical experiments based on
the non-hydrostatic meteorologicalmodel CReSS (Cloud Resolving StormSimulator) Version 2.3.
• No cloud microphysics (dry convection)

Altitude ( km )Computational domain
6560

Sponge Layer

4035

Sponge Layer

100kmGrid size
Boundary condition
Time step
Initial perturbation
Eddy diffusion

dx = 200m dz = 125m
lateral：Periodic Top and bottom：Fixed wall
HE-VI, Long : 1.0 s Short : 0.2 s
Random perturbation ( Maximum value : 10-3 K )1.5-order turbulent kinetic energy closure
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Thermal forcing
Shortwave heating rate is taken from Tomasko et al. (1980).

Longwave heating/cooling rate is taken from the one-dimensionalradiative-convective equilibrium calculation by Ikeda (2010).
Static stability calculated by Ikeda(2010)  used as the initial stateZero net heating
Non-zero net heating

Zero net heating
Shortwave
Longwave

Heating rate ( W m-3 )

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60

0.0

Altitude ( km )w (m/s)3.6-3.6

0.0
-0.5

50
45
01020304050
60
Horizontal distance ( km )

70

80

90

100

80

90

100

Potential temperature perturbation

60
Altitude ( km )

0.5

55

40

θ’ (K)
1.0

Vertical velocity

55
50
45
40

0

1020304050

Horizontal distance ( km )60

70

Heat flux
Heat is transported
upward by convection
60605045

Heat is transported
40
downward by-15-10
penetrative convection

Heat flux calculated byBaker et al. (1998)

Convective
heat flux
Altitude ( km )Altitude ( km )55

Net radiativeflux-505Flux (W m-2)

10155550

Convective
heat flux

4540-400-200

0200400

Flux (W m-2)

A part of the energy supplied to the convection zone by longwave heatingis transported downward by penetrative compressible convection.
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Modeling the latitudinal dependence and diurnalvariation• The clouds make a circuit ofVenus in 7.3 Earth days thatare the mean circulatory period
at 50km altitude.

Heating rate at local noon Shortwave heating changes
with a period of 7.5 Earth days.
Short wave heating

7.5 Earth day• A constant profile of longwave heating and cooling is given based on
the observations that the latitudinal and local time dependences of the15
temperature are small below the cloud top.

Latitudinal dependence and diurnal variation

RMS vertical velocity ( m s-1)

Diurnal variation of vertical velocity at 50km altitude
2.5
2.0
1.5
1.0

0°
30°
60°

0.5
0.0

00:00
LST

06:00
LST

12:00
LST

18:00
LST

24:00
LST The diurnal variation is more prominent in the low latitude than in the high latitude.
 The vertical velocity is depressed at daytime and at low latitude.

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Structure of convection
Equator (0°)

High latitude (60°)

Altitude (km)

12:00
LST
18:00
LST
00:00
LST
06:00
LST
Horizontal distance (km)Stronger and deeper convection occurs during nighttime rather than daytime,
17and at high latitudes rather than at low latitudes.

Amplitude of temperature fluctuation
Equator (0°)

High latitude (60°)
12:00
LST

Stronger gravity wave excitation at higher latitudes
00:00
LST18

Depth of neutral stability layer
Zonally-averaged static stability 14.6 Earth days
after start of the calculation

More deepening of theneutral stability layer
at higher latitudes
Initial
0°30°60°

Summary• The newly-developed Venus’s cloud-level convection model showedthat stronger and deeper convection occurs at high latitudes rather
than at low latitudes, and during nighttime rather than daytime. Thisis qualitatively consistent with the observed latitudinal tendencies of
the convective layer depth and the amplitudes of small-scale waves.
• In the Earth’s convection layer, incoming solar radiation is quickly
converted to heat which drives convection. In the case of Venus’s
cloud-level convection, on the other hand, the solar energy supplied
to the lower atmosphere is well mixed in latitude and longitude dueto the long radiative timescale before being delivered to the cloudlevel. Because of this difference the latitudinal tendency becomesdifferent.
• The suggested latitudinal tendency is expected to be common toplanets having a thick aerosol layer detached from the ground and aradiatively-thick lower atmosphere.
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