eart164: planetary atmospheresfnimmo/eart164/week4_clouds.pdff.nimmo eart164 spring 11 last week -...
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![Page 1: EART164: PLANETARY ATMOSPHERESfnimmo/eart164/Week4_Clouds.pdfF.Nimmo EART164 Spring 11 Last Week - Chemistry •Cycles: ozone, CO, SO 2 •Photodissociation and loss (CH 4, H 2 O etc.)](https://reader033.vdocument.in/reader033/viewer/2022052021/60353b19960c2108a61b7b36/html5/thumbnails/1.jpg)
F.Nimmo EART164 Spring 11
EART164: PLANETARY
ATMOSPHERES
Francis Nimmo
![Page 2: EART164: PLANETARY ATMOSPHERESfnimmo/eart164/Week4_Clouds.pdfF.Nimmo EART164 Spring 11 Last Week - Chemistry •Cycles: ozone, CO, SO 2 •Photodissociation and loss (CH 4, H 2 O etc.)](https://reader033.vdocument.in/reader033/viewer/2022052021/60353b19960c2108a61b7b36/html5/thumbnails/2.jpg)
F.Nimmo EART164 Spring 11
Last Week - Chemistry
• Cycles: ozone, CO, SO2
• Photodissociation and loss (CH4, H2O etc.)
• D/H ratios and water loss
• Noble gas ratios and atmospheric loss
(fractionation)
• Outgassing (40Ar, 4He)
• Dynamics can influence chemistry
• Non-solar gas giant compositions
• Titan’s problematic methane source
![Page 3: EART164: PLANETARY ATMOSPHERESfnimmo/eart164/Week4_Clouds.pdfF.Nimmo EART164 Spring 11 Last Week - Chemistry •Cycles: ozone, CO, SO 2 •Photodissociation and loss (CH 4, H 2 O etc.)](https://reader033.vdocument.in/reader033/viewer/2022052021/60353b19960c2108a61b7b36/html5/thumbnails/3.jpg)
F.Nimmo EART164 Spring 11
• Physics of cloud formation
– Vapour pressure, nucleation
• Clouds in practice
– Mars (CO2 + H2O), Venus (SO2), Earth (H2O)
– Titan (CH4), Gas giants, Exoplanets
• Dust
• Guest lecture (Patrick Chuang)
This Week – Clouds, Hazes, Dust
![Page 4: EART164: PLANETARY ATMOSPHERESfnimmo/eart164/Week4_Clouds.pdfF.Nimmo EART164 Spring 11 Last Week - Chemistry •Cycles: ozone, CO, SO 2 •Photodissociation and loss (CH 4, H 2 O etc.)](https://reader033.vdocument.in/reader033/viewer/2022052021/60353b19960c2108a61b7b36/html5/thumbnails/4.jpg)
F.Nimmo EART164 Spring 11
Schematic cloud formation T
P
Gas
Solid/liquid
Adiabat
Cloud
base
Phase
boundary
Clouds may consist of either
solid or liquid droplets
Lapse rate gets smaller when
condensation begins - why?
g dz = Cp dT + LH df
LH is the latent heat
![Page 5: EART164: PLANETARY ATMOSPHERESfnimmo/eart164/Week4_Clouds.pdfF.Nimmo EART164 Spring 11 Last Week - Chemistry •Cycles: ozone, CO, SO 2 •Photodissociation and loss (CH 4, H 2 O etc.)](https://reader033.vdocument.in/reader033/viewer/2022052021/60353b19960c2108a61b7b36/html5/thumbnails/5.jpg)
F.Nimmo EART164 Spring 11
• Condensation occurs when the partial pressure of
vapor in the atmosphere equals a particular value (the
saturation vapor pressure Ps) defined by the phase
boundary given by the Clausius-Clapeyron relation:
Vapour pressure
• This gives us ln(Ps)=a-b/T
• As condensation of a species proceeds, the partial
pressure drops and so T will need to decrease for
further condensation to proceed
• What happens when you boil water at high altitude?
2RT
PL
dT
dP sHs
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F.Nimmo EART164 Spring 11
Phase boundary
H2O
RT
LCP H
Lvap exp
E.g. water CL=3x107 bar, LH=50 kJ/mol
So at 200K, Ps=0.3 Pa, at 250 K, Ps=100 Pa
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F.Nimmo EART164 Spring 11
• Liquid droplets can form spontaneously from vapour
(homogeneous nucleation), but it can require a large
degree of supercooling
• In practice, nucleation is much easier if there are
contaminants (e.g. dust) present. This is
heterogeneous nucleation.
• In real atmospheres, nucleation sites (cloud
condensation nuclei, CCN) are usually present
• On Earth, pollution is one major source of CCN
• CCN are much smaller than raindrops (~0.1 mm)
Nucleation
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F.Nimmo EART164 Spring 11
• Crudely speaking, air
picks up water from
the ocean and
deposits it on land
• Equatorial easterly
winds mean that
western sides of
continents tend to be
cloud-free and very
dry
Earth Clouds
Global cloud-cover, averaged over
month of October 2009
Equatorial easterlies
What causes the equatorial easterlies (trade winds)?
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F.Nimmo EART164 Spring 11
• Clouds can have an enormous impact on albedo and hence
surface temperature
• E.g. Venus A=0.76 Earth A=0.4
• Venus receives less incident radiation than Earth!
• Clouds are typically not resolved in global circulation models
– but can be very important
• Sea surface warming will lead to more clouds, partly offsetting
the warming effect
• “cloud feedbacks remain the largest source of uncertainty in
climate sensitivity estimates” (IPCC 2007)
• How much extra cloud cover would be required to offset a 2K
increase in temperature?
Albedo and feedback
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F.Nimmo EART164 Spring 11
Venus
Thermal breakdown at 400K
H2SO4 SO3 + H2O
H2O + SO3 H2SO4
(condenses)
SO2, H2O
outgassing
H2O + SO3 H2SO4
Thermal breakdown
Clouds consist mostly of H2SO4 droplets
These break down at high temperatures –
lower atmosphere is cloud free
O + SO2 SO3
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F.Nimmo EART164 Spring 11
Mars Clouds Mars Express 2004
CO2 clouds
Observed in spacecraft images
Most clouds observed are water ice (very thin, cirrus-like)
Do not have significant effect on global energy budget (unlike Earth)
CO2 ice clouds have also been observed
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F.Nimmo EART164 Spring 11
Mars clouds
CO2
Mars lower
atmosphere
H2O
Poles
• CO2 clouds form only when cold – either at high
altitude (~100 km) or near poles
• H2O is not abundant (few precipitable microns)
• But H2O clouds are common where there is a source
of water (e.g. polar caps in spring)
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F.Nimmo EART164 Spring 11
Titan Atmospheric Structure
pole equator
200 k
m
Haze (smog)
Cirrus
Cumulus
10 k
m 30 k
m
Methane
rain
Methane
ice crystals
Clouds consist mostly of CH4 ice & droplets
Haze is a by-product of methane photochemistry high in the
atmosphere (long-chain hydrocarbons), ~0.1 mm
Haze layer
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F.Nimmo EART164 Spring 11
• Tropospheric methane clouds
• North pole, 2009 (equinox)
• Speeds ~ 5 m/s
Clouds & rain on Titan
PIA12811_full_m
ovie.mov
• Patches of surface look darker after clouds
form – suggests rainfall took place
• Distribution of clouds is observably changing
with seasons (moving past equinox)
• Titan has a dynamic “hydrological” cycle
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F.Nimmo EART164 Spring 11
Giant planet clouds A
ltit
ude
(km
)
Different cloud decks, depending
on condensation temperature
Colours are due to trace
constituents, probably
sulphur compounds
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F.Nimmo EART164 Spring 11
Exoplanet Clouds
Sudarsky et al. 2003
• I – Ammonia (<150 K)
• II – Water (<250 K)
• III – Cloudless (>350 K)
• IV – Alkali Metals (>900
K)
• V – Silicate (>1400 K)
Different classes of exoplanets predicted to have very different
optical & spectroscopic properties depending on what cloud
species are present
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F.Nimmo EART164 Spring 11
Dust on Mars
Dust has major control on energy budget of atmosphere
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F.Nimmo EART164 Spring 11
Dust lofting & settling
2gr
Ht
Global dust storms on Mars result from feedback: dust means
more energy absorbed in atmosphere, local increase in wind
strength, more dust lofted and so on . . .
Why don’t we get global dust storms on Earth?
-Oceans
-Wet atmosphere helps particles flocculate
How do we calculate the viscosity of a gas? 2
~rN
v
A
m
Sinking timescale:
For Mars, ~10-3 Pa s, H~15 km, r~10mm so t ~few months
Where does this come from?
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F.Nimmo EART164 Spring 11
Dust Devils on Mars
Phoenix image
Helpful in cleaning solar cells!
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F.Nimmo EART164 Spring 11
Moving dunes on Mars
Bridges et al. Nature 2012
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F.Nimmo EART164 Spring 11
Thermal effect of dust
)(
4
31)( 44 zTzT eq
Tropospheric Teq=160 K
Tropospheric warming due to dust
gives T=240 K (see diagram)
Implies ~5 (a bit high)
t =
3dr
4rrsIf d=20 km and r=10 mm, ~10-5
kg m-3 . What surface thickness
does this represent?
0.2 kg/m2 = 0.1 mm (not a lot!)
Dust
No dust
*
* We’ll discuss next week
(radiative transfer)
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F.Nimmo EART164 Spring 11
Key concepts • Saturation vapour pressure, Clausius-Clapeyron
• Moist vs. dry adiabat
• Cloud albedo effects
• Giant planet cloud stacks
• Dust sinking timescale and thermal effects
2gr
Ht
2RT
PL
dT
dP sHs
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F.Nimmo EART164 Spring 11
End of lecture
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F.Nimmo EART164 Spring 11
• Could talk about Zahnle style water
atmosphere and radiative heat loss 300 W/m2?
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F.Nimmo EART164 Spring 11
Giant planet atmospheric structure
• Note position and order of cloud decks