Physics of the Atmosphere Physik der Atmosphäre

SS 2010

Ulrich PlattInstitut f. Umweltphysik

R. 424Ulrich.Platt@iup.uni-heidelberg.de

Last Week• The planetary boundary layer is the layer where surface friction has an impact (τ ≠ 0). It

can be subdivided into different regimes:– Molecular-viscous layer governed by molecular diffusion– Prandl- layer, where shear stress is constant with altitude– Ekman- layer, where shear stress decreases with altitude

(until it is zero in the free atmosphere)• Basic assumption: Turbulent diffusion coefficient is proportional to altitude Logarithmic

wind profile• Water vapour has an impact on vertical stability not only due to the release of latent heat,

but also due to its lower density• The transport of scalar tracers in the boundary layer can be parameterised with the

transfer resistance R or the piston velocity v12:

• In the turbulent regime, the transfer resistance is proportional to the logarithmic ratio of the altitude difference

• Air/sea gas exchange is a very important issue in the chemistry and climate of the atmosphere (how much anthropogenic CO2 is taken up by the oceans?)

• It can be investigated using wind-wave facilities, such as the Aelotron at the IUP

2

1

z

12 12z 12

dz 1R or v

K( z) R

Contents

Topics

• Temperature and Radiation in the Stratosphere

• Stratospheric Dynamics

– Circulation

– Stratosphere – Troposphere Exchange

• Water Budget of the Stratosphere

• S in the Stratosphere: Junge Layer

The Structure of the Atmosphere

Ionosphäre

Heterosphäre

Homosphäre

Mesosphäre

Troposphäre

Thermosphäre

Tropopause

The Stratosphere

Mean Latitude Distribution of Temperature and Wind Velocity

(Source: NASA) White lines: Isolinies of zonal (east-west) wind velocity (m/s)

Mean Latitude Distribution of Actual and Potential Temperature

Drawn lines: Potential TemperatureDashed lines: Actual TemperatureHolton et al., 1995

The Stratospheric Ozone Layer

Radiation Heating and Cooling of the Atmosphere

Brasseur and Solomon, 2005(IUP-Book 1968)

local heating rates:– stratosphere ~ radiative equilibrium– troposphere ≠ radiative equilibrium - “convective adjustment”

Higher atmosphere is (mainly) cooled by LW and heated by SW radiation

Vertical Radiation Intensity Profile in the Atmosphere

1) For simplicity we first consider a (hypothetical isobaric atmosphere, i.e. c(z) = c0):

We obtain for I(z): 0 1c z z z0 0I z I e I e

With:= Optical Density

c = Trace gas concentration (e.g. particles m-3)  = Absorption cross section

I0 = Intensity outside the atmosphere

2) In reality, of course we have an exponential decrease:

d c ds

s

zz

0c z c e

Thus the Optical Density as function of height z:

s s

z ' z

z z0 0 s

z

z c e dz ' c z e

and the intensity:

z

zs0 sz c z e

0 0I z I e I e

The above equation is also known as Chapman Function.

The Chapman Function

Intensity I/I0

From a certain altitude (e.g. for  = 1, red line) we can consider the atmosphere as ‘black’.

0 sc z 5

Altitude/Km

0

5

10

15

20

25

30

35

40

0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00

Absorption of Radiation in the Atmosphere

Turbulent Diffusion Konstant KM, cm2s-1

Source: Brasseur and Solomon 1986

The Brewer-Dobson Circulation I

• In 1948: Alan Brewer discovers that stratospheric air above England is a lot drier than expected from local dew point temperature.

• latitudinal gradient of Θ: no direct advection of tropical air (radiative cooling rates would have to be unreasonably high)

• Stratosphere is NOT in radiative equilibrium due to BDC:

– ascending branch: radiative heating

– sinking branch: radiative cooling

• Dehydration of air entering the stratosphere: freeze-drying

The Brewer-Dobson circulation II

Brewer (1949)

Slow circulation from the (cold) equatorial tropopause to higher latitudes provides a supply of dry air to the entire stratosphere

Mean Air Mass Ages at different Latitudes and Altitudes

Waugh and Hall 2002

Alter in Jahren

The „Age“ of stratospheric Air

Air mass age (from CO2) as function of latitude at 20km

And as Fu. of latitude (5oS, 40oN, 65oN) and altitude

Atmospheric Motion and Mean Methane Mixing Ratio

Atmospheric Motion and Mean Methane Mixing Ratio II

Stratosphere-Troposphere Exchange at Mid-Latitudes

Holton et al., 1995

Stratosphere-Troposphere Exchange Global Picture

WMO (2003)

Very Short Lived Source Gases WMO (2003)

Freeze Drying the Stratosphere

• most efficient upward transport mechanism: deep convection

• tropical TP is VERY cold freeze drying

• tropical TP temp is lowest in NH winter minimum in specific humidity in tropical lower stratosphere in NH winter

Water in the Equatorial Stratosphere

Potential Vorticity (Ertel’s Vorticity)

• In absence of friction and diabatic processes (radiation, latent heat, ..) PV is conserved:

• measure of vertical stability and circulation• measure of ratio of absolute vorticity to effective depth of vortex• dynamical tracer of horizontal motion• large gradient of PV across the tropopause• 1 PVU = 10-6 m2 s-1 K kg-1

constp

gfPV

)( Relative Vorticity:

v

Coriolis Parameter:

2 sin

const

u

x y

f

3 3 2

2 2 3 3 2

1 m K m K m K m KPV

s s N m s N s Kg m s s Kg

Adiabatic flow over mountain range

uniform zonal flow

initial lifting of Θ0+dΘ layer

horizontal spread of vertical displacement at top of column

stretching of Θ0+dΘ layer

development of lee-wave due to changes in f

constp

gfPV

)(

Holton (1992)

Tropopause Definitions

• Focus on increase in stability

• Θ/PV:

– tropical TP: Θ=380K

– extratropics: 2 PVU

• WMO:

– lowest level at which dT/dz ≤ 2 K km-1

– and: dT/dz ≤ 2 K km-1 in “surrounding” 2km

• Ozone:

– altitude with first occurrence of [O3] > 0.1 ppm

Stratosphere-Troposphere Exchange• Tropics:

– deep, overshooting convection

• Extratropics:

– tropopause folds in jet stream regions

– cut-off lows

– isentropic exchange in lower-most stratosphere

• STE in models: hard problem, vertical resolution near TP has to be fairly high

Shapiro (1980) in Holton et al (1995)

Feb 27, 1800 UTC

Feb 29, 1800 UTCOwen R. Cooper

Owen R. Cooper

PV Contours

Source for Stratospheric NOx

N2O + O(1D) 2 NO (58%) N2 + O2 (42%)

N2O + hν N2 + O(1D)

Brasseur et al., 1999

Sulfur in the Stratosphere

• Sources:

– volcanic SO2 and sulfate aerosols

– OCS

• OCS chemistry:

(1) OCS + hv S + CO

S + O2 SO + O

(2) OCS + O SO + CO

SO + O2 SO2 + O

SO + NO2 SO2 + NO

• Formation of sulfate aerosols: Junge layer, condensation nuclei for PSC (discovered by Junge, 1961)

• Important for radiation balance and ozone chemistry

Pinatubo I

before

after

Photo: NASA

Pinatubo II

WMO (2003)

NASA

Pinatubo III

WMO (2003) Temperature increase in the stratosphere

The Global sulfur cycle

Brasseur et al., 1999

Global Atmospheric Chlorine Cycle

O rganic ch lorine (m ostly C H 3C l)

Industria l em issionsB iom ass

burningVolcanoes

C FC sTroposphericaccum .: 0 .56

Inorganic ch lorine com pounds (m ostly H C l)

C hloride ion in sea sa lt aerosol

S tra tospheric C hlorine com poundsaccum ulation: 0 .08

600

Sedim entation , P rec ipation 5400

W ave generation 6000

Precip ita tion 610

H C l 7

3

0.81

1.5

O ceanic em iss ions 2 .5

P recip ita tion,H O -reaction 5

0.03

0.24 0.19 P recip ita tion

units:1012 g Cl yr-1

Graedel and Crutzen [1993]

Summary

• The stratospheric temperature is determined by radiation balance

• Exchange between Strat. And Trop. Is determined by the Brewer-Dobson circulation and transport along isentropes

• The Brewer-Dobson circulation also determines the stratospheric water budget in first approximation (polar stratospheric cold trap)

• S in the Stratosphere: Junge Layer