Physics of the Atmosphere Physik der Atmosphäre

1. Physics of the Atmosphere Physik der Atmosphäre SS 2010 Ulrich Platt Institut f. Umweltphysik R. 424 Ulrich.Platt@iup.uni-heidelberg.de

2. 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

3. Contents

4. Topics • Temperature and Radiation in the Stratosphere • Stratospheric Dynamics • Circulation • Stratosphere – Troposphere Exchange • Water Budget of the Stratosphere • S in the Stratosphere: Junge Layer

5. Ionosphäre Heterosphäre Thermosphäre Mesosphäre Homosphäre Tropopause Troposphäre The Structure of the Atmosphere

6. The Stratosphere

7. Mean Latitude Distribution of Temperature and Wind Velocity White lines: Isolinies of zonal (east-west) wind velocity (m/s) (Source: NASA)

8. Mean Latitude Distribution of Actual and Potential Temperature Drawn lines: Potential Temperature Dashed lines: Actual Temperature Holton et al., 1995

9. The Stratospheric Ozone Layer

10. Radiation Heating and Cooling of the Atmosphere local heating rates: • stratosphere ~ radiative equilibrium • troposphere ≠ radiative equilibrium - “convective adjustment” Brasseur and Solomon, 2005 (IUP-Book 1968) Higher atmosphere is (mainly) cooled by LW and heated by SW radiation

11. 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): 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: Thus the Optical Density as function of height z: and the intensity: The above equation is also known as Chapman Function.

12. Altitude/Km 40 35 30 25 20 15 10 5 0 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00 The Chapman Function From a certain altitude (e.g. for  = 1, red line) we can consider the atmosphere as ‘black’. Intensity I/I0

13. Absorption of Radiation in the Atmosphere

14. Turbulent Diffusion Konstant KM, cm2s-1 Source: Brasseur and Solomon 1986

15. 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

16. The Brewer-Dobson circulation II Slow circulation from the (cold) equatorial tropopause to higher latitudes provides a supply of dry air to the entire stratosphere Brewer (1949)

17. Alter in Jahren Mean Air Mass Ages at different Latitudes and Altitudes Waugh and Hall 2002

18. 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

19. Atmospheric Motion and Mean Methane Mixing Ratio

20. Atmospheric Motion and Mean Methane Mixing Ratio II

21. Stratosphere-Troposphere Exchange at Mid-Latitudes Holton et al., 1995

22. Stratosphere-Troposphere Exchange Global Picture WMO (2003)

23. WMO (2003) Very Short Lived Source Gases

24. 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

25. Water in the Equatorial Stratosphere

26. 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

27. Adiabatic flow over mountain range stretching of Θ0+dΘ layer initial lifting of Θ0+dΘ layer Holton (1992) horizontal spread of vertical displacement at top of column development of lee-wave due to changes in f uniform zonal flow

28. 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

29. 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)

30. Feb 29, 1800 UTC Feb 27, 1800 UTC Owen R. Cooper

31. PV Contours Owen R. Cooper

32. Source for Stratospheric NOx N2O + O(1D)  2 NO (58%)  N2 + O2 (42%) N2O + hν  N2 + O(1D) Brasseur et al., 1999

33. 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

34. Pinatubo I before after Photo: NASA

35. Pinatubo II WMO (2003) NASA

36. Pinatubo III  Temperature increase in the stratosphere WMO (2003)

37. The Global sulfur cycle Brasseur et al., 1999

38. Global Atmospheric Chlorine Cycle Graedel and Crutzen [1993]

39. 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