Atmospheric transport and chemistry lecture

1. Atmospheric transport and chemistry lecture • Introduction • Fundamental concepts in atmospheric dynamics: Brewer-Dobson circulation and waves • Radiative transfer, heating and vertical transport

2. Brewer-Dobson circulation from ozone Dobson (1956) inferred the meridional circulation from the fact that maximum ozone (extratropics) is far away from production region (tropics) meridional gradient in ozone O2 photolysis [molec./cm3 sec] max ozone photochemical production max ozone concentration O3 nd [1012 molec./cm3] Johnston 1975

3. Brewer-Dobson circulation from H2O Brewer (1949) inferred the meridional circulation from the fact that stratosphere is very dry (compared to troposphere) water vapor vmr is determined by coldest temperature suggesting the tropical troppause as stratospheric entry point extratropical tropopause is to warm to explain dryness H2O VMR [10-6=ppm] ~7km/yr

4. The global picture: middle atmosphere dynamics

5. The global picture: middle atmosphere dynamics ozone production by photochemistry downward transport of ozone, photochemically stable photochemical decay ozone hole, chemical ozone loss

6. Atmospheric structure Fleming et al. 1988 cold tropical tropopause warm summer stratopause cold summer mesopause

7. Atmospheric structure mesospheric & stratospheric circulation Fleming et al. 1988 cold tropical tropopause cold summer mesopause

8. Radiative heating and temperature maximum heating in summer mesopause, but minimum T Near tropopause little meridional variation in rad. heating, but minimum T in tropics Importance of (zonal) winds (thermal balance) Atmospheric dynamics (waves) modify T/winds cooler warmer cooler summer winter London, 1980 • Diabatic heating (K per day): • Heating by UV absorption in O2 (>60 km) and O3 (<60 km) • Cooling by IR emission of CO2, H2O, and O2

9. Zonal mean wind and temperature Radiation produces wind (heating) and alters chemical constituents directly (photochemistry) Atmospheric wind (zonal mean flow) interacts with waves Waves deposit momentum and dissipate in the upper atmosphere and modify (seasonal variability) and modulate (interannual variability) T/wind distribution relevant for constituent transport Fleming et al. 1988 Westerlies (winter) Easterlies (summer) Wind reversal above mesopause

10. Cold summer mesopause and noctilucent clouds (NLC) Oslo, Norway • Aachen, Germany

11. Atmospheric dynamics (1) some of the basics is covered in Andrews et al. (1987) and Holton et al., (1992), see literature list +

12. Atmospheric dynamics (2)

13. Atmospheric dynamics (3)

14. Potential temperature distribution underworld: isentropes in contact with troposphere (tropopause crossing)  [K] /25  h [km] Adiabatic (fast) motion along isentropes (=const.) Slow ascent/descent across isentropes overworld =380 K underworld  T

15. Atmospheric dynamics (4)

16. Atmospheric dynamics (5)

17. Upper atmosphere dynamics (1) Surfaces of constant GPH (geopotential height) means that gravitational force (including centrifugal correction) is constant, e.g. no horizontal component due to gravity

18. Upper atmosphere dynamics (2) • (earth rotation angular • velocity=2/24h) 

19. Upper atmosphere dynamics (3) Strongest zonal winds (u) where meridional temperature gradients are largest (dT/d) In stratospheric models thermal wind balance are used to calculate wind fields from T (diagnostics), but it fails near the equator (f0, zero Coriolis foce)

20. Geostrophic balance Geostrophic winds follow contours of constant geopotential heigths Density of geopotential height contours are proportional to wind velocity Geopotential height in dekameter at 300 hPa (ca. 9 km altitude) maximum winds Wind measurements at 25 km by Doppler images HRDI/UARS (Ortland et al., 1996)

21. zonal mean wind: u stratopause jet subtropical westerly jet polar night jet (polar vortex) units: m/sec

22. Meridional circulation using zonal means (1) We reduce equation of motion from 3D to 2D by averaging over longitudes ---> meridional (latitude) and vertical direction

23. Meridional circulation using zonal means (2) u´v´ and v´´ are called eddy momentum flux and eddy heat flux terms

24. Meridional circulation using zonal means (3) Eddy heat flux term convergence and vertical motion nearly cancel with small net zonal mean diabatic heating (q) non-acceleration theorem (Andrews and McIntyre, 1976) atmospheric waves with constant amplitude (steady waves) and no dissipation do not induce a net meridional circulation Because of cancellation of terms the net meridional circulation is best to calculate from diabatic heating rates directly (Dunkerton 1978) TEM formulation vertical motion eddy heat flux convergence/ horizontal mixing • q=Q/Cp O´Neill 1980  TEM equation of motion

25. Meridional circulation using zonal means (4) The net residual circulation (v*,w*) can be now calculated via diabatic heating rates the residual circulation or diabatic circulation describes the Brewer-Dobson circulation The convergence of the Eliassen-Palm flux (E) describes the momentum transfer from atmospheric waves into the upper stratosphere that induces the diabatic circulation (not the radiative heating!!!) residual velocities  TEM equation of motion • Eliassen-Palm flux:

26. EP flux diagnostics • zonal momentum equation: • continuation equation: • diabatic momentum equation: • Eliassen-Palm Flux: Contours (DF): 1 m/sec/day 1979-1990 NMC (Randel 1992)

27. Atmospheric waves and residual circulation (1) • Propagation of Rossby waves into stratosphere (EP flux) (1) • Depositing easterly momentum (EP flux convergence) (2) • deceleration of westerlies (3) • Induction of residual circulation/ uplifiting in tropics and descent in polar region (4) Newman et al. (2001)

28. Atmospheric waves and residual circulation (1) • Wave propagation only in region of westerlies (summer: no wave propagation into easterlies) • Depositing easterly momentum (EP flux convergence) • stirring of airmasses (v*) • Adiabatic expansion in tropics (fast cooling) and adiabatic compression in extratropics (fast warming) • slow diabatic heating in tropics (w*>0) and cooling in extratropics (w*<0) towards radiative equilibrium (radiative relaxation) Wave driven diabatic circulation

29. EP flux divergence (DF) and T 254K@10hPa 210K@10hPa

30. EP flux divergence (DF) and zonal mean wind (u) Major stratospheric warming! -16 m/s@10hPa 80 m/s@10hPa Weak wave driving: • wave refracting into subtropics • strong polar night jet, low T High wave driving • wave propagating towards pole • weak/no polar vortex, high T

31. Major stratospheric warmings Statistics on major warmings with wind reversal at 10hPa and 60° Direct temperature response: cooling in tropics and warming in polar region in response to pulse in wave driving Enhanced convection in SH tropics (low =p/dt=-w) and reduced outgoing long wave radiation due to clouds (OLR) Kodera (2006)

32. Residual circulation (v*,w*) and age of stratospheric air age of stratospheric air (model) (Rosenlof 1995) diabatic circulation (Andrews et al. 1987) mesospheric circulation stratospheric circulation • >3.5 years (from 100hPa in tropics) stratospheric jets= transport barriers

33. Meridional circulation and long-lived tracers Other suitable long lived tracer: SF6, CO2, N2O Tracer with quasi-linear trends near the tropical tropopause and without significant sinks in the upper atmosphere are best suited to investigate age of air (Waugh and Hall, 2002) Tracer with constant mixing ratio

34. Meridional circulation and long-lived tracers Stratospheric age from SF6 (stratosphere) and CO2 (mesosphere) Test of climate and chemical transport model (models tend to be too young in air) Hall et al. 1999

35. ozone transport and chemistry in polar region Update Weber et al. 2003 Spring-to-fall ozone ratio controlled by planetary wave driving

36. Spring-to-fall ozone ratio controlled by planetary wave driving Wave driving of ozone transport March GOME October Update Weber et al. 2003 October

37. Wave driving of ozone chemistry GOME Sept 1-15 March 1-15 Update Weber et al. 2003 March 1-15 high PSC volume Low T low PSC volume High T

38. Seasonal cycle of wave driving and T Strong polar jet 90N 90S 90N • K Cold tropical lowermost stratosphere/tropopause cold polar vortex • Link between T variability in tropics to planetary wave driving (Yulaeva et al. 1994) • Link between planetary wave driving and Arctic T variability (Newmann et al. 2001)

39. Stratospheric water vapor entry and BD circulation Note eddy heat flux (momentuzm deposition) are added froim both hemispheres Stratospheric water vapor entry in tropics in NH winter anti-correlates with BD circulation strength (wave driving force) confirmation of Brewer‘s original concept (1949), which however was thought as Lagrangian motion (not as a result of a residual/diabatic circulation) In recent years stratosphere has become drier (climate change?) Dhomse et al. 2006

40. Tropospheric and stratospheric mode of variability NAO+: strong polar vortex (high PNJ speed) NAO – : weak polar vortex (low PNJ wind speed, strengthening of SJ) PNJ = polar night jet SJ=subtropical jet PNJ SJ NAO- NAO+ Hartmann et al. 2000 EP flux convergence Arosa O3

41. Mesopheric meridional circulation Mesospheric circulation is driven by gravity waves in both hemispheres Gravity aves break in both westerlies (winter) and easterlies (summer) Rossby waves mainly transfer angular momentum in lower stratosphere (driving stratospheric circulation), while gravity waves transfer momentum in the mesosphere (zbreak) and dissipate above the mesopause (energy and heat transfer) Pole-to-pole circulation (from summer hemisphere to winter hemisphere) with minimum temperature in summer mesopause region (~90 km) Wave driving (DF) in m/s day Brasseur et al. 2000 Lindzen 1981