CHAPTER 4: ATMOSPHERIC TRANSPORT

1. CHAPTER 4: ATMOSPHERIC TRANSPORT • Forces in the atmosphere: • Gravity • Pressure-gradient • Coriolis • Friction to R of direction of motion (NH) or L (SH) Equilibrium of forces: In vertical: barometric law In horizontal: geostrophic flow parallel to isobars gp P v P + DP gc In horizontal, near surface: flow tilted to region of low pressure gp P v gf P + DP gc

2. Air converges near the surface in low pressure centers, due to the modification of geostrophic flow under the influence of friction. Air diverges from high pressure centers. At altitude, the flows are reversed: divergence and convergence are associated with lows and highs respectively

3. TODAY’S US WEATHER MAP Highs, Lows, fronts, winds, cloud cover

4. HOT COLD COLD THE HADLEY CIRCULATION (1735): global sea breeze • Explains: • Intertropical Convergence Zone (ITCZ) • Wet tropics, dry poles • General direction of winds, easterly in the tropics and westerly at higher latitudes • Hadley thought that air parcels would tend to keep a constant angular velocity. • Meridional transport of air between Equator and poles results in strong winds in the longitudinal direction. • …but this does not account for the Coriolis force correctly.

5. TODAY’S GLOBAL INFRARED CLOUD MAP (intellicast.com) shows Intertropical Convergence Zone (ITCZ) as longitudinal band near Equator Bright colors indicate high cloud tops (low temperatures) Today

6. TROPICAL HADLEY CELL • Easterly “trade winds” in the tropics at low altitudes • Subtropical anticyclones at about 30o latitude

7. CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(January)

8. CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(July)

9. TIME SCALES FOR HORIZONTAL TRANSPORT(TROPOSPHERE) 1-2 months 2 weeks 1-2 months 1 year

10. SHORT QUESTIONS • Is pollution from California more likely to impact Hawaii in summer or in winter? Explain in terms of the seasonal variation of the general circulation. • In the movie The Day After Tomorrow, climatologist hero Jack Hall observes a mass of cold air from the upper troposphere descending rapidly to the surface and predicts that it will trigger an ice age over the United States. When another forecaster objects, “Won’t this air mass heat up as it sinks?”, our hero replies “It’s sinking too fast. It doesn’t have time”. Can our hero be right?

11. VERTICAL TRANSPORT: BUOYANCY Buoyancy: z+Dz Object (r) Fluid (r’) z Barometric law assumed T = T’ e gb = 0 (zero buoyancy) T T’ produces buoyant acceleration upward or downward

12. “Lapse rate” = -dT/dz ATMOSPHERIC LAPSE RATE AND STABILITY Consider an air parcel at z lifted to z+dz and released. It cools upon lifting (expansion). Assuming lifting to be adiabatic, the cooling follows the adiabatic lapse rateG : z G = 9.8 K km-1 stable z unstable • What happens following release depends on the local lapse rate –dTATM/dz: • -dTATM/dz > Ge upward buoyancy amplifies initial perturbation: atmosphere is unstable • -dTATM/dz = Ge zero buoyancy does not alter perturbation: atmosphere is neutral • -dTATM/dz < Ge downward buoyancy relaxes initial perturbation: atmosphere is stable • dTATM/dz > 0 (“inversion”): very stable ATM (observed) inversion unstable T The stability of the atmosphere against vertical mixing is solely determined by its lapse rate.

13. WHAT DETERMINES THE LAPSE RATE OF THE ATMOSPHERE? • An atmosphere left to evolve adiabatically from an initial state would eventually tend to neutral conditions (-dT/dz = G ) at equilibrium • Solar heating of surface and radiative cooling from the atmosphere disrupts that equilibrium and produces an unstable atmosphere: z z z final G ATM G ATM initial G T T T Initial equilibrium state: - dT/dz = G Solar heating of surface/radiative cooling of air: unstable atmosphere buoyant motions relax unstable atmosphere back towards –dT/dz = G • Fast vertical mixing in an unstable atmosphere maintains the lapse rate to G. • Observation of -dT/dz = Gis sure indicator of an unstable atmosphere.

14. IN CLOUDY AIR PARCEL, HEAT RELEASE FROM H2O CONDENSATION MODIFIES G Wet adiabatic lapse rate GW = 2-7 K km-1 z T RH 100% GW “Latent” heat release as H2O condenses GW = 2-7 K km-1 RH > 100%: Cloud forms G G = 9.8 K km-1

15. 4 3 Altitude, km 2 1 0 -20 -10 0 10 20 30 Temperature, oC cloud boundary layer

16. SUBSIDENCE INVERSION typically 2 km altitude

17. DIURNAL CYCLE OF SURFACE HEATING/COOLING:ventilation of urban pollution z Subsidence inversion MIDDAY 1 km G Mixing depth NIGHT 0 MORNING T NIGHT MORNING AFTERNOON

18. VERTICAL PROFILE OF TEMPERATUREMean values for 30oN, March Radiative cooling (ch.7) - 3 K km-1 Altitude, km 2 K km-1 Radiative heating: O3 + hn e O2 + O O + O2 + M e O3+M heat Radiative cooling (ch.7) Latent heat release - 6.5 K km-1 Surface heating

19. A well known air pollution problem is “fumigation” where areas downwind of a major pollution source with elevated stacks experience sudden bursts of very high pollutant concentrations in mid-morning. Can you explain this observation on the basis of atmospheric stability? • A persistent mystery in atmospheric chemistry is why the stratosphere is so dry (3-5 ppmv H2O). Based on water vapor concentrations observed just below the tropopause, one would expect the air entering the stratosphere to be moister, One theory is that very strong thunderstorms piercing through the tropopause can act as a “cold finger” for condensation of water and thereby remove water from the lower stratosphere. Explain how this would work. SHORT QUESTIONS

20. TYPICAL TIME SCALES FOR VERTICAL MIXING • Estimate time Dt to travel Dz by turbulent diffusion: ~ tropopause (10 km) 10 years 5 km 1 month 1 week 2 km “planetary boundary layer” 1 day 0 km