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
Summary of where we will end up in this lecture gc
In horizontal, near surface: flow tilted to region of low pressure gp P gf v
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
Consider the way mass is moved here—then discuss MASS FLOW: convergence and divergence / lows and highs / at the surface and aloft. Rain in lows, clear in highs—why is that?

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

4. 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.
HOT
COLD

5. TODAY’S GLOBAL INFRARED CLOUD MAP (intellicast.com)
shows Intertropical Convergence Zone (ITCZ) as longitudinal band near Equator
ITCZ: location, strength
Wet and Dry season in Amazônia
Strength and location of polar and subtropical jet streams
Images show colder temperatures as brighter colors
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
z+Dz
Object (r)
Fluid (r’) z
buoyancy force= (rho*g); gravity=(rho'*g); net=g*(rho' – rho) ; accel= net/rho (force per unit mass)
Barometric law assumed T = T’ e gb = 0 (zero buoyancy) T
T’ produces buoyant acceleration upward or downward

12. ATMOSPHERIC LAPSE RATE AND STABILITY
“Lapse rate” = -dT/dz
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 rate G : z
G = 9.8 K km-1
stable z
unstable
What happens following release depends on the local lapse rate –dTATM/dz:
-dTATM/dz > G e upward buoyancy amplifies initial perturbation: atmosphere is unstable
-dTATM/dz = G e zero buoyancy does not alter perturbation: atmosphere is neutral
-dTATM/dz < G e downward buoyancy relaxes initial perturbation: atmosphere is stable
dTATM/dz > 0 (“inversion”): very stable
inversion
ATM
(observed)
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
ATM G
final 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 = G is 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 cloud Altitude, km 2 boundary layer 1 -20 -10 0 10 20 30
Temperature, oC 1 4 2 3
Altitude, km
cloud
boundary
layer
A picture to illustrate. Not the same place/time, but the same phenomenon!!
Have the students work out: why the clouds start where they do, and why they stop.
Air is turbulent (cf. airplane take off and landing) below the cloud base and usually smooth above—why?

16. SUBSIDENCE INVERSION
typically
2 km altitude

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

18. VERTICAL PROFILE OF TEMPERATURE Mean 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. SHORT QUESTIONS
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.

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