CHAPTER 4: ATMOSPHERIC TRANSPORTCHAPTER 4: ATMOSPHERIC TRANSPORT

Forces in the atmosphere:

• Gravity • Pressure-gradient• Coriolis • Friction

g 1/ P pγ

2 sinc v to R of direction of motion (NH) or L (SH)kfγ v

Equilibrium of forces:

In vertical: barometric law

In horizontal: geostrophic flow parallel to isobars P

P + P

p

c

v

In horizontal, near surface: flow tilted to region of low pressure

P

P + Pc

vf

p

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

TODAY’S US WEATHER MAPTODAY’S US WEATHER MAPHighs, Lows, fronts, winds, cloud cover

THE HADLEY CIRCULATION (1735): global sea breezeTHE HADLEY CIRCULATION (1735): global sea breeze

HOT

COLD

COLD

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.

TODAY’S GLOBAL INFRARED CLOUD MAP (intellicast.com)TODAY’S GLOBAL INFRARED CLOUD MAP (intellicast.com)

TodayBright colors indicate high cloud tops (low temperatures)

shows Intertropical Convergence Zone (ITCZ) as longitudinal band near Equator

TROPICAL HADLEY CELLTROPICAL HADLEY CELL

• Easterly “trade winds” in the tropics at low altitudes• Subtropical anticyclones at about 30o latitude

CLIMATOLOGICAL SURFACE WINDS AND PRESSURESCLIMATOLOGICAL SURFACE WINDS AND PRESSURES(January)(January)

CLIMATOLOGICAL SURFACE WINDS AND PRESSURESCLIMATOLOGICAL SURFACE WINDS AND PRESSURES(July)(July)

TIME SCALES FOR HORIZONTAL TRANSPORTTIME SCALES FOR HORIZONTAL TRANSPORT(TROPOSPHERE)(TROPOSPHERE)

2 weeks1-2 months

1-2 months

1 year

SHORT QUESTIONSSHORT 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?

VERTICAL TRANSPORT: BUOYANCYVERTICAL TRANSPORT: BUOYANCY

Object (z

z+zFluid (’)

Buoyancy:

b pγ = γ - g

g

Barometric law assumed T = T’ b = 0 (zero buoyancy)

T T’ produces buoyant acceleration upward or downward

ATMOSPHERIC LAPSE RATE AND STABILITYATMOSPHERIC LAPSE RATE AND STABILITY

T

z

= 9.8 K km-1

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 :

z

“Lapse rate” = -dT/dz

-1/ 9.8 K kmp

gdT dz

C

ATM(observed)

What happens following release depends on the local lapse rate –dTATM/dz:• -dTATM/dz > upward buoyancy amplifies initial perturbation: atmosphere is unstable• -dTATM/dz = zero buoyancy does not alter perturbation: atmosphere is neutral• -dTATM/dz < downward buoyancy relaxes initial perturbation: atmosphere is stable• dTATM/dz > 0 (“inversion”): very stable

unstable

inversion

unstable

stable

The stability of the atmosphere against vertical mixing is solely determined by its lapse rate.

WHAT DETERMINES THE LAPSE RATE OF THE WHAT DETERMINES THE LAPSE RATE OF THE ATMOSPHERE?ATMOSPHERE?

• An atmosphere left to evolve adiabatically from an initial state would eventually tend to neutral conditions (-dT/dz = at equilibrium

• Solar heating of surface and radiative cooling from the atmosphere disrupts that equilibrium and produces an unstable atmosphere:

Initial equilibriumstate: - dT/dz =

z

T

z

T

Solar heating ofsurface/radiative cooling of air: unstable atmosphere

ATM

ATM

z

Tinitial

final

buoyant motions relaxunstable atmosphere back towards –dT/dz =

• Fast vertical mixing in an unstable atmosphere maintains the lapse rate to Observation of -dT/dz = is sure indicator of an unstable atmosphere.

IN CLOUDY AIR PARCEL, HEAT RELEASE FROM IN CLOUDY AIR PARCEL, HEAT RELEASE FROM HH22O CONDENSATION MODIFIES O CONDENSATION MODIFIES

RH > 100%:Cloud forms

“Latent” heat releaseas H2O condenses

9.8 K km-1

W2-7 K km-1

RH

100%

T

z

W

Wet adiabatic lapse rate W = 2-7 K km-1

-20 -10 0 10 20 30 Temperature, oC

0

1

4

2

3

Alt

itu

de,

km

cloud

boundarylayer

SUBSIDENCE INVERSIONSUBSIDENCE INVERSION

typically 2 km altitude

DIURNAL CYCLE OF SURFACE HEATING/COOLING:DIURNAL CYCLE OF SURFACE HEATING/COOLING:ventilation of urban pollutionventilation of urban pollution

z

T0

1 km

MIDDAY

NIGHT

MORNING

Mixingdepth

Subsidenceinversion

NIGHT MORNING AFTERNOON

VERTICAL PROFILE OF TEMPERATUREVERTICAL PROFILE OF TEMPERATUREMean values for 30Mean values for 30ooN, MarchN, March

Alt

itu

de,

km

Surface heating

Latent heat releaseRadiativecooling (ch.7) - 6.5 K km-1

2 K km-1

- 3 K km-1Radiativecooling (ch.7)

Radiative heating:O3 + hO2 + OO + O2 + M O3+M

heat

SHORT QUESTIONSSHORT 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.

TYPICAL TIME SCALES FOR VERTICAL MIXINGTYPICAL TIME SCALES FOR VERTICAL MIXING

• Estimate time t to travel z by turbulent diffusion:

2

5 2 -1 with 10 cm s2 z

z

zt K

K

0 km

2 km

1 day“planetaryboundary layer”

tropopause

5 km

(10 km)

1 week

1 month

10 years

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