CHAPTER 4: ATMOSPHERIC TRANSPORTCHAPTER 4: ATMOSPHERIC TRANSPORTForces in the atmosphere:

• Gravity g• Pressure-gradient• Coriolis • Friction

1 /p P 2 sinc v to R of direction of motion (NH) or L (SH)

f kv

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

Angular velocity ω = 2π/24hWind speed vLatitude Friction coefficient k

Betty Heidler hammer throw

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

Link to current weather map

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• Easterly trade winds in the tropics

But… Meridional transport of air between Equator and poles results in strong winds in the longitudinal direction because of conservation of angular momentum; this results eventually in unstable conditions.

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)

Global cloud cover this morning (visible)Global cloud cover this morning (visible)

Intellicast.com

Global cloud cover this morning (infrared)Global cloud cover this morning (infrared)Bright colors indicate the tallest cloudsBright colors indicate the tallest clouds

Intellicast.com

QuestionsQuestions

1. Would you expect winds to be generally stronger in winter or in summer?

2. The circular air motion of cylones is a consequence of the fictitious Coriolis force in our observation reference frame on a rotating sphere. Yet we are familiar with satellite images of hurricanes (tropical cyclones).  Why does the circular motion persist in the satellite reference frame?

Hurricane Sandy loop

3. Can tropical cyclones cross the Equator?

Cyclone tracks, 1985-2005

Satellites in geostationary orbit

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

VERTICAL TRANSPORT: BUOYANCYVERTICAL TRANSPORT: BUOYANCY

Object (z

z+zFluid (’)

Buoyancy acceleration (upward) :

b pγ = γ - g g

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

T T’ produces buoyant acceleration upward or downward

g

pγ

P(z) > P(z+Δz) pressure-gradient force on object directed upward

Consider an object (density ρ) immersed in a fluid (density ρ’):

For air, aM P

RT so ρ↑ as T↓

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.

TEMPERATURE SOUNDING AT CHATHAM, MATEMPERATURE SOUNDING AT CHATHAM, MAFeb 14, 2013 at 12Z (7 am)Feb 14, 2013 at 12Z (7 am)

weather.unisys.com

Temperature

Dew point

Aadiabaticlapse 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

PlanetaryBoundaryLayer (PBL)depth

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 + h→O2 + OO + O2 + M →O3+M

heat

TYPICAL TIME SCALES FOR VERTICAL MIXINGTYPICAL TIME SCALES FOR VERTICAL MIXING

0 km

2 km

1 dayplanetaryboundary layer

tropopause(10 km)

1 month

10 years

Questions

1. A sea-breeze circulation often produces a temperature inversion. Explain why.

2. A well known air pollution problem is "fumigation" where surface sites downwind of a major pollution source with elevated smokestacks experience sudden bursts of very high pollutant concentrations in mid-morning. Can you explain this observation on the basis of atmospheric stability?

3. 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. How would this work?

Los Angeles smog: sea breeze (“marine layer”) and strong subsidence inversion

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

PlanetaryBoundaryLayer (PBL)depth

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 + h→O2 + OO + O2 + M →O3+M

heat