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

Problem: does not account for Coriolis force. Meridional transport of air between Equator and poles would result in unstable longitudinal motion.

GLOBAL CLOUD AND PRECIPITATION MAPGLOBAL CLOUD AND PRECIPITATION MAP20 Feb 2003 @12Z (intellicast.com)20 Feb 2003 @12Z (intellicast.com)

       

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

QUESTIONSQUESTIONS

1. Is the general atmospheric circulation stronger (i.e., are the winds faster) in the winter or in the summer hemisphere?  2. Is pollution from North America more likely to affect Hawaii in winter or in summer? 3. Concentrations of CO2, krypton-85, and other gases emitted mainly in the northern hemisphere DECREASE with altitude in the northern hemisphere but INCREASE with altitude in the southern hemisphere. Explain.

VERTICAL TRANSPORT: BUOYANCYVERTICAL TRANSPORT: BUOYANCY

• What is buoyancy?

Object (z

z+zFluid (’)

Balance of forces:

buoyancy P gradient gravity

g

Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’

P gradient gravity T T’ would produce bouyant acceleration

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

EFFECT OF STABILITY ON VERTICAL STRUCTUREEFFECT OF STABILITY ON VERTICAL STRUCTURE

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 disrupts that equilibrium and produces an unstable atmosphere:

Initial equilibriumstate: - dT/dz =

z

T

z

T

Solar heating ofsurface: unstableatmosphere

ATM

ATM

z

Tinitial

final

buoyant motions relaxunstable atmosphere to –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

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

SUBSIDENCE INVERSIONSUBSIDENCE INVERSION

typically 2 km altitude

QUESTIONSQUESTIONS

1. An atmosphere with W < -dT/dz < - is called"conditionally unstable" (W is the wet adiabatic lapse rate). Why?  2. Kuwaiti oil fires during the Persian Gulf war produced largeclouds of soot a few km above the Earth's surface. Soot absorbs solarradiation. How would you effect such clouds to affect atmosphericstability? 3. Vertical profiles of concentrations of species emitted at thesurface often show a "C-shape", particularly in the tropics, with highconcentrations in the lower and upper troposphere and lowconcentrations in the middle troposphere. How would you explain such aprofile?

DIURNAL CYCLE OF SURFACE HEATING/COOLING:DIURNAL CYCLE OF SURFACE HEATING/COOLING:

z

T0

1 km

MIDDAY

NIGHT

MORNING

Mixingdepth

Subsidenceinversion

NIGHT MORNING AFTERNOON

FRONTSFRONTS

WARM FRONT:

WARM AIR COLD AIR

WIND Front boundary;inversion

COLD FRONT:

COLD AIRWARM AIR

WIND

inversion

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