THE HADLEY CIRCULATION (1735): global sea breeze

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THE HADLEY CIRCULATION (1735) global sea breeze

• 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.

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GLOBAL CLOUD AND PRECIPITATION MAP20 Feb 2003
_at_12Z (intellicast.com)
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TROPICAL HADLEY CELL

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

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CLIMATOLOGICAL SURFACE WINDS AND
PRESSURES(January)
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CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(July)
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TIME SCALES FOR HORIZONTAL TRANSPORT(TROPOSPHERE)
1-2 months
2 weeks
1-2 months
1 year
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QUESTIONS
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.
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VERTICAL TRANSPORT BUOYANCY

• What is buoyancy?

Balance of forces
zDz
Object (r)
Fluid (r)
z
Note Barometric law assumed a neutrally buoyant
atmosphere with T T
T
T would produce bouyant acceleration
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ATMOSPHERIC LAPSE RATE AND STABILITY
Lapse rate -dT/dz
Consider an air parcel at z lifted to zdz 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 gt 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 lt G e downward buoyancy relaxes
  initial perturbation atmosphere is stable
• dTATM/dz gt 0 (inversion) very stable

ATM (observed)
inversion
unstable
T
The stability of the atmosphere against vertical
mixing is solely determined by its lapse rate
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EFFECT OF STABILITY ON VERTICAL STRUCTURE
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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 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 unstable atmosphere
buoyant motions relax unstable atmosphere to
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.

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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 gt 100 Cloud forms
G
G 9.8 K km-1
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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
O3M
heat
Radiative cooling (ch.7)
Latent heat release
- 6.5 K km-1
Surface heating
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SUBSIDENCE INVERSION
typically 2 km altitude
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QUESTIONS
1. An atmosphere with GW lt -dT/dz lt - G is
called "conditionally unstable" (GW is the wet
adiabatic lapse rate). Why?    2. Kuwaiti oil
fires during the Persian Gulf war produced
large clouds of soot a few km above the Earth's
surface. Soot absorbs solar radiation. How
would you effect such clouds to affect
atmospheric stability?   3. Vertical profiles of
concentrations of species emitted at the surface
often show a "C-shape", particularly in the
tropics, with high concentrations in the lower
and upper troposphere and low concentrations in
the middle troposphere. How would you explain
such a profile?
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DIURNAL CYCLE OF SURFACE HEATING/COOLING
z
Subsidence inversion
MIDDAY
1 km
Mixing depth
NIGHT
0
MORNING
T
NIGHT
MORNING
AFTERNOON
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FRONTS
WARM FRONT
WIND
Front boundary inversion
WARM AIR
COLD AIR
COLD FRONT
WIND
WARM AIR
COLD AIR
inversion
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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