Transcript Special Lecture: Circulation

THE HADLEY CIRCULATION (1735): global sea breeze
COLD
Explains:
• Intertropical Convergence
Zone (ITCZ)
• Wet tropics, dry poles
HOT
COLD
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 MAP
20 Feb 2003 @12Z (intellicast.com)
TROPICAL HADLEY CELL
• Easterly “trade winds” in the tropics at low altitudes
• Subtropical anticyclones at about 30o latitude
CLIMATOLOGICAL SURFACE WINDS AND PRESSURES
(January)
CLIMATOLOGICAL SURFACE WINDS AND PRESSURES
(July)
TIME SCALES FOR HORIZONTAL TRANSPORT
(TROPOSPHERE)
1-2 months
2 weeks
1-2 months
1 year
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.
VERTICAL TRANSPORT: BUOYANCY
•
What is buoyancy?
Balance of forces:
 buoyancy   P  gradient   gravity
r  r

g
r
z+Dz
Fluid (r’)
Object (r)
z
Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’

T

P  gradient

gravity
)
T’ would produce bouyant acceleration
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
stable
G = 9.8 K km-1
g
G  dT / dz 
 9.8 K km-1
Cp
z
unstable
inversion
unstable
What happens following release depends on the
local lapse rate –dTATM/dz:
ATM
• -dTATM/dz > G e upward buoyancy amplifies
(observed) 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
T
initial perturbation: atmosphere is stable
• dTATM/dz > 0 (“inversion”): very stable
The stability of the atmosphere against vertical mixing is solely determined
by its lapse rate
EFFECT OF STABILITY ON VERTICAL STRUCTURE
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
ATM
G
z
final
G
ATM
T
Initial equilibrium
state: - dT/dz = G
G
initial
T
Solar heating of
surface: unstable
atmosphere
T
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.
IN CLOUDY AIR PARCEL, HEAT RELEASE FROM
H2O CONDENSATION MODIFIES G
Wet adiabatic lapse rate GW = 2-7 K km-1
z
T
RH
“Latent” heat release
as H2O condenses
RH > 100%:
Cloud forms
GW  2-7 K km-1
G  9.8 K
km-1
100%
GW
G
VERTICAL PROFILE OF TEMPERATURE
Mean values for 30oN, March
Altitude, km
Radiative
cooling (ch.7)
- 3 K km-1
2 K km-1
Radiative heating:
O3 + hn e O2 + O
O + O2 + M e O3+M
heat
Radiative
cooling (ch.7)
- 6.5 K km-1
Latent heat release
Surface heating
SUBSIDENCE INVERSION
typically
2 km altitude
QUESTIONS
1. An atmosphere with GW < -dT/dz < - 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?
DIURNAL CYCLE OF SURFACE HEATING/COOLING:
z
Subsidence
inversion
MIDDAY
1 km
Mixing
depth
NIGHT
0
MORNING
T
NIGHT
MORNING AFTERNOON
FRONTS
WARM FRONT:
WIND
WARM AIR
Front boundary;
inversion
COLD AIR
COLD FRONT:
WIND
WARM AIR
COLD AIR
inversion
TYPICAL TIME SCALES FOR VERTICAL MIXING
•
Estimate time Dt to travel Dz by turbulent diffusion:
Dz )

Dt 
2
2K z
with K z
5
10 cm s
tropopause
(10 km)
10 years
5 km
“planetary 2 km
boundary layer”
0 km
1 month
1 week
1 day
2 -1