Transcript Document

CHAPTER 4: ATMOSPHERIC TRANSPORT
Forces in the atmosphere:
• Gravity g
• Pressure-gradient γp   1/  P
• Coriolis  c  2v sin  to R of direction of motion (NH) or L (SH)
• Friction γ f  kv


Equilibrium of forces:
In vertical: barometric law
p
P
In horizontal: geostrophic flow parallel to isobars
v
c
P + DP
In horizontal, near surface: flow tilted to region of low pressure
p
f
v
c
P
P + DP
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 MAP
Highs, Lows, fronts, winds, cloud cover
THE HADLEY CIRCULATION (1735): global sea breeze
COLD
HOT
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)
shows Intertropical Convergence Zone (ITCZ) as longitudinal band near Equator
Bright colors indicate high cloud tops (low temperatures)
Today
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
SHORT 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: BUOYANCY
Buoyancy:
z+Dz
Fluid (’)
γb = γp - g
  

g

Object (
z
Barometric law assumed T = T’ e
T

b = 0 (zero buoyancy)
T’ produces buoyant acceleration upward or downward
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.
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 and radiative cooling from the atmosphere
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/radiative
cooling of air:
unstable atmosphere
T
buoyant motions relax
unstable atmosphere
back towards –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
4
Altitude, km
3
cloud
2
boundary
layer
1
0
-20
-10
0
10
Temperature, oC
20
30
SUBSIDENCE INVERSION
typically
2 km altitude
DIURNAL CYCLE OF SURFACE HEATING/COOLING:
ventilation of urban pollution
z
Subsidence
inversion
MIDDAY
1 km
G
Mixing
depth
0
NIGHT
MORNING
T
NIGHT
MORNING AFTERNOON
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
SHORT 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 MIXING
•
Estimate time Dt to travel Dz by turbulent diffusion:
Dz 

Dt 
2
2K z
with K z
~
105 cm2s-1
tropopause
(10 km)
10 years
5 km
“planetary 2 km
boundary layer”
0 km
1 month
1 week
1 day