Download Atmospheric transport

CHAPTER 4: ATMOSPHERIC TRANSPORT
Forces in the atmosphere:
• Gravity g
• Pressure-gradient  p   1/   P
• Coriolis  c  2v sin  to R of direction of motion (NH) or L (SH)
Angular velocity ω = 2π/24h
• Friction  f   kv
Wind speed v
Equilibrium of forces:
In vertical: barometric law
Latitude 
Friction coefficient k
Betty Heidler hammer throw
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
Link to current weather map
THE HADLEY CIRCULATION (1735): global sea breeze
COLD
HOT
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 CELL
• Easterly “trade winds” in the tropics at low altitudes
• Subtropical anticyclones at about 30o latitude
CLIMATOLOGICAL SURFACE WINDS AND PRESSURES
(January)
Global cloud cover this morning (visible)
Intellicast.com
Global cloud cover this morning (infrared)
Bright colors indicate the tallest clouds
Intellicast.com
Questions
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?
Satellites in
geostationary orbit
Cyclone tracks, 1985-2005
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
VERTICAL TRANSPORT: BUOYANCY
Consider an object (density ρ) immersed in a fluid (density ρ’):
γp
Fluid (’)
z+Dz
Object (
z
g
Buoyancy acceleration (upward) :
MaP

RT
For air,

  
γb = γp - g 
g

so ρ↑ as T↓
Barometric law assumes T = T’ e
T
P(z) > P(z+Δz) e pressure-gradient force
on object directed upward
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.
TEMPERATURE SOUNDING AT CHATHAM, MA
Feb 14, 2013 at 12Z (7 am)
Dew point
Temperature
Aadiabatic
lapse rate
weather.unisys.com
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
Planetary
Boundary
Layer (PBL)
depth
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 → O2 + O
O + O2 + M → O3+M
heat
Radiative
cooling (ch.7)
- 6.5 K km-1
Latent heat release
Surface heating
TYPICAL TIME SCALES FOR VERTICAL MIXING
tropopause
(10 km)
10 years
1 month
planetary 2 km
boundary layer
0 km
1 day
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 midmorning. 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:
ventilation of urban pollution
z
Planetary
Boundary
Layer (PBL)
depth
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 → O2 + O
O + O2 + M → O3+M
heat
Radiative
cooling (ch.7)
- 6.5 K km-1
Latent heat release
Surface heating