Transcript 03_Atmos_GenCircx

Atmospheric General Circulation
and Climate (Chap 6)
Atmospheric General Circulation
• Also: Global or planetary circulation.
• The general circulation (GC) of the atmosphere is the
totality of motions that characterize the large scale flow
of the atmosphere (e.g., Hadley cells, jet stream)
• Statistical description of wind, temperature, humidity,
precipitation, etc. The mean is an example.
• The GC can be simulated by numerically solving
equations of atmosphere (i.e., climate modeling)
Atmospheric General Circulation
• The energy for the GC comes from the uneven
distribution of solar radiation with respect to latitude
• The motion of the GC is modified by Earth’s rotation
(Coriolis force) and other physical effects (e.g., heating,
friction, turbulence)
• Effects of the GC:
– Rapid movement and exchange of properties
(compared to ocean or land)
– Redistribution of heat from equator to poles;
redistribution of moisture from oceans to land.
If Earth rotated very slowly
• The sun would heat the
equatorial regions more than
the poles.
• In response, a vertically
oriented circulation cell would
develop in each hemisphere
• Warm air rising at equator,
then radiatively cooling and
sinking at pole.
Lutgens and Tarbuck (2001)
Rotating Earth
• Coriolis force deflects
the flow to the right in
the Northern
Hemisphere and to the
left in the Southern
Hemisphere.
• Amount of deflection is
smallest at equator
and largest at poles.
Rotating Earth
• Idealized circulation on
rotating Earth has three
cells in each hemisphere.
• Coriolis turns the upper
branch of the Hadley
cells to the east,
meaning air does not
reach pole.
• Sinking air reaching
surface spreads out
following Corilois.
Lutgens and Tarbuck (2001)
Idealized Winds and Pressure
Lutgens and Tarbuck (2001)
“Mean Meridional Circulation” (MMC)
MMC
Ferrell
Cell
Hadley
Cell
Polar
Cell
Hadley and polar cells are “thermally direct”, meaning that warm
is is rising and cool air is sinking. Ferrell cells are “thermally
indirect”, weaker, and less well defined.
Two types of atmospheric jet streams
Eddy driven jet (EJ)
(Polar front jet)
More poleward jet
located at polar
front between
polar and Ferrel
cells (driven by
temperature
gradients)
Subtropical jet (SJ)
• At the poleward
boundary of the
Hadley cell
Subtropical jet: angular Momentum
• Parcels of air moving from equator toward pole in
upper branch of Hadley cell speed up to conserve
angular momentum (like ice skater bringing
arms toward body).
• Accounts for
subtropical jet
streams
uϕ (m/s)
ϕ (°N)
Subtropical jet: angular Momentum
• Theoretical wind speeds in the subtropical jet
stream (135 m/s) are much higher than the
observed ones (40 m/s).
• This is because northward motion in the Hadley
cells is slow (~1 m/s). It takes ~1 month for air
parcels to travel from equator to 30ºN.
• Energy losses by friction and other processes have
enough time to reduce the angular momentum of
the air parcels.
Atmospheric Jets
Subtropical jet (SJ)
Eddy driven jet (EJ)
(Polar front jet).
Often hard to
distinguish
Seasonal changes in pressure and wind
Subpolar low
Polar Highs
Equatorial Lows
Subtropical Highs
• Land-Sea contrast and
seasonal effects lead to
longitudinal variations.
Subpolar low(s)
January
Seasonal changes in pressure and wind
July
• Remember: This is a
climatology. A daily weather
map may show a quite
different picture.
Seasonal changes in pressure and wind
Notice that ITCZ shifts about 15 degrees into summer
hemisphere. As ITCZ arrives, it brings the rainy “monsoon”
season.
Seasonal changes in pressure and wind
ITCZ migration is associated
with changes in position
and strength of Hadley
Cells (cell descending in
winter hemisphere is
stronger).
July
30°S
Annual mean (also Fall and Spring)
15°S
0°
15°N
30°N
0°
15°N
30°N
January
30°S
15°S
Energy balance
“change = in – out”
Energy in atmosphere
Types of energy (J/kg)
Percent of total energy
Internal energy
due to T
70%
Potential energy
due to z
27%
Latent energy
due to phase
2.9%
Kinetic energy
due to motion
0.1%
Energy in atmosphere
• Internal energy (IE) and potential energy (PE)
constitute about 97% of the total energy.
• Although KE is small, it is still very important
because it is responsible for motions and
transports.
Energy is converted between
types by circulation. For example,
warm moist air rising in Hadley
cell is conversion from internal
and latent energy to potential
energy.
Atmospheric Energy Balance
Ea=energy of atmospheric column of unit area (1 m2) from
the surface to TOA.
RTOA≈ 0
Atmosphere
Rs
LP
Ea
SH
RTOA : Net radiation into the top of the atmosphere
Rs
: Net radiation into the surface
Ra = RTOA - Rs: net radiative heating of atmosphere
LP
: latent heat release during precipitation
SH
: sensible heat transfer from the surface
ΔFa
: energy leaving column horizontally
Atmospheric Energy Balance
Change in Ea over time is
DEa
= RTOA - Rs + LP + SH - DFa
Dt
=
Ra + LP + SH - DFa
In annual mean,
DFa = Ra + LP + SH
, so
RTOA≈ 0
Atmosphere
Rs
Ea
LP
SH
TS
Atmospheric General Circulation
• Atmosphere moves energy from surplus around equator
toward energy deficit poleward of 60°N and 60°S.
Annual mean zonal mean energy budget
from: Hartmann 1994
Atmospheric General Circulation
• Atmosphere moves energy from surplus around equator
toward energy deficit poleward of 60°N and 60°S.
Annual mean zonal mean energy budget
LP pattern
resembles P pattern
from: Hartmann 1994
Atmospheric General Circulation
• Atmosphere moves energy from surplus around equator
toward energy deficit poleward of 60°N and 60°S.
• Energy exported from
equatorial region
(positive ΔFa)
• Energy imported into
polar regions (negative
ΔFa)
Annual mean zonal mean energy budget
from: Hartmann 1994
Heat transport
• Northward
atmospheric energy
transport is by eddies
(areas of high and low
pressure) near 45°N
and S that move warm
air poleward and cool
air equatorward.
• Hadley cells export
energy from tropics.
Heat transport
• Poleward energy transport peaks where
eddies are most vigorous (~40°N and S)
from: Hartmann 1994
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Practice with
energy balance
1. Verify the energy balance
2. Why are radiative and turbulent heat fluxes
balanced or imbalanced at this latitude?
3. Draw a diagram illustrating what the value of
ΔFa means physically. How is this flow
maintained by the general circulation
(eddies/MMC)?
Walker circulation and monsoons
Walker circulation
• Intense convection over southeast Asian warm
pool, Africa, and South America generates eastwest oriented circulations in the tropics.
• Visible in annual means
• Weakening of Walker cell associated with El Nino
Walker cell
Monsoon circulation
• Thermally direct
circulation (warm air
rising)
• Summer: rising motion
and moisture
convergence over land
= heavy precipitation
• Winter: sinking motion
and moisture
divergence over land =
dry conditions
Monsoon circulations
• Monsoons visible over India, Africa, and the
southwestern US.
JAN
JUL