Transcript g104_class14_atmospheric circulation

Geography 104 - “Physical Geography of the World’s Oceans”
Atmospheric Circulation
- air moves because of pressure differences
- large scale air motion is influenced by Coriolis effect
- horizontal air motion drives ocean circulation
- atmosphere and ocean circulation have similarities
greatest solar heating is in equatorial region
more reflection at high latitudes
Earth’s radius = 6371 km
atmosphere’s thickness ~100 km
so figure not to scale
longer path through atmosphere
at high latitudes
solar radiation at Earth’s surface (W m-2)
cumulonimbus cloud
density, rising air, condensation
mean sea surface T & S
Coriolis Effect (summary)
- Motions in a rotating frame will appear deflected to the right
in the northern hemisphere
- Motions in a rotating frame will appear deflected to the left
in the southern hemisphere
- Motions are not deflected on the equator, greatest
deflection at poles, varies with latitude
vertical structure of atmosphere
air temperature
cools with height
in troposphere
fluid motion is driven by spatial pressure differences
atmospheric circulation in its simplest form
- solar heating maximum at equator decreases air density
- light moist air rises decreasing atmospheric mass, thus lowering pressure
- rising air cools adiabatically
- cooling causes condensation, rain and increases density
- air moves poleward cooling and increasing density
- heavy, cool, dry, air increases atmospheric mass, thus increasing pressure
- pressure difference gives rise to Hadley circulation
Ferrel’s three-cell model of atmospheric circulation
low pressure (warm/wet) at equator
high pressure (cold/dry) at poles
Ferrel’s three-cell model of atmospheric circulation
addition of Coriolis Effect gives global winds
global atmospheric circulation
tropical
vertical air velocity (July average)
rising air
sinking air
Hadley circulation is not zonally uniform
intertropical convergence zone (ITCZ)
more realistic global atmospheric circulation
has meandering subpolar fronts
tropical
Rossby waves dominate at midlatitudes
warm and cold conditions can migrate at mid-latitudes greatly
influencing weather patterns, especially in the United States
global pressure and winds
July
January
Geostrophic Flow
For large scale motion, pressure and Coriolis are
roughly in balance. This is referred to as
geostrophic balance and gives rise to geostrophic
flow.
Stated another way, geostrophic flow arises from
a balance of pressure and Coriolis “forces”.
Geostrophic flow is along lines of constant
pressure and its magnitude is proportional to
the pressure gradient.
global winds and pressure; why we live in the west
July
January
atmospheric circulation over North America
map shows both pressure contours and wind vectors
clockwise or anti-cyclonic rotation around a high in the northern hemisphere
atmospheric circulation over North America
map shows both pressure contours and wind vectors
counter-clockwise or cyclonic rotation around a low in the northern hemisphere
Sea Breeze – another mechanism for (local) winds
global winds and pressure
July
January
Indian Monsoons
Monsoon winds are not in
geostrophic balance because
winds are not along lines of
constant pressure. Thus,
pressure and Coriolis are not
in balance.
Northeast or Winter Monsoon
Southwest or Summer Monsoon
wind
P
C
Northeast - The system that
brought the rain and snow across
the Northeast yesterday will
continue to do so today, but
precipitation should generally be
lighter than it was on Tuesday.
West - Warm temperatures are
expected again across the West
with an upper level ridge
continuing to be the dominant
weather feature in the region. An
upper level trough will begin to
move into the West Coast
tomorrow night and Friday,
breaking down the ridge that's
been in place.
Readings (Ocean and Atmosphere):
Text Chapter 8 (pgs 138 – 151 and 154 - 160)
Reader pgs. 51 - 61
Reader Maps pgs 200 - 204
HW #2 assigned; Due Friday 31 Oct 2008
Midterm on Wednesday 5 Nov 2008