Download 02-21-2014 Global Wind Systems

REMINDERS:
Problem Set 3: Due Today
Midterm 2: Friday, February 28
- lecture material covering chapters 6, 7, and 15
(since first midterm and through Wed lecture)
- same Format as first midterm
UPCOMING REVIEW SESSIONS:
- Thursday, Feb 27, 6:30-8:00pm in HSS 1330
Thermal Circulations:
• Circulation set up by change in air temperature
Two columns with equal mass (same surface pressure)
- one warm and the other is cool
- warm column expands (air is less dense)
- cold column contracts (air is more dense)
warm air
atmospheric
columns of air
cold air
PA
PB
Psurf=1000mb
Psurf=1000mb
More mass above level Z on warm side compared to cold side
Since Pressure ∝ mass above:
at level Z, PB > PA
height = Z
THERMAL CIRCULATION
L
H
H
L
surface
COLD
WARM
THERMAL CIRCULATIONS
Sea/Land Breezes - Daily timescale
Monsoons - Seasonal timescale
Clicker Question
Set Frequency to "AD"
We know that the Earth is warmer at the equator and colder
at the poles. Assume for the moment, there is no Coriolis force.
Which way would you expect the surface wind to blow at X?
A
(A) Southerly (poleward)
X
D
B
(B) Westerly
Equator
C
(C) Northerly (equatorward)
(D) Easterly
Clicker Question
Set Frequency to "AD"
We know that the Earth is warmer at the equator and colder
at the poles. Assume for the moment, there is no Coriolis force.
Which way would you expect the surface wind to blow at X?
A
(A) Southerly (poleward)
X
D
B
(B) Westerly
Equator
C
(C) Northerly (equatorward)
(D) Easterly
Single Cell Model
Originally proposed by G. Hadley
"Hadley Cell"
- warm air at equator rises
- goes poleward at higher levels
- sinks at poles
- moves equatorward at the surface A (Very Large) Thermal Circulation
HOWEVER,
Due to the Earth's rotation, the single cell model is
too simplified and would result in extremely strong
winds due to Coriolis force and become unstable.
A more realistic view of the global circulation is
provided by the "3-Cell Model"
- Certainly not perfect, but does explain many
features of the global circulation system
NP
EQ
SP
3-Cell Model
- no land mass
- time averaged
- no seasons
Polar Cell
NP
Ferrel Cell
60
30
Hadley Cell
EQ
SP
3-­‐Cell Circula,on System Polar Cell
SURFACE
WINDS
NP
Ferrel Cell
60
30
Hadley Cell
EQ
SP
Polar Cell
NP
Ferrel Cell
60
30
Hadley Cell
EQ
TRADEWINDS
-Northeasterly in NH
SP
Polar Cell
NP
Westerlies
Ferrel Cell
60
- 30-60 Mid-latitudes
30
Hadley Cell
EQ
SP
Polar Cell
Polar
Easterlies
NP
Ferrel Cell
60
30
Hadley Cell
EQ
SP
Similar System of Cells
in Southern Hemisphere
Polar Cell
NP
Ferrel Cell
60
30
Hadley Cell
EQ
Hadley Cell
30
Ferrel Cell
60
SP
Polar Cell
Tradewinds
Island of Hawai'i
NE Tradewinds
Most rainfall on
east side of island
Satellite Image
Surface LOWS where
air is RISING
60
L
Polar Cell
NP
L
RISING AIR
Ferrel Cell
L
30
Hadley Cell
L
L
EQ
L
RISING AIR
Hadley Cell
30
60
L
L
Ferrel Cell
L
RISING AIR
SP
Polar Cell
SINKING AIR
Surface HIGHS where
air is SINKING
NP
H
L
60
Polar Cell
L
Ferrel Cell
L
SINKING AIR
H
30
H
H
Hadley Cell
L
L
EQ
L
Hadley Cell
H
30
H
H
SINKING AIR
60
L
L
Ferrel Cell
L
H
SINKING AIR
SP
Polar Cell
Clicker Question
Set Frequency to "AD"
In general, what sort of surface conditions would you expect at about 30° where the Hadley cell meets the Ferrell cell?
(A) stormy, cool and moist with much precipitation
(B) few storms, warm and dry with little precipitation
Clicker Question
Set Frequency to "AD"
In general, what sort of surface conditions would you expect at about 30° where the Hadley cell meets the Ferrell cell?
(A) stormy, cool and moist with much precipitation
(B) few storms, warm and dry with little precipitation
Desert Regions
Desert Regions
30°˜N
30°˜S
ITCZ
Intertropical Convergence
Zone (ITCZ): Region of
NP
Cumulus Towers and
H
Precipitation
60
L
L
H
30
Polar Cell
Ferrel Cell
L
H
H
Hadley Cell
ITCZ
L
L
EQ
L
Hadley Cell
H
30
60
H
L
L
H
Ferrel Cell
L
H
SP
Polar Cell
ITCZ
ITCZ
Polar Front: Where
Westerlies meet Polar
Easterlies
L
60
Polar Front
H
30
Polar Cell
NP
H
L
Ferrel Cell
L
H
H
Hadley Cell
ITCZ
L
L
EQ
L
Hadley Cell
H
30
H
H
Polar Front
60
L
L
Ferrel Cell
L
H
SP
Polar Cell
Simulated Water Planet Reality with Land Masses Because of land masses, subtropical highs and polar lows are not con,nuous bands Simulated Water Planet Reality with Land Masses par,cularly true in N. Hem where there is more land mass Because of land masses, subtropical highs and polar lows are not con,nuous bands Average Pressure Patterns in July
Average Pressure Patterns in January
In Northern Hemisphere: Highs Pacific High Bermuda High Lows Aleu,an Low Icelandic Low In Northern Hemisphere: Highs Lows Pacific High Aleu,an Low Bermuda High Icelandic Low weaken and strengthen and WINTER shiJ south shiJ south In Northern Hemisphere: Highs Lows Pacific High Aleu,an Low Bermuda High Icelandic Low weaken and strengthen and WINTER shiJ south shiJ south strengthen and weaken and SUMMER shiJ north shiJ north Semi-permanent Subtropical Highs
Pacific High
- Pacific High is an important factor
for California's climate
Bermuda High
U.S. West vs. East Coast Weather/Precipitation
34°N
34°N
U.S. West vs. East Coast Weather/Precipitation
Why so little
precipitation
during summer
in So Cal?
34°N
34°N
Clicker Question
Set Frequency to "AD"
When are the semi-permanent high pressure systems known as the
"Pacific High" and "Bermuda High" at their maximum intensity?
(A) Summer
(B) Winter
(C) Equal at all times of the year
Clicker Question
Set Frequency to "AD"
When are the semi-permanent high pressure systems known as the
"Pacific High" and "Bermuda High" at their maximum intensity?
(A) Summer
(B) Winter
(C) Equal at all times of the year
West US:
Prevailing wind is from the Northwest
bringing cool and moist air onto land.
Because air is cool, still not much actual
water vapor in air, so over land RH is low.
=> inhibits thunderstorm development
East US:
Prevailing wind is from the Southeast
bringing warm and moist air onto land.
Because air is warm there is large
amount of water vapor in air, so over land
RH is high.
=> good for thunderstorm development
West US:
Prevailing wind is from the Northwest
bringing cool and moist air onto land.
Because air is cool, still not much actual
water vapor in air, so over land RH is low.
=> inhibits thunderstorm development
East US:
Prevailing wind is from the Southeast
bringing warm and moist air onto land.
Because air is warm there is large
amount of water vapor in air, so over land
RH is high.
=> good for thunderstorm development
Also important: in summer ocean water off West coast (50-70°F)
much colder than water off East coast (70-85°F)