Transcript met60-topic01 - Department of Meteorology and Climate Science

MET 60
Fall, 2009
8/23/09
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Welcome Back Party!
This Thursday 2-4 in here
snacks, drinks, prizes
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Flow of classes in MET major…
• Freshman year: MET 10
– Qualitative survey (no math)
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Flow of classes in MET major…
• Freshman year: MET 10
– Qualitative survey (no math)
• Sophomore year: MET 60,61
– Quantitative survey (with math)
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Flow of classes in MET major…
• Freshman year: MET 10
– Qualitative survey (no math)
• Sophomore year: MET 60,61
– Quantitative survey (with math)
• Junior year: MET 121A,B (dynamics), 124
– Fluid dynamics (lots of math)
– Physical Met (math & physics)
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Flow of classes in MET major…
• Freshman year: MET 10
– Qualitative survey (no math)
• Sophomore year: MET 60,61
– Quantitative survey (with math)
• Junior year: MET 121A,B (dynamics), 124
– Fluid dynamics (lots of math)
– Physical Met (math & physics)
• Senior year: MET 171A,B (synoptics)
– Uses MET 121A,B etc. (no so mathy)
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Goals of MET 60, 61…
1)
Survey the material


2)
Show you that an understanding of the
atmosphere is gained by using principals of
physics, as well as math skills.



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More depth than in MET 10
Less depth than in MET 121,124 etc.
Often also need computing skills
Analysis of vast amounts of data
Computer simulation of atmospheric phenomena
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Review/overview
Atmospheric Science (Meteorology) – the study of
atmospheres, their structure, behavior and evolution.
Also the study of atmospheric phenomena – their
structures and behavior.
Mid-latitude storms, El Niño, Santa Ana wind,
thunderstorms, climate change etc.
A young science – still dynamic and growing!
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Review/overview
• Read Chapter 1 before Thursday class
– Thursday’s class this week is at 9am.
• Read Chapter 2 before next Tuesday class
• Everything in Cht. 1 will be “developed” in the rest of
the book!
• Let’s use the figures to guide us through…
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Forecasting (1.1):
Skill much improved since 1980 – why?
• Better observing, especially via satellites which can
measure:
– Visible (clouds/no clouds)
– IR (cloud top temperature) 
• cloud top height
• cloud depth
• Cloud type
• Better understanding of physical processes
• Improved modeling capabilities
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Ozone Hole (1.2):
Ozone Layer in lower stratosphere (Fig. 1.9) – a thin shell
of enhanced O3 concentrations
The Ozone “hole” refers to decreased concentrations in
this shell.
Due to man-made substances (e.g., CFCs) + natural
physical processes (p.191)
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Greenhouse gases (1.3):
Shows without a doubt that greenhouse gases levels are
rising (Co2, CH4 etc.)
Radiative heating theory suggests this should lead to
global warming (Cht. 4).
Sophisticated models also all suggest global warming
(Cht. 10).
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Geometry (1.4):
Assume earth is a pure sphere (it isn’t).
Be familiar with these symbols:
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
Longitude

Latitude
r
Radial distance out from center of earth
Re
Radius of earth (we also use the symbol “a”).
NOTE: r = Re + z, z = altitude above surface.
Mostly, r  Re
x
Distance increasing EAST
y
Distance increasing NORTH
z
Distance increasing UP
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Geometry continued:
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u
East-west wind.
u = dx/dt and u > 0 for EASTWARD (westerly)
motions.
v
North-south wind.
v = dy/dt and u > 0 for NORTHWARD (southerly)
motions.
w
Vertical wind.
w = dz/dt and w > 0 for UPWARD motions.
t
Time!
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Geometry continued:
The quantity
T
x
is a derivative.
It represents a gradient – how much “T” changes as “x”
changes.
If T varies in response to variations in x,y,z, and t (time),
then the derivative means “how much “T” changes as
“x” changes, while “y”, “z”, and “t” are not changing”.
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Geometry continued:
Remember the relationship:
dT T
T
T
T

u
v
w
dt
t
x
y
z
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The atmosphere is thin! (1.5):
Earth radius Re = 6371 km
Circumference is 2Re = ___________
Depth of the troposphere is roughly 10 km.
Deeper in tropics (warmer!) – shallower near poles (colder!)
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Clouds are pretty! (1.7):
Solid layer near coast.
Broken up further offshore.
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Density falls off exponentially with height (1.8):
The atmosphere is compressible – you can squish it!
It is denser at lower levels – less dense as we rise up.
Likewise the air pressure gets lower as we rise up.
Globally-averaged sea-level air pressure is _____________ mb
Or _____________ hPa.
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Density/pressure…
Recall from physics:
pressure = force per unit area
p =F/A
force = mass x acceleration
 F = mg,
And:
since g = acceleration due to gravity (9.81 m/s2)
Putting together:
p = mg / A
So air pressure is the weight (“mg”) of air above you.
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Density/pressure…
Air pressure is the weight of air above you.
As you go up, there is less air above you
 air pressure must decrease as you go up.
How rapidly?
To good approximation:
p ( z )  po ( z )e  z / H
where p(z) is the air pressure at altitude z meters above sea level
(ASL), and po is the air pressure at sea level (1000 mb), and H is called
the scale height – the depth over which pressure decreases by a factor
of “e” (i.e., about 2.73). In the troposphere, H  8 km.
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Layers of the atmosphere (1.9):
Layers are based on temperatures and lapse rates.
Average tropospheric lapse rate is…
6.5 C/km
(meaning it cools by 6.5 C every one km up you go).
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Layers of the atmosphere
Troposphere
•
•
•
•
•
•
•
All our weather
All our clouds (but…noctilucent clouds)
The jet stream (but…)
Air-sea interaction
Most of the water (vapor, liquid, solid)
Temperatures generally decrease with height
Except in an inversion – common in the Bay Area!
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Layers of the atmosphere
Stratosphere
•
•
•
•
•
Temperature stops dropping with height
Temperature starts increasing with height!
Where is the heat source?????????
Ozone!!!!!!!!!!!!
The ozone layer is in the stratosphere (around 25 km).
Tropopause
• Boundary between troposphere & stratosphere.
• Acts as a (leaky) lid to the troposphere (Fig. 1.10).
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Global winds and temperatures (1.11):
•
•
•
•
•
•
Note where it’s cold and warm:
Winter pole in trop & strat
Summer pole at mesopause!
Equatorial tropopause!
Warm at summer stratopause.
Complicated!
• Strong west  east jet in winter stratosphere.
• Weaker east  west jet in summer stratosphere.
• Westerly jets in troposphere
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Winds:
•
•
•
•
•
Storms of many scales embedded in the flow.
Largest storms are synoptic-scale waves.
Warm and cold fronts.
Mid-latitude cyclones (Fig. 1.12).
Arise due to an instability in the flow – baroclinic instability.
• Next: tropical cyclones = hurricanes (Fig. 1.13).
• Gain energy from the warm ocean.
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Winds:
• Winds blow whenever there are pressure gradients.
• We show pressures on weather maps via isobars = lines of constant
pressure.
• Strong pressure gradient  strong winds (isobars close together).
• Winds away from the surface blow roughly parallel to isobars except near the equator.
• Winds near the surface blow inward towards lower pressure.
• Northern hemisphere (NH) winds blow counterclockwise around a
“L” – and vice versa around a “H”.
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Global winds (1.15):
• Imagine an “aquaplanet” – water-covered, no mountains.
• Due to rotation and the sun’s heating, we would get:
–
–
–
–
Equatorial “L”
Sub-tropical “H”
Mid-latitude “L”
Polar “H”
• And…
– Trade winds
– Inter-tropical Convergence Zone (ITCZ…cloud band)
– Mid-latitude westerlies
– Polar easterlies
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Global winds (1.15):
• One impact of land masses is to break zones into cells.
• Example: summer “H” cells over oceans (e.g., Pacific High).
• Example: winter “L” cells over oceans (e.g., Icelandic Low,
Aleutian Low).
Observed winds (1.18 from satellites, 1.19 from “models”):
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Smaller-scale winds (1.15):
• Ahrens’ figure 7.2
• Motions all the way down to turbulence – Fig 1.23
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Precipitation (1.25, 1.26):
• Consider the relationship between rainy areas and surface air
motions.
• Consider the relationship between dry areas and surface air
motions.
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