Lecture11 oct16-bb
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Transcript of Lecture11 oct16-bb
Lecture 11
1
Gradient windAtmospheric boundary layer
Geostrophic Wind when the isobars are straight, parallel lines, and the
only two forces acting on a parcel are the PGF and the
CF, then the wind is called geostrophic wind• PGF and CF are equal in magnitude and
opposite in direction• Geostrophic winds are always parallel to
the isobars
Surface500 mb
500 mb
475 mb
CF
PGF Wind
H
L
2
Strength of Geostrophic Wind
geostrophic conditions are that PGF and CF are equal in magnitude and opposite in direction
2 singPGF CF V
2 sing
PGFV
500 mb
475 mb
CF
PGF Wind
H
L
3
strength of geostrophic wind is determined by PGF not by CF, why?
The Coriolis force can only change a parcel's direction, it CAN NOT affect its speed.
4
2 sing
PGFV
Centripetal force and
gradient wind
5
6
Gradient Wind
Geostrophic Wind
counter lines are relatively straight
Counter lines are curvature
Gradient WindCentripetal forceif you attach a string to a ball and swing it in a circular manner, then the force required to keep the ball moving in a circular path is called centripetal force
string
Cent
ripe
tal f
orce
V
2
V
centripital forcer
magnitude of the centripetal force is
V=speed of a parcel (object)r= distance from the center
direction of the centripetal force is directed inward, towards the axis of rotation 7
recall geostrophic wind
500 mb
475 mb
CF
PGFWind
H
L
8
Gradient Wind Centripetal force
Gradient wind is produced by the balance between PGF, CF and centripetal force
PGF
PG
F
PG
F
PGF PGF
PG
F
9
Free troposphereand
Boundary layer
10
above 850 mb level, the flow is either geostrophic wind or gradient wind(free troposphere)
near the surface about 1-2 km (boundary layer), we must include the effect of friction and therefore, the flow is no longer geostrophic or gradient balance
11
Free troposphere without friction
Boundary layer with friction
Q: As a result, which force becomes smaller, the PGF or the CF?
friction has a significant impact on wind in boundary layer (1-2 km)
the atmosphere above the boundary layer is called free troposphere
12
FrictionSlows down the wind, causes winds across isobars. Direction of friction is always in the opposite direction to the wind direction.
Four forces in the atmosphere
1. Pressure gradient force2. Coriolis force3. Centripetal force4. Friction
13
distancehigh lowP P
2 sin( )V 2V
r
kV
which force is not caused by wind?
PGF
which two forces do not affect wind speed?Coriolis force and centripetal
force
at the center of a surface low, the air converges, and then must rise
at the center of a surface high, the air diverges, and must come from aloft due to sinking motion
14
H L
Why temperature is the most important to
understand weather?
What is temperature definition?
Why earth’s radiation is infrared while solar radiation is short-wave?
Wien’s Law
max
3000( )
( )m
T K
The earth’s radiation energy is determined by what?
Stefan-Boltzmann Law4E T
The capacity of air for holding water vapor is determined by what?
0 5 10 15 20 25 30 35 40 45 500102030405060708090
100110120
Temperature (oC)
Satu
rati
on
vap
or
pre
ssu
re (
mb
)
T=30 oC
T=44 oC
T=10 oC
This curve indicates how much water vapor can be held in the air at a given temperature
2
*H OP
Temperature determines the capacity of holding water vapor in the air
2
17.27* 273.3( ) 0.6108
T
TH OP T e
Thermal circulation
Thermal circulation is generated by pressure gradient
Pressure gradient is produced by different heating
RuleCool surface air sinking high surface pressure
Warm surface air rising low surface pressure
Wind is blowing from high pressure to low pressure
CoolHigh P
WarmLow P
Wind
Where is clear sky?
Thermal circulation
1. sea breeze2. land breeze3. Monsoon4. valley breeze5. Katabatic winds6. Chinook7. Santa Ana winds8. Haboos
Scales of Atmospheric Motions
Time and space scale of atmospheric motions
Global scale 5000 km
Synoptic scale 2000 km
Mesoscale 20 km
Microscale 2m
Typical size
Typical life span
23
Sea breeze
Sea breeze1) assume land and sea have same temperature
in the morning (7 AM)2) also assume synoptic phenomena absent
Q: As solar radiation increases in the morning, which surface heats quicker and why?
Q: What is the resultant air flow?
3 PM
90oF 70oF
Wa
rm
Co
ld
Sea breeze
860 mb
900 mb
820 mb
isobars
780 mb
3 PM
90oF 70oF
Wa
rm
Sea breeze
860 mb
900 mb
820 mb
isobars
780 mb
H
LH
L
Co
ld