met 61 introduction to meteorology - lecture 10
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MET 61 Introduction to Meteorology - Lecture 10. Atmospheric Dynamics Dr. Eugene Cordero Ahrens: Chapter 9 W&H: Chapter 7, pg 271-296 Class Outline: Principle forces in the atmosphere Pressure gradient Coriolis Geostrophic wind. Atmospheric forces. - PowerPoint PPT PresentationTRANSCRIPT
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MET 61 Introduction to Meteorology - Lecture 10
Atmospheric DynamicsDr. Eugene CorderoAhrens: Chapter 9
W&H: Chapter 7, pg 271-296 Class Outline:
Principle forces in the atmosphere Pressure gradient Coriolis Geostrophic wind
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Atmospheric forcesAtmospheric forces
Fundamental Forces in the atmosphereFundamental Forces in the atmosphere– Pressure Gradient ForcePressure Gradient Force– GravityGravity– Rotation of the Earth Rotation of the Earth – FrictionFriction
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PressurePressure
Ultimately responsible Ultimately responsible for our weatherfor our weather
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Horizontal Pressure ChangesHorizontal Pressure Changes
Determines the direction and speed of winds:Determines the direction and speed of winds:– Predominate force in atmospheric flowsPredominate force in atmospheric flows
Can help explain general circulation of Can help explain general circulation of atmosphere.atmosphere.
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Pressure Gradient ForcePressure Gradient Force
Pressure gradient: Pressure gradient: – Dependent on spacing between isobarsDependent on spacing between isobars– Dense or tight clustering of isobars - strong or large Dense or tight clustering of isobars - strong or large
pressure gradientpressure gradient– Weak clustering of isobars - weak pressure gradientWeak clustering of isobars - weak pressure gradient
Pressure gradient directed from high to low pressurePressure gradient directed from high to low pressure
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Reading a weather mapReading a weather map
Orient yourself (location, date and time)Orient yourself (location, date and time)
Identify what you are looking atIdentify what you are looking at
Determine the interval of the fieldDetermine the interval of the field
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1.1. At what local time is this map valid?At what local time is this map valid?2.2. What fields are we looking at?What fields are we looking at?3.3. Indicate the direction of the pressure gradient force at points A-C.Indicate the direction of the pressure gradient force at points A-C.
• AA
• CC
• BB
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Atmospheric Thickness
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Hypsometric EquationHypsometric Equation Combination of ideal gas law with hydrostatic balance.
Relates atmospheric thickness with average temperature.
Thickness of atmosphere relates to difference between two atmospheric layers; z t (m) = thickness between two pressure levels
2
112 lnz
p
pT
g
Rz d
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knotsknots
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The rotation of the EarthThe rotation of the Earth
Rockets, migrating birds, and large scale weather Rockets, migrating birds, and large scale weather systems are all deflected due to the rotation of the systems are all deflected due to the rotation of the Earth.Earth.
The Earth’s rotation causes bothThe Earth’s rotation causes both– Translational movementTranslational movement– Rotational movement Rotational movement
The The Coriolis ForceCoriolis Force is the name of this rotational is the name of this rotational force that deflects motion.force that deflects motion.
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Coriolis ForceCoriolis Force
• Affects direction, not speed of object• Maximum at the poles• Zero at the equator (only translational
movement)
Fc=2v sin
- omega - Earth’s rotational rate =360 degrees/24 hours or
v - wind speed - latitude
2 radians/86400 seconds=7.27x10-5 s-1
Calculate Coriolis force for wind moving at
10m/s
2(7.27x10-5 s-1)(10m/s)(sin37)=8.8e-4 m/s2
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Coriolis Force
Take home message:– N. Hem - deflects air to the
right– S. Hem - deflects air to the
left– Relatively small
acceleration, thus requires long periods of time to influence motion.
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Geostrophic balance
Geostrophic balance is balance between:
Pressure gradient force and
Coriolis force
Result: flow of air is parallel to isobars
friction is assumed to be zero
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Geostrophic Wind exampleGeostrophic Wind example
L
H
1000 mb
1004 mb
1008 mb
Pressure Gradient Force
Geostrophic Wind
Coriolis Force
Northern Hemisphere
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• AA
• CC
• BB
1.1. Indicate with arrows the pressure gradient and Coriolis force at A, B and C.Indicate with arrows the pressure gradient and Coriolis force at A, B and C.2.2. Indicate the direction of the wind at each point.Indicate the direction of the wind at each point.3.3. Which point do you think the wind will be stronger?Which point do you think the wind will be stronger?
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Geostrophic WindGeostrophic Wind
• Assume friction is zero• Flow is parallel to isobars• Balance between pressure gradient and
Coriolis force
- density, f - Coriolis parameter (=2 sin Vg - geostrophic wind speedd – distance between isobars p – pressure difference
d
p
fVg
1
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http://www.met.sjsu.edu/weather/avn.html
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Estimate the geostrophic wind speed for this situation
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http://www.met.sjsu.edu/weather/avn.html
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Geostrophic Wind with Geostrophic Wind with FrictionFriction
L
H
1000 mb
1004 mb
1008 mb
Pressure Gradient Force
Geostrophic Wind
Coriolis Force
Northern Hemisphere
Friction
Friction decreases speed of wind, thusFriction decreases speed of wind, thusCoriolis force is weaker.Coriolis force is weaker.
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•A
•B
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•A
•B
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Terminology
Cyclone: Anticyclone:
At the surface, pressure cells are often closed. However, at higher altitudes, pressure cells are often elongated, forming ridges and troughs. – Low pressure systems -– High pressure systems -
refers to closed low pressure systemrefers to a closed high pressure system
Troughs
Ridges
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Upper atmosphere pressure gradients
Meteorologist often examine the upper level pressure gradients to determine the prevailing weather conditions.
However, it is not convenient to simply calculate the pressure gradient because of it’s dependence on density.
Rather, meteorologist calculate the height of a particular pressure surface. The slope of these heights determines the pressure gradient force.
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Quiz 3: Part A
1. Write down the component form of the Geostrophic wind.
2. Explain the difference between the total derivative and the local derivative, and show how these are different mathematically.
Pf
kVg 1ˆ
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Quiz 3: Part B
1. Write down the component form of the momentum equations. Be sure to show all your work (how you got each term).
2. For each term in your above equations, provide a physical description (explanation) for what it means.
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3. Indicate the temperature advection at points A and B. Justify your answer.
• B
• A
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Activity 9: Due April 11th
1.Use links found on the department web page to explore wind speed and direction from maps of model output such as shown in class. From these maps, calculate geostrophic wind using either pressure or height information (if you use height , then use equations given on pg 188-189 of Stull). Compare your answer with model wind information (isotachs). Show calculations and maps from at least two locations. 2. Compute by how much a soccer ball will be deflected during a 12m penalty kick due to the coriolis force.