12.810 Dynamics of the Atmospherepog.mit.edu/src/810/introduction_1.pdf · An Introduction to...
Transcript of 12.810 Dynamics of the Atmospherepog.mit.edu/src/810/introduction_1.pdf · An Introduction to...
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12.810 Dynamics of the Atmosphere
Course description: Discusses the dynamics of the atmosphere, with emphasis on the large scale.
Instructor: Paul O’Gorman Email: [email protected]: 54-1712
Questions: Drop by, email, and office hour (Wednesday?)
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Course Webpage http://www.mit.edu/~pog/810.html
Class times Tuesday and Thursday 10:30-12
Classroom 54-1623
Prerequisite Fluid dynamics of the atmosphere & ocean (12.800) or instructors permission
Logistics
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Grading: 1. Problem sets (4 in total, 60%)2. Project:
• writeup (≤12 pages+references, 20%)
• class presentations (~15 mins, 20%)
Problem Sets Policy: Collaboration is allowed, but students must write up the problem set on their own.
Project topics: Project topics should be decided at midterm. I will give out topics, or you can come up with a topic yourself in consultation with me.
Assessment: Problem sets and Project
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Schedule (will send out google calendar)
Projects: Project presentations will be in class in the last one or two weeks of semester
Project reports are due on the last day of class (May 12th)
Problem sets: Problem set 1 (out Mar 3rd; due Mar 17th) Problem set 2 (out Mar 17th; due Mar 31st) Problem set 3 (out Mar 31st; due Apr 14th) Problem set 4 (out Apr 14th; due May 5th) (longer problem set)
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Primary Textbook
Textbooks and other resources
An Introduction to Dynamic Meteorology, Holton and Hakim (5th edition)
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Secondary Textbook
Textbooks and other resources
Atmospheric and Oceanic Fluid Dynamics, Vallis (2nd edition)
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Other references:
Textbooks and other resources
Physics of Climate, Peixoto and Oort
Interactive plotting website: http://www.esrl.noaa.gov/psd/cgi-bin/data/composites/printpage.pl
Gridded datasets: www.cdc.noaa.gov/data/gridded/
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1.Hadley-cell dynamics for a zonally symmetric atmosphere (the role of eddies is discussed later in the course)
2.Internal gravity waves: propagation, effect on mean flow, forcing by mountains
3.Potential vorticity, quasigeostrophic dynamics, and Rossby waves includes omega equation for vertical motion
4.Growth of disturbances: wave activity and E-P fluxes, Charney-Stern condition, Eady model, non-modal growth
5.Energetics: concept of available potential energy
6.The general circulation: the role of eddies, transformed Eulerian mean, downward control
Course topics
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F
Plan for the remainder of this introduction
• Sources of observations
• Basic aspects of observed circulations and thermal structure
• Some motivating questions
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Sources of observations
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Observational data for studies of large-scale atmospheric dynamics
• Data sparseness in space and time is a major issue
• Often combine the data with an atmospheric general circulation model (GCM) using data assimilation (includes analysis and initialization)
• Goal is minimization of discrepancy between observations and model variables (e.g., 3D or 4D var)
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We will often refer to reanalysis products
• Reanalysis (e.g., NCEP, NCEP2, ERA40, ERA interim, ERA5, MERRA, NCEP CFSR, 20CR) means that GCM is held fixed over long time period, but observational inputs vary
• Very popular!
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Climate data guide
Timeline of observations assimilated in ERA interim
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Uppala et al, QJRM 2005
Timeline of observations assimilated in ERA40
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Uppala et al, QJRM 2005
Frequency of radiosonde reports
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Uppala et al, QJRM 2005
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Basic aspects of observed circulations and thermal structurewith some motivating questions
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Hadley cells, and subtropical and eddy-driven jets
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(T. Schneider, Fig.1,Ann. Rev. Earth Planet. Sci. 2006)
Mean meridional circulation and contours of angular momentum per unit mass (January; ERA40)
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(ERA40 reanalysis 1980-2001)
Zonal- and time-mean zonal wind (m/s)
Latitude
Sigm
a
2510
5
25
−60 −30 0 30 60
0.2
0.8
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(ERA40 reanalysis 1980-2001)
Latitude
Sigm
a
2510
5
25
−60 −30 0 30 60
0.2
0.8
Subtropical jet“Eddy driven” jet
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(ERA40 reanalysis 1980-2001)
Mean meridional streamfunction (1010 kg s-1): different seasons
Latitude
Sigm
a
4
2−20−4
−60 −30 0 30 60
0.2
0.8
Latitude
Sigm
a
24
−4
−2
−60 −30 0 30 60
0.2
0.8
December-January-February(DJF)
June-July-August(JJA)
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(ERA40 reanalysis 1980-2001)
SON
MAM
Latitude
Sigm
a
10
2−12
−4
−60 −30 0 30 60
0.2
0.8
Latitude
Sigm
a
12 2
−8−4
−60 −30 0 30 60
0.2
0.8
Mean meridional streamfunction (1010 kg s-1): different seasons
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(ERA40 reanalysis 1980-2001)
SON
MAM
Latitude
Sigm
a
10
2−12
−4
−60 −30 0 30 60
0.2
0.8
Latitude
Sigm
a
12 2
−8−4
−60 −30 0 30 60
0.2
0.8
Mean meridional streamfunction (1010 kg s-1): different seasons
2. What determinesextent of Hadley cells?
1. What determinesstrength of Hadley cells?
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Observed thermal structure and gravity wave dynamics
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(ERA40 reanalysis data 1980-2001)
Zonal and time mean temperature (K)
Latitude
Sigm
a
200
250290
260
−60 −30 0 30 60
0.2
0.8December-January-February
(DJF)
Latitude
Sigm
a
290
270
200
250
−60 −30 0 30 60
0.2
0.8
June-July-August(JJA)
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(ERA40 reanalysis data 1980-2001)
Zonal and time mean temperature (K)
Latitude
Sigm
a
200
250290
260
−60 −30 0 30 60
0.2
0.8
Latitude
Sigm
a
290
270
200
250
−60 −30 0 30 60
0.2
0.8
Note flat isotherms in tropics
December-January-February(DJF)
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(ERA40 reanalysis data 1980-2001)
Potential temperature (K)
DJF
JJA
Latitude
Sigm
a
280
260300
330
−60 −30 0 30 60
0.2
0.8
Latitude
Sigm
a
310
280270
−60 −30 0 30 60
0.2
0.8
Increase with height (implies
dry static stability)
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Static stability allows for internal gravity waves: here
forced by mountain
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Trapped lee waves downwind from Hawaiian Islands
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Questions:
1. What determines whether internal gravity waves are vertically propagating or trapped?
2. How do the waves affect the mean flow?
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Large-scale eddies:
Growth of baroclinic eddies, propagation of planetary waves, forcing of vertical motions
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Illustration of large-scale atmospheric flow: satellite water vapor imagery
Animation: Robert Simmon,NASAData: Seviri water vapor (IR)
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Transient (<1 week) eddies in midlatitudes in observations
Lim and Wallace 1991
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Why do eddies grow in midlatitudes?
Figure 5: [Holton Fig 8.10]
15
Most unstable wave in the Eady model
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Figure 5: [Holton Fig 8.10]
15
Why do eddies grow in midlatitudes?Most unstable wave in the Eady model
And what controls the magnitude of
the vertical velocity?
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How can we understand individual cyclogenesis events? (using potential vorticity!)
Ucellini chapter
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Planetary waves in June 2004150hPa 100hPa
How does the zonal wind control the vertical propagation of Rossby waves into the stratosphere?
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30hPa 10hPa
No planetary waves in the stratosphere at that time
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Zonal-mean zonal wind (Jun 2004): transition from westerly to easterly flow at 60hPa
Pres
sure
(hP
a)
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F
Role of eddies in the general circulation
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Eddies transport momentum: How does this determine the surface westerlies
and affect the Hadley cells?
Peixoto and Oort, Fig 11.7
Transient eddies
Total
Northward flux of momentum (m2/s2)
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(Pauluis et al, Science, 2008)
Why is the mean circulation very different when potential temperature is used as a vertical coordinate?
We will explore the closely related Transformed Eulerian Mean (TEM) framework