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Author Topic: What is baroclinicity?  (Read 5961 times)

Offline Umby

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What is baroclinicity?
« on: 29/09/2009 21:58:46 »
On 28/04/2008 I read an excellent reply by guest "paul.fr" on baroclinic waves that left me with some questions of my own.
As I understand baroclinic instability it is merely a surface thermal gradient or a boundary between different air masses (front).  This translates to an upper level jet above (around 300mb). Things are unstable but no cyclogenisis yet, right(1)? Then like a blasting cap hits and ignites dynamite, along comes a short-wave through the jet. And here's where I get lost: this causes warm air to advect up and into the cold air mass, and cold air advect down and into the warm air. Why(2)?  Is there a real life analogy to this process that might make it a little more intuitive(3)?  (Maybe some 3-D diagrams on the web that might make it more understandable). Also how can temperature contours be out of phase with pressure contours? Aren't temperature and pressure directly related i.e temperature drives the pressure(4)?
I have other questions regaring potential temperature and density but I have to make sure I am on the right track based on your answers. Thanks!
« Last Edit: 30/09/2009 14:34:42 by Umby »


 

Offline SkepticSam

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Re: What is baroclinicity?
« Reply #1 on: 30/09/2009 06:26:37 »
Quote
this causes warm air to advect up and into the cold air mass, and cold air advect down and into the warm air. Why(2)?


Isn't this to do with static instability and statically unstable vertical temperature gradients?

The air at lower altitude is warm (too warm) and the higher air is too cold. Convection results from this as a means of removing the excess heat and the sinking air is the now cooled air in a sort of overflow? Maybe.

Differences in latitudinal temperatures from pole to equator is baroclinic instability. The tropics heat up because of incoming solar radiation, you have a temperature gradient between the heated tropics / equator and the cold poles the causes creates an outflow of air to modify the temperatures at both extremes.

What do you think?

Links:
static stability [nofollow]

Quote
static stability—(Also called hydrostatic stability, vertical stability.) The ability of a fluid at rest to become turbulent or laminar due to the effects of buoyancy.
A fluid, such as air, tending to become or remain turbulent is said to be statically unstable; one tending to become or remain laminar is statically stable; and one on the borderline between the two (which might remain laminar or turbulent depending on its history) is statically neutral. The concept of static stability can also be applied to air not at rest by considering only the buoyant effects and neglecting all other shear and inertial effects of motion. However, if any of these other dynamic stability effects would indicate that the flow is dynamically unstable, then the flow will become turbulent regardless of the static stability. That is, turbulence has physical priority, when considering all possible measures of flow stability (e.g., the air is turbulent if any one or more of static, dynamic, inertial, barotropic, etc., effects indicates instability). Turbulence that forms in statically unstable air will act to reduce or eliminate the instability that caused it by moving less dense fluid up and more dense fluid down, and by creating a neutrally buoyant mixture. Thus, turbulence will tend to decay with time as static instabilities are eliminated, unless some outside forcing (such as heating of the bottom of a layer of air by contact with the warm ground during a sunny day) continually acts to destabilize the air. This latter mechanism is one of the reasons why the atmospheric boundary layer can be turbulent all day.
 

Offline SkepticSam

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What is baroclinicity?
« Reply #2 on: 30/09/2009 18:18:02 »
Oops. I wrote some nonsense about convection in the above post. No idea where that came from.

As you say, we are talking advection.

When you have a breakdown in the relationship between temperature and pressure accross the hemespheres the isotherms may not run parallel with the isobars and temperature will vary along the isobar. This causes the wind to blow at an angle to the isotherms resulting in either cold or warm air advection.

Warm air advection is when warm air is blown towards the poles and cold air advection is when cold air is blown towards the equator. This now leads to a "baroclinic condition". A condition that provides the energy for the creation of midlattitude depressions.  
 

Offline Umby

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What is baroclinicity?
« Reply #3 on: 30/09/2009 20:32:27 »
Thanks Sam that's starting to make more sense.  But why does a temperature gradient "on" a constant pressure surface cause wind?  As I write this there is this identical situation in Colorado (20:30 GMT 30/9/2009).  The isotherms south of the developing low are at angles with the height contours.  I can see it: the wind is blowing warm air north and cold air south (advecting), but I can intuitivley understand why.
 

Offline SkepticSam

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What is baroclinicity?
« Reply #4 on: 02/10/2009 05:36:16 »
Isn't it due to the "thermal wind" or geostropic wind sheer?

Here are a few links that may or may not help us with the question.

wind sheer - wikipedia [nofollow]

thermal wind - wikipedia [nofollow]

baroclinic instability - PDF File [nofollow]

Dynamics of rotating fluids [nofollow]

One other term that we have not used is "discontinuity" of temperature. A short descriptive is: "... The disconuity of temperature between different air masses disturbs the relationship between temperature and pressure accross each hemisphere. Isotherms may not run parallel with isobars. Temperature (and hence the thickness of the troposphere above) will now vary along an isobar. This means that the wind will now blow at an angle to the isotherms..." this is a baroclinic condition.   


 

Offline Umby

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What is baroclinicity?
« Reply #5 on: 05/10/2009 16:29:44 »
So (thermal) wind blows around a temperature mass synonymous with the way a geostrophic wind blows around a pressure center?
 

Offline SkepticSam

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What is baroclinicity?
« Reply #6 on: 05/10/2009 19:20:42 »
That's how I understand it. But I would advise you check with a tutor before accepting it as fact.
 

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What is baroclinicity?
« Reply #6 on: 05/10/2009 19:20:42 »

 

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