How do shortwave troughs form?

[TheDopplerEffect]

I understand the operational significance of shortwave troughs, but how do they form? Many resources describe a shortwave trough as being associated with a cold pool of air or cold front aloft. It makes sense that a cold pool results in a region of lowered heights -- a trough; however, from where does this cold pool aloft originate? Is it somehow related to ageostrophic winds (leading to thermal advection) in response to divergence aloft (at the 300 mb or 250 mb level)? Any explanation would be appreciated.

 

Thanks

[ohleary]

I understand the operational significance of shortwave troughs, but how do they form? Many resources describe a shortwave trough as being associated with a cold pool of air or cold front aloft. It makes sense that a cold pool results in a region of lowered heights -- a trough; however, from where does this cold pool aloft originate? Is it somehow related to ageostrophic winds (leading to thermal advection) in response to divergence aloft (at the 300 mb or 250 mb level)? Any explanation would be appreciated.

 

Thanks

 

Have you read this:

 

http://www.nwas.org/digest/papers/2006/Vol30No1/Pg17-Rochette.pdf

[TheDopplerEffect]

Have you read this:

 

http://www.nwas.org/digest/papers/2006/Vol30No1/Pg17-Rochette.pdf

Now I have. Thanks. It did a good job explaining ageostrophic winds and their significance, but it did not address the formation of shortwave troughs. How/why do shortwaves get generated within longwave patterns?

[Rainman]

Now I have. Thanks. It did a good job explaining ageostrophic winds and their significance, but it did not address the formation of shortwave troughs. How/why do shortwaves get generated within longwave patterns?

 

Any perturbation embedded within the flow whose behavior is not governed by planetary wave dynamics is essentially a shortwave.  The growth and evolution of such perturbations depends on their realization of available of baroclinic instability as they interact with the ambient environment.  An example of a perturbation developing within a longwave pattern might be a shear maximum (i.e. shear shortwave/shear max) developing along the cyclonic side of a jet streak.  Of course, the jet itself arises from a mass gradient that is due to temperature differences.  Another example could occur within the frontogenesis region that is often present in strong jet streaks.  The descending branch of a frontogenesis (or even frontolytic) circulation that takes place at 300mb, for example, will pull down stratospheric air, introducing a potential vorticity anomaly into the mean flow.  The behavior of that potential vorticity anomaly is governed by shortwave dynamics as it propagates through the longwave pattern.  Yet another example could simply be the movement of air across the front range of the Rocky Mountains.  This causes stretching along the z-axis, and due to conservation of angular momentum, a shortwave is born.  The poleward transport of any perturbation should also generate shortwaves due to conservation of momentum.

 

That was my valiant attempt at an answer.  The question is harder to answer than you'd think because we tend to simply think of the atmosphere as simply being full of perturbations which are just waiting to grow, which is essentially true.  As meteorologists, we concern ourselves with the growth of baroclinic waves, but not necessarily the ultimate origin of some particular packet of wave energy.  In the very broadest sense, they are a consequence of differentially heating a rotating fluid and the resultant poleward transport of energy (via 2nd law of thermodynamics), in addition to the large scale interaction/momentum transfer between the fluid and the land/mountains/sea (e.g. frictional torques, mountain torques, etc.)

 

I am no longer convinced that I am even qualified to answer your question and I think it made me question my own existence.

[ohleary]

Any perturbation embedded within the flow whose behavior is not governed by planetary wave dynamics is essentially a shortwave.  The growth and evolution of such perturbations depends on their realization of available of baroclinic instability as they interact with the ambient environment.  An example of a perturbation developing within a longwave pattern might be a shear maximum (i.e. shear shortwave/shear max) developing along the cyclonic side of a jet streak.  Of course, the jet itself arises from a mass gradient that is due to temperature differences.  Another example could occur within the frontogenesis region that is often present in strong jet streaks.  The descending branch of a frontogenesis (or even frontolytic) circulation that takes place at 300mb, for example, will pull down stratospheric air, introducing a potential vorticity anomaly into the mean flow.  The behavior of that potential vorticity anomaly is governed by shortwave dynamics as it propagates through the longwave pattern.  Yet another example could simply be the movement of air across the front range of the Rocky Mountains.  This causes stretching along the z-axis, and due to conservation of angular momentum, a shortwave is born.  The poleward transport of any perturbation should also generate shortwaves due to conservation of momentum.

 

That was my valiant attempt at an answer.  The question is harder to answer than you'd think because we tend to simply think of the atmosphere as simply being full of perturbations which are just waiting to grow, which is essentially true.  As meteorologists, we concern ourselves with the growth of baroclinic waves, but not necessarily the ultimate origin of some particular packet of wave energy.  In the very broadest sense, they are a consequence of differentially heating a rotating fluid and the resultant poleward transport of energy (via 2nd law of thermodynamics), in addition to the large scale interaction/momentum transfer between the fluid and the land/mountains/sea (e.g. frictional torques, mountain torques, etc.)

 

I am no longer convinced that I am even qualified to answer your question and I think it made me question my own existence.

 

[TheDopplerEffect]

Any perturbation embedded within the flow whose behavior is not governed by planetary wave dynamics is essentially a shortwave.  The growth and evolution of such perturbations depends on their realization of available of baroclinic instability as they interact with the ambient environment.  An example of a perturbation developing within a longwave pattern might be a shear maximum (i.e. shear shortwave/shear max) developing along the cyclonic side of a jet streak.  Of course, the jet itself arises from a mass gradient that is due to temperature differences.  Another example could occur within the frontogenesis region that is often present in strong jet streaks.  The descending branch of a frontogenesis (or even frontolytic) circulation that takes place at 300mb, for example, will pull down stratospheric air, introducing a potential vorticity anomaly into the mean flow.  The behavior of that potential vorticity anomaly is governed by shortwave dynamics as it propagates through the longwave pattern.  Yet another example could simply be the movement of air across the front range of the Rocky Mountains.  This causes stretching along the z-axis, and due to conservation of angular momentum, a shortwave is born.  The poleward transport of any perturbation should also generate shortwaves due to conservation of momentum.

 

That was my valiant attempt at an answer.  The question is harder to answer than you'd think because we tend to simply think of the atmosphere as simply being full of perturbations which are just waiting to grow, which is essentially true.  As meteorologists, we concern ourselves with the growth of baroclinic waves, but not necessarily the ultimate origin of some particular packet of wave energy.  In the very broadest sense, they are a consequence of differentially heating a rotating fluid and the resultant poleward transport of energy (via 2nd law of thermodynamics), in addition to the large scale interaction/momentum transfer between the fluid and the land/mountains/sea (e.g. frictional torques, mountain torques, etc.)

 

I am no longer convinced that I am even qualified to answer your question and I think it made me question my own existence.

 

I think I got it now. Thank you for the detailed explanation.