[ncl-talk] Computation of baroclinity in TC environments

George Vandenberghe george.vandenberghe at noaa.gov
Fri Feb 9 08:52:04 MST 2018

```Where the concept is relevant  (with an enormous number of caveats, yes)
is to answer the fundamental question, will TC interaction with
baroclinicity, induce transition to a growing or stable baroclinic
system (often observed) or will the TC perturbation just damp out (also
often observed) with a theoretically expected but hard to actually point
out, increase in baroclinic available potential energy for some future
perturbation to extract.

I am obviously not an expert either (and my M.S is 31 years stale)  and
I did sleep at home last night, not in a Holiday Inn Express but I think
it is a tractable forecast and theoretical modeling problem, again with
the caveats mentioned below.

Casually plugging this into a program without understanding the concepts
is worrisome, also agreed.

Thanks everyone for the response!

On 02/09/2018 10:25 AM, Dennis Shea wrote:
> a person and got the following response:
> --------
> Hi Dennis,
>
> Ha!  When I google Eady growth rate, the first thing that comes up is
> your NCL function and the second thing that comes up are some class
> notes that I wrote when I taught a course.  Apparently we are the
> state of knowledge on Eady growth rate, which is slightly concerning.
>
> Erm, for [1] I don't think the Eady model is intended to tell you
> something about tropical cyclone environments. It's really intended to
> describe the growth of mid-latitude eddies.  Since it's formulated on
> an f-plane, I don't think it's really appropriate for use in tropical
> environments.  That being said, the eady growth rate really is just a
> measure of baroclinicity so it might be fine to determine what the
> baroclinicity is in their tropical cyclone environments.
>
> For [2b] I'd say no too.  But I don't think using equivalent potential
> temperature would work either.  I think you'd have to include a
> diabatic heating term in the thermodynamic equation that's being
> solved to account for the release of heat associated with the
> convergence of moisture and precipitation that accompanies the growing
> wave.  WIth my expert googling skils, I came across this...
> https://iri.columbia.edu/~tippett/pubs/moist.pdf
> <https://iri.columbia.edu/%7Etippett/pubs/moist.pdf>
>
> For [2a] I don't really know the answer.  But I'm also not sure how
> you would measure the real world growth rates.  The eady model is
> intended to describe the growth of the normal modes and I think you
> could compare the predictions with a normal mode calculation using the
> real world basic state but I'm not sure how you would actually measure
> the growth rates of the real world.  I also think it might give you a
> dimensionless growth rate which tells you what scales dominate by
> growing fastest, but I'm not sure to what extent it is something that
> can be compared with the real world. In short, I don't really know.
> -----------
> HTH
> D
>
> On Wed, Feb 7, 2018 at 10:56 AM, George Vandenberghe
> <george.vandenberghe at noaa.gov <mailto:george.vandenberghe at noaa.gov>>
> wrote:
>
>     On 02/07/2018 12:47 PM, Prashanth Bhalachandran wrote:
>
>         Hello all,
>         I want to compare the baroclinity of two tropical cyclone
>         environments. What is the best way to do this using NCL?
>
>         I did some digging and found a function to calculate Eady
>         growth rate in v6.4 . I have never used it thus far, so I want
>         to know what the options are.
>
>         Thank you,
>         Prashanth
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>
>     Okay this is tangential to the topic but VERY INTERESTING in its
>     own right.  How well do the observed growth rates
>
>     for (presumably large scale because small length scale damps)
>     compare with the theoretical Eady growth rates
>
>     and (something I should know but don't) do Eady growth rates
>     incorporate the effect of moisture on stratification?
>
>
>
>
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