# [ncl-talk] Computation of baroclinity in TC environments

Dennis Shea shea at ucar.edu
Fri Feb 9 08:25:11 MST 2018

```I am not really knowledgeable about Eady Growth Rate (EGR) so  I asked 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

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> 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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