[ncl-talk] temperature advection

[Dennis Shea]

If the *global* grid is oriented South-to-North ( think:
([at(n+1)-lat(n-1)] ), I am not aware of any rational for multiplying by
-1.

The highly accurate spherical harmonic functions are used to calculate the
gradients. They require global, rectilinear (eg, gaussian, 1x1, 2x3...)
grids which are ordered South-to-North.

To be explicit, the WRF grids are curvilinear not rectilinear. Further, as
noted in the referenced post, they are not global.

---
A side-point, the announced 6.3.0 function 'advect_variable' was
inadvertently released as 'advect_var'. You can see the code via (search
for 'advect_var')

%> less $NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl

In the *SOON* to be released 6.4.0, users can access the function using
'advect_var' or 'advect_variable'

-------

The  6.4.0 release will have:

    http://www.ncl.ucar.edu/Applications/gradients.shtml

Note that this release will contain a new gradient function which will
allow gradients to be calculated on non-global grids. It uses less accurate
centered-finite-difference. The grids must be rectilinear and ordered
South-to-North


http://www.ncl.ucar.edu/Document/Functions/Contributed/grad_latlon_cfd.shtml

Hence, you could calculate the regional (say) temperature [T]  gradients
dT/dlat, dT/dlon, then multiply by u, v to get the advection.



On Thu, Feb 23, 2017 at 9:56 PM, Peter Gibson <peter.gibson at unsw.edu.au>
wrote:

> Hello,
>
>
> I am using the advect_variable function in NCL v6.3 on 3 hourly global
> reanalysis data to calculate horizontal temperature advection.
>
> The patterns of temperature advection I am seeing seem realistic, but
> appear to be the wrong sign (cold advection has positive values etc.)
>
>
> There is some discussion of multiplying by -1 to get the right sign here:
> http://mailman.ucar.edu/pipermail/wrf-users/2016/004188.html
>
>
> Can you confirm that it is indeed appropriate to multiply by -1?
>
>
> Thanks,
> Peter
>
>
>
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