Re: Problems with the gradient function

Hello,

I speculate that you are using ?CAM? data that has been interpolated to
pressure levels.
If NCL's "vinth2p" or "vinth2p_ecmwf" has been used to perform the
interpolation, then the
argument "kxtrp" was likely set to False.

Logical. False => no extrapolation when the pressure level is outside of
the range of psfc.
====
As noted in the documentation, the gradient function, which uses
spherical harmonics,
requires: (1) the data are global and (2) no missing values be present.
====

Solution:
[1] Regenerate the pressure level data with kxtrp=True
or
[2] Use
http://www.ncl.ucar.edu/Document/Functions/Built-in/poisson_grid_fill.shtml

*poisson_grid_fill*( x, True, 1, 1500, 1e-2, 0.6, 0)

on each variable prior to invoking the gradient function.

===
No matter what, always remember that these are extrapolated or
interpolated values.

=======================
Did you try the gradient function on the original hybrid level data and
*then* interpolate the budget quantities to pressure levels via vinth2p
or vinth2p_ecmwf??

Dood lock

Katie Holman wrote:
> Dear all,
>
> I am running into a problem with the gradient function. The gradient
> function is outputting missing values at pressure levels that are very
> close to the surface (1000mb, 925mb, and 850mb), which is negatively
> impacting a moisture budget I am trying to perform at one specific
> location. I know this is an artifact of the function because the
> specific humidity data I am using has data at each level except the
> surface for the specific lat/lon location. The problem is the missing
> values that are arising at levels above 1000mb. Is there a way to
> mitigate this somehow?
>
> I have tried to shrink the spatial domain of my data to remove high
> elevation areas. This has helped slightly. However, I am still running
> into missing values above 1000mb. The main goal is to obtain the
> gradient of specific humidity at one point only, so the size of the
> domain isn't a problem, assuming that there are enough data points to
> perform the differencing scheme. Is there a way to perform the
> gradient around missing values so as to not eliminate an entire level?
> Has anyone had this problem before or worked with the gradient function?
>
>
> Thank you,
> Katie Holman
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-- 
======================================================
Dennis J. Shea                  tel: 303-497-1361    |
P.O. Box 3000                   fax: 303-497-1333    |
Climate Analysis Section                             |
Climate & Global Dynamics Div.                       |
National Center for Atmospheric Research             |
Boulder, CO  80307                                   |
USA                        email: shea 'at' ucar.edu |
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