# Re: linear interpolation

Date: Wed, 23 Sep 2009 14:18:54 -0600

Hi Lei,
Use lonFlip to flip the longitudes of the data that run from -179.5:179.5:
http://www.ncl.ucar.edu/Document/Functions/Contributed/lonFlip.shtml

When using the regridding routines, always make sure that the
input/output latitudes/longitudes match in both range and direction.
Best regards,

Lei Meng wrote:
> Hi Dennis,
> Thanks for your suggestions and it works fine. Except that the
> longitudes from the two nc files do not match. The longitudes from one
> file are ranged from -179.5 to +179.5 and the ones from the other file
> are from 0 to 357. When I did the linear interpolation, only the values
> for longitudes 0 to 179.0 retain and there are no values for longitudes
> from -179.5 to 0. Any suggestions?
> Thank you for your help!
> Lei
>
>
> On Wed, Sep 23, 2009 at 2:13 PM, Dennis Shea <shea_at_ucar.edu
> <mailto:shea_at_ucar.edu>> wrote:
>
> To further what DaveB wrote:
>
> You did not state if the input grid was on hybrid levels or pressure
> levels.
> There are slightly different approaches that may be used.
>
> Generally, the vertical interpolation should be done first:
>
> X(12, 60,180,360) ===> Y(12, 28,180,360)
>
> Then, perform the horizontal interpolation:
>
> Y(12, 28,180,360) ===> Z(12, 28, 64,128)
> ===========
> NCL's Applications page is a good place to start:
>
> http://www.ncl.ucar.edu/Applications/
>
> ==================== VERTICAL INTERPOLATION ===============
> Under the "Data Analysis", Click: "Vertical Interpolation"
>
> http://www.ncl.ucar.edu/Applications/vert_interp.shtml
> If your input grid is on hybrid levels, Example 3 is
> closest to what you want to do.
>
> If the input is on pressure levels, the function "int2p"
>
>
> http://www.ncl.ucar.edu/Document/Functions/Built-in/int2p.shtml
> or, better, "int2p_Wrap", because it returns meta data
>
>
> http://www.ncl.ucar.edu/Document/Functions/Contributed/int2p_Wrap.shtml
>
> See Example 5
>
> yTemp = *int2p_Wrap*(p_in, x(time|:,lat|:,lon|:,pin|:), pout, linlog)
> printVarSummary(yTemp)
>
> YY = yTemp(time|:,iLev|:,LAT|:,LON|:) ; reorder for horizontal
> interpolation
> delete(yTemp)
>
> use, linint2_Wrap, or f2gsh_Wrap or area_hitolores for the grid
> interpolation.
>
>
>
>
>
> David Brown wrote:
>
> Hi Lei,
>
> First just a bit of terminology: you are talking about the
> dimensions of the variable not the attributes (which are
> metadata such as units or a descriptive long name for the
> variable). There are a number of functions you could use.
> Assuming you have coordinate values for the input and output
> lat and lon dimensions, and you are using version 5.1.0 of NCL,
> I would recommend the function area_hi2lores_Wrap. However, you
> could also use linint2_Wrap, or if the output grid is Gaussian
> ("T42") the function
> f2gsh_Wrap might be preferable.
> -dave
>
>
> On Sep 22, 2009, at 1:05 PM, Lei Meng wrote:
>
>
>
> Hi,
> I have a nc file with attributes (Time, Lev, Lat, Lon)
> (e.g., 12, 60,180,360) and I would like to convert it to a
> nc file with attributes (Time, iLev,iLat,iLon) (e.g., 12,
> 28, 64,128) using linear interpolation. Which functions
> should I use to interpolate the three attributes (ilev,
> ilat, ilon)? Thanks,
> Lei
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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 <http://ucar.edu> |
> ======================================================
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