# [gradsusr] Interpolating HRRR 15 minute GRIB to 5 minute

Sun Dec 18 09:51:24 EST 2016

```Just to clarify a misrepresentation … the lterp() function does indeed support interpolation in the Time dimension. From http://cola.gmu.edu/grads/gadoc/gradfunclterp.html:

"The source and dest expressions must have the same varying dimensions, which may be X, Y, or T. Interpolation is not performed in the Z or E dimension. The source and dest expressions may vary in 1 or 2 dimensions, unless you are using the aave or amean methods, in which case the grids must be 2-D with X and Y as the varying dimensions. If the domain of source is larger than the domain of dest, the returned result will have an expanded grid to cover the requested domain. For interpolation in the time dimension, you may interpolate (A) between monthly and yearly time axes, or (B) between minute, hourly, and daily time axes."

Lterp won’t work if the source expression is varying in three dimesions. If you want to interpolate 2D grids in time, you’ll essentially be creating a new data set with a higher frequency time axis, so you’ll have to be writing out grids to a file. In that case, it is probably simpler to create the algebraic expressions for the linear interpolation yourself, unless you want to ‘do the math’ described in Method 2 below.
—Jennifer

On Dec 17, 2016, at 9:54 PM, Jeff Duda <jeffduda319 at gmail.com<mailto:jeffduda319 at gmail.com>> wrote:

I've done stuff like this before. There's a few ways you can go. One of them is far more complicated than the other and probably not worth it.

Method 1:
-Use linear interpolation in time. Simple and quick, but if you're looking small scale features like precipitation or reflectivity, it's not going to give you accurate results.

Method 2:
-Estimate the temporal derivatives at the initial time, then integrate those forward to your sub-time steps. You can calculate things like advection, the Coriolis force, and vertical motion easy enough. Source and sinks for water vapor and heat will remain impossible, though. I'd have to "do the math" to be sure, but this method may end up giving you the same solution as method 1 would. In either case, I would recommend using method 1.

Good luck.

Jeff Duda

On Sat, Dec 17, 2016 at 2:32 PM, Brandon Schmidt <admin at wilmingtonwx.com<mailto:admin at wilmingtonwx.com>> wrote:
Hello all,

I'm currently looking for a solution to take an HRRR Sub-Hourly GRIB2 file with 4 time steps and interpolate it to have 12 time steps. I know this somehow involves the 'iterp' function but I can't seem to get it working. I'm barely even sure where to start...what must the destination control file contain to make this work? I imagine this can be done fairly easily, I'm just not sure how to do it myself. Any help would be greatly appreciated...thanks!

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Jeff Duda
Post-doctoral research fellow
University of Oklahoma School of Meteorology
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