Re: [idvusers] plotting true wind vectors

Hi Kathleen-

Kathleen Carroll wrote:
Is anyone aware of any bugs within IDV when trying to compute "true" wind
vectors from U10 and V10, (or in 3D using U and V)?  I have plotted some
actual observations on top of my model data and while, in some instances the
model data matches up fairly well with the observations, most of the time
the barbs are completely opposite the observed wind barbs.  I would assume
the "true" wind vectors would be plotted in meteorological coordinates (i.e.
the direction from which the wind is blowing).  I tried to find the formula
IDV uses to calculate these wind vectors, but was unsuccessful in doing so.

The true wind vectors should be in true meteorological coordinates as you suspect. However, the plotted observed wind vectors had a bug until recently where they were not being adjusted to the map projection. This has been fixed in the latest nightly builds (2.8a1). So there are a couple of reasons for what you are seeing:

1) The computation for true wind is wrong. This is computed in the Java class ucar.visad.quantities.GridRelativeHorizontalWind.cartesianHorizontalWind. However, for the staggered WRF grids (if that's what you are using), the calculations could be off. 2) The observed winds are being plotted incorrectly in the version of IDV that you are using. This would show up in a non-lat/lon projection or a rotated projection, particularly if it covered to polar region.
3) your model forecast is wrong. ;-)

Also, when sampling the data (using a middle click), is it sampling contours
or actual values at those points?  If I sample on my domain while having the
U10, V10, and true wind vectors (from U10 and V10) plotted, the true wind
vector value always matches the U10 component and appears to not take into
account the V10 component at all.  This may be part of my problem, but
without seeing the formula used it is hard to figure out where the problem
is.

It's only showing the u coordinate in the vector case. That's a known "issue" that we need to address. Do you want to see u/v or spd/dir? In all cases it should be showing the value at the grid points using the preferred sampling mode (nearest neighbor/weighted average) that you've set in your User Preferences.

I'd appreciate any insight, comments or solutions regarding these issues.

Don Murray
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Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
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