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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
Current content can be found at unidata.ucar.edu.
To learn about what's going on, see About the Archive Site.
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 ************************************************************* 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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