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[Support #YRR-987022]: Fwd: issue displaying 3D fields overlayed on terrain

Hi Randy-

> we have WRF output in netcdf that we are reading into
> IDV (actually its Mcidas-V). We have been trying to
> display the 2D terrain as a surface. Then, we want to
> display a cross section (east-west) as a function of
> height of let's say u-component of the wind. This also
> works, however, the bottom few model levels are
> displayed "below" the terrain surface. The wrf output
> is on a sigma level coordinate system and the terrain
> is in meters above sealevel. We believe IDV is
> plotting the data wrong because it is incorrectly
> calculating the heights of the sigma-levels. We know
> this because when we do an independent calculation
> outside of IDV the lowest layer in the model matches
> the terrain height. You will also notice that some of
> the data does not plot correctly east of the mountains
> that when displayed in matlab plots correctly.
> I am enclosing a image of the display rotated to show
> both the sub-terrain plotting and the missing stripe
> of data on the right side.
> 

The staggered WRF grids are problematic in many ways -
there is a lot of overhead in resampling from one staggering
to another, it uses a lot more memory because we have to
keep both the staggered grids and the normalized grids in
memory, etc.  We recommend that people use the WRF-Post options
to convert their data to pressure coordinates and calculate
the most commonly used met variables. But we also realize that 
some need to have the staggered coordinates so we allow for that.

For variables like U which are on a vertical Pressure coordinate, the height
is determined through a transformation of pressure to height.  The default
is to use the standard atmophere, but you can change to use a logarithmic
transform (Vis5D) in the Formats & Data section of the User Preferences.

The formula we use for calculating the height for the variables (e.g. W) on 
the staggered Z (height) coordinate is:

height(x,y,z) = (PH(x,y,z)+ PHB(x,y,z)) / 9.81


So, for comparison of the first 10 values in your file that are calculated vs 
HGT
we get:

HGT:
1575.897, 1580.329, 1584.752, 1590.211, 1596.429, 1602.936, 1609.786,
  1616.844, 1624.55, 1633.135

(W) (PH(x,y,z)+ PHB(x,y,z)) / 9.81:

1575.897, 1580.3292, 1584.7522, 1590.2109, 1596.4291, 1602.9358, 1609.7863
  1616.8439, 1624.5498, 1633.1349

(U) Pa -> m using standard atmosphere:

1524.0593, 1528.1156, 1532.2056, 1536.9418, 1542.4225, 1548.4596, 1554.9199,
  1561.5878, 1568.6985, 1576.4624

So, for the variables with a pressure vertical coordinate, you will see
some differences.  Plotting a cross section of W lines up pretty nicely
with the terrain.  

The blank line is a function of the sampling that we have to do between 
the native coordinates and the calculated lat/lon/alt values.  I don't
have a good solution for that.  

Don Murray


Ticket Details
===================
Ticket ID: YRR-987022
Department: Support IDV
Priority: Normal
Status: Open