Difference between revisions of "How to interpret WRF variables"
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* at the center of the front face - V point, staggered in Y | * at the center of the front face - V point, staggered in Y | ||
* at the center of the bottom face - W point, staggered in Z | * at the center of the bottom face - W point, staggered in Z | ||
| − | Scalar variables (such as thermodynamical variables, hence the theta: temperature, pressure,...) generally live at theta-points. One exception is the geopotential, which lives at W-points. | + | Scalar variables (such as thermodynamical variables, hence the theta: temperature, pressure,...) generally live at theta-points. One exception is the geopotential, which lives at W-points. |
| + | The documentation and comments of the code talk about theta-height (mid-level of the cell) and W-height (bottom of the cell). | ||
| − | == | + | ==Wind== |
| + | |||
| + | Components of the wind velocity vector '''U''', '''V''', '''W''' live at the corresponding points, hence the point names. These are in the geometrically horizontal and vertical directions. On flat ground, '''W''' on the first level ''k=1'' is zero. However, when the ground has nonzero slope, the vertical velocity components '''W''' is generally not zero, because the normal vector to the ground is not vertical. | ||
| + | |||
| + | ==Location== | ||
| + | |||
| + | The longitude and latitude of the theta and U nodes are given by arrays '''XLONG''' and '''XLAT''', which are set at initialization and then do not change. However the elevation has to be computed from the flow solution as [[wikipedia:Geopotential height|geopotential height]] by | ||
| + | : '''ELEVATION = (PHB + PH)/9.81"' | ||
| + | where '''PHB + PH''' is the [[wikipedia:geopotential|geopotential]]. '''PHB''' is constant, set at initialization, and '''PH''', called perturbation geopotential, varies with time. | ||
| + | |||
| + | PH and PHB live at the W-points, thus the geopotential height at the lowest level is the same as terrain height | ||
| + | : '''HGT(i,j) = PHB(i,j,1)/9.81 | ||
| + | (with the variable names and index order used in the NetCDF files, which is sometimes different from that in the WRF code itself; the mapping is established by the Registry) | ||
==Temperature== | ==Temperature== | ||
| − | |||
| − | |||
==Fire variables== | ==Fire variables== | ||
| − | |||
| − | |||
| − | |||
==See also== | ==See also== | ||
* [http://www.mmm.ucar.edu/wrf/users/docs/user_guide_V3/users_guide_chap5.htm#fields WRF output fields from the WRF-ARW user's guide] | * [http://www.mmm.ucar.edu/wrf/users/docs/user_guide_V3/users_guide_chap5.htm#fields WRF output fields from the WRF-ARW user's guide] | ||
| − | * [http://www.mmm.ucar.edu/wrf/users/docs/arw_v3.pdf A Description of the Advanced Research WRF Version 3 (June 2008)] Fig 7.3 page 59 | + | * Grid: [http://www.mmm.ucar.edu/wrf/users/docs/arw_v3.pdf A Description of the Advanced Research WRF Version 3 (June 2008)] Fig 7.3 page 59 |
| + | * [http://www.mmm.ucar.edu/wrf/WG2/WRF_conventions.html WRF Coding Conventions] | ||
| + | * [http://www.mmm.ucar.edu/wrf/users/tutorial/200807/WRF%20Registry%20and%20Examples.pdf WRF Registry and Examples] | ||
| + | * [http://www.mmm.ucar.edu/wrf/WG2/software_v2/ WRF v2 Software Tools and Documentation] | ||
| + | * [http://www.mmm.ucar.edu/wrf/WG2/software_2.0/registry_schaffer.pdf Description of WRF Registry] | ||
| + | |||
{{stub}} | {{stub}} | ||
[[Category:WRF-Fire]] | [[Category:WRF-Fire]] | ||
Revision as of 22:14, 7 August 2010
Grid
Grids in WRF are logically rectilinear. Variables for WRF cell indexed (i,j,k) can be located at one of 4 possible point. You can tell where by looking at the WRF registry or the attributes of NetCDF files, which WRF uses as I/O format. You can also tell by the fact that staggered variables have the staggered dimension larger by one, since there needs to be variable at the last boundary face.
- at the center of the cell - theta points, not staggered
- at the center of the left face - U point, staggered in X
- at the center of the front face - V point, staggered in Y
- at the center of the bottom face - W point, staggered in Z
Scalar variables (such as thermodynamical variables, hence the theta: temperature, pressure,...) generally live at theta-points. One exception is the geopotential, which lives at W-points. The documentation and comments of the code talk about theta-height (mid-level of the cell) and W-height (bottom of the cell).
Wind
Components of the wind velocity vector U, V, W live at the corresponding points, hence the point names. These are in the geometrically horizontal and vertical directions. On flat ground, W on the first level k=1 is zero. However, when the ground has nonzero slope, the vertical velocity components W is generally not zero, because the normal vector to the ground is not vertical.
Location
The longitude and latitude of the theta and U nodes are given by arrays XLONG and XLAT, which are set at initialization and then do not change. However the elevation has to be computed from the flow solution as geopotential height by
- ELEVATION = (PHB + PH)/9.81"'
where PHB + PH is the geopotential. PHB is constant, set at initialization, and PH, called perturbation geopotential, varies with time.
PH and PHB live at the W-points, thus the geopotential height at the lowest level is the same as terrain height
- HGT(i,j) = PHB(i,j,1)/9.81
(with the variable names and index order used in the NetCDF files, which is sometimes different from that in the WRF code itself; the mapping is established by the Registry)
Temperature
Fire variables
See also
- WRF output fields from the WRF-ARW user's guide
- Grid: A Description of the Advanced Research WRF Version 3 (June 2008) Fig 7.3 page 59
- WRF Coding Conventions
- WRF Registry and Examples
- WRF v2 Software Tools and Documentation
- Description of WRF Registry
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