:orphan:

Hybrid Vertical Coordinate
==========================

|

WRF Dynamics Contents
---------------------

`Dynamics Overview`_ |br|
`Hybrid Vertical Coordinate`_ |br|
Diffusion_ |br|
Damping_ |br|
Advection_ |br|
`Other Dynamics Options`_ |br|
`Lateral Boundary Conditions`_

.. _`Dynamics Overview`: dynamics.html
.. _`Hybrid Vertical Coordinate`: hybrid_vert_coord.html
.. _Diffusion: diffusion.html
.. _Damping: damping.html
.. _Advection: advection.html
.. _`Other Dynamics Options`: other_dynamics.html
.. _`Lateral Boundary Conditions`: lat_bdy_conditions.html 

|

Hybrid Vertical Coordinate
--------------------------

The Hybrid Vertical Coordinate (HVC) is the default coordinate for the WRF model. The HVC is terrain-following near the surface, and becomes isobaric at a pre-defined user level. Dry pressure is defined as

        |PDRY(i,j,k)| = |B(k)(PDRY SFC(i,j)-PTOP)| +(h(k)–B(k)) |(P0–PTOP)| + |PTOP|

.. |PDRY(i,j,k)| replace:: P\ :sub:`DRY`\ (i,j,k)
.. |B(k)(PDRY SFC(i,j)-PTOP)| replace:: B(k)(P\ :sub:`DRY SFC`\ (i,j)-P\ :sub:`TOP`\ )
.. |(P0–PTOP)| replace:: (P\ :sub:`0`\ -P\ :sub:`TOP`\ )
.. |PTOP| replace:: P\ :sub:`TOP`

where B(k) is a 1-D weighting array, computed internally.

    * When B(k)=h(k), this definition simplifies to a terrain-following coordinate.
    * When B(k)=0, this definition simplifies to an isobaric coordinate system.

The vertical value where the B(k) arrays transition to isobaric, |hC|, determines how many of the h layers (downward from the model lid) are isobaric. The default value for ETAC is set in the "Registry/registry.hyb_coord" file, and is safe for usage across the globe. The figure below shows the transition of coordinate surfaces from terrain-following to HVC under several values of ETAC.

.. image:: images/transition_tf_to_hvc.png

The transition of the h coordinate surfaces from terrain-following (TF) to isobaric is a function of the critical value of h at which the user requests that an isobaric surface be achieved. The fundamental property of the TF vs. HVC system is seen when tracing a horizontal line from any value on the “Weighting Term B(h)” axis. The degree of model coordinate “flatness,” for example, is the same in the TF system at h = 0.2 as in the HVC system for |hC| = 0.4 when the approximate value of h = 0.6.


.. |hC| replace:: h\ :sub:`C` 

|

The following depicts the vertical location of an h surface for an isobaric coordinate (top), a terrain following coordinate (middle), and a hybrid coordinate (bottom), given with a 2-D cross section for a given model lid (25 km is approximately 25 hPa), and for a given |hC| = 0.2. The depth of the atmosphere (m) and pressure are shown.


.. image:: images/h_sfc_isobaric.png
.. image:: images/h_sfc_tf.png
.. image:: images/h_sfc_hc.png



|

Prior to WRFV4.0, the vertical coordinate defaulted to terrain-following. To convert back to that option, set "hybrid_opt=0" in the &dynamics section of the namelist. real.exe and wrf.exe must both be run with the same "hybrid_opt" value. 

|

When using HVC output with post-processors, use either hydrostatic pressure (P_HYD) or total pressure (PB + P) for diagnostics and vertical interpolation.


|

|

|

|