Hybrid Vertical Coordinate


WRF Dynamics Contents

Dynamics Overview
Hybrid Vertical Coordinate
Diffusion
Damping
Advection
Other Dynamics Options
Lateral Boundary Conditions


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

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.

../_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.


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.

../_images/h_sfc_isobaric.png ../_images/h_sfc_tf.png ../_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.