The WRF Variational
Data Assimilation System (WRF-Var)
1. Introduction
The
MMM Division of NCAR supports a unified (global/regional, multi-model,
3/4D-Var) model-space variational data assimilation
system, WRF-Var, for use by NCAR staff and
collaborators, and is also freely available to the general community, together
with further documentation, test results, plans, etc., from the links below.
The
components of the WRF-Var system are shown in blue in
the sketch below, together with their relationship with rest of the WRF system.
2. Background
The
WRF-Var system has been developed in four major
stages.
a) The basic system (June 2003)
The
basic system contained the necessary infrastructure, but not all the
complexities (e.g., radiance assimilation, anisotropic recursive filters) of
subsequent research and advanced versions. The characteristics of the basic
WRF-Var system were:
i.
Incremental formulation.
ii.
MPI ready.
iii.
Flexible
observational F90 structures.
iv.
Weak balance
constraint through the choice of analysis variables.
v.
Un-staggered
grid.
vi.
Model space
solution.
vii.
3D- isotropic
but in homogeneous background error covariance matrix generated through the use
of recursive filters
viii.
Observation
operators and covariances will assume uncorrelated
observational errors.
ix.
Platform
specific quality control to be performed prior to analysis (WG12).
Multi-platform quality control to be performed within analysis system.
x.
Primary input
data to be in BUFR format.
xi.
Comprehensive
performance diagnostics.
xii.
Code management
using CVS.
b) Upgraded basic version (WRF 3DVAR
V2.0)
This
version was released in May 2004 as part of the WRFV2.0 package. It
incorporated the following additional capabilities:
i.
Re-ordering of
grid-points to i=x and k=1 at bottom (and removal of
MM5 I/O).
ii.
Variational quality
control via inner/outer loop structure.
iii.
Assimilation of
radar radial velocities.
iv.
Improved minimization
flexibility (option for conjugate gradient for linear inner-loop).
v.
Sophisticated
surface observation operators (uses PBL physics).
vi.
Changes to
permit WRF3D-Var to be cycled with WRF.
c) Research version (WRF-Var V2.2beta)
This
version was released in August 2005 as part of the WRFV2.1 package. It
incorporates the following additional capabilities:
i.
First-guess at
analysis time (FGAT) to make better use of asynoptic
observations in 3DVAR.
ii.
Radar
reflectivity assimilation capability (already in pre-operational trials in
Korea).
iii.
Prototype global
3D-Var capability.
iv.
A utility (gen_be) to enable users to calculate estimates of
background errors for their own domains.
v.
Porting to
Cray-X1 and Apple G4/G5 platforms.
vi.
Unification of
3D-Var and WRF in the V2.2 framework. This is in preparation for the 4D-Var
capability of WRF-Var that is under development.
d)
This
WRF-Var version, released in April 2008, is a part of
WRF 3.0. It incorporates the following additional capabilities:
i. Merge of WRF
and WRF-Var source code repositories.
ii. Inclusion
of various scripts to run an end-to-end WRF modeling system.
iii. Inclusion
of “Verification” package.
iv. Enhance
capability for the “gen_be” utility, such as
generation of background error using ensemble.
v. WRF-Var future released version.
vi. Inclusion
of documentation.
vii. Inclusion
of various NCL-based graphics utility.
viii. Upgrade for
observation and background error tuning.
ix. Important
bug fixes.
The
advanced version will incorporate new developments from the WRF-Var team. The upcoming features are 4DVar, direct
assimilation of satellite radiance, etc.
3. Process for inclusion of new capabilities in the WRF-Var repository
The
development of WRF-Var is a joint effort across a
number of international organizations. To avoid duplication of work, and to
coordinate multiple code changes, the following procedure has been adopted:
a)
Change proposed
with draft of methodology and impact, if any, in initial tests.
b)
If approved for potential
inclusion, reviewer assigned and schedule for development decided.
c)
Contributed code
tested in repository system (including standard tests and additional tests
which show the new capability in action).
d)
Developer/reviewer
present results to working group.
e)
When approved,
change is added to repository (including change file describing code changes
and impacts).
4. Summary of current status of development efforts
Version
3.0 of WRF-Var was released in April 2008. This
version is intended for use by knowledgeable, friendly users. A climatological estimate of the background errors may be
computed using the NMC method, and the code also has the provision to use
ensembles for computing background error estimates. The developers would
appreciate notification of any problems with the WRF-Var
code, but, because of resource constraints, cannot promise prompt response or
solution to the problems.
See
the WRF
Work
is now focusing on the research version of 3/4D-Var described above.
5. Links, Documentation
a) WRF-Var Tutorial - an introduction to running WRF-Var.
b) Advanced Research WRF
(ARW) Technical Note - see Chapter 9 for WRF-Var
(pdf).
c)
WRF User
Guide - see Chapter 6 for a description of WRF-Var.
d) MM5 3DVAR Homepage - useful for download, general documentation related
to the MM5 version of WRF 3DVAR.