Nesting

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Nesting
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Eta Model
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Hybrid and Eta Coordinates
Ptop
Ptop
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Pressure domain
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Sigma domain
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MSL
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Horizontal resolution of 12 km
12-km terrain
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WRF Model Family

• A Tale of Two Dynamical Cores

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Why WRF?

• An attempt to create a national mesoscale
  prediction system to be used by both operational
  and research communities.
• A new, state-of-the-art model that has good
  conservation characteristics (e.g., conservation
  of mass) and good numerics (so not too much
  numerical diffusion)
• A model that could parallelize well on many
  processors and easy to modify.
• Plug-compatible physics to foster improvements in
  model physics.
• Designed for grid spacings of 1-10 kmeta

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Two WRF Cores

• ARW (Advanced Research WRF) (aka Mass
  Core)developed at NCAR
• Non-hydrostatic Numerical Model (NMM) Core
  developed at NCEP
• Both work under the WRF IO Infrastructure

NMM
ARW
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The NCAR ARW Core Model (See www.wrf-model.org)

• Terrain following hydrostatic mass (p) vertical
  coordinate, arbitrary vertical resolution
• Arakawa C-grid, two-way nesting, any ratio
• 3rd order Runge-Kutta time-split differencing
• Conserves mass, entropy and scalars using up to
  6th order spatial differencing equ for fluxes
  (5th order upwind diff. is default)
• NCAR physics package (converted from MM5 and
  Eta), NOAH unified land-surface model, NCEP
  physics adapted too

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The NCEP Nonhydrostatic Mesoscale Model NMM
(Janjic et al. 2001)

• Hybrid sigma?pressure vertical coord.
• Arakawa E-grid, 31 nesting ratio
• Adams-Bashforth time differencing, time splitting
• Conserves kinetic energy, enstrophy and momentum
  using 2nd order differencing equation
• Separate set of equations for hydrostatic versus
  non-hydrostatic terms
• Modified Eta physics, Noah unified land-surface
  model, NCAR physics adapted too
• Parallelized within WRF infrastructure

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WRF Modeling System
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WRF Hierarchical Software Architecture

• Top-level Driver layer
• Isolates computer architecture concerns
• Manages execution over multiple nested domains
• Provides top level control over parallelism
• patch-decomposition
• inter-processor communication
• shared-memory parallelism
• Controls Input/Output
• Mediation Layer
• Specific calls to parallel mechanisms
• Low-Level Model layer
• Performs actual model computations
• Tile-callable
• Scientists insulated from parallelism
• General, fully reusable

Driver Layer
wrf
initial_
config
alloc
_and_configure
init
_domain
integrate
solve_interface
solve
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The National Weather Service dropped Eta ( old
NAM-North American mesoscale run) in June and
replace by WRF NMM (new NAM). The Air Force is
now switching from MM5 to WRF ARW. Most
universities using WRF ARW
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On June 13, 2006 starting with the 12 UTC model
run, NCEP will replace the forecast model used in
its North American Mesoscale (NAM) time slot.
Currently the the Eta forecast model is used for
the NAM, but on this date it will be replaced
with the Non-hydrostatic Mesoscale Model (NMM) in
the WRF framework The WRF/NMM with continue to
run over the same domain and same horizontal
resolution (12 km) as the Eta and its output will
be available at the same time. Specifics on the
differences between the Eta and WRF/NMM systems
are as follows 1. Model Changes - Replace
Eta prediction model with WRF version of the
Non-hydrostatic Meso Model (WRF-NMM) -
Extended model top pressure from 25 mb to 2 mb
- Replace Eta step-mountain vertical coordinate
with NMM hybrid sigma-pressure vertical
coordinate - Refined/retuned numerous aspects
of the Eta model physics for use in the NMM -
Replace Eta 3DVAR analysis system with the new
unified GSI analysis system that has been adapted
for application to the WRF-NMM
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WRF-NMM

• Same domain as Eta
• Sixty levels like Eta
• Essentially same physics as ETA
• Much better in terraindoesnt share the etas
  problems.

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12 UTC 19 June
15 UTC 19 June
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Round OneSubjective Impressions

• Surface and near surface wind and temperature
  fields are similar
• WRF has more intense, detailed, and more
  extensive precipitation structures.

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Round TwoObjective Verifications

• Both WRF and MM5 were verified against large
  array of surface observations over the Pacific
  Northwest.
• Model output was linearly interpolated to
  observation sites within the 12-km domain
  encompassing the Pacific Northwest.
• Will show statistics from 12 UTC March 29 to 12
  UTC June 6, 2005

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2- m Temperature Mean Absolute Error
oC
Forecast Hour
12-km domain, 12 UTC initialization, roughly
60,000 observations in each
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10-m Wind SpeedMean Absolute Error
kt
Forecast Hour
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Wind DirectionMean Absolute Error
Degrees
Forecast Hour
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Surface PressureMean Absolute Error
mb
Forecast Hour
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6-h PrecipitationMean Absolute Error
inch
Forecast Hour