Nesting

Eta Model

Eta CoordinateAnd Step Mountains

MSL

ground

= 1

Ptop = 0

Horizontal resolution of 12 km

12-km terrain

Drawbacks of the Eta Coordinate

• The failure to generate downslope wind storms in regions of complex terrain

• Weak boundary layer winds over elevated terrain when compared to observations

• The displacement of precipitation maxima too far toward the bottom of steeply sloping terrain as opposed to the observed location near the top half of the terrain slope

• The reduction in the number of vertical layers used to define the model atmosphere above elevated topography particularly within the boundary layer

WRF Model Family

A Tale of Two Dynamical Cores

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 km

Two WRF Cores• ARW (Advanced Research WRF) • developed at NCAR• Non-hydrostatic Numerical Model (NMM) Core developed at

NCEP• Both work under the WRF IO Infrastructure

NMM ARW

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

Terrain following vertical coordinate two-way nesting, any ratio Conserves mass, entropy and scalars using up to

6th order spatial differencing equ for fluxes. Very good numerics, less implicit smoothing in numerics.

NCAR physics package (converted from MM5 and Eta), NOAH unified land-surface model, NCEP physics adapted too

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

Hybrid sigmapressure vertical coord. 3:1 nesting ratio Conserves kinetic energy, enstrophy and

momentum using 2nd order differencing equation Modified Eta physics, Noah unified land-surface

model, NCAR physics adapted too Parallelized within WRF infrastructure

Hybrid and Eta Coordinates

ground MSL

ground

Pressure domain

Sigma domain

= 0

= 1 = 1

Ptop Ptop = 0

WRF Modeling System

Obs Data,Analyses

Post Processors,Verification

WRF Software Infrastructure

Dynamic Cores

Mass Core

NMM Core…

Standard Physics Interface

Physics Packages

StaticInitialization

3DVAR DataAssimilation

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

Mediation Layer

wrf

initial_config alloc_and_configure init_domain integrate

solve_interface

solve

Model Layer

Driver Layer

prep

filt

er

big_

step

deco

uple

adva

nce

uv

reco

uple

scal

ars

phys

ics

adva

nce

w

•The National Weather Service dropped Eta in 2006 as the NAM (North American Mesoscale) run and replaced it with WRF NMM.

•The Air Force uses WRF ARW.

•Most universities use WRF ARW

WRF-NMM

•Same domain as Eta•Sixty levels like Eta•Essentially same physics as ETA•Much better in terrain…doesn’t share the eta’s problems.•Clearly inferior synoptic initialization and synoptic forecast than GFS

NMM WRFNAM NMM upgrades December 2008, include

• GDAS (GFS analysis) as initial first guess. use of global analysis (GDAS) for first guess at t-12 hour (the start of the analysis cycle) improves the evolution of synoptic scale features in the new NAM-WRF. This is found consistently throughout the 84-hour forecast.

• Improved physics higher resolution snow analysis and changes to snow impact on surface energy budget, increased absorptivity of model clouds

NMM

• Generally inferior to GFS

Rapid Update Cycle-RUC

RUC• A major issue is how to assimilate and use the

rapidly increasing array of offtime or continuous observations (not a 00 and 12 UTC world anymore!

• Want very good analyses and very good short-term forecasts (1-3-6 hr)

• The RUC ingests and assimilates data hourly, and then makes short-term forecasts

• Uses the MAPS mesoscale model…which uses a hybrid sigma/isentropic vertical coordinate

• Resolution: 13 km and 50 levels

13km RUC

Improvements expected from 13km RUC- Improved near-surface forecasts- Improved precipitation forecasts- Better cloud/icing depiction- Improved frontal/turbulence forecasts

Terrain elevation - 100 m interval

NCEP computer upgrade allows RUC13 to run in same time as current RUC20

Observations used in RUCData Type ~Number Freq.--------------------------------------------------Rawinsonde 80 /12hNOAA profilers 30 / 1hVAD winds 110-130 / 1h Aircraft (V,temp) 1400-4500 / 1hSurface/METAR 1500-1700 / 1hSurface/METAR 1500-1700 / 1hBuoy/ship 100-150 / 1hGOES precip water 1500-3000 / 1hGOES cloud winds 1000-2500 / 1hGOES cloud-top pres 10 km res / 1hSSM/I precip water 1000-4000 / 6h--------------------------------------------------GPS precip water ~300 / 1hMesonet ~5000 / 1hMETAR-cloud-vis-wx ~1500 / 1h--------------------------------------------------

NC

EP

R

UC

20

op

era

tion

al

RUC13

(at NCEP June 2005)

Cloudanalysisvariables

RUC History – NCEP (NMC) implementations

1994 - First operational implementation of RUC- 60km resolution, 3-h cycle

1998 – 40km resolution, 1-h cycle, - cloud physics, land-sfc model

2002 – 20km resolution- addition of GOES cloud data in assimilation

2003 – Change to 3dVAR analysis from previous OI(April)

2004 – Vertical advection, land use (April)PBL-depth for surface assimilation

(September)

2005 – 13km resolution, new obs, new model physics(June)

2007 – WRF-based Rapid Refresh w/ GSI to replace RUC

More detailed coastline with 13km resolution

13km RUC 20km RUC

Soil moisture – 22z - 21 Feb 2005Dark blue = water

WRF RUC

• A new version of RUC has been developed, but not yet operational that uses the WRF model instead of the MAPS model.