Mesoscale Modeling

1
Mesoscale Modeling

• Review the tutorial at http//www.meted.ucar.edu/
  mesoprim/models/index.htm
• In class discussion on horizontal and vertical
  resolution, hydrostatic versus non hydrostatic,
  parameterization schemes, and boundary conditions
• Review of the WRF model
• A detailed description of the WRF model can be
  found at http//www.mmm.ucar.edu/wrf/users/

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WRF Model Overview -

• What are the important physical processes that
  are resolved in the WRF model?
• In the atmosphere, the equations of motion are
  solved for u,v,w
• Turbulence and turbulent mixing
• A thermodynamic equation to predict potential
  temperature
• Long and short wave radiation
• Clouds either explicitly resolved
  (qv,qc,qr,qi,qs,qg) or parameterized if the
  horizontal grid spacing is greater than about 10
  km
• A boundary layer scheme that handles the
  generation of boundary layers eddies that
  transport heat, moisture, and momentum through
  the boundary layer.
• A surface layer scheme handles the fluxes of
  heat, and moisture from the surface to the
  atmosphere. It also interacts with the radiation
  scheme as long/short wave radiation is emitted,
  absorbed, or scattered from the earths surface
• A land surface scheme that may contain a soil
  moisture model. It may also resolve the type of
  vegetation on the surface, ice, sea ice, and snow

3
WRF Model Overview -

• What do the model equations look like? Note that
  the equations below are NOT complete. For
  example, they do not contain any moisture
  variables.

4
WRF Model Overview -

• The complete model equations are then written in
  finite difference form to be solved numerically.
  What does the model grid look like?

(figures from the WRF-ARWV2 Users Guide)
5
WRF Model Overview -

• The Vertical Coordinate
• The eta surfaces are often packed close together
  where higher vertical resolution is required
  such as near the surface and up at tropopause
  level.

(figures from the WRF-ARWV2 Users Guide)
6
WRF Model Overview -

• To run the model, it needs to be initialized with
  the current atmospheric state. How is this done?
• Often with the analysis generated by a coarser,
  larger scale NWP model such as the Nam or GFS

Nam DOMAIN
WRF DOMAIN
7
WRF Model Overview -

• As the WRF model runs, it will periodically
  (often every 3 hours) need information from the
  coarse model (Nam) at the lateral boundaries to
  account for information passing into and out of
  the WRF model domain

Nam DOMAIN
Information such as u,v,w passed from Nam to WRF
model lateral boundary periodically during model
run
WRF DOMAIN
8
WRF Model Overview -

• Within WRF, and many other mesoscale models, you
  can nest a finer-scale grids within the outer
  domain

(figures from the WRF-ARWV2 Users Guide)
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WRF Model Overview -

• This is what the model grids would look like

(figures from the WRF-ARWV2 Users Guide)
10
WRF Model Overview -

• Physics options available within WRF
• Microphysics
• Surface layer physics
• Land surface model
• Boundary layer
• Long and short wave radiation
• Lets examine these in more detail

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WRF Model Overview -

• Microphysical Processes how does the model
  handle clouds and precipitation?

12
WRF Model Overview -

• Microphysical Processes how does the model
  handle clouds and precipitation?
• In WRF, you have 7 different options
• They all parameterize microphysical processes
  differently. The variables represented are not
  all the same for the different schemes.
• Lets run WRF using the Kessler warm rain scheme
  and the Lin ice scheme for a supercell simulation
• The prognostic variables for these schemes
  include qv,qc,qr,qi,qs,qg

(table from the WRF-ARWV2 Users Guide)
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WRF Model Overview -

• If the grid spacing is greater than say 5-10 km,
  the model resolution is to coarse to resolve
  convective cells explicitly. In this situation,
  clouds and precipitation are produced by cumulus
  parameterization schemes.
• Parameterization The representation, in a
  dynamical model, of physical effects in terms of
  admittedly oversimplified parameters, rather than
  realistically requiring such effect to be
  consequences of the dynamics of the system. (from
  Glossary of Meteorology online)
• In WRF, you have three cumulus parameterization
  processes to choose from

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WRF Model Overview -

• The surface layer
• The surface layer scheme handles the fluxes of
  heat, moisture and momentum from the model
  surface to the boundary layer above.
• There are two surface layer options available
  within WRF

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WRF Model Overview -

• The Land Surface Model (LSM)

8.4 Land-Surface Model The land-surface models
(LSMs) use atmospheric information from the
surface layer scheme, radiative forcing from the
radiation scheme, and precipitation forcing from
the microphysics and convective schemes, together
with internal information on the lands state
variables and land-surface properties, to provide
heat and moisture fluxes over land points and
sea-ice points. These fluxes provide a lower
boundary condition for the vertical transport
done in the PBL schemes (or the vertical
diffusion scheme in the case where a PBL scheme
is not run, such as in large-eddy mode). The
land-surface models have various degrees of
sophistication in dealing with thermal and
moisture fluxes in multiple layers of the soil
and also may handle vegetation, root, and canopy
effects and surface snow-cover prediction. The
land surface model provides no tendencies, but
does update the lands state variables which
include the ground (skin) temperature, soil
temperature profile, soil moisture profile, snow
cover, and possibly canopy properties. There is
no horizontal interaction between neighboring
points in the LSM, so it can be regarded as a
one-dimensional column model for each WRF land
grid-point, and many LSMs can be run in a
stand-alone mode. (text from the WRF-ARWV2 Users
Guide)
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WRF Model Overview -

• One of the land surface models available in WRF

Figure from RAP/NCAR
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WRF Model Overview -

• There are three land surface models available in
  WRF

(table from the WRF-ARWV2 Users Guide)
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WRF Model Overview -

• Boundary Layer Schemes
• In the real atmosphere, turbulent eddies
  transport heat, moisture, and momentum through
  the boundary layer
• Unless the horizontal grid spacing is a few
  hundred meters or less, these eddies are two
  small to be resolved in the model
• Therefore, the boundary layer scheme tries to
  parameterize these eddies and their role in
  transporting heat, moisture, and momentum.
• There are three boundary layer schemes available
  within WRF

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WRF Model Overview -

• Long and Short-wave Radiation

The radiation schemes provide atmospheric heating
due to radiative flux divergence and surface
downward longwave and shortwave radiation for the
ground heat budget. Longwave radiation includes
infrared or thermal radiation absorbed and
emitted by gases and surfaces. Upward longwave
radiative flux from the ground is determined by
the surface emissivity that in turn depends upon
land-use type, as well as the ground (skin)
temperature. Shortwave radiation includes visible
and surrounding wavelengths that make up the
solar spectrum. Hence, the only source is the
Sun, but processes include absorption,
reflection, and scattering in the atmosphere and
at surfaces. For shortwave radiation, the upward
flux is the reflection due to surface albedo.
Within the atmosphere the radiation responds to
model-predicted cloud and water vapor
distributions, as well as specified carbon
dioxide, ozone, and (optionally) trace gas
concentrations. All the radiation schemes in WRF
currently are column (one-dimensional) schemes,
so each column is treated independently, and the
fluxes correspond to those in infinite
horizontally uniform planes, which is a good
approximation if the vertical thickness of the
model layers is much less than the horizontal
grid length. This assumption would become less
accurate at high horizontal resolution.
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WRF Model Overview -

• Long and Short-wave Radiation
• There are currently 5 radiation schemes available
  in WRF