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/

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 earth’s 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

•Other Features:•Data Assimilation•WRF-Chem model•Grid nesting

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.

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)

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)

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:•You can run WRF as a global model.

WRF DOMAIN

Nam DOMAIN

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:

WRF DOMAIN

Nam DOMAIN

Information such as u,v,w passed from Nam to WRF model lateral boundary periodically during model run

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)

WRF Model Overview - This is what the model grids would look like:

(figures from the WRF-ARWV2 Users Guide)

WRF Model Overview - Physics options available within WRF•Microphysics•Cumulus parameterizations•Surface layer physics•Land surface model•Boundary layer•Long and short wave radiation

Let’s examine these in more detail:

WRF Model Overview - •Microphysical Processes – how does the model handle clouds and precipitation?

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.•Let’s 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-ARWV3 Users Guide)

Single moment

Mixing ratios and number distributions

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 four cumulus parameterization processes to choose from

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 three surface layer options available within WRF

WRF Model Overview -

•The Land Surface Model (LSM):

The land-surface models (LSMs) use atmospheric information from the surface layer scheme,radiative forcing from the radiation scheme, and precipitation forcing from the microphysicsand convective schemes, together with internal information on the land’s state variables andland-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 PBLschemes (or the vertical diffusion scheme in the case where a PBL scheme is not run, such as inlarge-eddy mode). [Note that large-eddy mode with interactive surface fluxes is not yet availablein the ARW, but is planned for the near future.] The land-surface models have various degrees ofsophistication in dealing with thermal and moisture fluxes in multiple layers of the soil and alsomay handle vegetation, root, and canopy effects and surface snow-cover prediction. The landsurfacemodel provides no tendencies, but does update the land’s state variables which includethe ground (skin) temperature, soil temperature profile, soil moisture profile, snow cover, andpossibly canopy properties. There is no horizontal interaction between neighboring points in theLSM, 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-ARWV3 Users Guide)

WRF Model Overview -

•One of the land surface models available in WRF:

Figure from RAP/NCAR

WRF has the option to couple an urban canopy model to this LSM• Street

canyons – urban geometry

• Shadowing from building and radiation reflection

• Urban winds in canopy layer

• Heat equation for roof wall and road surfaces.

WRF Model Overview -

•There are three land surface models available in WRF:

(table from the WRF-ARWV3 Users Guide)

Can couple an ocean mixed-layer model to this scheme (for hurricane modeling)

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 four boundary layer schemes available within WRF

WRF Model Overview -

•Long and Short-wave Radiation

The radiation schemes provide atmospheric heating due to radiative flux divergence and surfacedownward longwave and shortwave radiation for the ground heat budget. Longwave radiationincludes infrared or thermal radiation absorbed and emitted by gases and surfaces. Upwardlongwave radiative flux from the ground is determined by the surface emissivity that in turndepends upon land-use type, as well as the ground (skin) temperature. Shortwave radiationincludes visible and surrounding wavelengths that make up the solar spectrum. Hence, the onlysource is the Sun, but processes include absorption, reflection, and scattering in the atmosphereand 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 columnis treated independently, and the fluxes correspond to those in infinite horizontally uniformplanes, which is a good approximation if the vertical thickness of the model layers is much lessthan the horizontal grid length. This assumption would become less accurate at high horizontalresolution.

WRF Model Overview -

•Long and Short-wave Radiation•There are currently 7 radiation schemes available in WRF