WRF-ARW Physics Options

WRF-ARW allows for a diverse choice of physics parameterizations. The physics parameterizations are grouped into categories within WRF-ARW; they include: microphysics, cumulus parameterization, PBL, surface layer, radiation, and LSM. The following sections describe the physics parameterizations:

a. Microphysics Schemes

The Thompson, Field, Rasmussen, and Hall (2008) Scheme is a bulk

microphysical parameterization (BMP) within WRF-ARW and is used in this study for its robustness. The scheme predicts several hydrometeor species including cloud water, cloud ice, rain, snow, and graupel. The scheme is different from other BMPs within WRF-ARW because snow size distribution depends on both ice water content and

temperature and is represented using exponential and gamma distributions. In other BMP within WRF-ARW, snow is assumed spherical in shape and constant density, however the Thompson Scheme employs a non-spherical shape and density varies inversely with diameter, which is found in observations. It employs many techniques found in far more sophisticated spectral and bin schemes using look-up tables (Skamarock et al., 2008). Furthermore, the Thompson Scheme is used at the University of Washington (UW)

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Atmospheric Department, which produces forecasts over the northwestern United States at 4.0 and 1.33-km resolution. It should be noted that the lack operational predictability aerosols has limits the ability to model microphysical processes, clouds, and weather in general (Warner, 2011). Microphysical modeling is one of the more problematic

variables in atmospheric modeling. It is nearly impossible to initialize the microphysical variables and can only be roughly inferred based on satellite cloud ice and water imagery (Warner, 2011). Furthermore, vertical distributions of aerosol particles and their

horizontal spatial detail at cloud scale are unknown thus inhibit the model to adequately model microphysical variables (Warner, 2011). Figure 2.4 demonstrates the complexity of the microphysical processes that are represented within the model. For complete information on the microphysical scheme, please see Thompson et al. (2008).

Figure 2.4 Schematic showing microphysical processes of precipitation within the WRF-ARW, from Warner (2011).

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b. Cumulus Parameterization

Cumulus parameterization schemes are used to account for the sub-grid-scale updrafts and downdrafts that may be unresolved with large grid sizes. Cumulus parameterizations are theoretically only valid for coarser grid sizes (larger than 10 km) and should not be used in applications under this grid size because the model can resolve the convective eddies itself, such as grid sizes of 5 km or less (Skamarock et al., 2008). Currently, there is no solution on how to represent convection between 10-km resolutions and those that are needed to explicitly resolve convection (Warner, 2011). Because of the 4-km grid size adopted in this study, a cumulus parameterization is not enabled.

c. Planetary Boundary Layer and Surface Layer Schemes

The planetary boundary layer (PBL) is the lowest part of the atmosphere and is influenced greatly by the Earth’s surface. The PBL is the layer with the most friction in the atmosphere and thus can have a substantial impact on the resulting weather.

Therefore, the PBL is the most essential part of the atmosphere to model accurately. Turbulent eddies transport water vapor and heat upward from the surface and frictional stress exerted by the surface is transmitted via turbulence. Two forms of turbulence exist, buoyancy caused by convection and vertical shear of the horizontal wind component with height.

The PBL scheme within WRF is responsible for eddy transports in the entire atmospheric column. The PBL schemes determine the flux profiles within the well- mixed boundary layer and stable layer that provide atmospheric tendencies of

temperature, moisture, clouds, and horizontal momentum within the atmospheric column (Skamarock et al., 2008).

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The surface layer scheme calculates the amount of friction and exchange

coefficients for the LSM and the PBL scheme (Skamarock et al., 2008). The calculation of exchange coefficients is extremely important in modeling the land surface in Idaho (Sridhar, in revision) and thus this study uses the (itzl0nd) setting that will be discussed later. Surface layer schemes are tied to particular PBL schemes and are not currently interchangeable.

The Mellor-Yamada-Janjić (MYJ; Janjić, 2002) PBL coupled with the Eta surface layer (Janjić, 2002) scheme is used for this simulation. The Eta surface layer scheme is based on similarity theory and accounts for variable surface roughness height for temperature and humidity. The MYJ PBL scheme uses an upper limit on the master length scale that depends on the turbulent kinetic energy (TKE) and the buoyancy and shear of the driving flow (Skamarock et al., 2008). For more information, please see Janjić (2002).

d. Radiation Schemes

All radiation physics schemes within WRF are in the form of a one-dimensional column model with an assumption that the vertical grid resolution is much less than the horizontal grid resolution. Because the NARCCAP-CCSM WRF simulations were performed using the NCAR Community Atmospheric Model (CAM) radiation scheme (Collins et al., 2004), it is used in our simulation to ensure continuity.

The CAM Radiation (Collins et al., 2004) scheme is a spectral band scheme designed for climate simulations. The CAM shortwave (SW) accounts for 19 spectral bands in the parameterization scheme, while the CAM long wave (LW) accounts for two spectral bands. The scheme is suitable for regional climate simulations by having an

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ozone distribution that changes based on monthly zonal-mean climatological data. The scheme is versatile and interacts with clouds, uses cloud fractions, and has the ability to handle optical properties of aerosols and trace gases (Skamarock et al., 2008). Full details can be found in Collins et al. (2004).