Polar MM5

The polar-optimized MM5 demonstrated a much improved regional performance.

MM5Polar

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Quoted from: Bromwich, David H., John J. Cassano, Thomas Klein, Gunther Heinemann, Keith M. Hines, Konrad Steffen, and Jason E. Box. "Mesoscale modeling of katabatic winds over Greenland with the Polar MM5." Monthly Weather Review 129, no. 9 (2001): 2290-2309.https://doi.org/10.1175/1520-0493(2001)129%3C2290:MMOKWO%3E2.0.CO;2

The Polar MM5 model used for the simulations presented in this paper is based on version 2 of the PSU–NCAR MM5. A general description of this model is given by Dudhia (1993) and Grell et al. (1994). The model configuration used for the simulations presented in this paper is described below. In addition, a description of the changes made to the standard version of MM5 for use in polar regions is provided.

a. Polar MM5 dynamics and physics

The standard version of MM5 (version 2) allows for the use of either hydrostatic or nonhydrostatic governing dynamics. For the Greenland simulations presented in this paper the hydrostatic dynamics option is used since the hydrostatic approximation is valid for the model horizontal resolution used (40 km), and the hydrostatic option was found to run approximately 12% faster than the nonhydrostatic option. The hydrostatic version of the model includes three-dimensional prognostic equations for the horizontal components of the wind and temperature, and a two-dimensional prognostic equation for p* (defined as the surface pressure minus the pressure at the model top). Additional three-dimensional prognostic equations for the water vapor mixing ratio and the mixing ratio of various cloud species are also part of the model equations. Parameterizations for cloud microphysics and precipitation processes, cumulus convection, radiative transfer, and turbulence are included in the model, with multiple options available for the representation of many of these processes.

For the Polar MM5 simulations the large-scale (grid scale) cloud and precipitation processes are represented by the Reisner explicit microphysics parameterization (Reisner et al. 1998). This parameterization predicts the mixing ratio of cloud water and ice crystals as well as the rain and snow water mixing ratios, and allows for the presence of mixed phase clouds. Subgrid-scale clouds are parameterized with the Grell cumulus parameterization (Grell et al. 1994).

Excessive cloud cover was found to be a problem over the Antarctic in sensitivity simulations using an older version of MM5 (MM4) (Hines et al. 1997a,b), similar to results found by Manning and Davis (1997) for cold, high clouds over the continental United States. Replacement of the Fletcher (1962) equation for ice nuclei concentration with that of Meyers et al. (1992) in the MM5 explicit microphysics parameterizations, as suggested by Manning and Davis (1997), helped to eliminate this cloudy bias in polar simulations with MM5, and is now a standard option in the Polar MM5 model. This modification to the Reisner microphysics parameterization is used for all of the simulations presented here.

The Polar MM5 also uses a modified version of the NCAR community climate model, version 2 (CCM2), radiation parameterization (Hack et al. 1993) for prediction of the radiative transfer of shortwave and longwave radiation through the atmosphere. In the original version of this parameterization the cloud cover was predicted as a simple function of the grid-box relative humidity, with the cloud liquid water (CLW) path determined from the grid-box temperature. Sensitivity simulations revealed that this parameterization of cloud cover tended to significantly overestimate the CLW path, and thus the radiative effects of the clouds, which was particularly noticeable as large downwelling longwave radiation fluxes during the austral winter over the Antarctic ice sheet (Hines et al. 1997a,b). In order to resolve this problem, the predicted cloud water and ice mixing ratios from the Reisner explicit microphysics parameterization are used in the modified CCM2 radiation parameterization for determination of the radiative properties of the modeled cloud cover. This modification allows for a consistent treatment of the radiative and microphysical properties of the clouds and for the separate treatment of the radiative properties of liquid and ice phase cloud particles, similar to that in the CCM3 radiation parameterization (Kiehl et al. 1996) that is in part based on results discussed by Ebert and Curry (1992).

Turbulent fluxes in the atmosphere, and the turbulent fluxes between the atmosphere and the surface, are parameterized using the 1.5-order turbulence closure parameterization used in the National Centers for Environmental Prediction Eta Model (Janjić 1994). Heat transfer through the model substrate is predicted using a multilayer “soil” model. The thermal properties used in the soil model for snow and ice surface types are modified following Yen (1981). In addition, the number of substrate levels represented in the soil model was increased from six to eight, with an increase in the resolved substrate depth from 0.47 m to 1.91 m. Also, a sea ice surface type is added to the 13 surface types available in the standard version of MM5 (Hines et al. 1997a). The sea ice surface type allows for fractional sea ice cover in any oceanic grid point, with surface fluxes for the sea ice grid points calculated separately for the open water and sea ice portions of the grid point, which are then averaged before interacting with the overlying atmosphere. The sea ice thickness varies from 0.2 m to 0.95 m and is dependent on the hemisphere and sea ice fraction at the grid point (Table 3). Some surface characteristics for the surface types of interest in this study are listed in Table 3.

b. Model grid

MM5 is formulated using a staggered horizontal grid with a vertical σ-coordinate system that is defined in terms of pressure. The model domain used in this study consists of 100 grid points in the north–south direction and 110 grid points in the east–west direction, centered at 71°N latitude and 30°W longitude, with a horizontal resolution of 40 km (Fig. 1). The model terrain over Greenland is specified from the Ekholm (1996) digital elevation data (Table 3), since accurate representation of the terrain slope over the ice sheet is required for accurate katabatic wind simulations. The 40-km horizontal grid spacing used in the Polar MM5 adequately resolves the terrain slopes over all but the steepest margins of the ice sheet (Cassano and Parish 2000).

A total of 28 σ levels are used, of which seven are located within the lowest 400 m of the atmosphere, and the lowest atmospheric model level is located at a nominal height of 12 m AGL. This relatively high resolution near the surface is required to accurately represent the evolution of the shallow katabatic layer over the Greenland ice sheet. The model top is set at a constant pressure of 100 hPa.

c. Polar MM5 initial and boundary condition data

A list of the datasets used to initialize the Polar MM5, and those that are used to provide boundary conditions to the model during the simulations, are listed in Table 3. The 2.5° horizontal resolution ECMWF Tropical Ocean Global Atmosphere (TOGA) surface and upper-air data are used to provide the initial and boundary conditions for the model atmosphere. These data are interpolated to the Polar MM5 model grid using the standard preprocessing programs provided by NCAR for use with the MM5 modeling system. In addition the 1.125° ECMWF TOGA global surface analyses are used to specify the initial surface temperature [and sea surface temperature (SST)], deep soil temperature, and snow cover. Snow cover on the tundra grid points on Greenland is manually specified to match snow cover observations from the KABEG'97 field campaign. Sea ice cover is based on the SST specified with the higher-resolution surface data, and is considered to be present at all grid points with an SST < 271.7 K. Sea ice fraction for these grid points is determined based on climatological values given by Gloersen et al. (1992).

The Polar MM5 was used to produce short duration (48-h length) simulations of the atmospheric state over Greenland for April and May 1997. The model was initialized with the 0000 UTC ECMWF analyses for each day of the two-month period, with the 24–48-h forecast used for model verification, unless otherwise noted.

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The Ohio State University team (2019). Polar MM5, Model Item, OpenGMS, https://geomodeling.njnu.edu.cn/modelItem/45574373-f4dd-46ab-a0a0-154360c8f5e1
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