WRF ARW User's Guide - MMM - UCAR

should be written to may be indicated with the opt_output_from_geogrid_path
variable, and the format of these domain files may be changed with io_form_geogrid.
In the “geogrid” namelist record, the projection of the simulation domain is defined, as
are the size and location of all model grids. The map projection to be used for the model
domains is specified with the map_proj variable. Each of the four possible map
projections in the ARW are shown graphically in the full-page figure below, and the
namelist variables used to set the parameters of the projection are summarized in the
following table.
Map projection / value of map_proj Projection parameters
Lambert Conformal / 'lambert' truelat1
truelat2 (optional)
stand_lon
Mercator / 'mercator'
Polar stereographic / 'polar'
Regular latitude-longitude, or cylindrical
equidistant / 'lat-lon'
truelat1
truelat1
stand_lon
pole_lat
pole_lon
stand_lon
WPS
In the illustrations of the Lambert conformal, polar stereographic, and Mercator
projections, it may be seen that the so-called true latitude (or true latitudes, in the case of
the Lambert conformal), is the latitude at which the surface of projection intersects or is
tangent to the surface of the earth. At this latitude, there is no distortion in the distances
in the map projection, while at other latitudes, the distance on the surface of the earth is
related to the distance on the surface of projection by a map scale factor. Ideally, the map
projection and its accompanying parameters should be chosen to minimize the maximum
distortion within the area covered by the model grids, since a high amount of distortion,
evidenced by map scale factors significantly different from unity, can restrict the model
time step more than necessary. As a general guideline, the polar stereographic projection
is best suited for high-latitude WRF domains, the Lambert conformal projection is wellsuited
for mid-latitude domains, and the Mercator projection is good for low-latitude
domains or domains with predominantly west-east extent. The cylindrical equidistant
projection is required for global ARW simulations, although in its rotated aspect (i.e.,
when pole_lat, pole_lon, and stand_lon are changed from their default values) it can
also be well-suited for regional domains anywhere on the earth’s surface.
WRF-ARW V3: User’s Guide 3-9