LMDZ zoom parameters

The "Z" in LMDZ stands for "Zoom". This is because it is possible to define, instead of a regular longitude-latitude grid, a grid with varying mesh sizes along longitudes and latitudes.

Zoom parameters

In practice one defines a "zoomed region" of interest where the resolution will be highest (and homogeneous), whereas outside this region the resolution will be low (and also homogeneous, apart from a transition region between the two). This is achieved by specifying the following parameters (usually in gcm.def):

• The zoom center coordinates (in degrees), clon and clat
• The zoom factor along longitude(x)/latitude(y) directions, grossismx and grossismy. These ratio represent the ratio between meshes in the zoomed region versus meshes far outside of it.
• The fraction of the zoomed region (i.e. relative to the whole planet which is 360° in longitude and 180° in latitude) along longitude (x) and latitude (y), dzoomx and dzoomy. In practice the extension in longitude and latitude of the zoomed region will thus be of dzoomx*360° in longitude and dzoomy*180° in latitude
• The steepness of the mesh transition between the inner zoom and outer meshes, taux and tauy. Typically one should avoid a too sharp transition; tau ~3 is a typically maximum value if running without nudging; with strong nudging harsher transitions, e.g. tau~8 or even 10 can also work.
• It is advised to target, by adjusting number of points along longitude and latitude, to have something close to a "square" mesh (i.e. same extension, in degrees, along longitudes and latitudes) in the inner zoomed area.

Choosing an adequate time step for a zoomed simulation

The rule of thumb, starting from a "day_step" value that works for a given regular (non-zoomed) grid:

day_step(zoom) ~ day_step(regular)*max(grossismx,grossismy)

It is also advised to check what others users have successfully used, see e.g. LMDZ_zoom_collection

24/11/2021