4. Geometries

Again, fields objects (and their geometry attribute) are rather independent from the resource they originate. In the following, we handle geometries and fields coming from FA files, for convenience only.

>>> import epygram
>>> epygram.init_env()
>>> arp = epygram.formats.resource('icmsharpe+0000', 'r')  # an ARPEGE file
>>> aro = epygram.formats.resource('ICMSHAROM+0042', 'r')  # an AROME file

4.1. Horizontal geometries

The most usual handled fields are horizontal, 2D fields. epygram derive geometries in different classes depending on the kind of grid (Gauss grids, regular Lon/Lat, regular grids on plane projections, or unstructured grids).

>>> t_arp = arp.readfield('CLSTEMPERATURE')
>>> type(t_arp.geometry)
<class 'epygram.geometries.H2DGeometry.H2DGaussGeometry'>
>>> t_aro = arp.readfield('CLSTEMPERATURE')
>>> type(t_aro.geometry)
<class 'epygram.geometries.H2DGeometry.H2DProjectedGeometry'>

>>> g_arp = t_arp.geometry; g_aro = t_aro.geometry  # shortcuts
>>> g_arp.rectangular_grid  # reduced Gauss grids have a variable number of longitudes depending on the latitude
False
>>> g_aro.rectangular_grid
True
>>> g_arp.name
'rotated_reduced_gauss'
>>> g_aro.name
'lambert'

4.1.1. Noticeable attributes

• geometry.dimensions : grid dimensions (including C+I+E zones for LAM)
• geometry.grid : grid other parameters (positionning, definition, resolution)
• geometry.projection : parameters of projection, if appropriate
• geometry.vcoordinate : the vertical characteristics of a horizontal field

Note that lon/lat angles as parameters of a geometry are stored in epygram.util.Angle objects, in order to:

• ensure to keep bit-reproducibility of original values (as read in a given format)
• enable easy unit conversion (degrees, radians, (cos, sin) tuples...)

>>> g_aro.projection['reference_lat']._origin_unit
'radians'
>>> g_aro.projection['reference_lat'].get('degrees')
46.70000000000001
>>> g_aro.projection['reference_lat']._origin_value
0.815068760681
>>> numpy.radians(numpy.degrees(g_aro.projection['reference_lat']._origin_value))
0.81506876068135203

4.1.2. Most useful methods

• get grid, eventually directly a subzone of (LAM)

  >>> (lons, lats) = g_aro.get_lonlat_grid()
  >>> corners = g_aro.gimme_corners_ll(subzone='C')  # get the corners lon/lat of the C zone
  

• lon/lat <=> grid indexes

  >>> g_aro.ll2ij(1.25, 45.666)  # lon/lat to grid indexes
  (669.16059153010667, 673.76289192747686)
  >>> g_aro.ij2ll(669, 674)  # grid indexes to lon/lat
  (1.2472761341493812, 45.668753485393296)
  

• create a Basemap object (mpl_toolkits.basemap), as much appropriate to the geometry or with requested specificities

  >>> g_aro.make_basemap()
  >>> # or
  >>> g_arp.make_basemap(specificproj='kav7')
  

...

4.2. Vertical geometries

Vertical kind of geometries are usually built from either resources or 3D fields:

>>> p = aro.extractprofile('S*TEMPERATURE', 1.26, 45.3)
>>> type(p)
<class 'epygram.fields.V1DField.V1DField'>
>>> p.geometry.vcoordinate.typeoffirstfixedsurface
119
>>> # levels originate from FA historic file : 119 = GRIB2 code for hybrid-pressure vertical coordinate
>>> from epygram.geometries.VGeometry import hybridP2pressure  # vertical coordinate conversion functions
>>> vert_coord_as_pressure = hybridP2pressure(p.geometry.vcoordinate, Psurf=102500, vertical_mean='geometric')
>>> # and eventually
>>> p.geometry.vcoordinate = vert_coord_as_pressure  # but be careful: no consistency check is done here
>>> # actually, this kind of transforms is integrated:
>>> p = aro.extractprofile('S*TEMPERATURE', 1.26, 45.3, vertical_coordinate=100)
>>> p.geometry.vcoordinate.typeoffirstfixedsurface
100

Cf. http://apps.ecmwf.int/codes/grib/format/grib2/ctables/4/5 for vertical coordinate types.