Conventions#
UGRID Conventions#
The UGRID conventions provide a standard for storing unstructured grid model data in the form of a NetCDF file. UXarray uses the UGRID conventions as a foundation for representing unstructured grids.
More Info
For further information about the UGRID conventions, please refer to the official documentation
Elements & Dimensions#
An unstructured grid is composed of nodes, edges, and faces which either fully or partially cover some surface. In the context of climate modelling, this surface is the surface of the Earth, typically represented as a sphere.
For example, output from a global atmospheric model covers the entire sphere, while a global ocean model would not have any elements over land.
Nodes#
An unstructured grid contains n_node
corner nodes, which define the corners of each face. It may also
contain n_face
centroid nodes, which represent the center of each face, and n_edge
edge nodes, which represent the center of each edge.
Edges#
An unstructured grid contains n_edge
edges, which each connect two corner nodes to form an arc.
Faces#
An unstructured grid contains n_face
faces.
UXarray is built to support 2D flexible grids, meaning that each face can have a variable number of nodes surrounding it.
Each face can have an independent number of nodes that surround it, which is represented through the
descriptor variable n_nodes_per_face
, which itself has a dimension of n_face
The minimum
number of nodes per face is 3 (a triangle), with the maximum number being represented by the dimension
n_max_face_nodes
Coordinates#
Definitions#
Spherical Coordinates#
Coordinate 
Grid Attribute 
Dimensions 
Summary 

Node Longitude 


Longitude of each corner node 
Node Latitude 


Latitude of each corner node in degrees 
Edge Longitude 


Longitude of the center of each edge 
Edge Latitude 


Latitude of the center of each edge 
Face Longitude 


Longitude of the center of each face 
Face Latitude 


Latitude of the center of each face 
Note
All spherical coordinates are represented in degrees, with longitudes between (180°, 180°) and latitudes between (90°, 90°).
Cartesian Coordinates#
Coordinate 
Grid Attribute 
Dimensions 
Summary 

Node X 


X location of each corner node 
Node Y 


Y location of each corner node 
Node Z 


Z location of each corner node 
Edge X 


X location of the center of each edge 
Edge Y 


Y location of the center of each edge 
Edge Z 


Z location of the center of each edge 
Face X 


X location of the center of each face 
Face Y 


Y location of the center of each face 
Face Z 


Z location of the center of each face 
Note
All Cartesian coordinates are represented in meters.
Connectivity#
UXarray relies on connectivity variables to describe how various elements (i.e nodes, edges, faces) can be connected.
Connectivity 
Grid Attribute 
Dimensions 
Summary 

Face Node 


Indices of the nodes that make up each face 
Face Edge 


Indices of the edges that surround each face 
Face Face 


Indices of the faces that surround each face 
Edge Node 


Indices of the two nodes that make up each edge 
Edge Edge 


Indices of the edges that surround each edge 
Edge Face 


Indices of the faces that saddle each edge 
Node Edge 


Indices of the edges that surround each node 
Node Face 


Indices of the faces that surround each node 