MARS

Specifies the desired sub-area of the data to be retrieved.

area=<value>

If specified for fields, the area keyword will trigger some post-processing. The input fields to this operation can be

• spherical harmonics
• regular lat/lon grids
• global regular Gaussian grids
• global reduced Gaussian grids, including the octahedral reduced Gaussian grid
• wave fields

As the target field needs to be gridded it should be specified in the same request using the keyword grid. 

A horizontal sub-area is in general defined by a list of its latitude/longitude coordinates North/West/South/East in decimal degrees. If South > North, the values are automatically swapped. Latitude/longitude coordinates must be given as signed numbers as follows:

• Northern latitudes (north of the equator) must be given as positive numbers
• Southern latitudes (south of the equator) must be given as negative  numbers
• Eastern longitudes (east of the 0 degree meridian) must be given as positive numbers
• Western longitudes (west of the 0 degree meridian) must be given as negative numbers

If the longitude values overlap, the most eastern line of points will not be returned, e.g.

AREA=90/-180/-90/180, 
GRID=0.5/0.5

will return a field with the boundaries 90/-180/-90/179.5. 

Areas can be defined to wrap around the globe, e.g. in order to retrieve all longitudes but Europe one would specify area=75/60/10/-20.

MARS (with MIR) also supports shifted grids, with no point at latitude=0, longitude=0, e.g. 

AREA=89.5/0.5/-89.5/359.5, 
GRID=1.0/1.0

provided the differences between N-S and E-W limits are both divisible by the corresponding grid increments without remainder.

In this example, the black points represent the original grid with 1 degree spacing. The red points show the user defined area, which has the same spacing but shifted by half a degree. MARS will honour the user defined area and return the inner grid shown in green.

In cases where the differences between N-S and E-W limits are not divisible by the corresponding increments without remainder, MARS starts from the user-specified south-west corner and goes up to (but does not exceed) the north-east corner.  For example, a request specifying:

AREA=55.5/0.5/45.0/5.0, 
GRID=1.0/1.0

retrieves AREA=55.0/0.5/45.0/4.5 with the northern and eastern boundaries adjusted so that the area returned lies within that specified by the request and includes a point at latitude=0, longitude=0.

Area examples

For observations, the area keyword is used as a filter. 

Dissemination

The keyword is used as with MARS, but mandatory in dissemination.