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

value	comment
`North/West/South/East`	latitude/longitude coordinates of sub-area
`E`	specifies the European area `73.5/-27/33/45`
`G`	specifies the global area `90/0/-90/360` (default)

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.

If a global area with boundaries 90/-180/-90/180 is needed for a regular (ful) Gaussian grid (e.g., with grid=F640) or a octahedral (e.g.,with grid=O1280) or classical (e.g., with grid=N640) reduced Gaussian grid then the area should be specified with the eastern longitude slightly less than 180°, e.g.:

AREA=90/-180/90/179.999, GRID=O1280

This ensures all of the grid points at each latitude line are included and avoids potential problems when decoding the latitudes and longitudes. To ensure the most easterly longitude is included in the area, the value needs to be

greater than 180.0 - 90/<N> and less than 180.0 for the octahedral grid
greater than 180.0 - 90/(20 + <N>) and less than 180.0 for the classical reduced or full Gaussian grid.

where <N> is the Gaussian grid number.

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. Shifted Grid

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

`area=90/-180/-90/180`	`area=90/0/-90/360`
`area=75/-25/10/60`	`area=75/-15/30/42.5`
`area=73.5/-27/33/45 (equivalent to area=E)`	`area=40/-20/-40/60`
`area=50/-135/-60/-30`	`area=40/60/-60/180`