MARS/Dissemination
Specifies the desired sub-area of data to be extracted.
area=<value> |
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For fields the specification of the area keyword requests a 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
The target field needs to be gridded, therefore keyword grid should also be specified in the same request.
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 swapped and a warning issued; southern latitudes and western longitudes must be given as negative numbers:
- Latitude values must be given as signed numbers, with:
- north latitudes (i.e. north of the equator) being positive (e.g: 40.5)
- south latitudes (i.e. south of the equator) being negative (e.g: -50.5)
- Longitude values must be given as signed numbers, with:
- east longitudes (i.e. east of the 0 degree meridian) being positive (e.g: 35.0)
- west longitudes (i.e. west of the 0 degree meridian) being negative (e.g: -20.5)
A European area pre-defined as 73.5/-27/33/45 can be specified as letter "E".
value | comment |
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E | Same as |
North/West/South/East | latitude/longitude coordinates of sub-area |
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
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If the area and the specified grid spacing do not match, the area boundaries will be moved inwards onto an underlying global grid with a point at latitude=0, longitude=0.
For observations, the area keyword is used as a filter.
MIR can work with shifted grids, with no point at latitude=0, longitude=0. An example would be:
AREA=89.5/0.5/-89.5/359.5, |
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This is done automatically provided the differences between N-S and E-W limits are both divisible by the corresponding 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 with MIR 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.
NORTH = 90° | WEST = -180° | SOUTH = -90° | EAST = 179.5° |
NORTH = 75° | WEST = -25° | SOUTH = 10° | EAST = 60° |
NORTH = 75° | WEST = -15° | SOUTH = 30° | EAST = 42.5° |
NORTH = 70° | WEST = -20° | SOUTH = 35° | EAST = 20° |
NORTH = 75° | WEST = 10° | SOUTH = 30° | EAST = 40° |
NORTH = 40° | WEST = -20° | SOUTH = -40° | EAST = 60° |
NORTH = 50° | WEST = -135° | SOUTH = -60° | EAST = -35° |
NORTH = 40° | WEST = 60° | SOUTH = -60° | EAST = 180° |
NORTH = 80° | WEST = 115° | SOUTH = -5° | EAST = 180° |
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specifies a frame three grid points wide with the outer limits 74/60/10/-20 and inner limits 72/58/8/-18.