MARS/Dissemination

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

area=<value>

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". 

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.

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, 
GRID=1.0/1.0

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.

area=75/-25/10/60            area=75/-55/30/42.5

area=75/-55/30/42.5

specifies a frame three grid points wide with the outer limits 74/60/10/-20 and inner limits 72/58/8/-18.