The following sections give some information relevant to retrieving data efficiently.
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Request scheduling
The main archive implements a queueing system necessary to satisfy all type of requests: time-critical operational tasks, interactive users retrieving a few fields for visualisation, batch users retrieving Gigabytes of data, etc.
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The queueing system handles priorities: the request with the highest priority will be chosen next for execution. This priority is computed from the age of a request since it entered the queue. An artificial mechanism of ageing requests allows to prioritise operational work or interactive users. You can view the MARS queue online.
Data collocation
A request is scheduled more efficiently if it minimises the number of tapes it has to access in order to be satisfied. In order to do so, users must know how the data is organised.
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Different projects have different needs, and therefore these rules may vary. Users are encouraged to visit the MARS archive catalogue in order to inspect how much related data an archive object contains. The description above is the rule, but resources available at certain times might cause to break it, e.g. 1 month of Analysis may be in 2 files because at that particular month the MARS system was short of disk space and data had to be written to tape earlier than desired.
Post-processing
Any requested data manipulation or post-processing is carried out by the MARS client, except in the case of a local Member State's client where data is first processed at ECMWF prior to its transmission over the network. The post-processing is carried out by a set of routines present in the EMOSLIB library. Please, refer to the field interpolation software routines for in-depth details about such post-processing.
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Sub-area extraction is possible for regular Gaussian and latitude longitude fields (including wave). They cannot be applied if the resulting field is in spherical harmonics or reduced (quasi-regular) representation.
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Conversion of horizontal representations
The following conversions (interpolations) of horizontal representations are available:
- reduce the triangular truncation of spherical harmonic fields, e.g. resol=319
- change the resolution of Gaussian grids, e.g. grid=42
- change the resolution of latitude/longitude grids, e.g. grid=5/5
- convert spherical harmonic to Gaussian grids, e.g. grid=160, gaussian=reduced
- convert spherical harmonics to latitude/longitude grids, e.g. grid=2.5/2.5
- convert quasi-regular Gaussian grids to regular Gaussian grids, same or different resolution, e.g. grid=160, gaussian=regular
- convert Gaussian to latitude/longitude grids, e.g. grid=2.5/2.5
ECMWF GRIB data archived on latitude/longitude grids can be converted into NetCDF format, following the Climate and Forecast (CF) Metadata Conventions, by calling
mars -N
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Derived fields
Some fields are not archived but are derived from others. This is the case for wind components (U and V), which are derived from vorticity and divergence.
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