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MIR stands for Meteorological Interpolation and Regridding and is a interpolation and regridding tool and library. This new piece of software is replacing the veteran EMOSLIB when it comes to perform those operations in MARS.  Beyond this, MIR’s flexible design facilitates scalability improvements and additional features. These include efficiency gains, a high degree of user configurability, and support for a wider range of grids than in the current package.

You can try the new MIR-enabled MARS client with:

mars -m

You are encouraged to try this client and report back any issues.

Why are you changing?

The EMOSLIB interpolation package was written in the 1980s and much has changed since then: the model grid resolution has steadily increased, a variety of grid types have been introduced, and many new parameters have been added over the years, often associated with different processing requirements. Both software and hardware technologies, such as programming languages, design paradigms, supporting libraries and hardware architectures, have evolved significantly. Together with new numerical methods and ECMWF's improved understanding of user requirements, all these aspects have prompted ECMWF to design the new, extensible and easy-to-maintain MIR package

Do I need to change my MARS requests?

No, all existing valid MARS requests will also work when using MIR. the mars language stays the same, with the exception of some new keywords. 

Will I get different results?

MIR has gone through a thorough validation process and tests to ensure its quality and correctness. However, since the implementations and strategies used in both packages are different, you may see some differences when using MIR instead of EMOSLIB. How significant those differences will be will depend on each specific case.

Highlights and main differences

Interpolation methods and distance computation

By default, MIR uses a linear method (based on a triangular mesh) which replaces the EMOSLIB bilinear method (based on 4 points).

MIR will also compute distance in 3D whereas EMOSLIB computes distances on the 2D lat-lon grid. Consequently, different nearest points may be selected for the interpolation methods, and the nearest neighbour method should improve as a result.

Parameter classes

In MIR, parameters have been classified into groups, or "classes",  which have an associated default interpolation method. As a result some parameters will now have a significantly different default interpolation method compared to EMOSLIB.  Most parameters will use the default linear method in MIR, while some specific classes will use a different method:

MethodClasses affected
nearest neigbour
  • index parameters: associated to a table
  • spectra parameters: intensity as a function of discrete wavelengths
nearest with Land-Sea Mask
  • vegetation cover ratio parameters
Linear (default)
  • All the rest

Precipitation

MIR does not use any special processing for precipitation fields like EMOSLIB.

Sub-areas

MIR brings a number of improvements in the sub-area and cropping operations.

Consistency with fields from dissemination

MIR will now move the boundaries inwards onto an underlying global grid with a point at latitude=0, longitude=0. This behaviour is a change in respect to EMOSLIB, and it brings it closer to the product generation for dissemination.

Subareas on reduced Gaussian grids

EMOSLIB only supports sub-area operations on regular Gaussian or lat-lon grids. MIR is capable of cropping sub-areas and frames directly on global reduced Gaussian grids, including the octahedral reduced Gaussian grid.

Support for shifted grids

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 both the N-S and E-W boundary limits are divisible by the corresponding increments without remainder. This represents a change in respect to EMOSLIB, which would adjust the area to fit. In the previous example, it would go to the more conventional area 90/0/-90/359.

Land-Sea Mask

 

By default, MIR does not use land-sea mask processing for any parameters. However, users can specify the land-sea mask to use LSM processing for both input and output grids.

Note that as a consequence, MIR does not apply land-sea mask processing when using the nearest neighbour method, which was the case in EMOSLIB.

Spectral to grid point transformations

Concept of "RESOL=AV" when going to a lower resolution is different.  With MIR, RESOL=AV specifies that the transformation is made first to the corresponding octahedral reduced Gaussian grid (i.e., T1279 → O1280 or T639 → O640) followed by grid point interpolation to the user-specified grid.

Default truncation values for "RESOL=AUTO" ("autoresol") have also changed. MIR uses a formula to truncate the spectral series to correspond to the equivalent linear Gaussian grid, replacing the fixed table used in EMOSLIB.

Transformations to regular latitude-longitude grids uses an intermediate (octahedral or full ? - TBC) Gaussian grid (TBC)

This diagram illustrates an example transformation going from T1279 to a regular lat-lon 1.0/1.0 grid. Depending on the values of RESOL and GRID, MIR will follow different paths:

GreenRESOL=AV, GRID=1.0/1.0Transformation from T1279 to the corresponding intermediate reduced octahedral Gaussian O1280, before going to 1.0/1.0.
RedRESOL=AUTO, GRID=1.0/1.0

Direct transformation from T1279 to 1.0/1.0, with an automatic truncation to T179 or RESOL=179.

Blue

RESOL=179,

GRID=F90,

and then

GRID=1.0/1.0

Transformation from T1279 the F90 full (regular) Gaussian grid (with an automatic truncation to T179) followed by a grid-to-grid interpolation from the F90 grid to 1.0/1.0.


Usage of IFS spectral transform library

While EMOSLIB has its own implementation for the transformations from spherical harmonics to grid space, MIR makes use of the spectral transform library, called Trans from the IFS model. That means MIR behaviour is more consistent to what is done in the model, and will immediately benefit from any improvements made in Trans.

Waves

Due to the differences mentioned above, the nearest neighbour method selects different points due to the difference in the computation of the distance (3D versus 2D). Different treatment of neighbouring missing values produces a smoother interpolation close to coasts.

Also, note that all "Spectra-related" parameters have been classified and are interpolated in the same way as the 2D spectra, using a nearest neighbour in MIR instead of EMOSLIB's bilinear method.

Interpolation from limited-area wave data on a regular lat-lon grid, not working with EMOSLIB,  is now supported in MIR. 

Rotated grids

MIR does not support the 12-point interpolation method, used by EMOSLIB for rotated grids. Therefore, the default interpolation method used is the linear. Rotated spherical harmonics are not supported in MIR or EMOSLIB.

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