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.

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, the mars language stays the same, with the exception of some new keywords. All existing valid MARS requests will also work when using MIR.

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:

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 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.

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.