Method

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Method in brief

What is atmospheric blocking?

A block is a large, quasi-stationary high-pressure system that interrupts the usual west-to-east flow at mid and high latitudes for days to weeks. Blocks divert storms and are linked to persistent heat waves, cold spells and droughts. There is no single accepted definition. Indices differ in the variable they use (geopotential height, potential vorticity, wave activity) and in which features they require, so they often disagree on the events they detect.

What FAST-IBAN does differently

Physical sampling scale. Around each candidate point, 64 rays of 500 km are traced along great circles. A point is a high (low) candidate when at least 90 % of the ray end points have lower (higher) Z500. Gradient-based indices compare Z500 at a fixed latitude offset (e.g. 15°), which is not available near the pole. A distance measured along the Earth's surface does not have that limitation.

Shape of the pattern. Candidates are grouped into clusters, and highs are paired with the neighbouring lows using closed contours and directions:

Pattern	Configuration
Omega	A high flanked by two lows, resembling the Greek letter Ω
Rex	A high–low dipole

Most indices only check that the flow is reversed or persistent; the morphology is what makes a pattern recognisable as a block to a forecaster.

Determinism. The same input produces bit-identical output with any number of threads or MPI processes. This is checked automatically on every change.

What it is not

• Not a new index competing with established ones. It aims to be a transparent, fast and reproducible implementation that can be compared with them.
• Not a forecasting system. It diagnoses the fields it receives.
• Not a tracker yet. It works on instantaneous fields; temporal tracking of events is planned (see Status).

Full method description and references: README.