ICON-D2 Triangle Grid Explained – Why Germany's Model Crushes Coastal Forecasts
If you are still using GFS for your coastal wind forecast, you are missing the details. The German ICON model (specifically ICON-D2) has a secret weapon: Triangles.
Most weather models (like GFS) divide the world into squares (Latitude/Longitude grid). This works fine over the open ocean. But on the coast, squares are clumsy. They fail to capture the curve of a bay or the sharp edge of a cliff.
The Icosahedral Advantage
ICON stands for ICOsahedral Non-hydrostatic. It covers the globe in a mesh of millions of triangles.
Triangles are flexible. They tessellate (fit together) perfectly on a round sphere. Squares get distorted near the poles (the "singularity problem"). Triangles stay uniform.
This allows ICON to perform "grid nesting." It can zoom in on specific areas without breaking the math. The ICON-D2 version focuses on Europe with a resolution of 2.2 km.
Resolving the Coastline
Why does this matter for wingfoil wind?
Imagine a headland that sticks out 1 km into the sea.
- GFS (22 km squares): The model sees the headland as a flat blur. It averages the land and water. It predicts 12 knots.
- ICON (2.2 km triangles): The model sees the specific shape of the rock. It calculates the friction. It sees the acceleration around the tip. It predicts 20 knots.
We call this "crushing" the forecast. ICON resolves the acceleration zones (Venturi effects) that square grids simply delete.
Non-Hydrostatic Physics
The "Non-hydrostatic" part of the name is equally important. Standard models assume vertical equilibrium. ICON calculates vertical wind flow.
This allows it to predict thermal wind rising up a slope or a downdraft from a storm cloud. For wingfoil wind, which depends heavily on local thermals, this vertical calculation is vital.
When to Use ICON
ICON-D2 is a short-range model. It is best for the next 48 hours.
- Use GFS or ECMWF for planning your week.
- Use ICON for planning your session tomorrow morning.
Summary
Geometry matters. The triangular grid of ICON hugs the coastline tighter than the square grid of GFS. When hunting for wind gusts around a cape or thermal wind in a bay, trust the triangles.