Terrain and spatial units¶
Why not model every pixel?¶
A region such as Central Asia covers millions of terrain pixels. Running hourly physics on all of them for decades is costly, and unnecessary: two north-facing slopes at 3200 m in the same valley receive almost the same weather and build almost the same snowpack.
SnowMapper therefore follows the TopoSUB approach (Fiddes & Gruber, 2012). It groups pixels with similar terrain into terrain units (clusters), runs the physics once per unit, and maps the result back to every pixel of the unit.
From DEM to terrain parameters¶
The digital elevation model is the Copernicus GLO-90 (90 m) or GLO-30 (30 m) DEM, reprojected to an equal-area grid for the region (grid size from 30 m to 500 m). From it, each pixel gets:
| Parameter | Why it matters |
|---|---|
| Elevation | temperature, pressure, rain/snow split |
| Slope and aspect (as cos and sin) | how much direct sunshine the surface receives |
| Sky-view factor (SVF) | share of the sky visible from the pixel: diffuse sunshine and thermal radiation from the sky vs from surrounding slopes |
| Horizon angles | when surrounding terrain shades the sun |
| Position (x, y) | keeps units compact in space, so one unit does not mix two distant valleys with different weather |
Clustering¶
The parameters are standardised and weighted. Then k-means clustering sorts the pixels into (typically) 10 000 units. Position gets a high weight, so units are both terrain-similar and spatially coherent. Elevation weighs more than aspect because it drives the largest differences.
Each unit carries the mean terrain parameters of its pixels. It is then treated as one hillslope with that elevation, slope, aspect and sky view.
What this means for the map
- Effective map detail is a unit, typically a few km², smaller in steep terrain where the parameters change fast.
- Sharp edges along ridges are real: a north and a south slope belong to different units.
- A point (station, village, ski run) is represented by its unit, whose mean elevation can differ from the point's.
Masks and boundaries¶
Some boards are cut to a national border or a set of hydrological basins, with a small buffer. Outside the mask nothing is modelled. The meteogram says "No forecast at this point".
Glaciers¶
Units on glaciers (from the Randolph Glacier Inventory) are flagged. Where the glacier-enabled snow model is used, they get an ice layer. Elsewhere they are excluded from climatology and anomaly products. See Snow model.