Presets and playback
Presets
Wind and snowfall
Snow and time
Fence
Changing the fence restarts the run.
Layers
Model checks
The science
Blowing snow transport
Wind moves snow only when the friction velocity u* exceeds a threshold u*t (cohesion, grain bonds, moisture). Above it, grains hop by saltation and the mass flux per metre of crosswind width follows the empirical Kawamura law q = 2.78 ρa/g · (u* − u*t)(u* + u*t)2. The 10 m wind is converted with the logarithmic profile u(z) = (u*/κ) ln(z/z0), κ = 0.4, z0 = 1 mm.
Drift growth (Exner equation)
Snow is conserved: ρs ∂h/∂t = −∂q/∂x + precipitation. Where the wind slows (behind a fence) the capacity qcap(u*(x)) falls below the incoming flux and the surplus is deposited; where it speeds up the surface is eroded. Each cell passes on min(capacity, incoming + available snow), so the scheme conserves mass to rounding error.
Fence wake
The fence lowers the near-surface friction velocity downwind by a factor 1 − r0 e−(x−xf)/Lw with r0 = 0.9(1−p) and Lw ≈ 9 Heff; as the drift fills the fence, the effective height falls and the fence stops working. Solid fences trap a short deep drift, porous ones a long shallow drift.
Honest simplifications
This is a 1-D (along-wind) parametric model: no turbulence, no snow metamorphism or sintering, constant threshold, one grain class, and a fitted wake instead of a flow solver. It illustrates the mechanism, not a design tool for real fences.
What to try
Run Bare field (nothing is trapped: flux in equals flux out), then Solid fence and watch the drift grow downwind of the fence until it is full. Compare Slatted fence: a longer, lower drift. Raise the threshold with Wet cohesive snow and the transport stops. Check the mass residual in the readouts.