Stable-TC defaults: spline_order=3, dt-scaled wind-sea floor on by default#66
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…fault Default the validated stable tropical-cyclone configuration so a symmetric TC eye is steady and axisymmetric out of the box. - WaveGrowth2D: spline_order default 1 -> 3 (cubic deposition). Order 1 (CIC) and 2 (TSC) imprint a 4-fold grid-axis "cross" on a symmetric TC eye (issue #59); cubic restores axisymmetry. Tripolar grids auto-fall-back to order 1 with a warning (was a hard error), so the new default works on any grid. - ODESettings.windsea_alpha default 0.0 -> 0.005 (wind-sea-scaled reconstruction floor). Damps the remesh limit cycle at wind/calm interfaces and the rotating eyewall, energy-conserving for a resolved eyewall. - mapping_2D: the floor is now automatically dt-scaled, alpha_eff = windsea_alpha * min(1, dt/10min). Erosion per unit time scales as alpha/dt, so a constant alpha drains the field at small dt; the dt-scaling bounds erosion and prevents the small-dt collapse, making the floor safe to leave on by default. Guidance (docstrings): resolve the eyewall (dx <~ Rmax/20), pick dt so the wave Courant C_cg = c_g*dt/dx ~ 2-5 (advective C_U = U*dt/dx ~ 10-20) — the eyewall stability sweet spot. On an under-resolved eyewall the floor can still collapse the storm; resolve first. Full analysis: analysis/stability FINDINGS_eyewall_stability. NB: changes default numerics — existing test/regression baselines will shift and need re-blessing. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Summary
Make the validated stable tropical-cyclone configuration the default, so a stationary symmetric TC eye is both steady in time and axisymmetric in space out of the box. Derived from a 900+ run eyewall/discontinuity stability sweep (
analysis/stabilityin the parent repo).Changes
WaveGrowth2D:spline_orderdefault1 → 3(cubic deposition). Order 1 (CIC) / 2 (TSC) imprint a 4-fold grid-axis "cross" on a symmetric eye (#59); cubic restores axisymmetry. Tripolar grids auto-fall-back to order 1 with a warning (previously a hard error), so the new default is safe on any grid.ODESettings.windsea_alphadefault0.0 → 0.005— the wind-sea-scaled reconstruction floor that damps the remesh limit cycle at wind/calm interfaces and the rotating eyewall (energy-conserving for a resolved eyewall).mapping_2D: the floor is now automatically dt-scaled,α_eff = windsea_alpha · min(1, dt/10min). Erosion per unit time ∝α/dt, so a constantαdrains the field at smalldt; the dt-scaling bounds erosion and prevents the small-dt collapse, making the floor safe to leave on by default.Recommended regime (now in docstrings)
dx ≲ Rmax/20(≥40 ideal).dtso the wave CourantC_cg = c_g·dt/dx ≈ 2–5(advectiveC_U = U·dt/dx ≈ 10–20) — the eyewall stability sweet spot (both temporal pulsing and spatial asymmetry minimize atC_cg ≈ 3). Typicaldt ≈ 5–10 min.Validation
With these defaults (order 3,
windsea_alpha=0.005dt-scaled,dx ≲ Rmax/20,C_cg ≈ 3), a CAT3–5 storm eyewall reachescv ~ 1e-3(temporal) andasym ~ 1e-3(axisymmetry); wind/calm discontinuities are fully steady. Full write-up:analysis/stability/FINDINGS_eyewall_stability_courant.{md,pdf}.This changes default numerics. Existing test/regression baselines will shift and need re-blessing. Downstream code that omits
spline_order/windsea_alphawill get the new behavior; pass explicit values to keep the old behavior.🤖 Generated with Claude Code