# Access-ais3 **URL:** \ **Category:** Experiments\ **Created:** [23 July 2026 05:24 UTC](https://forum.access-hive.org.au/t/access-ais3/6709 "2026-07-23T05:24:56Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![justinh2002](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.access-hive.org.au/justinh2002/32/2853_2.png) [@justinh2002](https://forum.access-hive.org.au/u/justinh2002)\ **Post date:** [23 July 2026 05:24 UTC](https://forum.access-hive.org.au/t/access-ais3/6709/1 "2026-07-23T05:24:56Z")
> > > \# ACCESS-AIS3 inversion work log > > Diagnosis and repair of the present-day fricti…on / rheology inversion. > Goal: a well-initialised Antarctic state with \*\*grounded velocity RMSE \< 50 m/yr\*\* and > spatially-white residuals (the reference result achieved with shelf-only rheology > inversion plus a friction inversion, grounded B from temperature). > > \--- > > \## 1. Headline progress > > | configuration | grounded RMSE (m/yr) | notes | > |---|---|---| > | null model (\`RMS(v\_obs)\`) | 310.6 | baseline to beat | > | original setup | 323.5 | \*\*worse than null\*\* — model was effectively static | > | joint friction + B, 70 iters | 171.2 | 70 % of grounded within ±50 | > | \*\*friction-only, 200 iters\*\* | \*\*110.6\*\* | \*\*83 % within ±50; median residual −0.1\*\* | > > Residual structure at 110.6: > > \`\`\` > median -0.1 mean -4.9 std 110.5 |res| \< 50 : 83 % > grounded, \>100 km inland : RMSE 55.3 ratio 1.00 \<- already at target > grounded, 0-5 km from GL : RMSE 185.9 ratio 1.18 \<- grounding zone > Transantarctic Mountains : RMSE 269.6 ratio 2.00 \<- worst region > shelves (out-of-sample) : RMSE 237.9 ratio 1.03 > \`\`\` > > \*\*The error is highly concentrated: 9.01 % of grounded vertices carry 90.6 % of the total > squared error.\*\* The interior is already at the target; what remains is a small number of > identifiable regions. > > \--- > > \## 2. Bugs found and fixed > > \### 2.1 Friction coefficient initialised in a dead zone > With Budd \`p=q=3\`, \`tau\_b = C^2 · N · u^(1/3)\`, so \*\*\`u ∝ C^-6\`\*\*. The initial guess > \`C = 100\` put the model six orders into a region where \`∂u/∂C ≈ 0\`; grounded ice was > static (median velocity 0.0 m/yr against 71.9 observed). > > Fixed: \`C\_init = 1.8\`, bounds \`\[0.1, 10\]\`, \`control\_scaling\_factors = 1.8\`. > > \*\*Consequence:\*\* every earlier tuning result is void. The "best config" from the original > regularisation campaign was optimised against \`fastGrnd RMSE ≈ 498\`, which was literally > \`RMS(v\_obs) = 499.1\` — the metric never measured model skill because the model did not move. > > \### 2.2 \`rheology\_B\` was a uniform placeholder > \`cuffey(273.15 − 20) = 2.027e8\` everywhere, collapsing a ~17× real spatial viscosity range > (8.5e7 → 1.43e9 over the RACMO range) to one value. 51 % of grounded vertices were wrong by > \> 20 %. No friction inversion can correct this — friction acts on the bed, the error is in > the ice. > > Fixed in \`config/ais\_0.1\_param.py\`: B derived from the RACMO surface-temperature field. > Caveat retained in the code: surface temperature is a proxy for depth-averaged, biased stiff. > > \### 2.3 Wrong cost-function ID for friction regularisation > The friction L-curve used \*\*502\*\* (\`RheologyBbarAbsGradient\`), which is not a control in a > friction inversion — the term was inert and the L-curve flat. Fixed to \*\*501\*\* > (\`DragCoefficientAbsGradient\`) in \`config/ais\_0.1.py\`. > > \### 2.4 Element-based friction mask stripped drag from grounded ice > The zero-friction mask flagged \*all\* vertices of any element containing one floating > vertex, removing basal drag from a band of genuinely grounded ice at the grounding line. > 92.7 % of the velocity blow-up lived in that band. > > Fixed: vertex-based test \`ocean\_levelset \< 0\`. > > \### 2.5 Absolute-misfit weighting ignored the slow interior > \`cf101 = 1000 / cf103 = 0.1\` weights absolute error, so the slow interior contributed > almost nothing. Fixed: log-weighted \`cf101 = 10 / cf103 = 100\`. > > \--- > > \## 3. Dead ends (with evidence) > > \### 3.1 Friction regularisation (501) is unusable > Sweeping 501 upward produced \*\*five byte-identical non-starters\*\* — \`C moved 0.0 %\`, > roughness exactly 0.0000. > > Root cause: the friction field steps \`1.8 → ~0\` across the grounding line because floating > C is pinned. At the initial guess that single ring of elements (2.3 % of the mesh) supplied > \*\*100.0000 %\*\* of \`|grad C|^2\`; grounded ice contributed exactly \*\*0\*\*. Since floating C is > bounded to \`\[0,0\]\`, the penalty is irreducible — a fixed cost with a gradient ~10^7 times > the misfit gradient, which kills the m1qn3 line search for every \`501 \>= 1e2\`. > > Masking the grounding-line ring out of the functional removed the artefact (verified: > \`|grad C|^2\` at init \`1.423e5 → 0\`) and the runs then iterated 17–33 steps. But they still > stalled early with a \*\*ruined fit\*\* (RMSE 2900–3300, worse than null; C moved only ~25 %). > The smooth fields were smooth because they had barely moved, not because they were coherent. > > \*\*Conclusion:\*\* the data term wants sharp features at ice-stream margins, the smoothing term > fights them, and the line search cannot reconcile the two. Abandoned; 501 left inert. > > \### 3.2 Joint friction + B inversion is worse than friction-only > | | abs misfit (101) | log misfit (103) | grounded RMSE | > |---|---|---|---| > | joint (stopped, dxmin @ 70) | 684.1 | 1.399e6 | 171.2 | > | friction-only (200 iters) | 615.9 | 6.852e5 | \*\*110.6\*\* | > > B freedom let the optimiser \*\*stop early at a worse velocity fit\*\*. A previously-reported > "invariant fast-ice deficit of 0.70–0.74 across every configuration" was an artefact of > this early stopping — friction-only running to 200 iterations moved it to \*\*0.87\*\* on its own. > > \### 3.3 Dropping the thin-ice floor entirely > Removing the floor \*and\* excluding thin ice (H \< 50 m) from the misfit gave > \*\*RMSE 1917.4, maxvel 1.6e5\*\* — a runaway. The lower cost function was an illusion created > by the smaller misfit set: excluding thin ice did not fix it, it merely stopped constraining > it. Thin ice must stay \*\*in\*\* the misfit. > > \--- > > \## 4. The Transantarctic Mountains failure > > Confirmed geographically: 30,739 grounded vertices, \*\*RMSE 269.6, ratio 2.00\*\* (2× too fast). > > Residual binned by physical regime (baseline joint run): > > | regime | n | RMSE | ratio | > |---|---|---|---| > | N \< 1e5 Pa | 18,355 | 678.9 | \*\*14.56\*\* | > | H floored to 100 m | 71,578 | 368.7 | 2.19 | > | slope \> 0.1 | 88,152 | 357.7 | 2.56 | > | H 0–150 m | 85,783 | 342.8 | 1.93 | > | bed 1000–4000 m | 66,695 | 265.3 | 1.58 | > | \*real ice, H \> 100 m\* | 1,158,642 | 150.8 | 1.01 | > > Every failing regime is \*\*too fast\*\*, and they all describe thin ice on steep, high bedrock. > > \### Root cause: a self-inflicted geometry error (in the experiment scripts, not production) > The experiment scripts floored thickness at 100 m and then rebuilt > \`surface = bed + H\`, which \*\*raised the surface\*\* wherever real ice was thinner and corrupted > the \*\*surface slope\*\*. Since \`tau\_d = rho·g·H·grad(s)\`, that fabricates the driving stress — > worst exactly in the mountains. > > Production (\`config/ais\_0.1\_param.py:62\`) does it correctly, flooring at 10 m and absorbing > the change into the \*\*base\*\*, leaving the observed surface intact. > > \### Hypothesis that was refuted > Effective-pressure starvation was proposed as the cause and \*\*disproved\*\*: above sea level > \`N/overburden = 1.016\`, i.e. already correct. Genuine N-starvation exists but only in the > small 18k-vertex group (\`N/overburden = 0.007\`) responsible for the 14.56× outliers. > > \--- > > \## 5. Shelf rheology is inconsistent with the coupled model > > \`md.extract()\` imposes Dirichlet BCs from \`vel\_obs\` on \*\*every\*\* new boundary node > (\[\`Model.py:558-560\`\](../../../pyISSM/src/pyissm/model/Model.py)). For the floating-only > extraction used by the shelf inversion, that clamped \*\*both\*\* the grounding line and the > \*\*calving front\*\* to observations. The coupled run instead uses a \*\*Neumann\*\* shelf front. > > So B was fitted while artificially restrained; released, the shelf accelerates: > > | distance from GL (km) | RMSE | median residual | ratio | > |---|---|---|---| > | 0–5 | 227.4 | −2.9 | 1.08 | > | 15–30 | 260.9 | +7.5 | 1.03 | > | 60–120 | 343.4 | +108.6 | 1.19 | > | 120–250 | 296.5 | \*\*+226.9\*\* | \*\*1.26\*\* | > > The error \*\*grows\*\* with distance from the grounding line — i.e. it is worst at the calving > front, not inherited from grounded ice. > > \*\*A shelf-only domain cannot be made BC-consistent.\*\* Freeing the calving front on floating > ice (which has no basal drag) leaves the system nearly singular; the solve fails outright > (\`Recovery solver failed\`, 0 iterations). The original workflow only worked because the > all-Dirichlet extraction was propping it up — the very artefact that biased B soft. > > Therefore shelf B must be inverted \*\*inside the coupled domain\*\*, where the front BC and the > grounding-line inflow are both exactly what production uses. > > \--- > > \## 6. Production config status > > Committed: > \- \`ais\_0.1\_param.py\` — temperature-derived \`rheology\_B\` > \- \`ais\_0.1.py\` — 502 → 501 fix, \`extract\_friction\_inversion\_domain()\` (Neumann fronts), > \`friction\_coupling\` variable > > \*\*Still carrying known-wrong values:\*\* > > | parameter | production | validated | > |---|---|---| > | \`friction.coefficient\` init | 10.0 | \*\*1.8\*\* (bounds \`\[0.1, 10\]\`) | > | \`friction\_cf101 / cf103\` | 1000 / 0.1 | \*\*10 / 100\*\* | > > The \`1000 / 0.1\` weighting comes from the invalidated campaign (§2.1). Not yet promoted > because the configuration is still being revised. > > Correct as-is: thickness floor 10 m into the base; \`N\_floor\_frac = 0.07\`; Budd \`p=q=3\`; > \`coupling = 3\`. > > \--- > > \## 7. Open items > > 1. \*\*N-starved vertices\*\* — 18,355 with \`N/overburden = 0.007\`, running 14.56× too fast. > 2. \*\*Grounding zone\*\* — grounded ice 0–5 km from the GL is 18 % too fast (RMSE 185.9 vs 55.3 inland). > 3. \*\*Steep-slope tail\*\* — slope \> 0.1, 2.56× too fast. > 4. \*\*C field is noisy\*\* — roughness 0.164, 3 % at bounds. Not blocking the fit, but 501 > cannot fix it; would need a different smoother (e.g. inverting \`log C\`). > 5. \*\*Promote validated parameters\*\* to the production config. > 6. \*\*Forward / relaxation validation\*\* — not yet run. > > \## 8. Practical notes > > \- \`qcat -o \\` streams a running job's stdout; ISSM writes \`.outlog\` only at exit. > \- 192 cores (4 nodes, 760 GB) gives \*\*6.5×\*\* the throughput of 32 cores: 35.2 vs 5.45 > iterations/hour, essentially linear scaling. Memory \*rises\* with rank count > (130 GB @ 32 → 450 GB @ 192) due to halo duplication. > \- Friction-only inversions need ≥ 200 iterations; the 110.6 run hit the maxsteps ceiling > still improving. > \- Cost-function values are \*\*not comparable\*\* between runs with different misfit masks — > always compare grounded RMSE on a common mask.