Case 1 · 2023-11-30 — Afternoon convection, sparse gauges

The event. A weak afternoon convective episode on 30 November 2023, active 16–21 Z (area-mean max ~0.7 mm/h). The DA window ran 12–15 Z — 4 hourly 3D-Var cycles assimilating INEA Macaé radial velocity and reflectivity. The free forecast ran from 15 Z for 21 h (to 12 Z+1) on d01 → d02 → d03 via ndown.

Both branches (control and radar-cycled) use the same GFS 12Z pre-forecast as background — fair same-lineage baseline (no cold start, no spin-up mixing with the DA signal).

The nest picker placed d03 to the NE of the radar (centre −21.99°, −41.31°), climo <z> +0.32σ (indicative — DJF climatology applied to 30 Nov). The d03 box landed in a gauge-sparse region (10 stations) — the station verification there is empty; the signal lives on d02 and in the object-based metrics.

21-h accumulation scatter d02 — 21-h accumulated rainfall scatter — d02. MAE 20.5→14.6 mm (−29%).

case date

2023-11-30

afternoon event, 16–21 Z; weak (max ~0.7 mm/h area-mean)

climo <z>

+0.32σ

indicative — DJF climo applied to 30 Nov

DA cycles

12, 13, 14, 15 Z — hourly 3D-Var, rv + rf

headline

MAE d02 −29%

21-h accum 20.5→14.6 mm; MODE d03 centroid 51.6→10.6 km (1 hr matched)

Honest reading. Hourly point scores at d02 are slightly worse (POD ≥1 mm/h 0.30→0.20, BIAS rises 3.5–4.9) — DA injects moisture that over-forecasts light rain. Displacement-tolerant metrics improved: 21-h accum MAE −29% on d02; the one matched MODE hour shows d03 centroid collapsing from 51.6 to 10.6 km. The d03 box fell in a gauge-poor region (10 stations, no events recorded) — lesson: in weak cases the picker should consider observation density when choosing the fine-domain centre.

Radar DA case study — Macaé INEA, 30 November 2023 Afternoon weak convection · DA 12–15 Z · forecast 15 Z → 12 Z+1 · draft for WRF User Meeting 2026

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Where did the nest go?

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