Multi-Model Turbulence Analysis

Step 1: Shear-Based Severity

Step 2: GA-Calibrated Shear Thresholds

Wind shear magnitude determines the baseline severity:

Stability Cap & KHI Boost

When Ri ≥ 0.5 (stable or transitionally stable), severity is capped at LIGHT — never overridden to SMOOTH. Bulk Richardson numbers from NWP model layers (2,000–6,000 ft thick) can miss thin shear layers near fronts, low-level jets, and mountain waves where turbulence co-exists with a stable bulk layer (Sharman et al. 2006). Capping at LIGHT instead of clearing to SMOOTH prevents dangerous false clears.

Conversely, when Ri drops below 0.25 (dynamic instability / Kelvin-Helmholtz onset) above 5,000 ft AGL and shear is at least 4 kt/1000ft, severity is boosted by one level. This captures turbulence from KHI billows in environments near the tropopause or convective boundaries — a source of false negatives in shear-only models.

Boundary layer suppression: The KHI boost is suppressed below 5,000 ft AGL. Low Richardson numbers in the atmospheric boundary layer are routine mechanical mixing from surface friction and gusty winds — not the free-atmosphere dynamic instability that KHI detection targets. This prevents strong surface gusts (e.g., 40 kt) from triggering false SEVERE classifications in the lowest flight levels during approach and departure.

Severity band downgrading: Separately from the KHI boost suppression, any SEVERE turbulence band that lies entirely below 5,000 ft AGL is automatically downgraded to MODERATE and flagged as low-level wind shear. Strong surface gusts during approach and departure can produce high model-derived shear in the lowest layers, but this is a surface phenomenon — not the kind of en-route severe turbulence that warrants a SEVERE classification in your briefing. The 5,000 ft threshold is computed from airport elevation when available, so it works correctly at high-elevation airports like Denver (5,431 ft MSL → cap at ~10,431 ft MSL).

Conservative Max Consensus

For safety-critical aviation, PlaneWX uses a conservative max consensus: the highest severity predicted by any model is used, unless that model is an outlier by two or more categories (in which case it is tempered down by one level).

• Safety first: If the high-resolution HRRR detects severe turbulence that GFS and ECMWF smooth away, a median-based approach would report “Light” — a dangerous false clear that could lead a pilot into a KHI zone or rotor.
• Outlier protection: If one model says MODERATE and two say SMOOTH (a 2-category gap), the consensus is tempered to LIGHT rather than accepting the outlier value at face value.
• Per-model columns: You can still see each model's individual assessment in the table. If models disagree, the per-model columns make that visible so you can factor it into your decision.

At Each Sample Point, We Compute:

How It Works

• PlaneWX fetches a detailed terrain elevation profile along your route using a dedicated elevation API.
• Ridge orientation is estimated dynamically from the actual terrain gradient, with confidence predicates that assess gradient magnitude and bearing coherence across neighboring points. Low-confidence bearings trigger conservative fallbacks.
• Model sounding winds are interpolated at ridge level and decomposed into perpendicular-to-ridge and parallel-to-ridge components.
• Relief-scaled thresholds: Wind speed thresholds are dynamically adjusted based on terrain relief. Lower-relief terrain (Appalachians, coastal ranges) uses lower thresholds to avoid under-detection, while major barriers (Rockies, Sierra) retain higher thresholds. The formula is: base = 18 + (relief_ft / 1,000) × 4 kt, with SEVERE = base + 10 kt, MODERATE = base + 5 kt, LIGHT = base.
• Advisory tier: 15+ kt perpendicular flow over terrain with ≥2,000 ft relief generates a marginal mountain wave advisory in your briefing. This carries no WX Score penalty but alerts you to conditions where light to moderate mountain wave turbulence is possible near ridgelines.
• Sub-2,000 ft terrain promotion: Lower-relief terrain like the Appalachians, Ozarks, and coastal ranges (<2,000 ft crest-to-valley relief) can still produce significant turbulence when flow is blocked. When the Froude number indicates blocked flow (Fr < 0.60) and cross-ridge winds are ≥25 kt, severity is promoted by one level even over modest ridges. This prevents under-detection of real hazards in lower-relief terrain.
• Low-confidence ridge fallback: When terrain gradient is weak or ridge orientation cannot be determined with confidence, PlaneWX uses 70% of the total wind speed as a conservative estimate of the perpendicular component (the RMS value of |sin(θ)| over all possible ridge orientations). This avoids worst-case 100% assumptions that would over-alert, while still flagging genuine risk.
• Results feed into pre-flight checks, WX Score deductions, and the LLM briefing prompt as terrain-specific hazards.

Altitude-Banded Thresholds

When resolved shear exceeds the guard threshold for that altitude band, the Ri cap is bypassed and the shear-based severity stands:

Lower-altitude layers are relatively thinner and resolve shear more faithfully, so a higher threshold is appropriate. Upper layers are much thicker (~2,500–7,500 ft between pressure levels), heavily smoothing shear — a lower threshold protects weaker signals that likely represent stronger real-world shear.

Froude Number (Fr)

Fr = U_perp / (N × h), where all quantities are in SI units:

• U_perp — perpendicular wind component across the ridge in m/s (converted internally from knots; 1 kt = 0.514 m/s)
• N — Brunt-Väisälä frequency (atmospheric stability, s^-1), computed from potential temperature lapse between valley and crest
• h — effective ridge height (meters), max-min elevation within ±2.7 NM window, requiring ≥500 ft relief

Scorer Parameter Gate

Not all blocked-flow conditions produce rotors. Rotor turbulence requires trapped lee waves, which only form when the atmospheric profile supports wave trapping. PlaneWX computes the Scorer parameter (l²) from model soundings between ridge crest and ~4 km above crest to verify this condition:

l² = (N² / U²) − (1/U)(d²U/dz²)

• When min(l²) ≥ 0.25 between crest and crest+4 km, wave trapping is supported and the rotor alert stands.
• When min(l²) < 0.25, waves propagate vertically rather than trap — rotor formation is suppressed and the alert is removed. This prevents false alarms in common winter jet-stream scenarios where strong upper-level winds prevent wave trapping.
• When sounding data is insufficient to compute the Scorer parameter, the system falls through to the Froude-number analysis alone (conservative).

Complex Terrain Modifier

Simple 2D Froude analysis can over-predict rotor severity in complex 3D terrain where multiple ridge orientations exist (sinuous valleys, overlapping ridges). PlaneWX computes the circular variance of ridge-crest orientations within the analysis window. When variance exceeds 0.35 (indicating highly complex terrain), the Froude number is inflated by 25% to account for 3D flow-splitting effects that weaken rotors. Deeply blocked flows (Fr < 0.35) are exempt from this adjustment because 3D splitting can concentrate downslope flow in confined valleys.

Weight Categories

Normalization Examples

• Heavy aircraft (B747, B777, A330, A380): “Light” becomes “Moderate” and “Moderate” becomes “Severe” in your briefing.
• Medium aircraft (B737, A320, CRJ, E-jets, biz jets): “Light” becomes “Light-Moderate” and “Moderate” becomes “Moderate-Severe.”
• “Smooth” and “Negative” reports are not adjusted — smooth air is smooth regardless of aircraft weight.
• Reports from Light GA aircraft are not adjusted — they are already calibrated for your experience.
• Unknown aircraft types are flagged with a low-confidence note in the briefing. The intensity is displayed as-reported (not normalized), and the briefing explicitly notes that the aircraft type could not be identified. This helps you weigh the report appropriately.

Soft / Hard Limit System