# NEXRAD Clear-Air Stability Spike — Findings

**Date:** 2026-04-10
**Purpose:** Task 3.1 of the propagation modeling improvements plan.
**Question:** Can NEXRAD composite reflectivity texture be used as an observed BL stability feature?

## Data Source

IEM provides CONUS n0q composite reflectivity as PNG at 5-minute cadence:

```
https://mesonet.agron.iastate.edu/archive/data/YYYY/MM/DD/GIS/uscomp/n0q_YYYYMMDDHHmm.png
```

- Resolution: 12200 × 5400 pixels at 0.005°/pixel (~550m)
- Coverage: CONUS (-126°W to -65°W, 23°N to 50°N)
- Cadence: every 5 minutes
- Historical availability: confirmed back to at least 2022 (spot-checked)
- World file provides georeferencing
- File size: 2-4 MB per frame

## Visual Assessment

Downloaded dawn (10Z) vs afternoon (20Z) pairs for:
- 2022-06-11 (stable summer day)
- 2022-08-20 (convective August day, 382 QSOs in our DB)

**Visual gate: marginal.** The n0q product is primarily sensitive to precipitation-sized hydrometeors. The "smooth clear-air echoes" the meteorologist described are faint returns from insects, dust, and refractive index gradients in the BL — these show up on individual radar sites in clear-air mode but are largely thresholded out in the CONUS composite product.

## Compression-Ratio Proxy

PNG compression efficiency correlates inversely with image texture (more complex patterns compress less). As a very crude metric:

| Date | Dawn (10Z) | Afternoon (20Z) | Ratio |
|---|---|---|---|
| 2022-06-11 (stable) | 2.14 MB | 2.40 MB | 1.13 |
| 2022-08-20 (convective) | 2.30 MB | 4.18 MB | 1.81 |

The direction is correct: afternoon images have more texture than dawn, and the difference is much larger on convective days. This is consistent with the meteorologist's claim that BL turbulence produces spatially heterogeneous radar returns.

## Assessment

**The signal exists but the n0q CONUS composite may not be the right product.**

Better options to investigate:
1. **Individual radar Level III products** — higher sensitivity, preserve clear-air returns that the composite thresholds out. Available from IEM per station.
2. **MRMS (Multi-Radar Multi-Sensor)** — NOAA's seamless mosaic at 1 km resolution, preserves more low-reflectivity detail than the IEM composite.
3. **Level II raw data** — full volume scans. Maximum sensitivity but requires complex ingestion (not a PNG).

## Recommendation

**Park Phase 3 for now.** The concept is sound but the n0q CONUS composite isn't sensitive enough. Revisit when:
- We have bandwidth to spike MRMS or Level III per-station products
- Or when Phase 9 recalibration identifies BL turbulence as a weak spot in the Phase 2 features (Ri, θₑ, shear) and we need an observed check on the model-derived values

The Phase 2 turbulence features (computed from HRRR native profiles) are a good interim proxy. NEXRAD would add independent observational confirmation, but it's not the highest-priority data source to build right now.