Recalibrate refractivity thresholds for HRRR gradient distribution

Previous thresholds (-500 to -60) were calibrated for radiosonde data.
HRRR profiles have coarser vertical resolution, with gradients clustering
between -40 and -130 N/km (median -70). Nearly all grid points were
falling through to the default score of 42, wasting the refractivity
factor. New thresholds (-200 to -40) spread across HRRR percentiles.

algo.md

@@ -685,48 +685,21 @@ end
 
 ### 9. Refractivity Score — When Sounding/HRRR Data Available
 
-Best predictor when available. Binary duct detection is useless (54% baseline rate). Use continuous gradient magnitude and BL depth instead. Surface-based ducts (base=0) matter more than elevated ducts.
+Best predictor when available. Binary duct detection is useless (54% baseline rate). Use continuous gradient magnitude and BL depth instead.
 
-From sounding data: avg gradient -389 N/km for ducting events vs -123 for non-ducting. From HRRR: BL depth < 200m correlates with avg gradient of -93.7 (approaching super-refraction).
+Thresholds calibrated for HRRR-derived gradients which are coarser than radiosonde soundings. HRRR gradient distribution: p1=-230, p5=-162, p10=-130, p25=-94, p50=-70, p75=-53, p95=-40 N/km. Previous thresholds (-500 to -60) placed nearly all HRRR profiles in the default bucket.
 
-```elixir
-def score_refractivity(sounding_or_hrrr, band_config) do
-  cond do
-    sounding_or_hrrr == nil -> 50  # No data — neutral
+| Gradient (N/km) | Beneficial Score | Harmful Score | Condition |
+|---|---|---|---|
+| < -200 | 98 | 85 | Strong ducting (HRRR p1) |
+| < -150 | 92 | 80 | Enhanced refraction (HRRR p5) |
+| < -100 | 82 | 72 | Above-average refraction (HRRR p25) |
+| < -75 | 68 | 62 | Near-median gradient (HRRR p50) |
+| < -55 | 55 | 55 | Below-median (HRRR p75) |
+| < -40 | 48 | 48 | Weak gradient (HRRR p95) |
+| ≥ -40 | 42 | 42 | Standard/sub-refractive |
 
-    sounding_or_hrrr.min_dn_dh < -500 ->
-      # Strong ducting — gradient 3x non-ducting average
-      case band_config.humidity_effect do
-        :beneficial -> 98   # 10 GHz benefits most from ducting
-        :harmful    -> 85   # Higher bands benefit but absorption limits range
-      end
-
-    sounding_or_hrrr.min_dn_dh < -300 ->
-      # Moderate ducting — near ducting average of -389
-      case band_config.humidity_effect do
-        :beneficial -> 92
-        :harmful    -> 78
-      end
-
-    sounding_or_hrrr.min_dn_dh < -200 ->
-      80  # Enhanced refraction — above non-ducting average of -123
-
-    # BL depth signal: shallow BL at 12Z = strong surface duct potential
-    sounding_or_hrrr.bl_depth_m != nil and
-    sounding_or_hrrr.bl_depth_m < 300 ->
-      82  # Very shallow BL — strong inversion cap (12Z signature)
-
-    sounding_or_hrrr.min_dn_dh < -100 ->
-      65  # Mild enhancement
-
-    sounding_or_hrrr.min_dn_dh < -60 ->
-      55  # Slightly enhanced — within the broad "Enhanced" HRRR regime
-
-    true ->
-      42  # Standard/sub-refractive — HRRR shows only 5% of profiles here
-  end
-end
-```
+Shallow BL fallback: when gradient is unavailable but BL depth < 300m, score 82 (strong inversion cap).
 
 ---
 

lib/microwaveprop/propagation/band_config.ex

@@ -32,12 +32,15 @@ defmodule Microwaveprop.Propagation.BandConfig do
   @humidity_beneficial_thresholds [{4, 55}, {7, 70}, {10, 82}, {14, 90}, {18, 95}, {22, 88}]
   @humidity_beneficial_default 75
 
+  # Thresholds calibrated for HRRR-derived gradients (coarser than sounding data).
+  # HRRR percentiles: p1=-230, p5=-162, p10=-130, p25=-94, p50=-70, p75=-53, p95=-40
   @refractivity_thresholds [
-    {-500, 98, 85},
-    {-300, 92, 78},
-    {-200, 80, 80},
-    {-100, 65, 65},
-    {-60, 55, 55}
+    {-200, 98, 85},
+    {-150, 92, 80},
+    {-100, 82, 72},
+    {-75, 68, 62},
+    {-55, 55, 55},
+    {-40, 48, 48}
   ]
   @refractivity_default {42, 42}
   @shallow_bl_threshold_m 300

test/microwaveprop/propagation/band_config_test.exs

@@ -252,11 +252,12 @@ defmodule Microwaveprop.Propagation.BandConfigTest do
       thresholds = BandConfig.refractivity_thresholds()
 
       assert thresholds == [
-               {-500, 98, 85},
-               {-300, 92, 78},
-               {-200, 80, 80},
-               {-100, 65, 65},
-               {-60, 55, 55}
+               {-200, 98, 85},
+               {-150, 92, 80},
+               {-100, 82, 72},
+               {-75, 68, 62},
+               {-55, 55, 55},
+               {-40, 48, 48}
              ]
     end
   end

test/microwaveprop/propagation/scorer_test.exs

@@ -227,14 +227,14 @@ defmodule Microwaveprop.Propagation.ScorerTest do
       assert Scorer.score_refractivity(-600, 500, @band_10g) == 98
     end
 
-    test "strong ducting gradient returns 85 for harmful" do
-      # gradient < -500 -> 85 for harmful
-      assert Scorer.score_refractivity(-600, 500, @band_24g) == 85
+    test "strong gradient returns 85 for harmful" do
+      # gradient < -200 -> 85 for harmful (HRRR-calibrated)
+      assert Scorer.score_refractivity(-250, 500, @band_24g) == 85
     end
 
     test "moderate gradient returns appropriate score" do
-      # gradient < -300 but >= -500 -> 92 for beneficial
-      assert Scorer.score_refractivity(-400, 500, @band_10g) == 92
+      # gradient < -150 but >= -200 -> 92 for beneficial (HRRR-calibrated)
+      assert Scorer.score_refractivity(-160, 500, @band_10g) == 92
     end
 
     test "nil gradient returns 50" do