prop/test/microwaveprop/weather/skewt_params_test.exs

defmodule Microwaveprop.Weather.SkewtParamsTest do
  use ExUnit.Case, async: true

  alias Microwaveprop.Weather.SkewtParams

  # A moderately unstable summer-like profile. Surface ≈ 30 °C / 22 °C
  # dewpoint, gradually drier and colder aloft. Used as a baseline for
  # LCL/LFC/EL/CAPE assertions.
  @summer_profile [
    %{"pres" => 1000.0, "hght" => 100.0, "tmpc" => 30.0, "dwpc" => 22.0},
    %{"pres" => 925.0, "hght" => 800.0, "tmpc" => 24.0, "dwpc" => 19.0},
    %{"pres" => 850.0, "hght" => 1500.0, "tmpc" => 18.0, "dwpc" => 14.0},
    %{"pres" => 700.0, "hght" => 3100.0, "tmpc" => 8.0, "dwpc" => -2.0},
    %{"pres" => 500.0, "hght" => 5800.0, "tmpc" => -10.0, "dwpc" => -25.0},
    %{"pres" => 300.0, "hght" => 9300.0, "tmpc" => -38.0, "dwpc" => -55.0},
    %{"pres" => 200.0, "hght" => 12_000.0, "tmpc" => -60.0, "dwpc" => -80.0}
  ]

  # A capped / stable profile: subsidence inversion above the surface
  # mixed layer. Should produce 0 CAPE.
  @capped_profile [
    %{"pres" => 1000.0, "hght" => 100.0, "tmpc" => 25.0, "dwpc" => 18.0},
    %{"pres" => 925.0, "hght" => 800.0, "tmpc" => 28.0, "dwpc" => 5.0},
    %{"pres" => 850.0, "hght" => 1500.0, "tmpc" => 25.0, "dwpc" => -5.0},
    %{"pres" => 700.0, "hght" => 3100.0, "tmpc" => 12.0, "dwpc" => -15.0},
    %{"pres" => 500.0, "hght" => 5800.0, "tmpc" => -8.0, "dwpc" => -30.0}
  ]

  describe "lcl/3" do
    test "returns LCL pressure and temperature consistent with Bolton (1980)" do
      # Surface 30°C / 22°C dewpoint at 1000 mb → LCL ~880-920 mb,
      # T_LCL ~17-22°C. Tolerances stay loose because Bolton is an
      # approximation and the surface RH already drives most of the
      # answer.
      {p_lcl, t_lcl} = SkewtParams.lcl(30.0, 22.0, 1000.0)
      assert p_lcl >= 850.0 and p_lcl <= 940.0
      assert t_lcl >= 16.0 and t_lcl <= 23.0
    end

    test "LCL pressure equals surface pressure when air is saturated" do
      {p_lcl, t_lcl} = SkewtParams.lcl(20.0, 20.0, 1000.0)
      assert_in_delta p_lcl, 1000.0, 1.0
      assert_in_delta t_lcl, 20.0, 0.2
    end
  end

  describe "derive/1" do
    test "produces every field documented in the public spec for a non-trivial profile" do
      params = SkewtParams.derive(@summer_profile)

      # Surface-based parcel parameters present.
      assert is_map(params)
      assert is_number(params[:lcl_pressure_mb])
      assert is_number(params[:lcl_height_m])
      assert is_number(params[:sbcape])
      assert is_number(params[:sbcin])
      assert is_number(params[:lifted_index])
      # LFC/EL may be nil if no positive area exists — but for this
      # unstable summer profile they must resolve.
      assert is_number(params[:lfc_pressure_mb])
      assert is_number(params[:el_pressure_mb])
      # Cold-side milestones.
      assert is_number(params[:freezing_level_m])
      assert is_number(params[:wbz_m])
      # Layer lapse rates.
      assert is_number(params[:lapse_rate_700_500_c_per_km])
      assert is_number(params[:lapse_rate_sfc_3km_c_per_km])
      # Parcel trajectory used by SkewtSvg to draw the dashed grey line.
      assert is_list(params[:parcel_trace])
      assert length(params[:parcel_trace]) >= 2
    end

    test "summer profile is unstable: SBCAPE > 0, SBCIN <= 0, LFC < surface" do
      params = SkewtParams.derive(@summer_profile)

      assert params[:sbcape] > 0
      # CIN by convention is negative (or zero).
      assert params[:sbcin] <= 0
      assert params[:lfc_pressure_mb] < 1000.0
      assert params[:el_pressure_mb] < params[:lfc_pressure_mb]
      # Lifted index < 0 for an unstable summer atmosphere.
      assert params[:lifted_index] < 0
    end

    test "capped profile is stable: SBCAPE = 0, no LFC" do
      params = SkewtParams.derive(@capped_profile)
      assert params[:sbcape] == 0.0
      assert params[:lfc_pressure_mb] == nil
      assert params[:el_pressure_mb] == nil
      # Lifted index > 0 in a stable atmosphere.
      assert params[:lifted_index] > 0
    end

    test "freezing level is the height where T crosses 0 °C" do
      params = SkewtParams.derive(@summer_profile)
      # Profile goes 30→24→18→8→-10°C between 100 m and 5800 m.
      # 0°C falls between 700 mb (8°C, 3100 m) and 500 mb (-10°C, 5800 m).
      # Linear interpolation puts it ~4300 m.
      assert params[:freezing_level_m] >= 3700
      assert params[:freezing_level_m] <= 4900
    end

    test "lapse rate sfc-3km is the (T_sfc - T_3km) / 3 km gradient" do
      params = SkewtParams.derive(@summer_profile)
      # 30 °C at 100 m, ~9 °C at 3100 m → ~7 °C/km drop.
      assert params[:lapse_rate_sfc_3km_c_per_km] >= 5.5
      assert params[:lapse_rate_sfc_3km_c_per_km] <= 9.0
    end

    test "empty profile returns a fully-nil parameter map (no crash)" do
      params = SkewtParams.derive([])
      assert is_map(params)
      assert params[:sbcape] == nil
      assert params[:lcl_pressure_mb] == nil
      assert params[:parcel_trace] == []
    end
  end
end