Add check for whether P(z) is bijective or not

src/Atmos.jl

@@ -22,11 +22,17 @@ Linearly interpolate `var1d` from `origin_pressure` to `target_pressure`.
 Note: Values outside of the range are linearly extrapolated
 """
 function _resample_column!(dest, var1d, origin_pressure, target_pressure)
+    reverse_origin_pressure = reverse(origin_pressure)
+    isunique =
+        length(unique(reverse_origin_pressure)) ==
+        length(reverse_origin_pressure)
+    !(isunique && issorted(reverse_origin_pressure)) &&
+        error("P(z) is not bijective, cannot resample column")
     # Interpolations.jl require increasing knots, but pressure is decreasing, so
     # we have to reverse it
     var1d_of_P = Intp.extrapolate(
         Intp.interpolate(
-            (reverse(origin_pressure),),
+            (reverse_origin_pressure,),
             reverse(var1d),
             Intp.Gridded(Intp.Linear()),
         ),

test/test_Atmos.jl

@@ -181,6 +181,30 @@ import OrderedCollections: OrderedDict
         Zvar,
         Zvar,
     )
+
+    # P(z) is not bijective (not sorted)
+    pressure_var = ClimaAnalysis.OutputVar(
+        attribs,
+        Dict("z" => z_alt),
+        dim_attribs,
+        reverse(pdata),
+    )
+    @test_throws ErrorException ClimaAnalysis.Atmos.to_pressure_coordinates(
+        pressure_var,
+        pressure_var,
+    )
+    pressure = 300.0:-2.0:100.0 |> collect |> reverse
+    pressure[2] = 100.0
+    pressure_var = ClimaAnalysis.OutputVar(
+        attribs,
+        Dict("z" => z_alt),
+        dim_attribs,
+        pressure,
+    )
+    @test_throws ErrorException ClimaAnalysis.Atmos.to_pressure_coordinates(
+        pressure_var,
+        pressure_var,
+    )
 end
 
 @testset "RMSE of pressure coordinates" begin