introduce hard coded exner near surface pressure

Author: glwagner

src/Thermodynamics/atmosphere_thermodynamics.jl

@@ -318,9 +318,11 @@ end
     Rᵐ = mixture_gas_constant(state.q, thermo)
     cᵖᵐ = mixture_heat_capacity(state.q, thermo)
     inv_ϰᵐ = Rᵐ / cᵖᵐ
+
     pᵣ = reference_pressure(state.z, ref, thermo)
     FT = eltype(pᵣ)
-    pₑ₀ = convert(FT, 1e5) # hard-coded for now
+    pₑ₀ = convert(FT, 1e5) # "exner surface reference pressure" -- hard-coded for now
+
     return (pᵣ / pₑ₀)^inv_ϰᵐ
 end