Prognostic cloud fraction diagnosis from relative humidity and cloud water. More...

#include <AMReX_GpuControl.H>
#include <AMReX_Math.H>
#include <AMReX_FArrayBox.H>
#include <cmath>

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Functions
AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE amrex::Real	compute_relative_humidity (amrex::Real qv, amrex::Real T, amrex::Real P)
	Compute relative humidity from water vapor mixing ratio. More...

AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE amrex::Real	diagnose_cloud_fraction_from_rh_qc (amrex::Real rh, amrex::Real qc, amrex::Real rh_min, amrex::Real rh_max, amrex::Real qc_scale)
	Diagnose cloud fraction from relative humidity and cloud water. More...

Detailed Description

Prognostic cloud fraction diagnosis from relative humidity and cloud water.

Implements RH/qc-based cloud fraction diagnosis for use in TwoStream radiation.

Cloud fraction diagnosis formula: cf_rh(k) = max(0, min(1, (rh(k) - rh_min) / (rh_max - rh_min))) [linear ramp] cf_qc(k) = min(1, qc / qc_scale(k)) [qc contribution] cf_diag(k) = min(1, cf_rh(k) + cf_qc(k)) [combined with saturation]

References:

Function Documentation

◆ compute_relative_humidity()

AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE amrex::Real compute_relative_humidity	(	amrex::Real	qv,
		amrex::Real	T,
		amrex::Real	P
	)

Compute relative humidity from water vapor mixing ratio.

Approximate RH from qv and temperature using saturation mixing ratio. The saturation vapour pressure over liquid water is the Magnus formula with the WMO (2008) coefficients, applied to the temperature in Celsius.

Parameters

[in]	qv	Water vapor mixing ratio [kg/kg]
[in]	T	Temperature [K]
[in]	P	Pressure [Pa]

Returns: Relative humidity [0, 1]

 {
     // Guard against invalid inputs
     if (T <= 0.0 || P <= 0.0 || qv < 0.0 || !amrex::Math::isfinite(qv) ||
         !amrex::Math::isfinite(T) || !amrex::Math::isfinite(P)) {
         return 0.0;
     }
  
     // Saturation vapour pressure over liquid water from the Magnus formula,
     // with the WMO (2008) coefficients for the temperature in Celsius:
     //   e_sat = e0 * exp(a * tc / (tc + b)),  tc = T - T0
     // and the saturation mixing ratio qsat = epsilon * e_sat / max(e_sat, P - e_sat).
     const amrex::Real e0 = 611.2;  // Pa (saturation vapor pressure at 0°C)
     const amrex::Real T0 = 273.15; // K
     const amrex::Real a = 17.62;
     const amrex::Real b = 243.12;  // °C
  
     // tc + b <= 0 only below about 30 K, where e_sat tends to 0. Otherwise
     // arg < a, so e_sat stays below e0 * exp(a) and cannot overflow.
     const amrex::Real tc = T - T0;
     const amrex::Real arg = (tc + b > amrex::Real(0.0)) ? a * tc / (tc + b) : amrex::Real(-100.0);
     const amrex::Real e_sat = e0 * std::exp(arg);
  
     // Saturation mixing ratio. The denominator is floored at e_sat, as in
     // erf_qsatw, so qsat stays positive and bounded by epsilon when e_sat
     // approaches or exceeds P (hot or very low-pressure air).
     const amrex::Real epsilon = 0.622;  // ratio of molecular weights
     amrex::Real qsat = epsilon * e_sat / amrex::max(e_sat, P - e_sat);
     if (qsat <= 0.0) {
         qsat = 1.0e-6;  // fallback when e_sat underflows (unphysically cold air)
     }
  
     // RH = qv / qsat, clamped to [0, 1]
     amrex::Real rh = qv / qsat;
     if (rh < 0.0) rh = 0.0;
     if (rh > 1.0) rh = 1.0;
  
     return rh;
 }

Referenced by diagnose_cloud_fraction_prognostic(), and ParallelFor().

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◆ diagnose_cloud_fraction_from_rh_qc()

AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE amrex::Real diagnose_cloud_fraction_from_rh_qc	(	amrex::Real	rh,
		amrex::Real	qc,
		amrex::Real	rh_min,
		amrex::Real	rh_max,
		amrex::Real	qc_scale
	)

Diagnose cloud fraction from relative humidity and cloud water.

Computes diagnosed cloud fraction from RH and qc:

RH contribution: linear ramp from 0 at rh_min to 1 at rh_max
qc contribution: scaled by qc_scale coefficient
Combined: saturated blend (cf ≤ 1)

This is a GPU-safe inline function for device-side kernels.