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. Uses a simple saturation vapor pressure formula (Magnus or similar).

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;
     }
  
     // Simple saturation mixing ratio approximation (Magnus formula variant)
     // qsat = 0.622 * e_sat / (P - e_sat)
     // where e_sat = e0 * exp(a * (T - T0) / (T - b))
     // For simplicity, use approximate saturation vapor pressure
     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;  // K
  
     // Compute saturation vapor pressure (Magnus formula)
     amrex::Real arg = a * (T - T0) / (T - b);
     if (!amrex::Math::isfinite(arg) || arg > 100.0) {
         // Protect against overflow
         arg = 100.0;
     }
     amrex::Real e_sat = e0 * std::exp(arg);
  
     // Saturation mixing ratio: qsat = epsilon * e_sat / (P - e_sat)
     const amrex::Real epsilon = 0.622;  // ratio of molecular weights
     amrex::Real qsat = epsilon * e_sat / (P - e_sat);
     if (qsat < 0.0 || !amrex::Math::isfinite(qsat)) {
         qsat = 1.0e-6;  // fallback
     }
  
     // 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.