Constants and Units

Units

The following units are used in ERF:

Name

Units

Description

\(t\)

\(s\)

time

\(\rho\)

\(kg/m^3\)

density

\(\mathbf{u}\)

\(m/s\)

velocity

\(p\)

\(Pa\)

pressure

\(T\)

\(K\)

temperature

\(\theta\)

\(K\)

potential temperature

Constants

The following are ERF’s fixed reference values. They should not be confused with height-dependent reference-state fields or runtime inputs.

18 Selected physical constants
Symbol	Value	Meaning
\(R_d\)	287.0 J/(kg K)	Dry-air gas constant.
\(R_v\)	461.505 J/(kg K)	Water-vapor gas constant.
\(C_{p,d}\)	1004.5 J/(kg K)	Fixed reference dry-air heat capacity.
\(P_{00}\)	\(10^5\) Pa	Fixed reference pressure for potential temperature and the EOS (the C++ constant is named `p_0`); not the hydrostatic profile \(p_0(z)\).
\(g\)	9.81 m/s²	Magnitude of gravitational acceleration.
\(\Gamma\)	1.4	Fixed compressible EOS exponent, consistent with \(C_{p,d}/(C_{p,d}-R_d)\) at the reference constants.

The runtime option erf.c_p defaults to \(C_{p,d}\) but does not change the fixed EOS exponent \(\Gamma\). If erf.c_p is changed, thermodynamic conversions that use \(R_d/c_p\) can be inconsistent with inverse relations using the fixed exponent. See Governing Equations and Buoyancy.

These constants are defined in the file Source/ERF_Constants.H, which holds the thermodynamic and dynamical constants used throughout ERF. Two companion headers hold the rest:

Source/ERF_NumericalConstants.H – dimensionless numeric literals (zero, one, myhalf, …) and the mathematical constant \(\pi\). These carry no physical meaning; they exist so the code stays precision-agnostic between double and single builds. ERF_Constants.H includes this header.
Source/Microphysics/ERF_MicrophysicsConstants.H – constants used only by the moisture and cloud-physics code: hydrometeor densities, the temperature thresholds that partition condensate among the hydrometeor species, terminal fall-speed coefficients, autoconversion thresholds and collection efficiencies, size-distribution intercepts, and the latent heats of condensation, fusion and sublimation. This directory is on the include path for every ERF build, so any file may include it.