Physical constants

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Overview

PhysicalConstants collects fundamental physical constants used throughout Terrarium's process implementations. All constants are stored as fields of a single struct so that they are passed explicitly through the call graph — avoiding global state and keeping the code fully differentiable with Enzyme.jl. The struct is parametrically typed so that constants are automatically promoted to the model's numeric precision NF.

Terrarium.PhysicalConstants — Type

struct PhysicalConstants{NF}

A collection of general physical constants that do not (usually) need to be varied in parameter calibration.

Properties:

ρw: Density of water in kg/m^3
ρi: Density of ice in kg/m^3
ρₐ: Density of air at standard pressure and 0°C in kg/m^3
cₐ: Specific heat capacity of dry air at standard pressure and 0°C in J/(m^3*K)
Lsl: Sepcific latent heat of fusion of water in J/kg
Llg: Specific latent heat of vaporization of water in J/kg
Lsg: Specific latent heat of sublimation of water in J/kg
g: Gravitational constant in m/s^2
Tref: Reference temperature (0°C in Kelvin)
σ: Stefan-Boltzmann constant in J/(sm^2K^4)
κ: von Kármán constant
ε: Ratio of molecular weight of water vapor to dry air
Rₐ: Specific gas constant of air in J/(kg*K)
C_mass: Atomic mass of carbon [gC/mol]

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Default values follow standard references. Individual constants can be overridden at construction to support unit-testing or sensitivity studies.

Field	Symbol	Default	Units	Description
`ρw`	$\rho_w$	1000.0	kg/m³	Density of liquid water
`ρi`	$\rho_i$	916.2	kg/m³	Density of ice
`ρₐ`	$\rho_a$	1.293	kg/m³	Density of dry air at 0°C, 1 atm
`cₐ`	$c_a$	1005.7	J/(kg·K)	Specific heat of dry air
`Lsl`	$L_{sl}$	3.34×10⁵	J/kg	Latent heat of fusion
`Llg`	$L_{lv}$	2.257×10⁶	J/kg	Latent heat of vaporization
`Lsg`	$L_{sg}$	2.834×10⁶	J/kg	Latent heat of sublimation
`g`	$g$	9.80665	m/s²	Gravitational acceleration
`Tref`	$T_{\text{ref}}$	273.15	K	Reference temperature (0°C)
`σ`	$\sigma$	5.6704×10⁻⁸	W/(m²·K⁴)	Stefan-Boltzmann constant
`κ`	$\kappa$	0.4	—	von Kármán constant
`ε`	$\varepsilon$	0.622	—	Ratio of molecular weights $M_v / M_d$
`Rₐ`	$R_a$	287.058	J/(kg·K)	Specific gas constant of dry air
`C_mass`	—	12.0	gC/mol	Molar mass of carbon

Methods

Terrarium.celsius_to_kelvin — Function

celsius_to_kelvin(c::PhysicalConstants, T)

Convert the given temperature in °C to Kelvin based on the constant Tref.

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Terrarium.stefan_boltzmann — Function

stefan_boltzmann(c::PhysicalConstants, T, ϵ)

Stefan-Boltzmann law $M = \epsilon \sigma T^4$ where T is the surface temperature in Kelvin and ϵ is the emissivity.

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Terrarium.psychrometric_constant — Function

psychrometric_constant(c::PhysicalConstants, p)

Calcualte the psychrometric constant at the given atmospheric pressure p.

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Terrarium.compute_vpd — Function

compute_vpd(c, pres, q_air, T)

Computes the vapor pressure deficit over a surface at temperature T from the given surface pressure pres and specific humidity of air q_air.

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compute_vpd(
    i,
    j,
    grid,
    fields,
    atmos::Terrarium.AbstractAtmosphere,
    c::PhysicalConstants
) -> Any
compute_vpd(
    i,
    j,
    grid,
    fields,
    atmos::Terrarium.AbstractAtmosphere,
    c::PhysicalConstants,
    Ts
) -> Any

Computes the vapor pressure deficit over a surface at temperature Ts from the current atmospheric fields.

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