Constants: Atmospheric and Planetary Parameters
Summary
constants.F90 provides a number of controls that set the fundamental parameters of the planet being studied - it is used by nearly every other module in Isca. By default, Isca models a planet equivalent to Earth in terms of physical characteristics such as size, surface gravity and rotational period. In addition, water is set as the default condensate for any moist physics modules that may be used.
Planetary Parameters
Below are a set of controls that allow the physical size and rotation of a planet to be set in the namelist. This allows us to model a range of planets, including others in the Solar System such as Mars and Jupiter, in addition to exoplanets. Rotation can be set by two different parameters: a rotation rate omega can be specified; or alternatively a period in seconds can be given that is converted back to a value of omega.
Name |
Default |
Units |
Description |
|---|---|---|---|
|
\(6371\times 10^3\) |
m |
Radius of planet |
|
\(9.80\) |
ms \(^{-2}\) |
Surface gravitational acceleration |
|
\(7.2921150\times 10^{-5}\) |
rad \(\cdot\) s \(^{-1}\) |
Rotation rate of planet |
|
\(31557600\) |
s |
Orbital period of planet |
|
\(1368.22\) |
Wm \(^{-2}\) |
Stellar irradiance |
|
False |
n/a |
Modifies seconds per sol calculation (planetary solar day) |
By default the parameter seconds_per_sol (where earthday_multiple is false) is calculated using
whereas setting earthday_multiple to true modifies the calculation to
Note
Whilst the rotation and orbital rates are set within this module, other parameters associated with planetary motion such as axial tilt (obliquity) and eccentricity are controlled from the astronomy_mod module.
Atmospheric Parameters
Changing the basic atmospheric properties allows atmospheres with a wide range of compositions to be studied, including both terrestrial planets (e.g. Earth/Mars) and gas giants such as Jupiter. In the case of Earth, the reference surface pressure is taken to be the mean sea level pressure.
The dry air gas constant for any homogeneous atmosphere can be calculated from its chemical composition. It is calculated by dividing the universal gas constant \(R\) by the average molar mass of the atmosphere.
Name |
Default |
Units |
Description |
|---|---|---|---|
|
\(101325.0\) |
Pa / Nm \(^{-2}\) |
Mean (reference) surface pressure (SI) |
|
\(1.013250\times 10^6\) |
dyn \(\cdot\) cm \(^{-2}\) |
Mean (reference) surface pressure (cgs) |
|
\(287.04\) |
Jkg \(^{-1}\) |
Dry air gas constant |
|
\(2/7\) |
dimensionless |
Heat capacity ratio ( \(\gamma\) for an ideal gas) |
|
\(1.0\) |
dimensionless |
Humidity factor \(^\dagger\) |
\(^\dagger\): The humidity factor controls the atmospheric humidity content via the expression for saturation vapor pressure if do_simple: True is set within the idealized_moist_phys namelist.
Note
If the mean surface pressure value is changed here, it is necessary to also set reference_sea_level_press from the spectral_dynamics namelist, else the output file will not extend to the pressure specified.
Moist Atmospheres
We hope to be able to introduce controls for moist atmospheres in a future update to Isca. This will allow condensable species other than water to be easily modelled.
Relevant Modules
Since this module provides the definition of a number of physical constants, it is used by most other modules that exist within the Isca framework.