Units used in GNOME / PyGNOME

GNOME is a model of physical processes, and thus most of the things computed are in various physical units.

To keep things clean, the internal computational code is all handled in a consistent set of units. All values are stored in floating point types, generally 64 bit (double) unless otherwise noted. Almost all units are SI, but a few exceptions (legacy reasons…)

Conversion is done on I/O or occasionally within a class (i.e the class may store data in the units the user gave, but return values in the standard unit to other parts of the code) So unless otherwise noted, pass data into methods in the following standard units:

Time

In most places, time is represented by Python datetime and timedelta objects.

In a few cases (model timestep for instance) time is expressed in integer seconds

In the C / C++ code, time is stored as seconds in a C unsigned long, and datetimes are in seconds since 1904 (1904-01-01T00:00) – stored in a C unsigned long

Length

Lengths are in meters (m)

Mass

Mass is in kilograms (kg)

Volume

Volume is in cubic meters (m^3)

Density

Density is in kilograms per cubic meter (kg/m^3)

Velocity

Velocities are in meters per second (m/s)

Latitude-Longitude

Lat-long is in floating point degrees – range generally -360 to 360, so we can do stuff across the date line. It is up to the user to make sure all inputs are using the same system: -180–180 or 0–360.

Diffusion Coefficients

Diffusion coefficients (for both vertical and horizontal random diffusion) are given in units of square centimeters per second (cm^2/s) (note that this is not the SI unit, but it’s a long-standing tradition)

Droplet Diameter

Droplet Diameter is given and returned in meters (m)

Viscosity

Viscosity is usually kinematic viscosity, and is in units of square meters per second (m^2/s)

Salinity

Salinity is used in various calculations for sedimentation, wave formation, etc. Standard units is Practical Salinity Units (PSU) – more or less parts per thousand – e.g. fresh is 0, typical seawater is 35.

Surface Concentration

Surface concentration is in mass per area: grams per square meter. (g/m^2) – a deviation from SI.