The CM_Shared_Wind Module

The CM_Shared_Wind module defines the CMSharedWind class that is used py pycoxmunk for storing and computing data associated with wind speed and its effect upon sea surface reflectance. The CMSharedWind class contains numerous variables and helper functions, which are described here. In general, the user should only have need of initialising the class itself, with the helper functions being used internally by pycoxmunk.

The CMSharedWind class

Initialisiation

This class is initialised by the user with three arguments: Firstly, the scene geometry must be supplied via the CMSceneGeom class. The user must also supply u10 and v10, which define the U and V components of wind speed at 10m above the surface. These data are commonly output from numerical weather models such as NOAA and ECMWF’s operational forecasts, and are also available via widely-used reanalysis datasets such as ERA5. These two wind speeds can be provided each as a single float, which will result in a constant wind speed being used across the satellite image, or as arrays of the same shape as the satellite image, in which case the wind is assumed to be per-pixel.

The class also contains numerous other variables, but these are calculated internally and are not required to be provided by the user.

The simplest way to initialise CMSharedWind is to use the helper function from the main pycoxmunk.PyCoxMunk class:

from pycoxmunk import PyCoxMunk

# Initialise pycoxmunk
pcm = PyCoxMunk(scn, bnames)  # Where scn and bnames are defined elsewhere in your code

# Some function to retrieve wind
wind_u10, wind_v10 = get_wind()

# Initialise winds
pcm.setup_wind(wind_u10, wind_v10 )

You can also initialise the class manually:

from pycoxmunk.CM_SceneGeom import CMSceneGeom
from pycoxmunk.CM_Shared_Wind import CMSharedWind

# Some function to retrieve wind
wind_u10, wind_v10 = get_wind()

# Some function to set up the scene geometry
scn_geom = CMSceneGeom()

# Initialise winds
my_winds = CMSharedWind(scn_geom, wind_u10, wind_v10 )

The zeisse_ba function

The zeisse_ba function is used internally for computing a correction factor to the sea surface reflectance that is used at zenith angles greater than 70°. This process is described in Zeisse’s 1995 paper.

Arguments:

  • theta: Float or array, the zenith angle in radians.

  • cos_theta: Float or array, the cosine of the zenith angle.

  • ws: Float or array, the wind speed in m/s.

Returns:

  • ba: Float or array (depending on input variables), the ergodic cap area when the zenith angle is greater than 70° and wind speed is greater than 1m/s. Otherwise, the cosine of the zenith angle.

The calc_wind function

A helper function to calculate variables required by the pycoxmunk code that are not directly provided by the user. This function is called internally when the CMSharedWind class is initialised. Argument: - scenegeom: A CMSceneGeom type that contains the various satellite and solar geometry variables.

Returns:

  • Nothing

Nothing is returned by this function, but various internal variables relating to sea surface roughness and reflectance are calculated and stored within the CMSharedWind class.