The CM_Constants module

This module stores a variety of constants and other data used during processing. It also defines the WaterData class that stores the wavelength-dependant water properties.

The WaterData class

This class stores per-wavelength water properties, used within the code to compute - for example - the white cap reflectance.

Mandatory class attributes:

  • wavelength: Float, the wavelength, in micrometers, at which the remaining attributes are defined.

  • refrac_real: Float, he real component of the refractive index.

  • refrac_imag: Float, he imaginary component of the refractive index.

  • base_abs: Float, the base water absorption coefficient.

  • base_backscat: Float, the base water backscatter coefficient.

  • whitecap_refl: Float, the whitecap reflectance of water at this wavelength.

  • chl_a_coef: List of length 2, the Chlorophyll-A absorption coefficient defined as

  • total_abs: Float, the total water absorption.

  • total_backscat: Float, the total water backscatter.

  • clhabs: Float, the Chlorophyll-A absorption.

  • clhbsc: Float, the Chlorophyll-A backscatter.

The two values in chl_a_coef are used to compute absorption using the following equation:

\[abs_{chl} = coef_0 * ( 1 - e^{-1.61 * conc_{chl}}) + coef_1 * conc_{chl}\]

Where \(conc_{chl}\) is the Chlorophyll concentration in \(mg/m^3\).

At present, pycoxmunk contains predefined WaterData information for the following wavelengths: 0.47, 0.55, 0.65, 0.87, 1.24, 1.375, 1.6, 2.13, and 3.7 \(\mu m\)

The constants

Also defined in this module are several constants used throughout the processing:

  • dither_more: A float specifying a very small number, used to test if variables are significantly greater than zero.

  • chl_a_conc: The default Chlorophyll-A concentration, 0.18 \(mg/m^3\). In the future this will be replaced by data from ESA’s Ocean Color CCI.

  • n_air: The refractive index of air, defined as 1.00029

  • cm_min_wvl: The minimum wavelength for processing via pycoxmunk. Defined as 0.2 \(\mu m\)

  • cm_max_wvl: The maximum wavelength for processing via pycoxmunk. Defined as 4.5 \(\mu m\)

  • zeisse_coef: Coefficients used for computing the Zeisse correction, which is applied at high zenith angles.

  • n_quad_theta: The number of quadrature points for computing zenith component of the diffuse BRDF.

  • n_quad_phi: The number of quadrature points for computing azimuth component of the diffuse BRDF.

The default value of both n_quad_theta and n_quad_phi is 4. Increasing either or both of these beyond 4 will result in more accurate calculation of the diffuse BRDF terms but will have a significant impact on processing time and memory requirements.

Lastly, CM_DATA_DICT is a dictionary that stores WaterData information at all of the wavelengths specified earlier.