Light Extinction

The light_extinction section controls how incident shortwave radiation is partitioned into visible and non-visible wavebands and attenuated as it penetrates the water column. pyGOTM implements the two-band exponential scheme of Paulson and Simpson (1977) as used in GOTM.

The physics is implemented in pygotm.airsea.shortwave_radiation. The parameters feed into pygotm.gotm.runtime_params.RuntimeParams.light_A, light_g1, and light_g2.

Parsed by pygotm.config.settings.LightExtinctionSettings.

light_extinction:
  method: jerlov-i
  A:
    method: constant
    constant_value: 0.58
  g1:
    method: constant
    constant_value: 0.35
  g2:
    method: constant
    constant_value: 23.0

light_extinction.method

Type	string
Valid values	jerlov-i, jerlov-1-50m, jerlov-ia, jerlov-ib, jerlov-ii, jerlov-iii, custom
Default	"jerlov-i"

Selects the water-type preset or enables manual tuning.

Jerlov (1968) classified natural seawater into optical water types based on measured spectral transmittance. Each type has known values for the partition coefficient A and the two e-folding depths g1 and g2.

Jerlov water type presets

Method token	A	g1 (m)	g2 (m)
jerlov-i	0.58	0.35	23.0
jerlov-1-50m	0.68	1.20	28.0
jerlov-ia	0.62	0.60	20.0
jerlov-ib	0.67	1.00	17.0
jerlov-ii	0.77	1.50	14.0
jerlov-iii	0.78	1.40	7.9

When a preset is chosen, the values of A, g1, and g2 in the YAML are overridden by the preset constants.

custom

Use the values of A, g1, and g2 as specified in the YAML. Each can be a constant or a time-varying file input.

light_extinction.A

Non-visible fraction of shortwave radiation (near-infrared band).

Follows the InputSetting pattern (method: constant | file).

Units	dimensionless (fraction, 0–1)
Default	0.7

The remaining fraction \(1-A\) is the visible (photosynthetically active radiation, PAR) band. Both bands decay exponentially with depth.

The total shortwave irradiance at depth z (below surface) is:

\[I(z) = I_0 \left[ A \, e^{-z/g_1} + (1-A) \, e^{-z/g_2} \right]\]

where \(I_0\) is the surface irradiance after albedo correction.

light_extinction.g1

E-folding attenuation depth of the non-visible (red/near-infrared) band.

Follows the InputSetting pattern.

Units	m
Range	> 0.0
Default	0.4

Shorter g1 values indicate more turbid water (higher non-visible attenuation). For clear open-ocean water (Jerlov type I), g1 ≈ 0.35 m.

light_extinction.g2

E-folding attenuation depth of the visible (blue/green) band.

Follows the InputSetting pattern.

Units	m
Range	> 0.0
Default	8.0

Larger g2 values represent clearer water. For clear open-ocean water (Jerlov type I), g2 ≈ 23 m.

Note

When method: custom, all three parameters (A, g1, g2) can be read from a time-varying file to represent seasonal phytoplankton blooms or changing turbidity. Use column to select the appropriate column from a shared extinction data file.

The conventional three-column extinction file layout used in the reference cases is:

# Columns: A [-]   g1 [m]   g2 [m]
1976/04/06 06:00:00    0.6200000    0.6000000    22.47189
1976/04/06 12:00:00    0.6200000    0.6000000    22.35083

YAML reference:

light_extinction:
  method: custom
  A:  { method: file, file: extinction.dat, column: 1 }
  g1: { method: file, file: extinction.dat, column: 2 }
  g2: { method: file, file: extinction.dat, column: 3 }

See Timeseries (Scalar) File for the complete timeseries file format specification.

References

Jerlov, N. G. (1968). Optical Oceanography. Elsevier.

Paulson, C. A., and J. J. Simpson (1977). Irradiance measurements in the upper ocean. J. Phys. Oceanogr., 7, 952–956.