Continuum

The MT_CKD continuum is a fortran program which calculates the continuum absorption due to water vapour, nitrogen, oxygen, carbon dioxide, and ozone over the wavenumber range 0-20,000 cm^-1.

Basic Usage

The MT_CKD continuum can be added to a model atmosphere as a constituent. See the example in Quick Start.

The atmosphere must contain constituents that define the H2O, CO2, and O3 VMR profiles. By default, it is assumed that these species have been added to the atmosphere with the keys “H2O”, “CO2”, and “O3”. Different key names can be used by setting h2o_name, co2_name, and o3_name in the initialization of sasktran2_ext.continuum.MTCKDContinuum. Temperature and pressure profiles must also be set in the atmosphere.

Note

The continuum calculated by MT_CKD accounts for contributions that are further than 25 cm^-1 from each line. When using the continuum it is recommended to leave the parameter line_contribution_width in the optical properties that do line-by-line calculations, such as sasktran2.optical.hitran.HITRANAbsorber and sasktran2.optical.hitran.AERLineAbsorber, at its default value of 25.

Weighting Functions

Weighting functions are calculated with respect to pressure and temperature as well as H2O, CO2, and O3 VMR. In the output the weighting functions have continuum in their variable names to indicate they are portion of the species derivative resulting from the continuum. For example, if we perform a calculation with the continuum

import numpy as np
import sasktran2 as sk
from sasktran2_ext.continuum import MTCKDContinuum

config = sk.Config()
config.emission_source = sk.EmissionSource.Standard
config.single_scatter_source = sk.SingleScatterSource.NoSource

altitude_grid = np.arange(0, 65001, 1000.0)

geometry = sk.Geometry1D(
    0.6,
    0,
    6327000,
    altitude_grid,
    sk.InterpolationMethod.LinearInterpolation,
    sk.GeometryType.Spherical,
)

viewing_geo = sk.ViewingGeometry()
viewing_geo.add_ray(sk.GroundViewingSolar(0.6, 0, 0.8, 200000))

wavenum = np.arange(530, 550, 0.01)

atmosphere = sk.Atmosphere(geometry, config, wavenumber_cminv=wavenum)
sk.climatology.us76.add_us76_standard_atmosphere(atmosphere)
for species in ["H2O", "O3", "CO2"]:
    hitran_db = sk.database.HITRANDatabase(
        molecule=species,
        start_wavenumber=530,
        end_wavenumber=550,
        wavenumber_resolution=0.01,
        reduction_factor=1,
        backend="sasktran2",
        profile="voigt"
    )
    atmosphere[species] = sk.climatology.mipas.constituent(species, hitran_db)
atmosphere["emission"] = sk.constituent.ThermalEmission()
atmosphere["surface_emission"] = sk.constituent.SurfaceThermalEmission(300, 0.9)
atmosphere["continuum"] = MTCKDContinuum()

engine = sk.Engine(config, geometry, viewing_geo)
output = engine.calculate_radiance(atmosphere)

the output will be

print(output)

<xarray.Dataset> Size: 11MB
Dimensions:                            (wavelength: 2000, los: 1, stokes: 1,
                                        altitude: 66, H2O_altitude: 50,
                                        O3_altitude: 50, CO2_altitude: 50)
Coordinates:
  * wavelength                         (wavelength) float64 16kB 1.887e+04 .....
  * stokes                             (stokes) <U1 4B 'I'
Dimensions without coordinates: los, altitude, H2O_altitude, O3_altitude,
                                CO2_altitude
Data variables: (12/14)
    radiance                           (wavelength, los, stokes) float64 16kB ...
    wf_temperature_k                   (altitude, wavelength, los, stokes) float64 1MB ...
    wf_H2O_vmr                         (H2O_altitude, wavelength, los, stokes) float64 800kB ...
    wf_pressure_pa                     (altitude, wavelength, los, stokes) float64 1MB ...
    wf_specific_humidity               (altitude, wavelength, los, stokes) float64 1MB ...
    wf_O3_vmr                          (O3_altitude, wavelength, los, stokes) float64 800kB ...
    ...                                 ...
    wf_continuum_pressure_pa           (altitude, wavelength, los, stokes) float64 1MB ...
    wf_continuum_temperature_k         (altitude, wavelength, los, stokes) float64 1MB ...
    wf_continuum_H2O_vmr               (altitude, wavelength, los, stokes) float64 1MB ...
    wf_CO2_vmr                         (CO2_altitude, wavelength, los, stokes) float64 800kB ...
    wf_surface_emission_temperature_k  (wavelength, los, stokes) float64 16kB ...
    wf_surface_emission_emissivity     (wavelength, los, stokes) float64 16kB ...

Notice there are two weighting functions related to ozone, wf_O3_vmr and wf_continuum_O3_vmr. The first contains the ozone weighting function contribution due to line-by-line absorption and the second is the contribution from continuum absorption.