RTE: optical properties

Class ty_optical_props in module mo_optical_props in the RTE library provides a representation of the optical properties of the atmosphere. The following examples assume that a variable op of type(ty_optical_props) has been declared.

Spectral discretization

Optical properties are defined by their spectral dependence which is described during initialization using routine

  function init(band_lims_wvn, band_lims_gpt, name) result(err_message)
    real(wp), dimension(:,:),   intent(in   ) :: band_lims_wvn
    integer,  dimension(:,:), &
                      optional, intent(in   ) :: band_lims_gpt
    character(len=*), optional, intent(in   ) :: name
    character(len = 128)                      :: err_message

Array band_lims_wvn with extents (2, number-of-bands) describes the number of bands beginning and ending wavenumber of each band (in MKS units i.e. inverse meters). Optional argument band_lims_gpt describes the beginning and ending g-point of each band; if this array isn't provided it's assumed values are available by band. An optional name may be useful in debugging.

Variables of type ty_optical_props can copy the spectral discretization from a previously-initialized variable e.g. err_msg = op2%init(op1).

Real-valued functions op%get_band_lims_wavenumber() and op%get_band_lims_wavelength() return arrays of extent (2, number-of-bands) containing the spectral limits of each band expressed in wavelength or wavenumber = 1/wavelength (again in MKS units).

Procedure op%is_initialized() returns a logical value indicating if the spectral discretization has been provided. op%get_nband() and op%get_ngpt() return the (integer) number of band and g-points; if these numbers are the same the optical properties are defined by band.

Integer functions op%convert_band2gpt() and op%convert_gpt2band() provide a map between individual bands and g-points. Function op%get_gpoint_bands() returns an integer of length op%get_ngpt() with the band number to which each g-point belongs (i.e. it's the vector version of op%convert_gpt2band()).

Comparison functions op1%bands_are_equal(op2) and op1%gpoints_are_equal(op2) return logical values indicating whether the twi sets of optical properties share the same band or g-point discretization.

Optical property values

Class ty_optical_props_arry allows optical properties to be expressed as arrays: discrete values on a domain with dimensions column, layer, g-points. Class ty_optical_props_arry extends class ty_optical_props and inherits all the capabilities described above.

Class ty_optical_props_arry is abstract. Three concrete sub-classes extend this type:

1. ty_optical_props_1scl contains the data field tau with dimensions ncol, nlay, ngpt representing optical depth. This class is useful for describing problems without scattering.
2. ty_optical_props_2str contains the data fields tau, ssa, g with dimensions ncol, nlay, ngpt representing optical depth, single-scattering albedo, and asymmetry parameter. This class is used to describe scattering problems for which the two-stream approximation is adequate.
3. ty_optical_props_nstr contains the data fields tau, ssa, p. Variable p has leading dimension nmom to accommodate the moments of a phase function for use in higher-order calculations.

Variables of class ty_optical_props_arry must be allocated as well as initialized before use. Because the argument list is slightly different among the sub-classes each allocation routine has a distinct name. Users must call one of err_msg = op%alloc_1scl(ncol, nlay), err_msg = op%alloc_2str(ncol, nlay) or err_msg = op%alloc_nstr(nmom, ncol, nlay).

Initialization from an existing variable of type ty_optical_props can be combined with allocation: err_msg = op%alloc_2str(ncol, nlay, op1), etc.

Integer functions op%get_ncol() and op%get_nlay() return the number of columns or layers in the domain. Class ty_optical_props_nstr has a get_nmom() function.

Calling err_msg = op%delta_scale(for) will delta-scale the optical properties to remove strong forward peaks in the phase function. Optional argument for is an real array of forward-scattering fractions with the same extents as op%tau. If this argument is omitted the assumption for = g*g is made.

Optical properties can be added together: err_msg = op1%increment(op2) adds the optical properties of op1 to those of op2 making reasonable assumptions if the two variables are not of the same class.

err_msg = op%validate() checks to be sure that all values are valid: optical depth not less than 0, single-scattering albedo between 0 and 1, asymmetry parameter between -1 and 1.

err_msg = op%subset(start, n) extracts columns start to start+n-1 from the class into variable subset making reasonable assumptions if the variables are of different classes.