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mie_derivs_ln.pro

Purpose

The mie_derivs_ln.pro procedure performs Mie scattering calculations on aerosol with a log normal size distribution and provides analytically derived derivatives, with respect to the properties of the distribution, of all output parameters. The procedure provides the distribution of scattering radiation (in the form of the Mie intensity functions), the volume extinction and scattering coefficients and their derivatives.

A the derivation of expressions for the analytical derivatives of Mie scattering terms is covered by:
Grainger, R.G., J. Lucas, G.E. Thomas, G. Ewan, "The Calculation of Mie Derivatives", Appl. Opt., 43(28), 5286-5393, 2004.

Calling the procedure

mie_derivs_ln.pro is an IDL procedure and can be called with the following command line from the IDL prompt (as long as the source file lies within your IDL_PATH environment variable):

mie_derivs_ln, N, Rm, S, wavenumber, Cm[, Dqv=Dqv] [, Bext][, Bsca][, dBextdN][, dBextdRm][, dBextdS] [, dBscadN][, dBscadRm][, dBscadS][, i1][, i2] [, di1dN][, di1dRm][, di1dS][, di2dN][, di2dRm][, di2dS]

The input parameters in the above call are:

  • N: Number density of the particle distribution.
  • Rm: Median radius of the particle distribution (in microns).
  • S: Spread of the distribution, such that the standard deviation of ln(r) is ln(S)
  • wavenumber: Wavenumber of light, defined as 1/wavelength. This must be a positive scalar and it's units should match those of Rm (ie. if Rm is in microns, wavenumber should be in 1/microns).
  • Cm: Complex refractive index of the particle(s). Only one refractive index value can be used in each call to the procedure and must take the form complex(a,-b) (where a is the real part of the refractive index, and b is the imaginary (or absorptive) part, and is either zero or negative).
  • Dqv: Cosines of the scattering angles at which to calculate the intensity functions etc. If specified it must be vector of type float or double (although it can have only one element).
The output parameters are:
  • Bext: Volume extinction coefficient.
  • Bsca: Volume scattering coefficient.
  • dBextdN: Derivative of the extinction coefficient wrt the number density.
  • dBextdRm: Derivative of the extinction coefficient wrt the mean radius.
  • dBextdS: Derivative of the extinction coefficient wrt the spread.
  • dBscadN: Derivative of the scattering coefficient wrt the number density.
  • dBscadRm: Derivative of the scattering coefficient wrt the mean radius.
  • dBscadS: Derivative of the scattering coefficient wrt the spread.
Note: All of the above output variables are scalars of type double.
  • i1: First intensity function - intensity of light polarised in the plane perpendicular to the directions of incident light propagation and observation. Only calculated if Dqv is specified.
  • i2: Second intensity function - intensity of light polarised in the plane parallel to the directions of incident light propagation and observation. Only calculated if Dqv is specified.
  • di1dN: Derivatives of the first intensity function wrt the number density.
  • di1dRm: Derivatives of the first intensity function wrt the mean radius.
  • di1dS: Derivatives of the first intensity function wrt the spread.
  • di2dN: Derivatives of the second intensity function wrt the number density.
  • di2dRm: Derivatives of the second intensity function wrt the mean radius.
  • di2dS: Derivatives of the second intensity function wrt the spread.
Note: All of these output variables will be vectors of type double, with the same number of elements as Dqv and will only be calculated if Dqv is specified.

Limitations and dependences

It should be noted that is a very new routine and has yet to be extensively tested.

This procedure calls the mie_single.pro procedure.As a result of this, any distribution that contains particles with size parameters greater than 12000 will result in the programme exiting with the error message "Error: Size Parameter Overflow in Mie".

This procedure also calls the quadrature.pro and shift_quadrature.pro procedures (click to download). Quadrature is called with the command line:
quadrature, 'g', Npts, abs, wght
the string 'g' can be either 'g' for Gaussian quadrature, 'l' for Lobatto quadrature, 'r' for Radau quadrature, 's' for Simson's rule or 't' for the Trapezium rule. Npts specifies the number of quadrature points required and the routine returns the abscissa and weighting values in abs and wght respectively.

shift_quadrature.pro is called with the command line:
shift_quadrature,abscissa,weights,A,B,new_abscissa,new_weights
and simply transforms the abscissa and weights from the interval [-1,1] to [A,B], place the new values in new_abscissa and new_weights.

Download source code

The source code for this routine and supporting routines is part of the EODG Mie scattering distribution which may be downloaded as a gzipped tar file here.

Maintained by Greg McGarragh

Earth Observation Data Group, Department of Physics, University of Oxford. Page last updated: @22:29 GMT 21-Sep-2016