# -*- coding: utf-8 -*-
"""
pyradarmet.attenuation
======================
Functions to calculate coefficients used with attenuation calculations.
References
----------
Battan (1973), Radar Observations of the Atmosphere.
Doviak and Zrnic (1993), Doppler Radar and Weather Observations.
"""
import numpy as np
def abs_coeff(D, lam, m):
"""
Absorption coefficient of a spherical particle. Unitless.
Doviak and Zrnic (1993), Eqn 3.14a or Battan (1973), Eqn 6.6
Parameters
----------
D : float or array
Particle diameter [m]
lam : float
Radar wavelength [m]
m : float
Complex refractive index [unitless], in the form 7.14 - 2.89j
Notes
-----
An example from Battan (1973) is for water at 0C m=7.14-2.89j for a
wavelength of 3.21 cm and for ice m=1.78-0.0024j for
wavelength range from 1-10 cm.
See Battan (1973) Ch.4 , Tables 4.1 and 4.2 for values from
Gunn and East (1954).
Also see Doviak and Zrnic (1993), Fig. 3.3 caption.
"""
Km = (m**2 - 1) / (m**2 + 2)
Qa = (np.pi**2 * np.asarray(D)**3 / lam) * np.imag(-1 * Km)
return Qa
def scat_coeff(D, lam, m):
"""
Scattering coefficient of a spherical particle. Unitless.
Doviak and Zrnic (1993), Eqn 3.14b or Battan (1973), Eqn 6.5
Parameters
----------
D : float or array
Particle diameter [m]
lam : float
Radar wavelength [m]
m : float
Complex refractive index [unitless]
Notes
-----
An example from Battan (1973) is for water at 0C m=7.14-2.89j for a
wavelength of 3.21 cm and for ice m=1.78-0.0024j for
wavelength range from 1-10 cm.
See Battan (1973) Ch.4 , Tables 4.1 and 4.2 for values from
Gunn and East (1954).
Also see Doviak and Zrnic (1993), Fig. 3.3 caption.
"""
Km = (m**2 - 1) / (m**2 + 2)
Qs = (2 * np.pi**5 * np.asarray(D)**6 / (3 * lam**4) * (np.absolute(Km))**2)
return Qs
def ext_coeff(D, lam, m):
"""
Extinction coefficient of a spherical particle. Unitless.
Doviak and Zrnic (1993), Eqn 3.14b or Battan (1973), Eqn 6.5
Parameters
----------
D : float or array
Particle diameter [m]
lam : float
Radar wavelength [m]
m : float
Complex refractive index [unitless]
USAGE::
-----
Qe = ext_coeff(D,lam,m)
NOTES::
-----
An example from Battan (1973) is for water at 0C m=7.14-2.89j for a
wavelength of 3.21 cm and for ice m=1.78-0.0024j for
wavelength range from 1-10 cm.
See Battan (1973) Ch.4 , Tables 4.1 and 4.2 for values from
Gunn and East (1954).
Also see Doviak and Zrnic (1993), Fig. 3.3 caption.
"""
Qa = abs_coeff(np.asarray(D), lam, m)
Qs = scat_coeff(np.asarray(D), lam, m)
Qe = Qa + Qs
return Qe
