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3.2.1. snowProperties - an interface to snowflake library

The snowScatt package provides access to the microphysical and scattering properties of an extensive library of snow particle models. The main component of the snowScatt package is the core snow library and the interface to access it. snowScatt uses the structural properties of aggregates of various types to simulate the microphysical (size, mass, area, aspect ratio) and scattering (absorption, scattering, phase function) properties of realistically shaped snowflakes.

snowScatt._compute.backscatVel(diameters, wavelength, properties, ref_index=None, temperature=None, massScattering=None, theta=0.0, velocity_model='Boehm92', kwargsVelocity={}, massVelocity=None, areaVelocity=None)

Convenience function for the implementation of a simple Radar Doppler simulator. It computes backscattering fast as the backscatter function, but it also returns the array of velocities since they are already extracted from the library

Parameters:

• diameters (array-like or scalar double) – Snowflake diameters [meters] for which the scattering and microphysical properties are to compute. The number of passed diameters will define the number of different particles (Nparticles).
• wavelength (array-like or scalar double) – Wavelength in the vacuum [meters] of the incident electromagnetic wave. If an array-like is passed than it must have length=Nparticles.
• properties (string) – Label that defines the type of particle. Call snowLibrary.info() for a list of available snowflake properties
• ref_index (array-like or scalar complex (optional)) – Refractive index of the ice. If an array-like is passed than it must have length=Nparticles. If not set than the temperature attribute must be set
• temperature (array-like or scalar double (optional)) – Ambient temperature. Ignored if ref_index is set. Computes the complex refractive index of ice for the requested wavelength according to the Iwabuchi et al. (2011) model. If an array-like is passed than it must have length=Nparticles.
• massScattering (array-like or scalar double (optional)) – Mass of the snowflake particles. If an array-like is passed than it must have length=Nparticles. If left unset the mass is calculated from the snowLibrary properties. Override only the mass for the scattering computation; in order to override also the one for the fallspeed simulation set the massVelocity parameter
• theta (scalar double (optional default is 0.0 zenith)) – Polar incidence angle [radians] defaults to 0.0 (zenith-pointing). The incidence angle is used only to compute the effective size of the particle along the propagation direction (together with the information on particle aspect ratio). It ranges from 0 to pi. The code consider the snowflake overall shape to be spheroidal and thus there will be simmetry around pi/2 for this angle.
• velocity_model (string (optional default Boehm92)) – string identifing the fallspeed velocity model. Currently available models are Boehm92 (default), HeymsfieldWestbrook10, KhvorostyanovCurry05, Boehm89. Look at the documentation of the _fallSpeedModels and the _readProperties modules for a complete list
• kwargsVelocity (dictionary (optional default parameters in _constants.py)) – dictionary of additional arguments to pass to the velocity model. For example all models accept rho_air and nu_air as additional arguments to set air density and viscosity (default values set in _constants.py module). Boehm92 additional accept as_ratio, HeymsfieldWestbrook05 the k parameter for laminar/turbulent flow parametrization. Look at the documentation of the _fallSpeedModels module for a better explanation of the specific arguments for each fallspeed model.
• massVelocity (array-like(Nparticles) or scalar [kilograms]) – if not None overloads the mass computed with “properties” to calculate the fallspeed. The return value of mass is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like density exceeding the one of same sized solid ice).
• areaVelocity (array-like(Nparticles) or scalar [meters**2]) – if not None overloads the area computed with “properties” to calculate the fallspeed. The return value of area is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like exceeding the area of same sized full disk).

Returns:

• Cbck (array(Nparticles) - double) – Radar backscattering cross section (meters**2)
• vel (array(Nparticles) - double) – Terminal fallspeed (meters/second) as computed from the specified hydrodynamic model

snowScatt._compute.backscatter(diameters, wavelength, properties, ref_index=None, temperature=None, massScattering=None, theta=0.0)

Python interface to the fast computation of radar backscattering cross section using SSRGA. Unlike the ordinary SSRGA used by calcProperties, this function only solve the RGA problem for the backscattering direction, avoiding the calculation of the full phase function and the integration over the solid angle needed to compute the total scattering cross section. It is expected to provide an improvement in computational performances in the order of Nangle.

Parameters:

• diameters (array-like or scalar double) – Snowflake diameters [meters] for which the scattering and microphysical properties are to compute. The number of passed diameters will define the number of different particles (Nparticles).
• wavelength (array-like or scalar double) – Wavelength in the vacuum [meters] of the incident electromagnetic wave. If an array-like is passed than it must have length=Nparticles.
• properties (string) – Label that defines the type of particle. Call snowLibrary.info() for a list of available snowflake properties
• ref_index (array-like or scalar complex (optional)) – Refractive index of the ice. If an array-like is passed than it must have length=Nparticles. If not set than the temperature attribute must be set
• temperature (array-like or scalar double (optional)) – Ambient temperature. Ignored if ref_index is set. Computes the complex refractive index of ice for the requested wavelength according to the Iwabuchi et al. (2011) model. If an array-like is passed than it must have length=Nparticles.
• massScattering (array-like or scalar double (optional)) – Mass of the snowflake particles. If an array-like is passed than it must have length=Nparticles. If left unset the mass is calculated from the snowLibrary properties.
• theta (scalar double (optional default is 0.0 zenith)) – Polar incidence angle [radians] defaults to 0.0 (zenith-pointing). The incidence angle is used only to compute the effective size of the particle along the propagation direction (together with the information on particle aspect ratio). It ranges from 0 to pi. The code consider the snowflake overall shape to be spheroidal and thus there will be simmetry around pi/2 for this angle.

Returns:

Cbck – Radar backscattering cross section (meters**2)

Return type:

array(Nparticles) - double

snowScatt._compute.calcProperties(diameters, wavelength, properties, ref_index=None, temperature=None, massScattering=None, theta=0.0, Nangles=181, velocity_model='Boehm92', kwargsVelocity={}, massVelocity=None, areaVelocity=None)

This is the main function of the snowScatt module. It is a python interface to the snowLibrary and the low level C functions that compute the SSRGA scattering properties. It can be invoked for either single particles or an array of particles. The number of particles for which the properties are computed is determined by the length of the argument “diameters”. In case the other arguments are scalars, they are used as fixed parameters for all the different diameters. Otherwise, if they are arrays, they must be of the same length of the diameters argument; in this case each diameter will be computed with its own set of parameters. Some parameters need to be fixed for all computations. Those are the properties label, the incidence angle theta and the number of angular subdivisions of the 0-pi range for the phase function Nangles.

Parameters:

• diameters (array-like or scalar double) – Snowflake diameters [meters] for which the scattering and microphysical properties are to compute. The number of passed diameters will define the number of different particles (Nparticles).
• wavelength (array-like or scalar double) – Wavelength in the vacuum [meters] of the incident electromagnetic wave. If an array-like is passed than it must have length=Nparticles.
• properties (string) – Label that defines the type of particle. Call snowLibrary.info() for a list of available snowflake properties
• ref_index (array-like or scalar complex (optional)) – Refractive index of the ice. If an array-like is passed than it must have length=Nparticles. If not set than the temperature attribute must be set
• temperature (array-like or scalar double (optional)) – Ambient temperature. Ignored if ref_index is set. Computes the complex refractive index of ice for the requested wavelength according to the Iwabuchi et al. (2011) model. If an array-like is passed than it must have length=Nparticles.
• massScattering (array-like or scalar double (optional)) – Mass of the snowflake particles. If an array-like is passed than it must have length=Nparticles. If left unset the mass is calculated from the snowLibrary properties. Override only the mass for the scattering computation; in order to override also the one for the fallspeed simulation set the overloadMass parameter.
• theta (scalar double (optional default is 0.0 zenith)) – Polar incidence angle [radians] defaults to 0.0 (zenith-pointing). The incidence angle is used only to compute the effective size of the particle along the propagation direction (together with the information on particle aspect ratio). It ranges from 0 to pi. The code consider the snowflake overall shape to be spheroidal and thus there will be simmetry around pi/2 for this angle.
• Nangles (scalar integer (optional default is 181)) – Number of angles to calculate the phase function interval (0 to pi extremes always included). The default value is 181 meaning that the phase function is calculated with a 1 degree resolution. The scattering properties are calculated by integrating the phase function so increasing Nangle will also increase the computation accuracy
• velocity_model (string) – string identifing the fallspeed velocity model. Currently available models are Boehm92 (default), HeymsfieldWestbrook10, KhvorostyanovCurry05, Boehm89. Look at the documentation of the _fallSpeedModels and the _readProperties modules for a complete list
• kwargsVelocity (dictionary) – dictionary of additional arguments to pass to the velocity model. For example all models accept rho_air and nu_air as additional arguments to set air density and viscosity (default values set in _constants.py module). Boehm92 additional accept as_ratio, HeymsfieldWestbrook05 the k parameter for laminar/turbulent flow parametrization. Look at the documentation of the _fallSpeedModels module for a better explanation of the specific arguments for each fallspeed model.
• massVelocity (array-like(Nparticles) or scalar [kilograms]) – if not None overloads the mass computed with “properties” to calculate the fallspeed. The return value of mass is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like density exceeding the one of same sized solid ice).
• areaVelocity (array-like(Nparticles) or scalar [meters**2]) – if not None overloads the area computed with “properties” to calculate the fallspeed. The return value of area is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like exceeding the area of same sized full disk).

Returns:

• Cext (array(Nparticles) - double) – Extinction cross section (meters**2)
• Cabs (array(Nparticles) - double) – Absorption cross section (meters**2)
• Csca (array(Nparticles) - double) – Total scattering cross section (meters**2)
• Cbck (array(Nparticles) - double) – Radar backscattering cross section (meters**2)
• asym (array(Nparticles) - double) – asymmetry parameter (dimensionless)
• phase (2D array(Nparticles, Nangles) - double) – Normalized phase function. int(phase*sin(th)dth) = 1
• mass_prop (array(Nparticles) - double) – Particle mass as assumed by the snowLibrary properties (kilograms). It can be used to compare how much the user defined masses dfeviates from the average mass of the snowflakes for which the SSRGA parameters have been derived
• vel (array(Nparticles) - double) – Terminal fallspeed (meters/second) as computed from the specified hydrodynamic model.
• area (array(Nparticles) - double) – Snowflake vertical projected area [meters**2] computed from the average snow properties fit

Raises:

AttributeError : if neither ref_index nor temperature are defined

snowScatt._compute.snowMassVelocityArea(diameters, properties, velocity_model='Boehm92', kwargsVelocity={}, massVelocity=None, areaVelocity=None)

Convenience function to extract directly mass, velocity and area for a specific set of sizes and a particle in the database. Through the overloadMass and overloadArea parameters the user can also tune the mass and the area to values different from the one included in the library. If both quantities are user defined, the ‘properties’ argument becomes unnecessary for the computation of the fallspeed.

diameters : array-like(Nparticles) or scalar double [meters]

Snowflake diameters [meters] for which the scattering and microphysical properties are to compute. The number of passed diameters will define the number of different particles (Nparticles).

properties : string

Label that defines the type of particle. Call snowLibrary.info() for a list of available snowflake properties

velocity_model : string

string identifing the fallspeed velocity model. Currently available models are Boehm92 (default), HeymsfieldWestbrook10, KhvorostyanovCurry05, Boehm89. Look at the documentation of the _fallSpeedModels and the _readProperties modules for a complete list

kwargsVelocity : dictionary

dictionary of additional arguments to pass to the velocity model. For example all models accept rho_air and nu_air as additional arguments to set air density and viscosity (default values set in _constants.py module). Boehm92 additional accept as_ratio, HeymsfieldWestbrook05 the k parameter for laminar/turbulent flow parametrization. Look at the documentation of the _fallSpeedModels module for a better explanation of the specific arguments for each fallspeed model.

massVelocity : array-like(Nparticles) or scalar [kilograms]

if not None overloads the mass computed with “properties” to calculate the fallspeed. The return value of mass is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like density exceeding the one of same sized solid ice).

areaVelocity : array-like(Nparticles) or scalar [meters**2]

if not None overloads the area computed with “properties” to calculate the fallspeed. The return value of area is still the one computed by the internal library. The code does not check if unphysical values are passed from the user (like exceeding the area of same sized full disk).

Returns:

• mass (array(Nparticles) - double) – Particle mass as assumed by the snowLibrary properties (kilograms). It is derived from a power-law fit capped by the sphere maximum density of solid ice
• vel (array(Nparticles) - double) – Average terminal fallspeed (meters/second). Computed by default using the Boehm model (1992).
• area (array(Nparticles) - double) – Snowflake vertical projected area [meters**2] computed from the average snow properties fit. It is derived from a power-law fit capped the maximum area projected by a full disk of the smae diameter