Microphysics Flexible

The MicrophysicsFlexible.jl module relies on the extension defined in ext/CloudyExt.jl, based on a flexible N-moment microphysics scheme built in the external package Cloudy.jl. This option currently handles warm-rain processes including coalescence, condensation/evaporation, and sedimentation (terminal velocity). Unlike typical moment-based schemes which distinguish between categories such as rain and cloud, and which determine rates of conversion between categories (the canonical autoconversion, accretion, and self-collection), this option gives the user the flexibility to define as many or as few moments as they please, with these coalescence-based processes being solved directly without relying on conversion rates. Likewise, the rate of condensation/evaporation is defined through the rate of diffusion of water vapor to/from the surface of droplets defined by the subdistributions which underpin the method. The user has not only the flexibility to specify the number of moments (and therefore the complexity/accuracy) to use, but also the assumed size distributions corresponding to these moments. For instance, one might define a 5-moment implementation using an Exponential mode for smaller cloud droplets, plus a Gamma mode for larger rain droplets. Or, more creatively, perhaps a 12-moment implementation comprised of four Gamma modes.

Options for dynamics and size distributions are under continuous development in the Cloudy.jl package, thus only the default and suggested use cases are described in detail here.

Moments and Sub-Distributions

The prognostic variables of this parameterization are a set of N moments, which can be further divided into P sets of moments, each of which correponds to a subdistribution p. By design these moments begin at order 0 and increase as integers up to the maximum number of parameters for the chosen subdistribution. The first three such default moments have interpretable meanings:

• $M_0$ - the number density of droplets [1/m^3]
• $M_1$ - the mass density of droplets [kg/m^3]
• $M_2$ - proportional to the radar reflectivity [kg^2/m^3]

and can be converted to more canonical definitions of q_liq and q_rai through numerical integration.

When the user wishes to use more than 2 or 3 total variables to represent the system, these moments must be divided between $P > 1$ sub-distributions, each of which assumes the form of a particular mathematical distribution, such as an Exponential, Lognormal, or Monodisperse (each of which has two parameters), or a Gamma distribution (which takes 3 parameters).

Loading the extension

The package Cloudy.jl and its dependencies are not loaded by default when using CloudMicrophysics.jl. Rather, one must specify:

using CloudMicrophysics
using Cloudy

from the Julia REPL. Upon recognizing that Cloudy.jl is being loaded, the extension CloudyExt.jl will then be loaded and overwrite the function stubs defined in src/MicrophysicsFlexible.jl.

Setting up a system

All the details from the number of moments and type of subdistributions, to the parameterizations of coalescence, condensation, and sedimentation are defined through the CLSetup (CLoudySetup) mutable struct. This struct is mutable specifically because certain of its components, such as backend-computed coalescence tendencies, are updated prior to being passed to the timestepper. The components of a CLSetup object and their defaults are further described below.