# Precipitation model choices in Atmos.jl

The precipitation model in Atmos.jl describes the behavior of precipitating water in the atmosphere (i.e. rain and snow). There are two options available: NoPrecipitation and RainModel.

## NoPrecipitation

The NoPrecipitation model assumes there is no precipitating water present in any form in the simulation. It does not add any precipitation related variables to state variables. This model can be used (i) without defining any precipitation related source terms, (ii) or using the 0-moment microphysics scheme. In the first case ρ q_tot (total water specific humidity) is not removed during the simulation. In the second case ρ q_tot is removed if it exceeds a threshold.

## RainModel

The RainModel assumes that precipitating water is present but only in the form of rain (liquid-phase precipitation). It adds ρ q_rai (air density times total rain water specific humidity) to state variables. It uses a subset of source terms from the 1-moment microphysics scheme that parameterize processes relevant to liquid-phase clouds (autoconverion, accretion, and rain evaporation).

## RainSnowModel

The RainSnowModel assumes that precipitating water is present in the form of rain (liquid-phase precipitation) and snow (ice-phase precipitation). It adds ρ q_rai (air density times total rain water specific humidity) and ρ q_sno (air density times total snow water specific humidity) to state variables. It uses source terms from the 1-moment microphysics scheme.