Clouds and precipitation consist of many types of water particles ('hydrometeors'): cloud droplets, rain drops, ice crystals, hail stones, snowflakes…. The particle sizes range from micrometres for cloud droplets to millimetres, sometimes centimeter for snowflakes.
Many things can happen in the cloud. For example, water vapour condenses on a cloud droplet; colliding drops merge and form a larger drop coagulation; snow particles collect duplets. These interactions are called cloud microphysical processes.
Cloud proceses can be modelled by two approaches.
bin approach:
bulk approach:
In this example we see huge differences between the models – these are the changes introduced when using another distribution function or different predicted moments (N – number, L -- mass, Z – reflectivity).
We improve bulk models and validate their results with bin model results as benchmark.
When formulating the equations for the bulk approach, some choices have to be made:
The consequences of these decisions and assumptions are best studied in simple models.