Fluid mechanics use simple concepts that help understanding and appreciate balances stemming from the governing fluid motion equations. Such concepts highlight underlying physical processes and provide guidance for physical interpretations. Classical examples for the atmospheric and oceanic fluid dynamics are the hydrostatic balance, the cyclostrophy, or the geostrophy. This presentation will discuss simple balances appropriate for the cloud dynamics that we refer to as the "nimbostrophy". The simplest one is to assume that air flow within a cloud maintains water saturation (or ice saturation in very cold temperatures). Such an assumption privides a simple and computationally efficient methodology in cloud modeling. It does not allow, however, to appropriately model cloud microphysics, especially the role of cloud condensation nuclei and ice-forming nuclei in cloud and precipitation formation. The more accurate approach is to assume that the supersaturation inside a cloud is in a quasi-equilibrium between the forcing due to cloud updraft and the sink due growth of cloud particles (droplets or ice crystals). This pedagogical presentation will discuss the two approaches in the context of key cloud physics problems in modeling and predicting weather and climate.
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