You are quite right a SCWD is a droplet of water that exists at or below 0 degrees c.
It occurs because for freezing to occur and thence a change of state with its subsequent release of latent heat,a hygroscopic nuclei needs to exist.You see , in this state it is "unstable".When it locates this nuclei ie an ice particle ,it latches onto it and becomes stable.
At the top of a CB you will find virtually no SCWD , but plenty if ice particles.Why no SCWD?Because they have latched onto all the ice that exists.
Dug out from a while ago,hope it helps!
Autothrottle