Towards an enhanced droplet activation scheme for multi-moment bulk microphysics schemes

Initial droplet spectra produced upon activation impact the ensuing chain of microphysical processes and therefore play a crucial role in cloud evolution. This work re-examines dependencies of newly formed cloud droplet size distribution (CDSD) characteristics on environmental and aerosol properties via parcel model simulations that serve as the basis for a multi-moment bulk microphysics droplet activation scheme suitable for a cloud-resolving model (CRM). It is found that applying a fixed size threshold to define activated droplets versus employing physical considerations can lead to erroneous activation and overly broad CDSDs for high aerosol concentration and weak updraft conditions. Aerosol distributions characterized by larger median sizes and/or increased solubility can result in greater activated droplet numbers, whereas impacts of these parameters on CDSD spectral width depend on both aerosol number concentration and updraft velocity. An expansion of the activation scheme to include CDSD spectral width is proposed to aid efforts to extend high-order moment prediction to cloud droplet categories in CRMs as well as better represent variability in the activation process on the cloud scale.