Replacing the Meyers et al. formula in bulk ice microphysics schemes in Canadian mesoscale models

A substantial fraction of atmospheric clouds contain all ice or are mixed phase. Such clouds affect the earth's radiation balance and hydrological cycle. They can exist both at high and lower altitudes, but the initiation and formation mechanisms of these clouds are not well understood. At temperatures (T > -40^oC), ice particles are produced as a result of heterogeneous nucleation in the presence of sub-micron size insoluble aerosol particles that serve as ice nuclei. At temperatures (T < -40^oC), however, supercooled liquid drops freeze instantaneously and become ice particles and this is considered as homogeneous nucleation. It is believed that there are at least four different modes of heterogeneous nucleation such as contact, immersion, deposition, and condensation freezing depending on how the liquid droplets and water vapor interact with the aerosol particles. However, it is not clear which modes are more relevant to natural atmospheric ice particle production. The observed ice concentration in clouds normally exceeds the measured ice nucleus concentration by several order of magnitude, and this has been attributed to secondary ice multiplication mechanisms such as break up during collision or shattering and splintering during riming. However, many of the current Numerical Weather Prediction (NWP) and General Circulation models (GCM) cloud ice microphysics schemes use diagnostic parameterizations for ice nuclei number concentrations based on Fletcher, Meyers or Cooper. The bulk ice microphysics schemes used by models vary slightly, but are generally similar. For simplicity, in this paper, we will focus on the Kong and Yau scheme (KY). This scheme has been implemented in the MC2 model. In this scheme, there are several processes such as ice nucleation, ice deposition/sublimation, melting, riming, and sedimentation. The total ice particle concentration is estimated based on the Meyers et al formula and the ice particles are assumed to be spherical with a density of pure ice. In this paper, the validity of the KY scheme will be tested based on in-situ aircraft measurements of ice particle spectra and other microphysical variables in stratiform clouds during several field projects. A development of a new ice microphysics scheme based on measured ice particle spectra will be discussed. This new scheme will be tested in MC2 and some preliminary results will be presented.