Properties of tropical and midlatitude ice cloud particle ensembles. Part II: Applications for mesoscale and climate models

This is the second part of a study that characterizes several bulk properties of ice particle populations sampled in synoptically generated midlatitude and convectively generated tropical ice clouds, for the purpose of developing empirical and analytical relationships that describe microphysical properties for use in mesoscale and climate models. The purpose of this paper is to examine the interrelationships between the mass, area, and fall velocity properties of the particle size distributions, and the dependence of these properties on temperature. Gamma distributions of the form N = N₀D^μ_e^-λD are fitted to the measured particle size distributions (PSDs) over sizes (D) from as small as 10 μm to as large as 1.5 cm. Exponential distributions (μ = 0) are also fitted to the PSD. The intercept parameter N₀ and the slope λ are directly related, and decrease monotonically with increasing temperature. The μ values for the gamma fits tend from positive values at large λ to negative values at small λ. The maximum measured diameter D_max increases with decreasing λ. The N₀ values from the midlatitude clouds are about an order of magnitude lower than those for the tropical PSDs at the same temperatures. Bulk properties are derived from the fitted PSDs. The ice water contents (IWC) are about an order of magnitude higher for the tropical than for the midlatitude clouds. The median mass diameter (D_m) and the effective diameter (D_c) each increase with temperature, and are found to be related to each other. Several aspects related to the modeling of ice particle sedimentation in general circulation models (GCMs), and the relationship of these velocities to other bulk properties, are investigated. On average, the median mass-weighted terminal velocity (V_m) increase weakly with temperature. Correlations between V_m and IWC are also weak. It is found that for a given particle ensemble, most of the ice mass is contained within a relatively narrow range of fall velocities, although the values of V_m can be appreciable. Calculations reveal that the fallout of particles that dominate the extinction cannot be ignored, except at temperatures below -50°C. Also, the effective diameter is found to be strongly related to the ensemble mean V_m, perhaps allowing the two variables to be linked in GCMs.