Dominant Ice Cloud Microphysics Characterized by Simultaneous Observations of Radar Reflectivity and Doppler Velocity

This study proposes a novel process-oriented analysis method using a joint probability density function of the equivalent radar reflectivity factor (Ze) and Doppler velocity (υd) on a common logarithmic scale (Ze-log10 υd diagram) to characterize dominant microphysical processes in cirrus clouds. The method focuses on the linear relationship between Ze and the ice terminal velocity (υt) on a common logarithmic scale due to their physical definition. Assuming υt ≈ υd in non-convective clouds, the ratio of changes in Ze to changes in log10 υd, denoted as the slope (ΔZe/Δlog10 υd), is found to be explained by microphysical properties. Consequently, the principal component of the Ze-log10 υd diagram, sampled within a cloud layer, represents the slope in the cloud. This representative slope serves as a quantitative index to characterize key cloud microphysical processes within the cloud. In cirrus clouds, the slope is shown to be sensitive to the microphysical balance between aggregation and vapor deposition, supported by cloud microphysics theory. Ground-based radar observations reveal that the representative slope generally increases with atmospheric temperature, reflecting shifts in dominant microphysical processes. Furthermore, to elucidate differences in the representative slope, sensitivity experiments with a high-resolution climate model are performed using numerical simulations. These numerical experiments demonstrate that overestimated aggregation or suppressed vapor deposition each reduces the representative slope in distinct ways, as expected. The proposed method provides new insights into the cloud microphysics governing high clouds.