Attribution of riming and aggregation processes by application of the vertical distribution of particle shape (VDPS) and spectral retrieval techniques to cloud radar observations

Abstract. Advancing the understanding of mixed-phase cloud microphysical growth processes requires a thorough detection of the transition processes from pristine hydrometeor states toward aggregates, rimed particles and graupel. In this study, a versatile combination of techniques is applied to detect and characterize aggregated and strongly rimed hydrometeors even under harsh atmospheric conditions such as the presence of orographic gravity waves. The approach combines dual-frequency observations from vertical-stare Doppler cloud radars as well as measurements from a polarimetric scanning cloud radar. Core of the approach are profiles of the Vertical Distribution of Particle Shape (VDPS) method that serve as a proxy for the presence of columnar, isometric, or prolate cloud particles. At height levels within the VDPS-based shape profiles where isometric particles are identified, Doppler spectra and dual-wavelength vertical-stare cloud radar observations are used to discriminate the occurrence of aggregation or graupel formation.

The underlying dataset was acquired in the framework of the 3-year field experiment, “Dynamic Aerosols Clouds and Precipitation Observation in the Pristine Environment in the Southern Ocean” (DACAPO-PESO) at the southern hemispheric midlatitude site of Punta Arenas, Chile (53° S, 71° W). The frequent presence of layers of supercooled liquid water and the permanent occurrence of orographic gravity waves motivate a strong interest to understand the formation of precipitation and the role of aggregation and riming at this site. Therefore, two case studies of both strong riming events and aggregation processes from the DACAPO-PESO campaign are presented to demonstrate the potential of combining the new VDPS retrieval with spectral methods, which analyze particle fall velocity and the coexistence of multiple particle types. We found that the identification of layers of supercooled liquid water is essential to pin down regions of riming in the observed cloud systems. In consequence, considering the general notion of the excess of liquid water in clouds over the southern hemisphere midlatitudes, our study serves as a preliminary investigation into the occurrence of riming and aggregation processes above Punta Arenas.