Combined Analysis of Triple-Frequency Cloud Radar and Polarimetric X-Band Radar Observations of Snow and Ice Microphysics

Microphysical processes related to snowfall, such as depositional growth, riming, and aggregation, belong to the currently least understood topics in cloud physics. As a consequence of increasing model resolution and more complex microphysical parameterizations, there is an urgent need for comprehensive observational constraints.

Numerous studies have demonstrated the potential of radar polarimetry to provide valuable information about prevalent microphysical processes. Recent analyses of novel snow and ice particle scattering models suggested that a combination of three radar frequencies which cover the Rayleigh and Mie scattering regime could help to distinguish between certain classes of snowfall such as aggregates or graupel-like particles. Comparisons to in-situ data confirmed a close relation of the triple-frequency signatures and habit classes, particle size distribution parameters, and particle densities. Finally, radar Doppler spectra obtained from vertically pointing radars have been shown to bear potential to separate processes if the hydrometeors involved have different terminal fall velocities.

A first attempt to combine the advantages of all three methods has been recently made in the field experiment TRIple-frequency and Polarimetric radar Experiment for improving process observation of winter precipitation (TRIPEx). The two-month campaign took place during the winter 2015/2016 at the Research Center Jülich (Germany) and has been made possible thanks to the cooperation of the University of Cologne and Bonn, Karlsruhe Institute of Technology, and the Research Center Jülich. Vertically pointing ground-based triple-frequency (X-, Ka-, and W-band) Doppler radar observations were combined with observations from two nearby scanning polarimetric X-band radars. We will present a joint analysis of the different radar observables caused by aggregation, riming and typical intensive depositional growth zones (e.g. dendrites or needles) observed in frontal mid-latitude clouds. In some situations we find the different radar methods in close agreement e.g. indicating the presence of newly grown ice particles. However, we also discover the clear added value of combining the different methods for certain processes. For example, some process starts to become visible first in Doppler spectra and only in a later stage, when the spectral signatures become stationary, multi-frequency and polarimetric variables begin to change.

These initial results show clearly the benefit of synergistically combining the different observation approaches and reveal new avenues how to better constrain ice microphysical processes, such as depositional growth, aggregation, and riming using a combination of different radar observations.