Investigating ice microphysical processes in the dendritic growth zone by combining Monte-Carlo Lagrangian particle modelling with multi-frequency polarimetric radar observations

Abstract. The dendritic growth zone (DGZ) is associated with distinct polarimetric and multi-frequency radar signatures, yet the governing microphysical processes remain uncertain. We analyse characteristic DGZ observations showing a concurrent increase in dual-wavelength ratio (DWR), enhanced specific differential phase shift (KDP) and the maximum of the spectrally resolved ZDR (sZDR_max​), a pronounced reduction in mean Doppler velocity (MDV), and the occurrence of a secondary Doppler spectral mode near −15 °C.

To investigate the governing processes, radar observations are combined with the Lagrangian particle-based Monte Carlo model McSnow, which includes an updated ice habit scheme and a new fragmentation parametrization. Forward radar simulations use a discrete dipole approximation scattering database.

The simulations show that enhanced sZDR_max​ requires local formation of dendritic or plate-like crystals near −15 °C; sedimentation of pre-existing particles alone cannot reproduce the signal. The observed KDP enhancement is only reproduced when secondary ice production via collisional fragmentation is included, which also strengthens aggregation-related signatures. The reduction in mean Doppler velocity is explained by a habit change and aggregation of sedimenting columnar ice particles. Together, these signatures provide the most diagnostic constraints on DGZ microphysical processes identified so far.

This study demonstrates that multi-frequency polarimetric radar observations combined with Monte Carlo Lagrangian particle simulations can disentangle competing ice microphysical processes in the DGZ. The results identify collisional fragmentation as a key unifying mechanism, with the DGZ radar fingerprint emerging from the interplay of depositional growth, aggregation, and secondary ice production.