Studying ice particle growth processes in mixed-phase clouds using spectral polarimetric radar measurements

Clouds are a prominent part of the Earth hydrological cycle. In the mid latitudes, the ice phase of clouds is highly involved in the formation of precipitation. The ice particles in the clouds fall to earth either as snow flakes, in the winter month, or melting crystals that become rain drops. An efficient growth process is the interaction of ice crystals and supercooled liquid ater droplets in so called mixed-phase clouds. Mixed phase cloud systems contain both - ice crystals and super cooled cloud droplets - in the same volume of air. The interaction of ice and liquid phase leads to an enhanced growth of ice crystals and, therefore, enhances the amount of precipitation. However, such processes are still not fully understood. This work hows that such complex microphysical processes in mixed-phase clouds can be observed using state of the art ground based radar techniques. Analyzing spectral polarimetric radar data, different signatures of particle growth processes can be identified.

The results presented are based on measurements obtained with the Transportable Atmospheric Radar (TARA) during the ACCEPT campaign (Analysis of the Composition of Clouds with Extended Polarization Techniques), in autumn 2014, Cabauw, the Netherlands. TARA is an S-band radar profiler that has full Doppler and spectral polarimetric measurement capabilities. TARAs unique three-beam configuration is also able to retrieve the full 3-D velocity vector. Because the high temporal and spatial resolutions and its configurations TARA can capture the complexity of cloud dynamics and microphysical variabilities involved in mixed-phase cloud systems.

A new retrieval technique was applied to several case studies to qualitatively analyze ice particle growth processes within mixed phase cloud systems. These results demonstrate that using radar data re-arranged along fall streak, the interpretation of Doppler spectra and polarization parameters can improve. Based on synergetic measurements obtained during the ACCEPT campaign it was possible to detect possible to detect supercooled liquid water layers within the cloud system and relate them to TARA observations. Therefore, it was possible to even identify different growth processes, like particle riming, generation of the new particles, and particle diffusional growth within the TARA measurements. This demonstrates, that in order to observe ice particle growth processes within complex systems adequate radar technology and state of the art retrieval algorithms are required. Moreover, the ice particle growth processes within cloud systems can be linked directly to the increased rain intensities using along fall streak rearranged radar data.

The last objective of the thesis is the extension of the spectral polarimetric measurement capabilities of TARA and the estimate of the differential phase and the specific differential phase in the spectral domain. These two parameters are frequently used to improve rain estimation, hydrometeor classifications and, currently, more and more to improve microphysical process understanding, e.g. the onset of the aggregation of ice particles. So far, the parameters are used only as integrated moments. Nevertheless, the work demonstrates that further work has to be done to completely understand the microphysical information of these spectral resolved parameters.

Overall, this work demonstrates that spectral polarimetric radar data can be used to improve the microphysical process understanding. The presented work also shows that spectral polarimetric radar data can be used to estimate quantitative icrophysical
properties related to ice particle growth.