How do ice crystal habits and size distribution characteristics affect 95 GHz radar signals?

The potential of ground-based polarimetric and Doppler cloud radar measurements to improve information about ice cloud microphysics is evaluated. Using the discrete dipole approximation (DDA), backscattering at 95 GHz (3.16 mm) has been computed for various ice crystal shapes. Crystal sizes up to 2 mm in maximum dimension in various orientation scenarios have been modelled. Different size distribution types have been considered representing the same fixed ice water content.

Based on the modelling results, the effect of ice crystal habits and size distribution characteristics on radar observables as co-polar reflectivity (Zhh), linear depolarization ratio (LDR), and differential reflectivity (ZDR) as well as difference reflectivity (ZDP) can be investigated. Technical radar aspects such as scanning capability and cross polarization isolation requirements are assessed.

For the polarimetric radar parameters, it is found that the co-polar reflectivity (Zhh) is dominated by the size distribution, whereas the linear depolarisation ratio (LDR) is governed by the crystal shape and orientation but almost inde-pendent of size. The results indicate that if crystals have a preferred orientation, LDR is a useful observable for differentiating between the two major crystal types of columnar and planar shapes. However, the corresponding cross-polar backscatter intensities are theoretically and technically difficult to achieve, the later being subject to the system sensitivity.

The differential reflectivity (ZDR) and difference reflectivity (ZDP) involve only co-polar intensities. For an antenna elevation angle of 45°, these parameters are promising for discriminating horizontally aligned pristine crystals from randomly oriented ones or irregular aggregates.

Further improvements in derivation of ice cloud microphysics are expected by incorporating Doppler information from a vertically pointing antenna. For this approach, crystal terminal velocities (vD) are calculated by using theoretically based power-law expressions predicting fall speeds in terms of ice crystal maximum dimension.

The results are judged with regard to the implications for a synergy of cloud radar and lidar measurements.