Determination of shape and orientation of ice crystals in mixed-phase clouds based on observations from polarimetric cloud radar

A continuous estimation of ice-crystal shape in mixed-phase clouds is one of the major problems in the remote sensing of the atmosphere. First, knowledge about the shape allows for improving the accuracy of the existing size and concentration retrievals based on vertical observations from a cloud radar and a lidar. Second, in current weather prediction models the assumed shape of ice crystals defines the mass-area-terminal velocity relationship and the depositional growth rate of ice particles. Third, information about the ice shape enables a continuous estimation of ice-particle size distribution based on complete Doppler spectra measured by vertically pointed cloud radars with the use of known size-velocity dependencies.

We employed the newly developed 35-GHz cloud radar of type MIRA-35 with the hybrid polarimetric mode to estimate shape and orientation parameters of ice crystals in mixed-phase clouds. The radar permits to measure spectrally resolved polarimetric variables. The retrieval technique is based on elevation dependencies of differential reflectivity and correlation coefficient. To get the estimation of the shape and orientation we compared the measured polarimetric variables with those calculated with the use of available spheroidal scattering models.

In this poster we present several case studies based on measurements from the field campaign ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques). The measurements took place at the CESAR site, Cabauw, Netherlands, in October – November 2014. All the presented cases correspond to observed clouds with different temperatures at the top: -5, -9, -16, and -22 C. We also show that the retrieved shape is in a good agreement with long-term laboratory studies on the ice crystals grown above water saturation in free-fall chambers.