by Lukas Pfitzenmaier

Mixed-phase clouds are frequently observed in the atmospheric temperature range between -40 and 0 °C where water droplets and ice crystals can coexist. The composition of these mixed-phase clouds, i. e., the partitioning of liquid water and ice, plays a crucial role in the formation of precipitation and in the cloud radiative effect. The passage of crystals through layers of supercooled liquid water affects ice crystal growth and shape. Thus, to observe the evolution of the microphysical properties of ice crystals through mixed-phase layers, knowledge about their size, mass, and shape is required. The proposed project aims at improving techniques for phase detection and estimation of crystal shape at all stages, from the cloud formation to the precipitation of ice particles. Information about the shape of ice crystals is a prerequisite for the determination of size and mass of ice crystals, because the shape determines the relationship between measured particle fall velocity and the unknown particle size.

ACCEPT flyer

At the CESAR observatory, the 3-GHz radar TARA of the TU Delft is operated to retrieve particle shape of large precipitation particles. At this frequency, the signal is not contaminated by reflection from supercooled water droplets and is not attenuated, providing direct information on ice crystal properties even for optically thick clouds. However, at such a frequency, freshly formed particles cannot be detected. Additionally, the zenith-pointing lidar CAELI provides polarization measurements which allow the detection of liquid layers up to heights where the lidar beam is extinguished.

At TROPOS a new technique was developed in cooperation with METEK GmbH, Elmshorn, Germany, to investigate the shape of ice crystals with a cloud radar operating in so-called STAR (Simultaneous Transmit And Receive) mode at a wavelength of 35 GHz. Observations at the much shorter wavelength allow to estimate the shape of even freshly formed ice particles in optically thin clouds but suffer from attenuation and sensitivity to liquid water that affect the radar output for optically thicker clouds. In addition, co-located measurements of the off-zenith pointing polarization-lidar Polly-XT of TROPOS will be exploited to detect layers of horizontally aligned planar crystals, causing specular reflections which affect only the signal measured with the zenith-pointing CAELI.

The set of instruments will been completed during a vertical pointing 35 GHz cloud radar, microwave radiometer, ceilometer ,and distrometer will be operating and radiosounds will be launched during the campaign too.

In the ACCEPT campaign – Analysis of the Composition of mixed-phase Clouds with Extended Polarization Techniques – the multi-frequency complementarity provided by the unique, novel set of instruments to retrieve ice crystal microphysical properties in a large size range from micrometer- to cm-scales will be investigated. In addition, the inter-calibration of the three deployed radar systems will be evaluated.