Combined observations of turbulence and ice properties in mixed-phase layered clouds with Doppler lidar and cloud radar

For the continuous observation of aerosols, clouds, and their interactions the multi-instrument measurement platform LACROS (Leipzig Aerosol and Cloud Remote Observations System) was set-up at the Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany (51.3 °N, 12.4 °E). LACROS combines the abilities of advanced Raman/Depolarization lidars, a cloud radar, a Doppler lidar, a microwave radiometer and several auxiliary measurement systems to provide information about aerosols and clouds. The lidars mainly sense small droplets at the predominantly liquid cloud bases, simultaneously cloud radars deliver information about the presence and the properties of falling ice particles.

A comprehensive statistics of vertical motion patterns at cloud base was collected with a Doppler lidar during UDINE (Up- and Downdrafts in Drop and Ice Nucleation Experiment) at Leipzig and SAMUM-2 (Saharan Mineral Dust Experiment) at Praia, Cape Verde (15° N, 23.5 °W). The results of both campaigns are evaluated together, allowing to compare the properties of mid-latitudinal and sub-tropical layered clouds. Combined with simultaneous measurements of powerful depolarization lidars and cloud radars detailed insights into ice-formation processes are obtained. The statistics of vertical velocity at cloud base is presented together with the simultaneously measured properties of ice crystals falling from the cloud base. Basic ice crystal properties (e.g. ice water content) are evaluated with the help of a Z-T-parametrization.

The combination of lidars and radars is used to draw a coherent picture of the physical processes within turbulent cloud layers in the middle troposphere. In this way height, phase (liquid, mixed-phase or ice) and vertical velocity at cloud bases of mid-level layered clouds are probed at once. Layered clouds are ideal targets for the observation and investigation of cloud droplet freezing processes: They can be penetrated completely by both lidar and radar and show narrow constrains on environmental variables like pressure, temperature, etc... Knowledge about the complete vertical wind field around such a cloud system and inside its turbulent layers is needed for the quantification and understanding of the physical processes driving cloud formation and sustainment. Large-scale vertical motions are important for the generation of clouds (e.g. gravity waves), small scale turbulence is the dominant driver for the microphysical properties of a cloud layer. Formation of ice crystals via contact or immersion freezing, the generation of large droplets, support or inhibition of the Wegener-Bergeron-Findeisen process and the recreation of liquid drops in the presence of ice crystals critically depend on vertical air motion and its statistics. Despite this need for measurements there is currently no instrument that is specially designed to measure vertical wind fields at cloud base. Cloud radars do not sense the small droplets and the signal from Doppler lidars is contaminated by artifacts introduced by the large signal gradient at cloud base and laser pulse chirp. Methods were developed to reduce these artifacts and accurately measure vertical air velocity in the vicinity of clouds - especially at cloud bases.