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Free keywords:
cloud microphysics; atmospheric turbulence; energy dissipation rate
Abstract:
Clouds are crucial to the Earth’s radiation budget and energy balance. Inadequately resolved cloud processes are a major source of uncertainty in weather forecasts and climate prediction. Due to their turbulent nature, cloud dynamics span a vast range of spatial and temporal scales from μm to km and ms to h, respectively. Together with phase transitions, the transport of heat and moisture as well as cloud droplet-turbulence interaction, the multiscale property of turbulence poses a huge challenge for unraveling cloud processes. Deployed by kite-stabilized and helium-filled aerostats, the so-called helikites, two specially designed instruments, the Max-Planck-Cloudkite + (MPCK+) and the mini-Max-Planck-Cloudkite (mini-MPCK), measure the atmospheric state and flow velocity as well as cloud microphysical properties. During EUREC4A field campaign in the Caribbean from January to February 2020 above the Atlantic Ocean, both the MPCK+ and the mini-MPCK profiled the atmospheric boundary layer of the trade-wind region and sampled clouds. In this thesis, we report measurements of the energy dissipation rate and cloud droplet statistics in the trade-wind region. The energy dissipation rate is one of the most fundamental turbulence characteristics and is estimated from one-dimensional velocity time-records. To benchmark different methods for estimating the energy dissipation rate, each method is compared to the ground-truth reference of direct numerical simulation (DNS) of stationary homogeneous isotropic turbulence at different Taylor-scale Reynolds numbers between 74 and 321. The impact of finite turbulence intensity and misalignment between the probe orientation and the mean flow direction is systematically studied and expressed by analytical expressions. Additionally, the effect of a finite averaging window and its Rλ dependence is captured by scaling arguments which are compared to hot-wire measurements from the Max Planck Variable Density Turbulence Tunnel with Taylor-scale Reynolds numbers between 147 and 5864. Both atmospheric turbulence and cloud droplet-turbulence interaction is investigated with the help of energy dissipation rate estimates from, in total, 197 h record of scientific data. This thesis examines the spatial distribution of cloud droplets in trade-wind cumuli, the onset of warm-rain initiation and the altitude dependence of the cloud droplet size distribution, which are analyzed based on 144 h of cloud droplet records. Furthermore, turbulence characteristics of the boundary layer, its stability and isotropy on inertial length scales are determined.
