i. Theoretical and numerical studies on small-scale turbulence and dispersed phase dynamics.
A Lagrangian way of representation has been adopted for water droplets during Navier-Stokes DNS unsteady simulations of cloud boundaries. Statistical analysis has been carried out for droplet growth rate in different spatial regions. The most notable point is that the unstable mixing confining the cloud region hosts a notable acceleration of the droplet population dynamics.
The numerical investigation of the fluid dynamics of interfaces (TNTI) between turbulent and non-turbulent flows in stratified environment that can be representative of cloud/free-air boundaries was carried out. Results revealed the existence of both detrainment and entrainment events across the TNTI.
Direct Numerical Simulation (DNS) code SPARKLE was used along with a cloud microphysics routine. The cloud-edge mixing was represented as close to real-life clouds as possible and this included an actively growing cloud, the negatively buoyant subsiding shell and an environment. Results revealed a buoyancy driven shell with the in-shell mean velocity being passive and slaved to the buoyancy.
ii. Experimental part
Characterization and validation of a shadowgraph imaging device utilized to measure cloud droplets. For this a correction method for sample volume calculation was developed and minimum detection size limits of the shadowgraph instrument were established.
Lagrangian framework was employed by means of numerical tools and experiments to analyse the motion of numerical inertial particles crossing stratified interfaces and develop a parametric expression for the additional force exerted on particles due to variations in the density field.
To investigate how turbulence affects collision and coalescence processes, within the study an in-situ Lagrangian particle tracking experiment was performed. The experimental setting was located on top of the environmental research station Schneefernerhaus, at 2650 m altitude, just below the peak of Mt. Zugspitze in the German Alps.
iii. Links to cloud and weather/climate modeling
The focus was primarily put on the impact of sea surface temperature on the aggregation of deep convective clouds.
The impact of Sea Surface Temperature (SST) heterogeneities on the aggregation of convective clouds was investigated with the use of 3D cloud-resolving simulations of radiative/convective equilibrium.
iv.Telemetry-Instrumentation-Sensors
Prototype of an innovative cloud radio sonde. The mini ultralight smart balloon is a strategic new kind of expendable low cost, small (max 30 cm), light (about 20 g), environmental friendly (Mater-Bi.) radio probe embedding a microprocessor, IMU, GPS and sensors for the measurement inside clouds of velocity, acceleration, pressure, temperature, humidity and aerosol concentration fluctuations. To be released in the atmosphere by Unmanned Aerial Vehicles.
An ultrafast temperature, velocity and humidity probe was developed by following the design of the NSTAP (nanoscale thermal anemometry probe) family, cf. [Fan et al., Exp. Fluids, vol. 56:138, 2015]. The innovative design of the NSTAP probes makes them prime candidates for the development of ultrafast (with a frequency response of up to 100kHz) probes to measure statistics of multiphase flows such as clouds.
A drop generator capable of rapid creation of liquid droplets of sizes 5-50 μm, similar to those typically found in warm clouds was taken on to investigate experimentally generation, coalescence and dynamics of the droplets.
