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Cloud-MicroPhysics-Turbulence-Telemetry: An inter-multidisciplinary training network for enhancing the understanding and modeling of atmospheric clouds

Periodic Reporting for period 2 - COMPLETE (Cloud-MicroPhysics-Turbulence-Telemetry: An inter-multidisciplinary training network for enhancing the understanding and modeling of atmospheric clouds)

Reporting period: 2018-06-01 to 2021-01-31

Issue being addressed:
• characterization of the direct interactions between cloud dynamics, thermodynamics and microphysics at the meter scale, • development of new telemetry methods for in-situ measurements of the aerosol, turbulence, temperature, pressure and chemical content, • improve the cloud edges/interface models to better prediction of the mixing efficiency, temperature inversion, clear air entrainment/moist air detrainment, • transfer the knowledge acquired by field and laboratory Lagrangian measurements and DNS modelling towards the climate/weather modelling community

Importance for Society.
Clouds have been termed the “big bad player in global warming” (Kerr, Science 2009) and are listed among the most urgent scientific problems requiring attention by the Intergovernmental Panel on Climate Change. Remaining in the European context, just think of what now frequently happens: devastating storms and hailstorms, floods, storm surges. An important technology we have developed to detect what is happening inside the clouds is a small green expendable radioprobe that floats inside them to monitor local weather conditions. This is also important in the case of clouds formed following the accidental release of dangerous substances from industrial, or similar, plants.

Overall Objectives.
So far, understanding of clouds has come through the study of two phenomena: cloud microphysical processes in non-turbulent flows, and large-scale cloud circulation and dynamics. Since ambiguities related to representation of clouds in climate models prevail, explorative observations are still needed. The challenge is to establish connections across this range of scales to combine knowledge and training across vastly different scientific and engineering disciplines.
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.
Important findings extending the state of the art are as follows:
-Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell that grows with time. A self-similar regime is observed for first- and second-order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified.
-It is observed that the evaporation and condensation have a dramatically different weight inside the homogeneous cloudy region and the interfacial anisotropic mixing region. It is observed that the dynamics of drop collisions is highly effected by the turbulence structure of the host region. There is an increased probability of collisions in the interfacial layer hat houses intense anisotropic velocity fluctuations.
-A reconstruction of sub-grid scales in large eddy simulation (LES) of turbulent flows in stratocumulus clouds was conceived. The approach is based on the fractality assumption of turbulent velocity field.
-In the clear-sky regions, the formation of a higher pressure drives a near-surface flow from clear-sky to the cloudy regions. This flow transports moisture to an already moist region, dries further the clear-sky regions, and prevents cloud formation there. This process leads to the expansion of the clear-sky regions, thus confining convective clouds to a smaller moist region.
-An experimental method for measuring in situ the influence of small-scale turbulence in cloud formation and producing an in-field cloud Lagrangian dataset was developed by means of an innovative ultralight green expendable radioprobe. The electronic system design, biodegradable in part, lightweight (≈20 g), and expendable, has been successfully carried out. NOTE: atmospheric radiosondes currently in use are not able to passively float inside clouds.

Outcomes of the project are made available to the wider researcher community and the society in line with the Open Research Data policies of the EU. The publication of the data has been designed to adhere as much as possible to the FAIR principles.
Collision kernel: polydisperse drop population inside a nearly homogeneous warm cloud portion
Motion of polydisperse water drops inside a simulated interface between cloud top and clear air
LES sim.s of velocity, horizontal-vertical sections of a portion of cloud which include the top
Mixing Diagrams: droplet populations within air and water vapour inside a cloud top
Near-surface air temperature (colors, K) and cloud water (gray shades), sim.s with central hot spot
First in-field experiment with balloon and radioprobe
Collision kernel: poly-disperse drop population inside the interface with ambient air
Boundaries subsiding shell: red neg. buoyant mixture, blue region neg.vertical velocity
Column relative humidity sim.s: a. uniform surface temp, b. sea surface temp anomaly hot spot
In-field experiment with COMPLETE mini green radio and VAISALA probes
Mini-green-expendable radioprobe system architecture
Caption, see below image, S. Shamekh et al, 2020
Motion of polydisperse water drops inside cloud top
Turbulent Numerical sim: Supersaturation visualisation, cloud top, mild unstable stratification
H2020COMPLETE radioprobe board: sensors, radiomodule, microcontroller, power module