COMPLETE ITN - ETN NETWORK

The recent paper has been published on Physics of Fluids with a following title:

"Diffusion of turbulence following both stable and unstable step stratification perturbations" 

   Physics of Fluids (2022); https://doi.org/10.1063/5.0090042

     Luca Gallana, Shahbozbek Abdunabiev, Mina Golshan,  Daniela Tordella

The evolution of a two-phase, air and unsaturated water vapor, time decaying, shearless, turbulent layer has been studied in the presence of both stable and unstable perturbations of the normal temperature lapse rate. The top interface between a warm vapor cloud and clear air in the absence of water droplets was considered as the reference dynamics. Direct, three dimensional, numerical simulations were performed within a 6m x 6m wide and 12m high domain, which was hypothesized to be located close to an interface between the warm cloud and clear air. The Taylor micro-scale Reynolds’ number was 250 inside the cloud portion. The squared Froude’s number varied over intervals of [0.4; 981.6] and [-4.0; -19.6]. A sufficiently intense stratification was observed to change the mixing dynamics. The formation of a sub layer inside the shearless layer was observed. The sub-layer, under a stable thermal stratification condition, behaved like a pit of kinetic energy. However, it was observed that kinetic energy transient growth took place under unstable conditions, which led to the formation of an energy peak just below the center of the shearless layer. The scaling law of the energy time variation inside the interface region was quantified: this is an algebraic law with an exponent that depends on the perturbation stratification intensity. The presence of an unstable stratification increased the differences in statistical behavior among the longitudinal velocity derivatives, compared with the unstratified case. Since the mixing process is suppressed in stable cases, small-scale anisotropy is also suppressed.

The paper on the microphysical time scales has been accepted and is already published on Physics of Fluids (AIP).

Recent results have shown that there is an acceleration in the spread of the size distribution of droplet populations in the region bordering the cloud and undersaturated ambient. We have analyzed the supersaturation balance in this region, which is typically a highly intermittent shearless turbulent mixing layer, under a condition where there is no mean updraft. We have investigated the evolution of the cloud - clear air interface and of the droplets therein via direct numerical simulations. We have compared horizontal averages of the phase relaxation, evaporation, reaction and condensation times within the cloud-clear air interface for the size distributions of the initial monodispersed and polydisperse droplets. For the monodisperse population, a clustering of the values of the reaction, phase and evaporation times, that is around 20-30 seconds, is observed in the central area of the mixing layer, just before the location where the maximum value of the supersaturation turbulent flux occurs. This clustering of values is similar for the polydisperse population but also includes the condensation time. The mismatch between the time derivative of the supersaturation and the condensation term in the interfacial mixing layer is correlated with the planar covariance of the horizontal longitudinal velocity derivatives of the carrier air flow and the supersaturation field, thus suggesting that a quasi-linear relationship may exist between these quantities.

The supersaturation local balance equation is considered at a cloud boundary, and the structure of its turbulent production term is inferred from numerical experiments. The transient decay of an unstable-stratified, shearless-mixing layer at a warm cloud top is modeled as an initial value problem, in which the turbulent velocity, the active scalars, and the droplet dynamics are solved concurrently. The evolution of the cloud-clear air interface and the droplets therein is investigated via direct numerical simulations. Both initial monodisperse and polydisperse droplet size distribution cases are treated. We attempt to model the turbulent production of supersaturation - defined as the difference between the time derivative of the supersaturation and the condensation terms - in the mixing layer. The planar averages of the supersaturation production term are clearly correlated with the planar covariance of the computed horizontal velocity derivative and supersaturation fields, thus suggesting that a quasi-linear relationship may exist between these quantities. The value of the non-dimensional proportionality constant is deduced by i) modeling the production term as the interaction of two sinusoidal perturbations with local and global small-scale frequencies, and ii) spatially averaging the production term and the covariance of the supersaturation and the longitudinal velocity derivatives. It has been verified that the value of such proportionality constant does not vary considerably throughout the transient decay and is not sensitive to the initial droplet distribution dispersity.

Recently the development of biodegradable mini-radiosondes to measure fluctuations in pressure, temperature, humidity, velocity within warm clouds was defended by Giovanni Cipri under Prof. Tordella's supervisor.

The microphysical processes that take place within warm clouds are a source of uncertainty for current meteorology: several gaps exist in the knowledge of the growth of droplets in the clouds and the subsequent development of rain. One of the factors that makes it very uncertain is the role that turbulence plays in the microphysical processes that involve the growth of droplets: CCN (Cloud Condensation Nuclei), condensation, collision. The great knowledge lacks that there are on the subject are probably due to the limited amount of data taken in the field, with sensors transmitting data from the clouds. Even less has been, until now, the data captured by a probe capable of faithfully following the trajectories of the current in the clouds, that is in a Lagrangian way. The COMPLETE project, a Horizon2020 program, was created to broaden the knowledge science has about the interaction between turbulence (from small to large scales) and microphysical phenomena in the clouds, thanks to the use of a floating, disposable, biodegradable, radio probe capable of transmitting data in real-time to a ground receiver on pressure, temperature, humidity, trajectory, and speed. In particular, in this thesis, after having discussed and illustrated the current gaps in knowledge on the subject, it is intended to report: the main components of the radiosonde; the operating principle; the assembly procedures optimized by the author in the IIT laboratories; the first data captured by the probe.

A simplified cloud-clear air interface is studied through a direct numerical simulation on a grid of 512 × 512 × 1024 points. The interface is simulated through a time-decaying turbulent shearless mixing layer between two homogeneous and isotropic turbulent fields with different turbulent kinetic energy. The Navier-Stokes equations in the Boussinesq approximation are solved for an incompressible fluid together with the advection-diffusion equation for the water vapor density, seen as a passive scalar. Water droplets dynamics are taken into account through the solution of the droplet's momentum equations together with the water droplets growth equation. The main water particle growth mechanisms are the water vapor diffusional growth and the collision coalescence growth which are both considered in the code. The feedbacks of the water droplets dynamics on the velocity, temperature, and vapor density fields are taken into account. Two simulations have been carried out. The first simulation describes a situation in which the cloud region (the high energy region) is supersaturated and the interface is initially saturated and the Taylor microscale Reynolds number is Reλ = 43. The second simulation analyses the case in which the cloud region is saturated and the interface is subsaturate, while the Taylor microscale Reynolds number is slightly higher than in the first simulation, Reλ = 53. In this work not only the main features of the particular turbulent shearless mixing simulated are described but also the temperature and the water vapor density transport across the mixing layer are analyzed together with the water droplets dynamics. In particular, the role of turbulence in advecting the inertial particles is investigated through the visualization of the clustering phenomenon. The time evolution of the droplet size distribution spectrum has been analyzed for both simulations. The aim of this work is the study of the water droplet dynamic in a saturated and supersaturated turbulent environment and the effect of the entrainment on water droplets at the cloud-clear air interface. In the saturated case, a very strong reduction in the liquid water content due to the intense evaporation is observed, while in the supersaturated case an increase in the liquid water content can be seen. The droplet size distribution analysis showed the iii same trends, a strong decrease of the mean droplet radius is observed for the saturated case and a slight increase of the mean radius is seen for the supersaturated case. Finally from the visualization of the water droplets' spatial distribution, a significant clustering is observed. Furthermore, it is shown that the water droplets concentrate in the low vorticity intensity regions. Only two eddy turnover time was simulated and a significant droplet growth cannot be appreciated, but the results obtained agree with the results in the literature.

POLITO group has published a paper in the International Journal of Multiphase Flow (IJMF). The paper concentrates on intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments.

Atmosphere 2020, vol. 11, 199.