M. Manfrin

Influence of the Sonic Anemometer Temperature Calibration on Turbulent Heat-Flux Measurements

The speed of sound in moist air is discussed and a more accurate value for the coefficient of the linear dependence of sonic temperature on specific humidity is proposed. An analysis of speed-of-sound data measured by three sonic anemometers in a climate chamber and in the field shows that the temperature response of each instrument significantly influences not only the determination of sonic temperature, but also its fluctuations. The corresponding relative contribution to the error in the evaluation of the temperature fluctuations and the turbulent heat fluxes can be as high as 40%. The calibration procedure is discussed and a method of correction is proposed.

Experimental study of higher-order moments in shear-driven boundary layers with rotation

The results of laboratory wall turbulence experiments on a shear-driven rotating boundary layer are presented. The experiments were carried out in the Turin University Laboratory rotating water tank. The flow was generated by changing the rotation speed of the platform and measured by means of particle image velocimetry. In order to analyse the influence of the rotation and of surface roughness, different cases were examined. Several rotation periods were considered. The measurements were performed both over a smooth surface and over a rough-to-smooth transition. Mean flows and the higher-order moments of the velocity probability density function are shown and discussed together with a comparison of the different experimental cases, theory and large-eddy simulations.

Inspection of high-concentration CO2 events at the Plateau Rosa Alpine station

The Plateau Rosa Alpine station (Italy) has collected atmospheric concentrations of carbon dioxide since 1989. If the complete set of hourly data is observed, two distinct and exceptional very high concentration events are evident for February 2004. Similar and almost contemporary peaks were registered at the European high–altitude stations of Zugspitze–Schneefernerhaus and Sonnblick in the Alps, and at Mt. Cimone in the Northern Apennines. A regional meteorological model (the Weather Research and Forecast) was applied over a medium–high resolution grid to study the evolution of the meteorological fields and to identify the trajectories of the polluted air masses during the CO2 observed peaks. The results show that, during both episodes, atmospheric circulation conveyed highly polluted air from the European plains to the Alpine stations. This conclusion has been also confirmed through concentration measurements of the atmospheric trace gases in the same area.

Water Tank Simulation of a Dense Fluid Release

The turbulent velocity field of a dense fluid release has been measured in a water tank experiment carried out in the TURLAB laboratory in Turin (Italy). A vertical density driven current was created releasing a saline solution in a water tank with no mean flow. The experiment reproduces in physical similarity, based on the density Froud number, the release of a dense gas in the atmosphere and the PIV technique has been used to analyse the buoyancy generated velocity field. The high temporal and spatial resolution of the measurements gives a deep insight to the problems of the bouncing of dense gases and of the creation of the outflow velocity at the ground. The experimental findings are used to test and improve an original method for the dispersion of a positively and negatively buoyant plume. The method is based on the idea of Alessandrini and Ferrero (Phys. A 388:1375–1387, 2009) for the treatment of a background substance entrainment into the plume and it consists on the introduction of two fictitious scalars inside the Lagrangian Model SPRAY. The fictitious scalars represent the density and momentum difference between the plume and the environment air that naturally takes into account the interaction between the plume and the environment. As a consequence, no more particles than those inside the plume are released to simulate the entrainment of the background air temperature. In this way the entrainment is properly simulated and the plume sink is calculated from the local property of the flow. The approach is wholly Lagrangian.

Physical simulation of atmospheric microbursts

A laboratory simulation of atmospheric microbursts is presented. The physical model of the atmospheric phenomenon is reproduced at a reduced scale in a rotating tank (TURLab, Italy), the similitude is based on the Froude number. Different experiments were carried out varying the Rossby number, and the analysis of four significant cases is presented. The velocity, vorticity, and turbulent kinetic energy fields are evaluated together with the swirling strength analysis. The comparison with the natural prototype is eventually shown and discussed.

Neutral Saturated Lapse Rate: An Experimental Check from CALJET-1998 and PACJET-2001

The lapse rates of high-resolution temperature profiles during nearly neutral, saturated conditions are compared with the saturated adiabatic lapse rate and with that proposed by Richiardone and Giusti. A good agreement between the latter and the mean value of the observed lapse rate is found, whereas the saturated adiabatic lapse rate differs significantly, confirming experimentally that it is not completely correct to assess the moist neutrality from a comparison with the saturated adiabatic lapse rate. The lapse-rate distribution supports the hypothesis that the lapse-rate statistics is a local collection of saturated adiabatic lapse rates in a background normal distribution centered around the neutrality.