Franco Prodi

Measurements of diffusiophoretic velocities of aerosol particles in the transition region

Abstract Measurements of thermophoretic velocities of NaCl particles have been performed by injecting the aerosol as a thin sheet into a laminar flow of clean gas, with a temperature gradient established across. The experimental apparatus and the procedure are described. Particles deposit on a cold plate and are observed with an electron microscope. The dependence of the thermophoretic velocity on particle size is evident. Particles with a Knudsen number rising to unity approach Waldmans theory and Brocks theoretical curve is not far out for diameters below 1 μm.

Multiwavelength sun-photometers for accurate measurements of atmospheric extinction in the visible and near-IR spectral range

A new multiwavelength sun-photometer has been developed to obtain accurate measurements of the atmospheric attenuation of direct solar radiation at seven window wavelengths in the visible and near-infrared spectral range. The instruments optical features and performance are described. The photometer was calibrated by following a careful procedure based on the Langley method. An additional eighth bandpass filter was used to obtain measurements of the precipitable water. Five examples of this instrument have been constructed and calibrated for a network of mountain stations in Southern Italy to monitor Sahara dust transport episodes.

Chemical characterization of cloud episodes at a ridge site in Tuscan Appennines, Italy

Cloudwater samples were collected from November 1992 to March 1995 in Vallombrosa, a mountain site of the Tuscan Apennines (central Italy). Chemical analyses show that all examined inorganic ions contributed significantly to the total ionic content (TIC). The ratio SO42−/NO4− ranged from 0.92 to 3.46 and was >1 for 86% of samples. There is a wide range in the chemical composition of the cloudwater. The total ionic content ranged from 640 to 7476 μeq l−1 and pH from 3.17 to 6.22. The liquid water content (LWC) ranged from 0.06 to 0.94 g m−3 and electrical conductivity from 47 to 485 μΩ−1. The total ionic content decreases while the liquid water content increases. Also analyzed were soluble trace metals (Fe, Pb, Cu, Mn, Cd, Al), synthetic anionic surfactants and the methanesulphonic acid. Chemical analyses evidenced in some cases a high concentration of organic matter. The meteorological analysis for a few samples of individual passages was carried out for the possibility of establishing a correspondence between meteorological events and chemical composition. The sources (marine, crustal and anthropogenic) of chemical components were deduced.

Experimental measurements on thermophoresis in the transition region

Abstract Measurements of thermophoretic velocities of carnauba wax, polystyrene latex, silver particles and sodium chloride have been performed by injecting the aerosol as a thin sheet into a laminar flow of clean gas, with a temperature gradient established across it. These measurements have enabled us to compare the experimental data with the known theories. The aerosol radius range was 0.11– 0.83 μm . The measured reduced thermophoretic velocity was found to depend only on the Knudsen number, in the range 0.09–0.7, and not on the gas/particle conductivity ratio.

Cloud systems leading to flood events in Europe: an overview and classification

Over recent years most European countries have experienced destructive precipitation events, many of which have generated floods resulting in the loss of lives and economic damage. Rather surprisingly, Europe has no common database for flood events, which continue to be monitored by different national and/or local authorities, with different criteria. In the light of this situation, the main aim of the present work is to propose a classification scheme for a European-level flood database, in which severe flood episodes are related to meteorological conditions and mesoscale settings. The meteorological systems relating to the most severe events (22 events) over a five-year period (1992–1996) were analysed using Meteosat imagery and conventional meteorological data. Most of the observed events were related to extra tropical cyclone development. The results are discussed in terms of conceptual models currently used for extra tropical cyclones: the structure and dynamics of Atlantic cyclones can very often be interpreted in terms of well- known conceptual models, while characteristics of Mediterranean systems are not fully rendered by current descriptions. Our study underpins the need to improve the knowledge of Mediterranean systems, possibly introducing new conceptual models and/or refining the current ones. Copyright

Rainfall estimation by combining radar and infrared satellite data for nowcasting purposes

The aim is to evaluate the use of infrared satellite precipitation estimates for nowcasting purposes in the context of a real-time flood-warning scheme. A radar-based calibration technique is described which is applied to the Negri–Adler–Wetzel scheme. This procedure employs radar data over a defined calibration area to estimate, for each satellite image, actual rain-rates to be used in the Negri–Adler–Wetzel scheme. Calibrated satellite estimates obtained from this procedure can be used to diagnose areas of precipitation beyond radar range, thus allowing an extension of precipitation nowcasting lead time. Calibrated estimates are compared with radar rainfall measurements and results are discussed for various sizes of integration area. Calibration reduces consistently both bias and variance of the error of the original Negri–Adler–Wetzel estimates, even for integration areas as small as 2000 km2. This indicates the capabilities of the new technique for nowcasting purposes over mediumsized river basins. Copyright

Analysis of polarization radar returns from ice clouds

Using a modified T-matrix code, some polarimetric single-scattering radar parameters (Zh,v, LDRh,v, qhv, ZDR and dhv) from populations of ice crystals in ice phase at 94 GHz, modeled with axisymmetric prolate and oblate spheroidal shapes for a C-size distribution with different a parameter (a=0, 1, 2) and characteristic dimension Lm varying from 0.1 to 1.8 mm, have been computed. Some of the results for different radar elevation angles and different orientation distribution for fixed water content are shown. Deeper analysis has been carried out for pure extensive radar polarimetric variables; all of them are strongly dependent on the shapes (characterised by the aspect ratio), the canting angle and the radar elevation angle. Quantities like ZDR or dhv at side incidence or LDRh and qhv at vertical incidence can be used to investigate the preferred orientation of the particles and, in some cases, their habits. We analyze scatterplots using couples of pure extensive variables. The scatterplots with the most evident clustering properties for the different habits seem to be those in the (ZDR [v=0� ], dhv [v=0� ]), in the (ZDR [v=0� ], LDRh [v=90� ]) and in the (ZDR [v=0� ], qhv [v=90� ]) plane. Among these, the most appealing one seems to be that involving ZDR and qhv variables. To avoid the problem of having simultaneous measurements with a side and a vertical-looking radar, we believe that measurements of these two extensive variables using a radar with an elevation angle around 45� can be an effective instrument to identify different habits. In particular, this general idea can be useful for future spaceborne polarimetric radars involved in the studies of high ice clouds. It is also believed that these results can be used in next challenge of developing probabilistic and expert methods for identifying hydrometeor types by W-band radars. D 2001 Elsevier Science B.V. All rights reserved.

Effects of Altitude on Maximum Raindrop Size and Fall Velocity as Limited by Collisional Breakup

AbstractCoalescence and breakup of drops are recognized as the main mechanisms determining raindrop size distributions on the ground. Full knowledge of these processes is hindered by the challenging difficulties both in the laboratory and tunnel experiments and during observations in the open air.In real rain breakup is mainly due to collision between drops of different sizes (collisional breakup) and occurs when the collisional kinetic energy (CKE) is not absorbed by the colliding drops. In this work, the authors observe and measure the dependence on altitude of the occurrence of collisional breakup in real rainfall events, and then estimate the corresponding limit terminal velocities of drops and their size when breakup significantly takes place.Data from Pludix, an X-band microwave disdrometer, were collected at three locations at different elevations: collisional breakup position in the power spectrum of Pludix increases toward higher frequencies with increasing altitude. Terminal velocities and sizes...

Thermophoretic measurements in presence of thermal stress convection in aerosols in microgravity conditions of drop tower

The aim of the work is getting reference data on thermophoretic motion eliminating gravity-induced perturbation, developing new instrumentation and procedures. A series of experiments on measuring phoretic velocities was performed in the Bremen drop tower providing 4.7 s of high quality microgravity conditions, which allowed making negligible particle sedimentation and buoyancy driven convection. Motion of aerosol particles was observed simultaneously at low resolution to control nongravity convective motion in the cell and at high resolution by the digital holographic velocimeter in order to register particle three-dimensional trajectories. By choosing appropriate cell size and experimental procedures the heat and mass transfer relaxation processes were reduced to less than 0.3 s thus allowing measurements of particle velocities during more than 4 s. Side-wall temperature creep created convective motion in the cell. Its influence was suppressed by choosing sufficiently flat cell geometry. The values of the measured thermophoretic velocities for Knudsen number in the range 0.047–0.89 were found to be between predictions of the classical models of Talbot et al [1] on one hand and Yamamoto and Ishiara [2] on the other hand. Particles of different thermal conductivities (paraffin and NaCl) had about the same velocities. No negative thermophoresis was observed at these conditions for NaCl.

Time variability in rainfal events observed by Pludix

Abstract Pludix is a raingage-disdrometer based on the analysis of an X-band Continuous Wave radar signal backscattered by hydrometeors. The Doppler shift of the individual falling particle, which is supposed to randomly cross the radar volume, is detected. The 2048 readings of the signal are inverted to generate a hydrometeor size distribution subdivided into 21 bands. Pludix operates integrating 60-sec. backscattered signal and so it provides a more detailed information with respect to the classical tipping bucket rain gage. Moreover its possible to detect the actual rainfall rate as an indirect product. Since September I999 the instrument has been in operation aside with a tipping bucket and a weighting scale rain gauge downtown Bologna. More than 200 rainfall events have been detected and subgroup of 21 events has been investigated. The spectral intensity of the signal is analysed with different algorithms for both time variability and precipitation type. A network of Pludix seems to be the right tool for investigating the space variability of rain in addition to time variability.