1,721,026 research outputs found
Measurements of optical characteristics of dilute suspensions and dry deposits of clay particles with a polar nephelometer
No full textOptical characteristics and dimensional ranges of Clay Spur Montmorillonite particles have been determined by means of light-scattering experiments. We have taken advantage of an Aerodynamic Sampler that allows dimensional separation of the particles on a filter that is then made transparent in acetone vapours
Monodisperse droplet distribution in the ice accretion process
No full textIn the atmospheric ice accretion process, the spreading of the impinging droplet is an important factor in determining the deposit density and morphology.
In this work we present low-density ice aggregates grown in laboratory by impinging supercooled monodisperse droplets in comparisons with results of ballistic numerical model
Operational rainfall estimation using METEOSAT infrared imagery: its potential and drawbacks
No full textThe performances of two rainfall-estimation methods based upon the infrared channels of geostationary satellites are examined for very-short-range weather forecasting. A flood-hazard monitoring application in the Arno River Basin in Central Italy in November 1987 is presented. Results demonstrate that the rain-estimation techniques applied in the nowcasting environment are a powerful aid to the forecaster. Evidence is also provided that rain estimates must be correctly interpreted in terms of the structure of precipitation systems to avoid the considerable errors that can stem from routine, purely numerical applications. -Author
Measurements of diffusiophoretic velocities of aerosol particles in the transition region
No full textThe diffusiophoretic velocities of carnauba wax and paraffin aerosol particles have been measured by injecting them as a thin sheet into a laminar flow of clean gas, with a water vapour concentration gradient established across it. In the range 0.1-1 of the Knudsen number, the measured velocities was found to increase with the diameter of aerosol particles. The experimental results were in better agreement with the Derjaguin and Yalamov theory (In G.M. Hidy, JR, Brock (Eds.), International reviews of aerosol physics and chemistry vol. 3 (Topics in current aerosol research, Part. 2). Pergamon Press, Oxford, 1972, p. 384.) than with those of Waldmann and Schmitt (In C.N. Davies (Ed.), Aerosol Science, Academic Press, London, 1966, pp. 137-162.), and Beresnev et at. (J. Aerosol Sci. 30 (Suppl. 1) (1999) S761)
Cloud systems leading to flood events in Europe: an overview and classification
No full textOver 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
Atmospheric precipitation analysis using both disdrometric and satellite data for desertificaton studies.
No full textIn the study of desertification processes, an important role is obviously played by the atmospheric precipitation investigation. The Italian project RIADE (Ricerca Integrata per l’Applicazione di tecnologie e processi innovativi per la lotta alla Desertificazione), has been setup to study the atmospheric precipitation in southern Italy and islands (Sicily and Sardinia), that can be potentially affected by desertification phenomena. A comparison with observation from different sensors having different resolution has been realized. The analysis has been conducted in a quantitative way using the new generation X-band microwave raingage-disdrometer named PLUDIX, installed in three different Italian sites. PLUDIX data have been used both for rainfall events detection and for a climatological analysis of the rain events. Moreover, through rainfall rate (R) and drop size distribution (DSD) parameters estimation, the instrument has allowed a microphysical characterization of rain at the ground and the discrimination between convective and stratiform events. We also used satellite data from Meteosat-7, SSMI (Special Sensor Microwave Imager) and MODIS (Moderate Resolution Imaging Spectro-radiometer) for a qualitative analysis of the precipitation systems. Because of the different spatial scale of the disdrometer and satellites, satellite data was used for the temporal scale analysis and for the cloud characterization (quality, type and structure of the rain clouds) of the selected events, as well as for the comprehension of the spatial extension of each rain event (genesis and evolution). We have also linked the observed events to the corresponding conceptual models in the Mediterranean area
Multiparametric radar/microwave precipitation retrieval
No full textThe use of polarisation difference over the land for three cases of different nature has been investigated: Poldirad (14/07/1997, strong convective event), Graz (10/07/1997, convective event) and Chilbolton (27/06/1997, stratified event).
The correlation between vertical and horizontal polarisation Tbs differences at 85 and 19 GHz and ZH and ZDR as measured by the available radar has been evaluated. The radar domain has been classified in three rain classes by means of the ZH values: no rain (030 dBZ).
It was found that the 19 GHz polarisation differences decrease when Zdr increase. On the contrary, 85 GHz channels Tb differences are not related to Zdr variations. At 85 GHz the increase of Zdr results in an average increase of the polarisation difference from the satellite; this implies a rather strong correlation between the rain layer at the ground (as seen by radar) and the ice layer on the top of the cloud as seen at 85 GHz
RAIN-RATE ESTIMATION FROM SEVIRI/MSG AND AMSR-E/AQUA. VALIDATION AND COMPARISON BY USING U.K. WEATHER RADARS
No full textThe satellite rainfall estimation algorithm here proposed is based on a statistical approach (Artificial Neural Network: ANN): it needs only radiation satellite measurements as input data and provides, at 5 km of spatial resolution, surface rain-rate classification onto five classes of precipitation: [less than 1/32] mm/h (no rain), [1/32, 0.125] mm/h (slight rain), [0.125, 0.5] mm/h (slight/moderate rain), [0.5, 2.0] mm/h (moderate), [more than 2.0] (heavy rain).
The algorithm works for U.K. area, daytime and summer season and adopts a cascade method where at first a rain no-rain classification is computed and then a similar yes-no classification is computed for the other pair of classes. It has been developed and validated with the use of U.K. weather radar rainfall estimates for both SEVIRI (on the geosynchronous Meteosat-8 satellite) and AMSR-E (on the low earth orbit AQUA satellite) sensors.
To assess the performance against radar rainfall estimation some skill indicators are computed: the Equitable Threat Score (ETS) and BIAS are used for pair of classes of precipitation whereas the Heidke Skill Score (HSS) is used for the four raining classes.
The validation procedure (over U.K. area and for June, July and August 2004, at noon time), shown that the nine channels SEVIRI classifier provides performances very close to the ones provided by the twelve channels AMSR-E classifier. The advantage of using AMSR-E measurements is more evident when only sea area is considered.
The analysis also show which are the best sets of channels (among the nine SEVIRI channels and the twelve AMSR-E channels) that give the most important contribute to the above performances
Relationship between the hydrometeor size distribution and cloud microphysical properties using disdrometric data.
No full textOne of the most complete descriptions of rain is given by its drop size distribution (DSD). Several studies on the DSD, over different climatic regions and in different parts of the world, have been encouraged in the past, since the early work of Marshall and Palmer (1948), by the different applications of the DSD and variety of instruments related to the DSD applications. The problem of accurately describing the DSD temporal evolution and finding analytical functions which fit the observed DSD remains, however, a major obstacle in cloud physics. Only a few works focus the attention on mid-latitude continental rain. Most of them concern the study of tropical rains. Moreover there is an almost total lack of studies performed in Italy. This work aims at testing the significance of the existent DSD models and parameterizations in the different meteorological conditions in Ferrara (northern Italy), by using a Joss and Waldvogel disdrometer and, after a critical review of previous works and approaches, to propose modifications and improvements. In particular, two recent normalization approaches (one and two-parameter) to the DSD are tested. Case studies of the DSDs and rainfall integral parameters time variability are also analysed in order to characterize microphysically the Ferrara events, given the total lack of studies in this area. This aspect also allows a comparison of the results with the ones that have recently appeared in the literature. The identification between convective (C) and stratiform (S) precipitation involves a variety of subtleties. In the past considerable effort has gone into the development of methods for separating these two types of rainfall, especially ones based on radar images and wind-profilers data. A disdrometer approach involves the distinction between the properties of the DSDs. In this work, to face the specific characteristics of continental mid-latitude rain, a new C/S discrimination algorithm is developed
Experimental measurements on thermophoresis in the transition region
No full textMeasurements of thermophoretic velocities of carnuba 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 mum. 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
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