Guido Visconti

Installation of a high-sensitivity laser strainmeter in a tunnel in central Italy

A new laser interferometer for geophysical purposes is being operated in an underground laboratory in central Italy. A simple electro-optical technique has been used to limit the costs. The main characteristics of this instrument are: high sensitivity (Δl/l≈10−12), fast sampling rate (up to 6 kHz), large dynamic range (unbounded in principle), the capability of following strain rates as high as 7×10−5 s−1, and good reliability (two failures in nearly two years).

Analyses of the Precipitation Pattern on the Alpine Region Using Different Cumulus Convection Parameterizations

Abstract The analysis of several precipitation events occurring during June 1990 in the Alpine region is performed using the Pennsylvania State University–National Center for Atmospheric Research Fifth-Generation Mesoscale Model, version 1. A high-resolution dataset provided by Monitoring Precipitation Activity in the Padana Region observational campaign (June 1990) is used to verify the model forecast. Comparisons between model simulations, using different cumulus convective schemes associated with either an explicit computation of cloud water and rain (EXP) or a nonconvective scheme (NEXP), have been performed. The comparisons of EXP versus NEXP give indications of the ability of a cumulus scheme to handle nonconvective precipitation. On the other hand, comparing the schemes allows for evaluation of the ability to reproduce total and convective precipitation. The results show that the amount and the areal extent of the total precipitation are well reproduced if a cumulus scheme is associated with EXP; t...

Effects of the PDO Phase on the Tropical Belt Width

Recent studies have shown that the tropical belt (TB) has progressively expanded since at least the late 1970s. This trend has been largely attributed to the radiative forcing due to greenhouse gas (GHG) increase and stratospheric ozone depletion, even if an influence of sea surface temperature (SST) anomalies has been also suggested. The impact of the Pacific decadal oscillation (PDO) on the TB width is investigated in this work. The study is performed by using both Atmospheric Model Intercomparison Project (AMIP) and idealized simulations, produced by the NCAR Community Atmosphere Model, version 3 (CAM3) GCM and reanalysis data [40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), ERA-Interim, and Modern-Era Retrospective Analysis for Research and Applications (MERRA)]. Reanalyses show that a switch of the PDO from a positive to a negative phase can lead to a significant TB expansion during the equinoxes. This effect, indicating a possible PDO contribution to the widening that characterized the TB width during the last decades, is not correctly reproduced by model simulations. Deficiencies in the sensitivity of model-simulated convective processes to SST anomalies are suggested as a possible cause of the TB widening underestimation.

Equilibrium temperatures of small particles in the Earth's upper atmosphere (50–110 km)

Abstract Aerosols absorb radiation from the solar and planetary radiation fields, exchange sensible heat by collisions with the ambient gas, and emit thermal radiation. If changes in phase are possible, they also lose or acquire latent heat. As a result of equilibrium between source and loss terms, particles acquire a temperature Tp that can be substantially different from the ambient gas temperature T g∗ Differences Tp − Tg, in excess of 100 K are obtained above 60 km in daytime, depending on size, refractive index, and planetary albedo; negative differences can be obtained at night. The relative contribution of the energy exchange terms also varies; in particular, the fraction of absorbed radiation transferred into the ambient gas by collisions changes significantly with particle size and altitude. The attainment of high Tps may have important consequences in several upper atmospheric phenomena, such as the sublimation of alkali metals ( Fiocco and Visconti , 1973), the formation of noctilucent clouds, and other surface-related effects. The dependence of the heating rate, characterizing the exchange of energy between particles and gas, on various factors such as the planetary albedo may have a bearing on upper atmospheric behavior when sufficient particle concentrations exist.

Raman lidar observations of cloud liquid water

We report the design and the performances of a Raman lidar for long-term monitoring of tropospheric aerosol backscattering and extinction coefficients, water vapor mixing ratio, and cloud liquid water. We focus on the systems capabilities of detecting Raman backscattering from cloud liquid water. After describing the system components, along with the current limitations and options for improvement, we report examples of observations in the case of low-level cumulus clouds. The measurements of the cloud liquid water content, as well as the estimations of the cloud droplet effective radii and number densities, obtained by combining the extinction coefficient and cloud water content within the clouds, are critically discussed.

Sensitivity of stratospheric ozone to heterogeneous chemistry on sulfate aerosols

The effects of sulfate aerosols on stratospheric ozone have been studied with a 2D model. The model includes a comprehensive chemical code for homogeneous and heterogeneous reactions; both background and volcanic aerosols have been taken into account according to different scenarios. The basic temperature field is prescribed, while the perturbation introduced by the presence of volcanic aerosols is predicted. Increase of total chlorine in the stratosphere causes a well known ozone depletion by itself, but. the effects could be highly enhanced in presence of a large amount of volcanic aerosols that affect the balance of nitrogen and chlorine reservoirs through heterogeneous chemical reactions. For a volcanic eruption similar to El Chichon we show that the maximum O3 depletion (7% for a 2.5ppbv amount of stratospheric chlorine and 12% by doubling Cl) is to be expected at high latitudes in spring, because of a large chemical destruction acting in situ.

Arctic Sea Ice Reduction and Extreme Climate Events over the Mediterranean Region

During the last decade, Arctic sea ice cover has experienced an accelerated decline that has been suggested to drive the increased occurrence of extremely cold winter events over continental Europe. Observations and modeling studies seem to support the idea that Mediterranean climate is also changing. In this work, the authors estimate potential effects on the Mediterranean Basin, during the winter period, of Arctic sea ice reduction. Two sets of simulations have been performed by prescribing different values of sea ice concentrations (50% and 20%) on the Barents‐Kara Seas in the NCAR Community Atmosphere Model, version 3 (CAM3), as representative of idealized present and future sea ice conditions. Global model simulations have then been used to run the Abdus Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model, version 4 (RegCM4), over central Europe and the Mediterranean domain. Simulations provide evidence for a large-scale atmospheric circulation response to sea ice reduction, resembling the negative phaseof theArctic Oscillation (AO)andcharacterizedby awaveactivityfluxfromtheNorth Atlantictoward the Mediterranean Basin, during winter months. An increase in the occurrence and intensity of extreme cold events, over continental Europe, and extreme precipitation events, over the entire Mediterranean Basin, was found.Inparticular, simulationssuggestan increased riskofwinterfloodingin southernItaly,Greece,andthe Iberian Peninsula.

Continuous lidar measurements of stratospheric aerosols and ozone after the Pinatubo eruption Part II: Time evolution of ozone profiles and of aerosol properties

Two lidar systems, an aerosol lidar and an O3 Differential Absorption Lidar (DIAL), have been routinely operated at the same site (L‧Aquila, Italy; 42°N, 13°E) since August 1991. The multiwavelength analysis of the lidar signals allows to retrieve parameters related to equivalent aerosol size distributions and their optical properties. These are needed to correct the ozone DIAL profiles from the disturbance introduced by the stratospheric volcanic aerosols. The method and the confidence of the retrieved ozone profiles are discussed in a companion paper. Here we present the whole measurement series of ozone and backscattering ratio profiles during the period from August 1991 to December 1992. In addition, for some observations, the mode radius and the dispersion of the representative aerosol size distribution are reported. The time evolutions of aerosol surface area density and mass mixing ratio are also discussed within the uncertainties of the retrieval algorithm.

Continuous lidar measurements of stratospheric aerosols and ozone after the Pinatubo eruption. Part I: Dial ozone retrieval in presence of stratospheric aerosol layers

We report stratospheric DIAL (DIfferential Absorption Lidar) ozone observations in presence of large amount of volcanic aerosols, which strongly affect the inversion of the backscattering signals. The retrieval method is based on the use of three wavelengths, two from the O3 DIAL (308 and 351nm) and a third one from an aerosol lidar (589nm) operated at the same site (L‧Aquila, Italy; 42°N,13°E). For each measurement session the relationships between the backscattering ratios at different wavelengths are fitted with an appropriate size distribution of the aerosols. The aerosol optical properties determined with this procedure allow to correct the ozone profiles. The ozone data are compared whenever possible with balloon ozonesonde measurements taken at S.Pietro Capofiume (Italy, 45°N, 11°E) or Hohenpeissenberg (Germany, 48°N, 11°E) and they generally show a good agreement. The ozone profiles can be retrieved with an indetermination that, within the aerosol layer, ranges between ±15 to ±35%. A systematic intercomparison is reported with data of other stations to assess the reliability of the method.

High-Resolution Weather Forecasting over Complex Orography: Sensitivity to the Assimilation of Conventional Data

Abstract Weather forecasting for regions with complex orography, as the Alps, presents several challenges and the task becomes even more difficult when high resolution is required. Moreover, for the Alpine region, some of the problems are due to the lack of observations especially over the Mediterranean Sea. A possibility for improving forecasts is to reuse assimilation techniques locally. In this paper, results obtained through data assimilation are presented: objective analysis (OA) of observations and data analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) are used together to generate a new set of mesoscale initial (ICs) and boundary conditions (BCs). In particular, OA is applied to surface data and radiosoundings using two methods: Cressman and multiquadric. The sensitivity of the weather forecast to the number of upper-air stations assimilated by OA is tested using data from the Piedmont flood (4–6 November 1994). At first, a comparison is made between ICs, obtained through...