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Guido Visconti

Abstract

It has already been shown that a decrease of the oxygen level in the atmosphere of the earth would lower significantly the temperature of the exosphere, thus limiting the escape of hydrogen. The reduction of the escape flux would have important consequences on the evolution of oxygen in the last two billion years if O2 comes entirely from photodissociated water. The evaluation of the escape flux is possible only if the mixing ratios of atomic and molecular hydrogen are known at the homopause. In this paper a detailed photo-chemical model including transport by eddy and molecular diffusion is developed for an oxygen level of 0.1 PAL (preexisting atmospheric level). Jeans flux is assumed to be the dominant escape mechanism. The exospheric temperatures used in the model are 500 and 1000 K representing the expected range of variability at 0.1 PAL. It is shown that for stratospheric mixing ratios of total hydrogen comparable to those observed in our epoch, the escape flux is reduced to about 5 × 106 cm−2 s−1 for 5 ppm and 500 K and to 2.3 × 108 for 10 ppm and 1000 K. For comparison results are presented at 1 PAL for exospheric temperatures of 500 and 1000 K. As a by-product ozone densities resulting from such models are also presented taking into account both odd hydrogen chemistry and other possible sinks. Within the assumptions of low exospheric temperature and stratospheric humidity, it is argued that the role of water photodissociation as one of the major sources of atmospheric oxygen might have been overestimated.

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Giovanna Alimandi and Guido Visconti

Abstract

The dependence of radiative damping rates in the stratosphere is investigated as a function of the distribution of active radiative gases and the wavelength of the temperature perturbation.

Damping rates are found to depend considerably on the scale of the temperature perturbation. The same radiative model used to calculate damping rates is employed to study the effects of more accurate radiative calculations on the amplitude of long-wavelength Green modes in the stratosphere. A quasi-geostrophic β-plane model is used for this purpose and the results show a quite large reduction of the amplitudes of the modes up to wavenumber 3. These results are qualitatively discussed by taking into account a nonlinear damping mechanism and the dependence of the Newtonian cooling coefficient on the scale of the temperature perturbation.

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Giovanni Pitari and Guido Visconti

Abstract

A simple scheme is proposed to account for the Rayleigh scattering effects on photodissociation rates. The method is based on combining the reflectivity and transmission effects of two layers—one above and one below the height considered. Reflectivities are computed using existing data and tables on Rayleigh scattering. Computations using this method are reported and compared with other results obtained with a very detailed radiative scheme. The accuracy of the results and the short computing time make the method suitable for chemical-dynamical models of the atmosphere.

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Guido Visconti, Marco Verdecchia, and Giovanni Pitari

Abstract

A two-dimensional model has been integrated for two years to study the evolution of the El Chichón aerosol cloud in the stratosphere, starting about three months after the eruption. Initial conditions for the backscattering ratios are taken from airborne lidar measurements, while observations taken at Mauna Loa are used to estimate the initial size distribution for the aerosols. Aerosols have been treated as a passive tracer, because the small changes in the stratospheric dynamics due to the aerosol interaction with solar and longwave radiation can only induce marginal effects on large scale transport. We have also completely neglected microphysical processes like coagulation as well as photochemical effects on chemically reacting species. Results are shown for aerosol extinction mixing ratios, optical thickness, mass column density and size distribution. Extensive comparison with available lidar measurements are also presented and discussed. Major discrepancies are noted between the measured optical thickness and the one predicted by the model with the former being systematically higher. Neglect of coagulation and nucleation prevents formation of large particles at high altitude which are depleted in the simulation by sedimentation. The interhemispheric asymmetry is overestimated by the model with much more aerosol being transported in the Northern Hemisphere than in the Southern Hemisphere. Other differences are found in the sudden changes in the aerosol distribution. It is argued that two dimensional models are not suitable to simulate sporadic events and that microphysics should be taken into account even several months after the eruption.

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Claudia Faccani, Rossella Ferretti, and Guido Visconti

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 the data assimilation, and the surface data; then a few weather forecast experiments, using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), are performed to assess the impact of the data assimilation on the forecast. The results show a measurable improvement in the high-resolution precipitation forecast. It is also shown that this technique can be used for high-resolution real-time forecasts.

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Barbara Grassi, Gianluca Redaelli, and Guido Visconti

Abstract

The response of the Southern Hemisphere (SH) polar atmosphere to the tropical sea surface temperature (SST) during the 2002 winter–spring season is investigated by using a general circulation model (GCM). The SH stratospheric winter of 2002 was particularly unusual, characterized by a weaker-than-normal polar vortex during the whole season. It also registered, at the end of September, the first major warming yet observed in the SH. This event is unexpected in the SH, and it is supposed to be induced by a “preconditioning” of the polar vortex starting at the beginning of the winter. Atmospheric GCM experiments with prescribed SST boundary conditions are performed. The sensitivity of the Antarctic dynamics to the tropical SST of 2002 (a year characterized by an El Niño event of moderate intensity) is studied, and the uniqueness of the 2002 tropical oceanic condition is investigated through the comparison of the simulated response of the climatic system to 2002 and 1997 tropical SST (1997 being a year with a strong El Niño event). Model results highlight a primary role played by the tropical SST of 2002 in the development of the peculiar characteristics of the Antarctic dynamics during the winter months that appears to be a necessary condition for the generation of the anomalous destabilization of the polar vortex during the following spring. Results for June 2002 show a strong generation of vertically propagating waves resulting from the tropical SST that, through the perturbation of the westerly jet at middle latitudes, produces a preconditioning of the polar vortex by affecting the wave refraction index. The particular structure of the tropical SST anomalies during the winter of 2002 is thought to have influenced the subsequent preconditioning of the stratospheric vortex.

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Barbara Grassi, Gianluca Redaelli, and Guido Visconti

Abstract

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 phase of the Arctic Oscillation (AO) and characterized by a wave activity flux from the North Atlantic toward 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. In particular, simulations suggest an increased risk of winter flooding in southern Italy, Greece, and the Iberian Peninsula.

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Rossella Ferretti, Tiziana Paolucci, W. Zheng, Guido Visconti, and P. Bonelli

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; the differences between the simulations performed using EXP and NEXP are reduced if the precipitation is driven by a strong large-scale forcing such as a front. The comparison of the cumulus convection parameterizations highlights the different responses of the schemes to the meteorological situation. When the explicit computation of cloud water and rain is used, a good localization of the rain cells and a fair estimation of the amount of precipitation are obtained using either the Kain–Fritsch or the Grell cumulus convection parameterizations. On the other hand, the Anthes–Kuo scheme produces a strong overestimation of the precipitation regardless of the meteorological forcing and with both EXP and NEXP. The bias and the threat score for the cases analyzed confirm this finding.

Sensitivities to different initial conditions for the same case show that the precipitation forecast depends on the strength of the signal contained in the initial conditions.

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Barbara Grassi, Gianluca Redaelli, Pablo Osvaldo Canziani, and Guido Visconti

Abstract

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.

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