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  • Author or Editor: V. Levizzani x
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E. Cattani, A. Merino, and V. Levizzani

Abstract

East Africa experienced in the 2001–11 time period some of the worst drought events to date, culminating in the high-impact drought of 2010/11. Long-term monitoring of precipitation is thus essential, and satellite-based precipitation products can help in coping with the relatively sparse rain gauge ground networks of this area of the world. However, the complex topography and the marked geographic variability of precipitation in the region make precipitation retrieval from satellites problematic and product validation and intercomparison necessary. Six state-of-the-art monthly satellite precipitation products over East Africa during the 2001–09 time frame are evaluated. Eight areas (clusters) are identified by investigating the precipitation seasonality through the Global Precipitation Climatology Centre (GPCC) climatological gauge data. Seasonality was fully reproduced by satellite data in each of the GPCC-identified clusters. Not surprisingly, complex terrain (mountain regions in particular) represents a challenge for satellite precipitation estimates, as demonstrated by the standard deviations of the six-product ensemble. A further confirmation comes from the comparison between satellite estimates and rain gauge measurements as a function of terrain elevation. The 3B42 product performs best, although the satellite–gauge comparative analysis was not completely independent since a few of the products include a rain gauge bias correction.

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R. M. Rasmussen, V. Levizzani, and H. R. Pruppacher

Abstract

The internal and external heat transfer of a melting spherical ice particles less than 500 μm radius has been investigated theoretically. The effect of an internal circulation and eccentric location of the ice core was modeled. These two effects combined to reduce total melting times by ∼10%. However, this still left a 10–15% difference between theoretical and experimental melting times which could not be explained by experimental error. The external heat transfer was subsequently investigated, and it is postulated that: 1) surface irregularities and nonsphericity, 2) rear eddy shedding, and 3) nonsteady motions, are able to increase the external ventilation coefficient by a factor of two, and thus account for the observed discrepancy in melting times.

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R. M. Rasmussen, V. Levizzani, and H. R. Pruppacher

Abstract

An experimental and theoretical study has been performed on the melting of spherical ice particles between 3 and 20 mm in diameter. For the experimental study the UCLA Cloud Tunnel was employed in determining the melting rate, the mode of melting, the shedding rate, and the hydrodynamic behavior of the melting ice particles. Our experimental results demonstrate that the melting mode of ice particles can be grouped into distinct categories depending on the Reynolds number. For these categories, comparison was made to various theoretical expressions reported in literature and to our own formulations. These comparisons show that experiment and the appropriate theory agree within experimental error.

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S. Davolio, S. Della Fera, S. Laviola, M. M. Miglietta, and V. Levizzani

Abstract

The Mediterranean storm “Vaia” developed within a typical autumn synoptic circulation, generally associated with heavy rain conditions over the western Mediterranean Sea basin. Intense precipitation was responsible for floods over Italy between 27 and 30 October 2018, and the storm was accompanied by explosive cyclogenesis, storm surge, and extremely intense wind gusts that caused casualties and extensive damage, especially to the Alpine forests. This study investigates the contribution of different moisture sources to the extreme precipitation by means of numerical model simulations using the Bologna Limited Area Model (BOLAM). In particular, the attention is focused on the significant amount of water vapor transported into the Mediterranean basin from the Atlantic Ocean tropical area and organized along a narrow corridor across the African continent. First, a newly developed detection algorithm is applied to identify this transport as an atmospheric river (AR). Then, the implementation of an atmospheric water budget diagnostic, supported by sensitivity experiments, allows us to assess the role of the AR in terms of water supply to the precipitation systems. Although the transport of moisture from remote regions is known to be an important ingredient for the onset of heavy precipitation in the Mediterranean, the role of ARs, already identified in correspondence with some of these events, has not been deeply investigated and quantified yet in this specific area. The results demonstrate that the AR was critical for determining the magnitude of this heavy precipitation episode over Italy, whereas evaporation from the sea played a secondary role, especially for precipitation over the Alps.

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R. Rasmussen, C. Walcek, H.R. Pruppacher, S.K. Mitra, J. Lew, V. Levizzani, P.K. Wang, and U. Barth

Abstract

Results are presented of a recent wind tunnel experiment in which electrically unchanged water drops of 1000–3000 μm equivalent radius were freely suspended in the vertical air stream of the UCLA Cloud Tunnel. During their suspension, the drops were exposed to external, vertical electric fields of 0–90 volts cm-1. The change in the drop shape with drop size and with electric field strength was noted and is discussed in the light of theoretical work cited in literature which does not take into account the feedback effects between the electric forces of an external electric field and the hydrodynamic forms due to the flow past the drop. In contrast, the present wind tunnel study, documented by photographs from a 16 mm motion picture film, recorded the shape of the water drop in response to both hydrodynamic as well as electric forces.

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