Regimes in moist stratified flows over isolated topography: numerical experiments

Moist flows over simply shaped 3D mountains have been studied in numerical simulations made with a mesoscale meteorological model. Our aim is to examine the possible existence of multiple solutions, searching for different solutions depending on the path followed by the system in the parameter space. Results from three different sets of experiments are discussed here. In the first set of simulations, the height of the mountain has been progressively changed in time. In the second group of experiments, the humidity of the air flowing over the obstacle has been increased in time by adding a source term to the equation of evolution of moisture. The case of advection of moist air towards an obstacle, initially embedded in dry air, has been studied as a third type of flow. A dependence on the past history of the flow seems to characterise some types of system evolution, leading to different flow regimes over the obstacle. The experiments indicate that this result is mainly a consequence of changes of state of water, associated with the presence of humidity inthe atmosphere. These effects are emphasised in the case of an elliptical mountain, with its longer axis perpendicular to the main flow. In the three different sets of experiments presented here, evident differences with simulations where flow parameters are kept constant from the beginning persist in the flow regimes, also for periods of time much longer than the characteristic time of evolution towards stationary solutions

Modelling local winds over the Salento peninsula

A three-day mesoscale numerical simulation has been performed over the narrow Salento peninsula (south-eastern Italy) during summer conditions characterised by weak synoptic forcing. These atmospheric conditions favour the development of complex sea-breeze systems and convergence zones on the peninsula. The aim of this work is to investigate the ability of an atmospheric mesoscale model to reproduce the surface fields of meteorological variables in the presence of local-scale forcing and breeze circulations, which are fundamental in applications such as air pollution modelling and nowcasting. The modelled fields have been compared with available surface measurements and sodar data. Results indicate that the model can simulate the general mean wind field in a realistic way. The diurnal evolution of the wind is well reproduced and the maximum deviations mostly occur during the night, being associated with calm conditions. Statistical analysis indicates that the typical mean bias is found to be about 1 m s−1 for hourly averaged wind speed, less than 20° for wind direction and about 1°C for temperature. The root mean square error (rmse) varies from 1 to 3 m s−1 for wind speed, from 50° to 70° for wind direction, and is about 2.4°C for temperature. All the values of the numerical indexes are within ranges which are characteristic of those found for other state-of-the-art models applied to similar cases studies. Despite a good overall agreement between predictions and observations, some discrepancies were found in the individual profiles due both to the limited spatial representation of the local details and to the complex wind field which makes the space–time matching between the model and the observations quite critical. The structures of the thermal mixed layer and the breeze convergence zone are similar to numerical studies relative to more idealised conditions. Copyright © 2004 Royal Meteorological Societ

Numerical simulations of a tornadic supercell over the Mediterranean

On 28 November 2012, a multivortex EF3 tornado occurred in southeastern Italy causing one fatality and estimated damage of 60 million euros. At approximately 1050 LT (0950 UTC), this tornado, which initially formed in association with a supercell thunderstorm over the Ionian Sea, moved inland. The environment where the tornadic supercell developed was characterized by large vertical wind shear in the lowest 1 km of the atmosphere and moderate conditional instability. Mesoscale-model numerical simulations show that it is possible to produce a simulated supercell thunderstorm with a track, change in intensity, and evolution similar to the actual one that spawned the tornado in Taranto, southern Italy. The genesis of the simulated supercell is due to a combination of mesoscale meteorological features: warm low-level air advected toward the Ionian Sea, combined with midlevel cooling due to an approaching trough, increased the potential instability; the intense vertical shear favored the possibility of supercell development; and boundary layer rolls over the Ionian Sea moved in phase with the cells produced by the orography of Calabria to supply ascent, moisture, and heat to the convection. An unusual feature of the present case is the central role of the orography, which was verified in a sensitivity experiment where it was reduced by 80%.Peer ReviewedPostprint (published version

The role of subsidence in a weakly unstable marine boundary layer: a case study

The diurnal evolution of a cloud free, marine boundary layer is studied by means of experimental measurements and numerical simulations. Experimental data belong to an investigation of the mixing height over inner Danish waters. The mixed-layer height measured over the sea is generally nearly constant, and does not exhibit the diurnal cycle characteristic of boundary layers over land. A case study, during summer, showing an anomalous development of the mixed layer under unstable and nearly neutral atmospheric conditions, is selected in the campaign. Subsidence is identified as the main physical mechanism causing the sudden decrease in the mixing layer height. This is quantified by comparing radiosounding profiles with data from numerical simulations of a mesoscale model, and a large-eddy simulation model. Subsidence not only affects the mixing layer height, but also the turbulent fluctuations within it. By analyzing wind and scalar spectra, the role of subsidence is further investigated and a more complete interpretation of the experimental results emerges

Effect of a positive sea surface temperature anomaly on a Mediterranean tornadic supercell

Extreme events represent a topic of paramount importance and a challenge for modelling investigations. Due to the need of high-resolution models, the study of severe localized convective phenomena is even more critical, especially in relation to changes in forcing factors, such as sea surface temperatures (SSTs), in future climate scenarios. Here, we analyze the effect of changes in SSTs on the intensity of a tornadic supercell in the Mediterranean through modelling investigations. We show dramatic (nonlinear) changes for updraft helicity and vertical velocity, which measure the intensity of the supercell, even for variations of SST only of¿+¿/-1¿K.Peer ReviewedPostprint (published version

Influence of meteorological phenomena on worldwide aircraft accidents in the period 1967-2010

This is the pre-peer reviewed version of the following article: Mazon, J., Rojas, J. I., Lozano, M., Pino, D., Prats, X. and Miglietta, M. M. (2017), Influence of meteorological phenomena on worldwide aircraft accidents, 1967–2010. Met. Apps. doi:10.1002/met.1686, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/met.1686/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Based on the information available in databases from relevant national and international organizations from 1967 to 2010, an Aviation Weather Accidents Database (AWAD) was built. According to the AWAD, the weather is the primary cause in a growing percentage of annual aircraft accidents: from about 40% in 1967 to almost 50% in 2010. While the absolute number of fatalities and injured people due to aircraft accidents has decreased significantly, the percentage of fatalities and injured people in accidents attributed to the weather shows a slight increase in the studied period. The influence of turbulence, clear air turbulence, wind shear, low visibility, rain, icing, snow and storms on aircraft accidents was analysed, considering the different phases of flight, the meteorological seasons of the year and the spatial distribution over four zones of the Earth. These zones were defined following meteorological and climatological criteria, instead of using the typical political criteria. A major part of the accidents and accidents attributed to the weather occur in latitudes between 12º and 38º in both hemispheres. It is concluded that actions aimed at reducing the risk associated with low visibility, rain and turbulence, in this order, should have priority to achieve the most significant improvements in air transport safety.Postprint (author's final draft

An updated “climatology” of tornadoes and waterspouts in Italy

Ten years of tornadoes (TR) and waterspouts (WS) in Italy are analysed in terms of geographical, seasonal, monthly, diurnal, and rating distribution. Starting from the European severe weather database, a comprehensive data set is developed for the period 2007–2016, which includes 707 WS and 371 TR. The category of WS includes many weak events but also some intense vortices, able to produce significant damages as they make landfall. WS develop mainly near the Italian coasts exposed to westerly flows (Tyrrhenian and Apulia region Ionian coast); 25% of them makes landfall and becomes TR. The majority of WS develops in autumn (43%), followed by summer (33%). The average density is 0.9 events per 100 km of coastline per year, although there is a strong subregional variation, with peaks of around 5 in some spots along the Tyrrhenian coast. TR originate from WS in about half of cases; the average density of TR is 1.23 events per 104 km2 per year, which is comparable with other Mediterranean regions. The occurrence of TR is more frequent in summer, followed by autumn; however, limiting the analysis to TR originated inland, the number of events is maximum in summer and late spring. The latter result suggests a distinction of “continental” cases, mainly affecting northern Italy in late spring and summer, and “maritime” cases, which affect mainly the peninsular regions in late summer and autumn. The highest density of TR was reported along the coasts of Lazio and Tuscany, in the Venetian plain, in the southern part of Apulia: in these regions, the density of events is comparable with that of the U.S. states with the highest TR rates. In contrast, the probability of significant TR in any Italian region is much smaller than that of the U.S. states with the highest risk. © 2018 Royal Meteorological Societ

Mechanisms for convection development in a long-lasting heavy precipitation event over southeastern Italy

Observational data and outputs from the Weather Research and Forecasting (WRF) model are used to investigate a heavy precipitation event that occurred on 12 and 13 November 2004 in Apulia and Basilicata, southeastern Italy. The event lasted for more than 24. h and featured two large rainfall peaks, with values up to 250. mm in one day, recorded in two different phases over two distinct areas. The analysis indicates that a low-level jet stream (LLJS) induced by the large-scale pattern maintained a convectively unstable environment and advected moist air masses at very low levels throughout the event. During the first phase, the orography provided the lifting mechanism to develop convection. During the second phase, the convergence developed in the low levels was sufficient to maintain a quasi-stationary linear mesoscale convective system over the nearly flat terrain of Salento peninsula. High-resolution numerical outputs highlight that the shift of precipitation from the first to the second maximum was caused by the passage of a weak mesoscale cyclone and of an upper level short-wave trough. The delay in the modeled evolution of these subsynoptic features is considered responsible of the incorrect timing of the simulated precipitation.The study provides further evidence of the close relationship between the occurrence of a moist and unstable southerly LLJS and heavy precipitation over the Italian Ionian regions

Numerical simulations of a tornadic supercell over the Mediterranean

On 28 November 2012, a multivortex EF3 tornado occurred in southeastern Italy causing one fatality and estimated damage of 60 million euros. At approximately 1050 LT (0950 UTC), this tornado, which initially formed in association with a supercell thunderstorm over the Ionian Sea, moved inland. The environment where the tornadic supercell developed was characterized by large vertical wind shear in the lowest 1 km of the atmosphere and moderate conditional instability. Mesoscale-model numerical simulations show that it is possible to produce a simulated supercell thunderstorm with a track, change in intensity, and evolution similar to the actual one that spawned the tornado in Taranto, southern Italy. The genesis of the simulated supercell is due to a combination of mesoscale meteorological features: warm low-level air advected toward the Ionian Sea, combined with midlevel cooling due to an approaching trough, increased the potential instability; the intense vertical shear favored the possibility of supercell development; and boundary layer rolls over the Ionian Sea moved in phase with the cells produced by the orography of Calabria to supply ascent, moisture, and heat to the convection. An unusual feature of the present case is the central role of the orography, which was verified in a sensitivity experiment where it was reduced by 80%.Peer Reviewe