scholarly journals A numerical study to investigate the roles of former Hurricane Leslie, orography and evaporative cooling in the 2018 Aude heavy-precipitation event

2021 ◽  
Vol 2 (3) ◽  
pp. 795-818
Author(s):  
Marc Mandement ◽  
Olivier Caumont
Keyword(s):  
Mediterranean Sea ◽  
Evaporative Cooling ◽  
Heavy Precipitation ◽  
Balearic Islands ◽  
Precipitation Event ◽  
Near Surface ◽  
Low Level ◽  
Stationary Front ◽  
Mesoscale Convective ◽  
The Mediterranean

Abstract. In south-eastern France, the Mediterranean coast is regularly affected by heavy-precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. At synoptic scale, the former Hurricane Leslie was involved in the formation of a Mediterranean surface low that channelled conditionally unstable air towards the coast. At mesoscale, convective cells focused west of a decaying cold front that became quasi-stationary and downwind of the terrain. To investigate the roles of the moisture provided by Leslie, orography and evaporative cooling among the physical processes that led to the location and intensity of the observed rainfall, numerical simulations are run at 1 km and 500 m horizontal grid spacing and evaluated with independent near-surface analyses including novel crowd-sourced observations of personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updraughts mainly originated from areas east of the Balearic Islands, over the Mediterranean Sea, whereas in a second part, an increasing number originated from Leslie's remnants. Air masses from areas east of the Balearic Islands appeared as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moistening mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department did not play any substantial role in the stationarity of the quasi-stationary front. Regarding lifting mechanisms, the advection of conditionally unstable air by a low-level jet towards the quasi-stationary front, confined to altitudes below 2 km, reactivated convection along and downwind of the front. Most of the air parcels found inside strong updraughts near the location of the maximum rainfall were lifted above the quasi-stationary front. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting were maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation was primarily driven by the location of the quasi-stationary front and secondarily by the location of convective bands downwind of orography.

scholarly journals A numerical study to investigate the roles of former hurricane Leslie, orography, and evaporative cooling in the 2018 Aude heavy precipitation event

2020 ◽  
Author(s):  
Marc Mandement ◽  
Olivier Caumont
Keyword(s):  
Mediterranean Sea ◽  
Cold Front ◽  
Evaporative Cooling ◽  
Heavy Precipitation ◽  
Precipitation Event ◽  
Maximum Rainfall ◽  
Near Surface ◽  
Low Level ◽  
Mesoscale Convective ◽  
The Mediterranean

Abstract. In southeastern France, the Mediterranean coast is regularly affected by heavy precipitation events. On 14–15 October 2018, in the Aude department, a back-building quasi-stationary mesoscale convective system produced up to about 300 mm of rain in 11 h. The synoptic situation was perturbed by the former hurricane Leslie, involved in the formation of a Mediterranean surface low that focused the convective activity. At mesoscale, convective cells focused west of a quasi-stationary cold front and downwind of the terrain. To investigate the roles of Leslie, orography and evaporative cooling in the processes that led to the observed rainfall, numerical simulations are run and evaluated with near-surface analyses comprising standard and personal weather stations. Simulations show that, in a first part of the event, low-level conditionally unstable air parcels found inside strong updrafts mainly originate from the Mediterranean Sea, east of 4.5° E, whereas in a second part, an increasing number originates from Leslie's remnants. Air masses from east of 4.5° E appear as the first supplier of moisture over the entire event. Still, Leslie contributed to substantially moisten mid-levels over the Aude department, diminishing evaporation processes. Thus, the evaporative cooling over the Aude department does not play any substantial role in the stationarity of the cold front. Regarding lifting mechanisms, most of the air parcels found inside strong updrafts near the location of the maximum rainfall are lifted above the cold front, attesting its key role in focusing convection. Downwind of the Albera Massif, mountains bordering the Mediterranean Sea, cells formed by orographic lifting seem to be maintained by low-level leeward convergence, mountain lee waves and a favourable directional wind shear; when terrain is flattened, rainfall is substantially reduced. The location of the exceptional precipitation appears to be driven primarily by the location of the quasi-stationary cold front and secondarily by the location of convective bands downwind of the orography.

Strong Cross-Barrier Flow under Stable Conditions Producing Intense Winter Orographic Precipitation: A Case Study over the Subtropical Central Andes

2009 ◽  
Vol 24 (4) ◽  
pp. 1009-1031 ◽  
Author(s):  
Maximiliano Viale ◽  
Federico A. Norte
Keyword(s):  
Heavy Precipitation ◽  
Stable Stratification ◽  
Central Andes ◽  
Precipitation Event ◽  
Windward Side ◽  
Orographic Precipitation ◽  
Near Surface ◽  
Low Level ◽  
Precipitation Enhancement ◽  
Subtropical Central Andes

Abstract The most intense orographic precipitation event over the subtropical central Andes (36°–30°S) during winter 2005 was examined using observational data and a regional model simulation. The Eta-Programa Regional de Meteorología (PRM) model forecast was evaluated and used to explore the airflow structure that generated this heavy precipitation event, with a focus on orographic influences. Even though the model did not realistically reproduce any near-surface variables, nor the precipitation shadow in the leeside lowlands, its reliable forecast of heavy precipitation over the windward side and the wind fields suggests that it can be used as a valuable forecasting tool for such events in the region. The synoptic flow of the 26–29 August 2005 storm responded to a well-defined dipole from low to upper levels with anomalous low (high) geopotential heights at midlatitudes (subtropical) latitudes located off the southeast Pacific coast, resulting in a large meridional geopotential height gradient that drove a strong anomalous cross-barrier flow. Precipitation enhancement in the Andes was observed during the entire event; however, the highest rates were in the prefrontal sector under the low-level stable stratification and cross-barrier winds exceeding 2.5 standard deviations (σ) from the climatological monthly mean. The combination of strong cross-mountain winds with the stable stratification in the air mass of a frontal system, impinging on the high Andes range, appears to be the major factor in determining the flow structure that produced the pattern of precipitation enhancement, with uplift maximized near mountaintops and low-level blocking upwindleading to the formation of a low-level along-barrier jet. Additionally, only the upstream wind anomalies for the 15 heaviest events over a 10-yr (1967–76) period were investigated. They exhibited strong anomalous northwesterly winds for 14 of the 15 events, whereas for the remaining event there were no available observations to evaluate. Thus, these anomalies may also be exploited for forecasting capabilities.

scholarly journals Origin of the moisture feeding the Heavy Precipitating Systems over Southeastern France

2011 ◽  
Vol 11 (4) ◽  
pp. 1163-1178 ◽  
Author(s):  
F. Duffourg ◽  
V. Ducrocq
Keyword(s):  
Mediterranean Sea ◽  
Mediterranean Region ◽  
Heavy Precipitation ◽  
Horizontal Resolution ◽  
Northwestern Part ◽  
Low Level ◽  
Relative Contribution ◽  
The Mediterranean ◽  
Passive Tracers ◽  
Northwestern Mediterranean

Abstract. In the Northwestern Mediterranean region, large amounts of precipitation can accumulate over the coasts in less than a day. The present study aims at characterising the origin and the pathways of the moisture feeding such heavy precipitation. The ten Heavy Precipitating Events (HPEs) that occurred over the French Mediterranean region during the autumns of 2008 and 2009 are simulated with the non-hydrostatic research numerical model Meso-NH at 2.5 km, 10 km and 40 km horizontal resolution. Using eulerian on-line passive tracers, high-resolution simulations (2.5 km horizontal resolution) show that the heavy precipitating systems are fed by a south-southwesterly to easterly low-level moist flow. It is typically 1000 m deep and remains almost unchanged all along an event. This low-level feeding flow crosses the most northwestern part of the Mediterranean in 5 to 10 h. Larger-scale simulations (40 km and 10 km horizontal resolution) show that the moisture of the low-level feeding flow is provided by both evaporation of the Mediterranean Sea within the last 2 days before the HPE triggering and transport from remote sources in the lower half of the troposphere over more than 3 to 4 days. Local Mediterranean moisture is gained along the air mass low-level progress towards the Northwestern Mediterranean basin following two main branches along the Spanish coast and west of Sardinia. The Mediterranean Sea is the main moisture source when anticyclonic conditions prevail during the last 3 or 4 days before the HPE. When cyclonic conditions prevail before the HPE, the relative contribution of local and remote sources is more balanced. Remote moisture comes most of the time from the Atlantic Ocean. African tropical moisture is a less frequent but larger remote source.

scholarly journals Evaluation of Two Cloud-Resolving Models Using Bin or Bulk Microphysics Representation for the HyMeX-IOP7a Heavy Precipitation Event

Atmosphere ◽  
2020 ◽  
Vol 11 (11) ◽  
pp. 1177
Author(s):  
Diana Arteaga ◽  
Céline Planche ◽  
Christina Kagkara ◽  
Wolfram Wobrock ◽  
Sandra Banson ◽  
...  
Keyword(s):  
Hydrological Cycle ◽  
Wrf Model ◽  
Heavy Precipitation ◽  
Sensitivity Study ◽  
Precipitation Event ◽  
Convective System ◽  
Wind Fields ◽  
Massif Central ◽  
Surface Precipitation ◽  
Low Level

The Mediterranean region is frequently affected in autumn by heavy precipitation that causes flash-floods or landslides leading to important material damage and casualties. Within the framework of the international HyMeX program (HYdrological cycle in Mediterranean EXperiment), this study aims to evaluate the capabilities of two models, WRF (Weather Research and Forecasting) and DESCAM (DEtailed SCAvenging Model), which use two different representations of the microphysics to reproduce the observed atmospheric properties (thermodynamics, wind fields, radar reflectivities and precipitation features) of the HyMeX-IOP7a intense precipitating event (26 September 2012). The DESCAM model, which uses a bin resolved representation of the microphysics, shows results comparable to the observations for the precipitation field at the surface. On the contrary, the simulations made with the WRF model using a bulk representation of the microphysics (either the Thompson scheme or the Morrison scheme), commonly employed in NWP models, reproduce neither the intensity nor the distribution of the observed precipitation—the rain amount is overestimated and the most intense cell is shifted to the East. The different simulation results show that the divergence in the surface precipitation features seems to be due to different mechanisms involved in the onset of the precipitating system: the convective system is triggered by the topography of the Cévennes mountains (i.e., south-eastern part of the Massif Central) in DESCAM and by a low-level flux convergence in WRF. A sensitivity study indicates that the microphysics properties have impacted the thermodynamics and dynamics fields inducing the low-level wind convergence simulated with WRF for this HyMeX event.

scholarly journals The snow storm of 8 March 2010 in Catalonia (Spain): a paradigmatic wet-snow event with a high societal impact

2014 ◽  
Vol 14 (2) ◽  
pp. 427-441 ◽  
Author(s):  
M. C. Llasat ◽  
M. Turco ◽  
P. Quintana-Seguí ◽  
M. Llasat-Botija
Keyword(s):  
Meteorological Factors ◽  
Coastal Region ◽  
Heavy Precipitation ◽  
Precipitation Event ◽  
Societal Impact ◽  
Strong Wind ◽  
Near Surface ◽  
Wet Snow ◽  
Snow Storm ◽  
Meteorological Features

Abstract. A heavy precipitation event swept over Catalonia (NE Spain) on 8 March 2010, with a total amount that exceeded 100 mm locally and snowfall of more than 60 cm near the coast. Unusual for this region and at this time of the year, this snowfall event affected mainly the coastal region and was accompanied by thunderstorms and strong wind gusts in some areas. Most of the damage was due to "wet snow", a kind of snow that favours accretion on power lines and causes line-breaking and subsequent interruption of the electricity supply. This paper conducts an interdisciplinary analysis of the event to show its great societal impact and the role played by the recently developed social networks (it has been called the first "Snowfall 2.0"), as well to analyse the meteorological factors associated with the major damage, and to propose an indicator that could summarise them. With this aim, the paper introduces the event and its societal impact and compares it with other important snowfalls that have affected the Catalan coast, using the PRESSGAMA database. The second part of the paper shows the event's main meteorological features and analyses the near-surface atmospheric variables responsible for the major damage through the application of the SAFRAN (Système d'analyse fournissant des renseignements atmosphériques à la neige) mesoscale analysis, which, together with the proposed "wind, wet-snow index" (WWSI), allows to estimate the severity of the event. This snow storm provides further evidence of our vulnerability to natural hazards and highlights the importance of a multidisciplinary approach in analysing societal impact and the meteorological factors responsible for this kind of event.

scholarly journals Mapping Flood-Related Mortality in the Mediterranean Basin. Results from the MEFF v2.0 DB

Water ◽  
2019 ◽  
Vol 11 (10) ◽  
pp. 2196 ◽  
Author(s):  
Vinet ◽  
Bigot ◽  
Petrucci ◽  
Papagiannaki ◽  
Llasat ◽  
...  
Keyword(s):  
Population Density ◽  
Mediterranean Sea ◽  
Mediterranean Basin ◽  
Balearic Islands ◽  
Fatality Rate ◽  
Southern France ◽  
Rainfall Frequency ◽  
The Mediterranean ◽  
The Mediterranean Basin ◽  
Related Mortality

Recent events in Western Attica in Greece (24 deaths in November 2017), in the Balearic Islands (13 deaths in October 2018), and in southern France (15 deaths in October 2018) show that flood-related mortality remains a major concern in Mediterranean countries facing flash floods. Over the past several years, many initiatives have arisen to create databases on flood-related mortality. An international initiative started in 2011 pooling regional and national databases on flood mortality from region and/or countries bordering the Mediterranean Sea. The MEditerranean Flood Fatality Database (MEFF DB) brings together, in 2018, six Mediterranean regions/countries: Catalonia (Spain), Balearic Islands (Spain), Southern France, Calabria (Italy), Greece, and Turkey, and covers the period 1980–2018. MEFF DB is on progress and, every year, new data are included, but for this study, we kept only the preliminary data that were geolocated and validated on 31st of December 2018. This research introduces a new step in the analysis of flood-related mortality and follows the statistical description of the MEFF DB already published. The goals of this paper are to draw the spatial distribution of flood mortality through a geographical information system (GIS) at different spatial scales: country, NUTS 3 (Nomenclature of Territorial Units for Statistics. Level 3) regions, catchment areas, and grid. A fatality rate (F: number of deaths/year/million of inhabitants) is created to help this analysis. Then, we try to relate mortality to basic (human or physical) drivers such as population density, rainfall seasonality, or rainfall frequency across the Mediterranean Basin. The mapping of F shows a negative mortality gradient between the western and the eastern parts of the Mediterranean Sea. The south of France appears to be the most affected region. The maps also highlight the seasonality of flood-related deaths with the same west–east gradient. It confirms that flood mortality follows the climatological seasonal patterns across the Mediterranean Basin. Flood-related fatalities mainly occur during the early fall season in the western part of the Mediterranean area, while the Easter Basin is affected later, in November or during the winter season. Eastern Turkey introduces another pattern, as mortality is more severe in summer. Mortality maps are then compared with factors that potentially contribute to the occurrence of flood fatalities, such as precipitation intensity (rainfall hazard), to explain geographical differences in the fatality rate. The density of a fatal event is correlated to the population density and the rainfall frequency. Conversely, the average number of deaths per event depends on other factors such as prevention or crisis management.

scholarly journals Near-Surface Thermodynamic Sensitivities in Simulated Extreme-Rain-Producing Mesoscale Convective Systems

2017 ◽  
Vol 145 (6) ◽  
pp. 2177-2200 ◽  
Author(s):  
Russ S. Schumacher ◽  
John M. Peters
Keyword(s):  
Water Vapor ◽  
Initial Conditions ◽  
Weather Prediction ◽  
Mesoscale Convective Systems ◽  
Cold Pool ◽  
Near Surface ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Outflow Boundary

Abstract This study investigates the influences of low-level atmospheric water vapor on the precipitation produced by simulated warm-season midlatitude mesoscale convective systems (MCSs). In a series of semi-idealized numerical model experiments using initial conditions gleaned from composite environments from observed cases, small increases in moisture were applied to the model initial conditions over a layer either 600 m or 1 km deep. The precipitation produced by the MCS increased with larger moisture perturbations as expected, but the rainfall changes were disproportionate to the magnitude of the moisture perturbations. The experiment with the largest perturbation had a water vapor mixing ratio increase of approximately 2 g kg−1 over the lowest 1 km, corresponding to a 3.4% increase in vertically integrated water vapor, and the area-integrated MCS precipitation in this experiment increased by nearly 60% over the control. The locations of the heaviest rainfall also changed in response to differences in the strength and depth of the convectively generated cold pool. The MCSs in environments with larger initial moisture perturbations developed stronger cold pools, and the convection remained close to the outflow boundary, whereas the convective line was displaced farther behind the outflow boundary in the control and the simulations with smaller moisture perturbations. The high sensitivity of both the amount and location of MCS rainfall to small changes in low-level moisture demonstrates how small moisture errors in numerical weather prediction models may lead to large errors in their forecasts of MCS placement and behavior.

scholarly journals Comparison of Eulerian and Lagrangian moisture source diagnostics – the flood event in eastern Europe in May 2010

2013 ◽  
Vol 13 (11) ◽  
pp. 29333-29373 ◽  
Author(s):  
A. Winschall ◽  
S. Pfahl ◽  
H. Sodemann ◽  
H. Wernli
Keyword(s):  
Eastern Europe ◽  
Land Surface ◽  
Heavy Precipitation ◽  
Precipitation Event ◽  
Eulerian Model ◽  
Heavy Precipitation Event ◽  
Moisture Source ◽  
Moisture Sources ◽  
Source Regions ◽  
The Mediterranean

Abstract. Moisture convergence from different sources is an important prerequisite for a heavy precipitation event. The contributions from different source regions can, however, hardly be quantified from observations, and their assessment based on model results is complex. Two conceptually different numerical methods are widely used for the quantification of moisture sources: Lagrangian approaches based on the analysis of humidity variations along backward trajectories and Eulerian methods based on the implementation of moisture tracers into a numerical model. In this study the moisture sources for a high-impact heavy precipitation event that affected eastern Europe in May 2010 are studied with both Eulerian and Lagrangian moisture source diagnostics. The precipitation event was connected to a cyclone that developed over northern Africa, moved over the Mediterranean towards eastern Europe and induced transport of moist air towards the Carpathian mountains. Heavy precipitation and major flooding occurred in Poland, the Czech Republic and Slovakia between 16 and 18 May 2010. The Lagrangian and Eulerian diagnostics consistently indicate a~wide spatial and temporal range of moisture sources contributing to the event. The most important source is local evapotranspiration from the European land surface, followed by moisture from the North Atlantic. Further relevant contributions come from tropical Western Africa (10–20° N). Contrary to expectations, the Mediterranean Sea contributes only about 10% to the precipitation event. A detailed analysis of exemplary trajectories corroborates the general consistency of the two approaches, and underlines their complementarity. The Lagrangian method allows for mapping out moisture source regions with computational efficiency, whereas the more elaborate Eulerian model requires predefined moisture sources, but includes also processes such as precipitation, evaporation and turbulent mixing. However, in the Eulerian model, uncertainty concerning the relative importance of remote versus local moisture sources arises from different options to parameterise moisture tagging at the surface. Ultimately a more sophisticated parameterisation scheme will be required to reduce this uncertainty.

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