International audienceThe Mediterranean region is frequently affected by heavy precipitation events associated with flash floods, landslides, and mudslides that cause hundreds of millions of euros in damages per year and often, casualties. A major field campaign was devoted to heavy precipitation and flash floods from 5 September to 6 November 2012 within the framework of the 10-year international HyMeX (Hydrological cycle in the Mediterranean Experiment) dedicated to the hydrological cycle and related high-impact events. The 2- month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy, and Spain. The observation strategy of the field experiment was devised to improve our knowledge on the following key components leading to heavy precipitation and flash flooding in the region: i) the marine atmospheric flows that transport moist and conditionally unstable air towards the coasts; ii) the Mediterranean Sea acting as a moisture and energy source; iii) the dynamics and microphysics of the convective systems producing heavy precipitation; iv) the hydrological processes during flash floods. This article provides the rationale for developing this first HyMeX field experiment and an overview of its design and execution. Highlights of some Intense Observation Periods illustrate the potential of the unique datasets collected for process understanding, model improvement and data assimilation

Numerical studies by the authors and others of the 1994 Piedmont flood show that the orographically modified flow was a critical element for the production of extraordinary rainfall. To uncover the precise mechanism of orographic rainfall occurring in full-physics MM5 simulations of the 1994 Piedmont flood, the authors carried out simulations with the same real-data initial and boundary conditions, but with the real orography replaced by an idealized one. With excellent agreement between real and idealized orography on the rainfall rate versus time in the Piedmont area, analysis of the idealized-orography simulation provides a clear picture of the model's mechanism of orographically induced rainfall. As noted in previous studies of the 1994 Piedmont case, a moist saturated airflow has a reduced effective static stability and tends to flow over the mountains, while an unsaturated airstream is stable and tries to flow around (toward the left in the Northern Hemisphere). In the 1994 Piedmont case, there was a strong horizontal gradient of moisture; thus the western moist part of the airstream flows over, while the eastern drier part is deflected westward around the obstacle, and so a convergence is produced between the airstreams. This effect is explored using a simple version of MM5 wherein the flow, moisture distribution, and idealized orography are varied within the observed range. Quantitative as well as qualitative rainfall rates and flow features of the full-physics MM5 simulations are captured with the simple model.

SUMMARYAlthough the large-scale ow was similar, important differences in mesoscale atmospheric structure made the difference between moderately intense rain in Intensive Observing Period (IOP) 2b and relatively light rain in IOP 8 in the Lago Maggiore Target Area (LMTA). This conclusion is supported here through analysis of data from, and numerical simulations of, these two cases. Analysis of the large-scale data shows that there was in both cases a moist tongue of southerly ow moving from the west to the east of the Alpine south side. Mesoscale data analysis and numerical simulations suggest that the most important difference between the two cases was the presence of a cold stable air mass in the Po Valley in IOP 8 which persisted through the period in which the large-scale moist tongue was progressing eastward and prevented the most humid air from reaching the LMTA. Another difference contributing to the greater rainfall in the LMTA in IOP 2b was the development of conditional instability (with associated convective rain) due to the effect of the Alps on the eastward passage of the moist tongue; in IOP 8 the atmosphere remained locally stable throughout the period.

The UV Raman lidar system (BASIL), operational at University of Basilicata (Potenza-Italy) and capable to perform high-resolution and accurate measurements of atmospheric temperature and water vapour based on the application of the rotational and vibrational Raman lidar techniques in the UV, was recently involved in the LAUNCH 2005 experiment (International Lindenberg campaign for assessment of humidity and cloud profiling systems and its impact on high-resolution modelling) held from 12 September to 31 October 2005. A thorough description of technical characteristics, measurements capabilities and performances of BASIL is given in the paper. Measurements were continuously run between 1 and 3 October 2005, covering a dry stratospheric intrusion episode associated with a tropopause folding event. The measurements in this paper represent the first simultaneous Raman Lidar measurements of atmospheric temperature and water vapour mixing ratio, and consequently relative humidity, reported for an extensive observation period (32 hours). The use of water vapour to trace intruded stratospheric air allows to clearly identify a dry structure (approx. 1 km thick) originated in the stratosphere and descending in the free troposphere down to ~ 3 km. A similar feature is present in the temperature field, with lower temperature values detected within the dry air tongue. Relative humidity measurements reveal values as small as 0.5-1 % within the intruded air. The stratospheric origin of the observed dry layer has been verified by the application of a Lagrangian trajectory model. The subsidence of the intruding heavy dry air may be responsible for the gravity wave activity observed beneath the dry layer. Lidar measurements have been compared with the output of both the PSU/NCAR Mesoscale Model (MM5) and the European Center for Medium range Weather Forecasting (ECMWF) global model. Comparisons in term of water vapour reveal the capability of MM5 to reproduce the dynamical structures associated with the stratospheric intrusion episode and simulate the deep penetration into the troposphere of the dry intruded layer. Moreover, lidar measurements of potential temperature are compared with MM5 output, while potential vorticity from both ECMWF and MM5 is compared with estimates obtained combining MM5 model vorticity and lidar measurements of potential temperature

Abstract. The Special Observation Period (SOP1

Passive air samplers (polyurethane foam disks) were deployed on an altitudinal transect in the rural Italian Alps to investigate the potential influence of forest cover on air concentrations. Samplers were exposed overtwo periods, each of several weeks, either in clearings or in forests. In the first period, there was high leaf coverage (high leaf area index, LAI); in the second, the LAI was low after the autumnal leaf fall. PCBs sequestered in the PUF generally declined with altitude, for example, in the clearings PCBs-28, 52, 90/101, 118, and 138, all showed statistically significant declines (p < 0.05). The mass of HCB sequestered increased with altitude, evidence of cold condensation. Ratios of the forest:clearing concentrations were calculated; this ratio expresses the filtering ability of forests to deplete air concentrations compared to the adjacent clearings. During the high LAI sampling period, these depletion factors ranged between 0.93 and 0.54 and were inversely correlated with temperature-corrected log K0A. This relationship was notobserved during the low LAI sampling period. The depletion factors were normalized using the LAI to give a density independent depletion factor (DIDF). The slopes of the correlations with K0A were comparable for broadleaf or coniferous forests at different altitudes, suggesting that leaf surfaces determine the exchanges with air. Broadleaf forests at 1000 and 1400 m showed similar behavior, while a conifer forest at 1800 m gave depletion factors which were higher by about a factor of 2. It is suggested that DIDF can be used in regional environmental fate models to estimate the contribution of forests to contaminant fate.

A case study of the 1994 Piedmont flood is carried out by performing several numerical experiments using the MM5 model. We report on sensitivity tests and analyses that suggest the following description of the events leading to the flood: rising motion ahead of an upper-level trough, progressing from Spain towards Italy, initiated deep convection within the conditionally unstable air existing over the Mediterranean. Thus, a band of intense precipitation and lowlevel southerly winds formed and moved eastward with the upper trough. As the rain band and strong southerlies encountered the western Alps, moist air was lifted, resulting in large amounts of rainfall. The associated latent-heat-related upward motion produced a strong positive vorticity anomaly over the western Alps with associated low-level easterlies in the Po Valley which appear to have played a major role in retarding the generally eastward progress of the rain band over Piedmont.

High Impact Weather (HIW), particularly Heavy Precipitation Events (HPE), are common phenomena affecting the western Mediterranean (WMED) especially in the autumn period. Understanding and evaluating the capability to adequately represent such events in model simulations is one of the main goals of the Hydrological cycle in the Mediterranean Experiment (HyMeX) and the main motivation of this investigation. In order to gain a better knowledge of the model representation of HPE and related processes we perform a seamless multi‐model intercomparison at the event scale. Limited‐area model runs (grid spacing from 2 to 20 km) at weather and climate time‐scales are considered, four with parametrized and five with explicit convection. The performance of the nine models is compared by analysing precipitation, as well as convection‐relevant parameters. An Intensive Observation Period (IOP12) from the HyMeX‐SOP1 (Special Observation Period) is used to illustrate the results. During IOP12, HPE affected the northwestern Mediterranean region, from Spain to Italy, as a consequence of Mesoscale Convective Systems (MCSs) which initiated and intensified in the area of investigation. Results show that: (i) the timing of the maximum precipitation seems to be linked to the representation of large‐scale conditions rather than differences among models; (ii) Convection Permitting Models (CPMs) exhibit differences among each other, but better represent the short‐intense convective events. All four convection‐parametrized models produce a large number of weak and long‐lasting events. Regional Climate Models (RCMs) capture the occurrence of the event but produce notably lower precipitation amounts and hourly intensities than CPMs and Numerical Weather Prediction (NWP) models with parametrized convection; (iii) these differences do not seem to come from mean moisture or Convective Available Potential Energy (CAPE) which are in the same range for all models, but rather from differences in the variability and vertical distribution of moisture and the triggering of deep convection.