Boundary layer development and summer circulation in Southern Portugal

The summer circulation in Southwest Iberia is studied with data from a field experiment and numerical simulations by the non-hydrostatic mesoscale model, Meso-NH. The model is initialized and forced by the ARPEGE numerical weather prediction model. Surface fields and parameters were obtained from a number of different sources and validated against observations and long period integrations of the land surface model. The numerical simulations are found to capture most of the features found in surface and upper air observations, indicating the importance of the sea-breeze circulation in the diurnal cycle of the atmospheric boundary layer in regions more than 100 km away from the coast. The 3D nature of the summer atmospheric circulation over South Portugal is examined using results from a real case simulation of two summer typical days. Budget analysis in the 3D model are also shown, and used to clarify the relative importance of turbulent fluxes and horizontal advection in the dynamics of the boundary layer. This article was written in 2001. For various reasons it was never published, even though part of its content is published in Portuguese in the first author's PhD thesis, defended in 2006. The work was performed with an old version of Meso-NH, but we are convinced that their main results are up to date, which justifies its publication in English in an international scientific journal. Meanwhile, one of the authors, Joel Noilhan passed away too soon. The publication of this manuscript is also a small tribute to Joel and his scientific legacy

Impact of coherent eddies on airborne measurements of vertical turbulent fluxes

During the Hydrological-Atmospheric Pilot Experiment (HAPEX)-Sahel, which took place in Niger in the transitional period between the wet and dry seasons, two French aircraft probed the Sahelian boundary layer to measure sensible and latent heat fluxes. The measurements over the Niamey area often revealed organised structures of a few km scale that were associated with both thermals and dry intrusions. We study the impact of these coherent structures using a single day’s aircraft-measured fluxes and a numerical simulation of that day with a mesoscale model. The numerical simulation at high horizontal resolution (250 m) contains structures that evolve from streaks in the early morning to cells by noon. This simulation shows distribution, variance and skewness similar to the observations. In particular, the numerical simulation shows dry intrusions that can penetrate deeply into the atmospheric boundary layer (ABL), and even reach the surface in some cases, which is in accordance with the observed highly negatively skewed water vapour fluctuations. Dry intrusions and thermals organised at a few km scale give skewed flux statistics and can introduce large errors in measured fluxes. We use the numerical simulation to: (i) evaluate the contribution of the organised structures to the total flux, and (ii) estimate the impact of the organised structures on the systematic and random errors resulting from the 1D sampling of the aircraft as opposed to the 2D numerical simulation estimate. We find a significant contribution by the organised structures to the total resolved fluxes. When rolls occur, and for a leg length of about 30 times the ABL depth, the 1D sampled flux is shown to be sometimes 20% lower than the corresponding 2D flux when the 1D sampling direction is the same as the main axis of the rolls, whereas the systematic error is much lower when the direction of the leg is transverse to the rolls. In the case of cells, an underestimate of around 10% can still be observed with the 1D approach independent of direction, due to poor sampling of the energy-containing scales

Hydro-meteorological modelling of the Rhone basin: general presentation and objectives.

International audienceThe paper presents the French national effort undertaken these 5 last years in order to build a hydrological modelisation of the Rhone catchment, coupling the surface and the atmosphere at regional scale. The modelling strategy is based on the coupling of the operational surface model (the ISBA SVAT scheme), the snow model (CROCUS) of Meteo-France and the distributed hydrological model MODCOU developed at Centre d'Informatique G�ologique de l'Ecole Nationale Sup�rieure des Mines de Paris (CIG/ENSMP). As a first step, the coupled model (called in the following CIRSE) uses prescribed atmospheric forcing deduced from meteorological analysis. Several high resolution databases on a fourteen-year period have been constituted. The first part of the article presents the meteorological forcing database, the discharges database and the soil and vegetation maps. Then, the first results of CIRSE model and its validation on the riverflows are shown. As the coupled model was proved to be able to simulate present hydrology characteristics, it was finally used to conduct a preliminary climate change impact study. The impact of surface air temperature and precipitation variations on the hydrological cycle in a doubling CO2 scenario simulated by the Meteo-France climate General Circulation Model (GCM) are shown

Timepix3 as solid-state time-projection chamber in particle and nuclear physics

Timepix3 devices are hybrid pixel detectors developed within the Medipix3 collaboration at CERN providing a simultaneous measurement of energy (ToT) and time of arrival (ToA) in each of its 256×256 pixels (pixel pitch: 55 µm). The timestamp resolution below 2 ns allows a measurement of charge carrier drift times, so that particle trajectories can be reconstructed in 3D on a microscopic level (z-resolution: 30-60 µm). The 3D trajectory reconstruction methodology developed elsewhere is validated against simulated data providing ground truth information of the incident angles. The detector response functions and the achievable track angular resolutions are determined. For the first time, data taken with Timepix3 in the MoEDAL experiment are presented. After extracting singly charged minimum ionizing particle (MIP) tracks from the mixed radiation field using characteristic track features, their impact angles are evaluated. The directionality of the MIP radiation field is shown in elevation angle (θ) versus azimuthal angle (ϕ) maps, "unfolded" using the simulated detector responses to an omnidirectional radiation field.ISSN:1824-803