Sedimentation cycle of a freshwater tidal flat in the St. Lawrence estuary

Cap-Tourmente tidal flat is located on the north shore of the St. Lawrence middle estuary, 50 km downstream from Quebec city, Canada. Seasonal variations in sedient accretion were studied during the period without ice from May to December, using horizontal plates in conjunction with vertical stakes. The sedimentological regime which is characterized by very rapid changes, can be divided into four periods. There are two periods of intense erosion in May and October–November; during these months, erosion is initiated by walking and beak probing of very large flocks of snow geese. During the summer, very rapid accretion occurs for approximately 100 days; during this period, the sedimentation rate is 2 mm per day in the lower section of the tidal marsh which is covered with vegetation. Among the numerous factors influencing the sedimentation cycle, the very gradual slope of the upper part of the flat (0.25%) seems to be one of the most important because it results in a very large area (550 m wide) which is protected from strong tidal currents. This latter area is covered with dense and high vegetation which provides an effective protection against erosion unitl the snow geese arrive in October and raze it in a few days. The presence of vegetation reduces the velocities of tidal currents, particularly during ebb and modifies their direction for both flood and ebb, orienting them perpendicularly to the main tidal flow.     

Relative Influence of Urban And Orographic Effects for Low Wind Conditions in the Paris Area

The relative roles of land-use and orography on the meteorological fields in the Paris area are studied by means of numerical simulations of two Special Observing Periods (SOP), of the ECLAP experiment. Sensitivity experiments have been performed with flat orography to investigate the strength of the urban effects, and with a uniform land-use surface to replace the surface heterogeneities, to isolate the orographic forcing. Comparisons of the simulated fields with a reference simulation including all forcing are analysed. It is found that during these two SOP of spring-time conditions, despite the low terrain elevation, the orographic effect is the main forcing of the dynamic field but that although the land use is the dominant factor reproducing the thermal evolution of the boundary layer, the orography alone also plays a role in the temperature pattern with slope flows in and out of the Paris basin.     

Mesoscale Meteorological Simulations In Paris: Comparisons With Observations During The Experiment Eclap

This paper analyses the meteorological numerical simulation in Paris during two days (March 10 and 13, 1995) observed during ECLAP (Etude de la Couche Limite en Agglomération Parisienne– the boundary-layer study of the Paris area), a recent measurement campaign over the Paris area, in which both in-situ and remote-sensing devices at urban and rural locations have been deployed in addition to the operational meteorological network. The chosen days are well documented and correspond to different meteorological situations leading to pollution events (regarding wind direction, cloud cover, surface and synoptic inversion strength).Comparisons of three-dimensional mesoscale model results with the ECLAP observation data are presented, focusing on the temperature field and turbulent fluxes along with the urban heat island and the evolution of the mixing height. It is shown that the model correctly reproduces the main characteristics of the temperature field, its diurnal evolution (near the ground and higher up) especially the gradient between Paris and the surrounding countryside. The mixing-height evolutions are in a good agreement with the observations and the model is also able to reproduce the turbulent features of the atmosphere. The small difference observed between the urban and rural mixing-height evolution is also well reproduced.