No abstract

The Sahel is prone to intense soil erosion, and the dust emission flux is very sensitive to the surface wind speed. In this study, we use high‐frequency observations acquired across the Sahel to assess the ability of three global reanalyses (ERA‐interim, NCEP‐CFSR and MERRA) to capture the observed surface wind events that are critical to wind erosion. ERA‐Interim is shown to perform best. However, all three reanalyses present a too flat annual cycle, with important season‐dependent biases: they overestimate the surface wind during dry season nights and underestimate it during spring and monsoon season days. More importantly, the strongest wind speeds, observed in the morning and during deep convective events, are systematically underestimated. As analyzed wind fields are one of the main inputs of many dust emission models, their too low fraction of high wind speeds will lead to major errors in dust emission simulations.

?? 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Urbanized valleys are particularly vulnerable to particulate air pollution during the winter, when ground-based stable layers or cold-air pools persist over the valley floor. We examine whether the temporal variability of PM10 concentration in the section of the Arve River Valley between Cluses and Servoz in the French Alps can be explained by the temporal variability of the valley heat deficit, a bulk measure of atmospheric stability within the valley. We do this on the basis of temperature profile and ground-based PM10 concentration data collected during wintertime with a temporal resolution of one hour or finer, as part of the Passy-2015 field campaign conducted around Passy in this section of valley. The valley heat deficit was highly correlated with PM10 concentration on a daily time scale. The hourly variability of PM10 concentrations was more complex and cannot be explained solely by the hourly variability of the valley heat deficit. The interplay of the diurnal cycles of emissions and local dynamics is demonstrated and a drainage mechanism for observed nocturnal dilution of near-surface PM10 concentrations is proposed

No abstract

Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial distribution and temporal evolution of pollutant concentrations. They can be very complex and difficult to represent in numerical weather prediction models, particularly under stable conditions. Better knowledge of these local winds from observations is also a prerequisite to improving air quality prediction capability. This paper analyses local winds during the Passy-2015 field experiment that took place in a section of the Arve river valley, near Chamonix-Mont-Blanc. This location is one of the worst places in France regarding air quality. The wind analysis, which is mainly based on scanning Doppler lidar data sampling a persistent temperature inversion episode, reveals features consistent with the higher pollutant concentrations observed in this section of the valley as well as their spatial heterogeneities. In particular, an elevated down-valley jet is observed at night in the northern half of the valley, which, combined with a weak daytime up-valley wind, leads to very poor ventilation of the lowest layers. A northeast-southwest gradient in ventilation is observed on a daily-average, and is consistent with the PM10 heterogeneities observed within the valley.

No abstract

This study addresses the atmospheric boundary layer dynamics in the Grenoble valleys during persistent inversions, for 5 months during the 2006-2007 winter. During a persistent inversion, the boundary layer contains a layer with a positive vertical temperature gradient over a few days. Temperature data recorded on the valley sidewalls are first used. A bulk measure of the boundary layer stability, based upon the temperature difference between the valley top and the valley bottom, is introduced and a criterion is proposed to detect persistent inversions. We show that this criterion is equivalently expressed in terms of the heat deficit inside the boundary layer. Nine episodes are detected and coincide with the PM 10 -polluted periods of the 2006-2007 winter. Secondly, the five strongest and longest persistent inversions are simulated using the MesoNH model. Focus is made on the stagnation stage of the episode, during which the inversion exhibits a diurnal cycle that does not significantly evolve from day to day. Whatever the episode, the inversion develops from the ground over a height of about 1200 m, with a nighttime temperature strength reaching 20 K. The boundary-layer dynamics within the inversion layer are fully decoupled from the (anticyclonic, weak) synoptic flow, independent from the synoptic-wind direction and similar whatever the episode. This implies that these dynamics are controlled by thermal winds and solely depends upon the geometry of the topography and upon the radiative cooling of the ground. Finally, a 2-day high-resolution simulation is made for the strongest case, representative of any persistent inversion. The flow pattern displays a well-defined spatial structure, with a vertical layering resulting from the superposition of the down-valley winds flowing from the different valleys surrounding Grenoble. This pattern persists all day long over a shallow convective layer of about 50 m forming above the ground during the reduced daytime period. Within this shallow layer, convection triggers a weak up-valley wind. Ventilation and stagnation areas in the surface layer are also computed, providing insights for air quality studies. The main characteristics of these persistent inversions are comparable to the most extreme wintertime inversions recorded in the Grand Canyon, Arizona.

Abstract:The dynamics of the atmospheric boundary layer in an alpine valley at night or in winter is dominated by katabatic (or down-slope) flows. As predicted by McNider (1982) oscillations along the slope are expected to occur if the fluid is stably-stratified, as a result of buoyancy and adiabatic cooling/warming effects. Internal gravity waves must also be generated by the katabatic flows because of the stable stratification. The aim of the present paper is to identify and characterize the oscillations in the katabatic flow as well as the internal gravity wave field emitted by this flow. Numerical simulations with the ARPS code are performed for this purpose, for an idealized configuration of the Chamonix valley. We show that the oscillations near the slope are non propagating motions, whose period is well predicted by the single particle model of McNider (1982) and equal to 10 to 11 mn. As for the wave field, its frequency is close to 0.85N, where N is the value of the Brunt-Väisälä frequency in the generation region of the waves, consistently with previous academic studies of wave emission by turbulent motions in a stratified fluid. This leads to a wave period of 7 to 8 mn.