Fabienne Lohou

Observation of the Diurnal Cycle in the Low Troposphere of West Africa

Abstract The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground. Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s−1. This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor fro...

Regional transport and dilution during high-pollution episodes in southern France: Summary of findings from the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE)

In the French Mediterranean basin the large city of Marseille and its industrialized suburbs (oil plants in the Fos-Berre area) are major pollutant sources that cause frequent and hazardous pollution episodes, especially in summer when intense solar heating enhances the photochemical activity and when the sea breeze circulation redistributes pollutants farther north in the countryside. This paper summarizes the findings of 5 years of research on the sea breeze in southern France and related mesoscale transport and dilution of pollutants within the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE) program held in June and July 2001. This paper provides an overview of the experimental and numerical challenges identified before the ESCOMPTE field experiment and summarizes the key findings made in observation, simulation, and theory. We specifically address the role of large-scale atmospheric circulation to local ozone vertical distribution and the mesoscale processes driving horizontal advection of pollutants and vertical transport and mixing via entrainment at the top of the sea breeze or at the front and venting along the sloped terrain. The crucial importance of the interactions between processes of various spatial and temporal scales is thus highlighted. The advances in numerical modeling and forecasting of sea breeze events and ozone pollution episodes in southern France are also underlined. Finally, we conclude and point out some open research questions needing further investigation.In the French Mediterranean basin the large city of Marseille and its industrialized suburbs (oil plants in the Fos-Berre area) are major pollutant sources that cause frequent and hazardous pollution episodes, especially in summer when intense solar heating enhances the photochemical activity and when the sea breeze circulation redistributes pollutants farther north in the countryside. This paper summarizes the findings of 5 years of research on the sea breeze in southern France and related mesoscale transport and dilution of pollutants within the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE) program held in June and July 2001. This paper provides an overview of the experimental and numerical challenges identified before the ESCOMPTE field experiment and summarizes the key findings made in observation, simulation, and theory. We specifically address the role of large-scale atmospheric circulation to local ozone vertical distribution and the mesoscale processes driving horizontal advection of pollutants and vertical transport and mixing via entrainment at the top of the sea breeze or at the front and venting along the sloped terrain. The crucial importance of the interactions between processes of various spatial and temporal scales is thus highlighted. The advances in numerical modeling and forecasting of sea breeze events and ozone pollution episodes in southern France are also underlined. Finally, we conclude and point out some open research questions needing further investigation.

The DACCIWA project: dynamics-aerosol-chemistry-cloud interactions in West Africa

Massive economic and population growth, and urbanization are expected to lead to a tripling of anthropogenic emissions in southern West Africa (SWA) between 2000 and 2030. However, the impacts of this on human health, ecosystems, food security, and the regional climate are largely unknown. An integrated assessment is challenging due to (a) a superposition of regional effects with global climate change, (b) a strong dependence on the variable West African monsoon, (c) incomplete scientific understanding of interactions between emissions, clouds, radiation, precipitation, and regional circulations, and (d) a lack of observations. This article provides an overview of the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) project. DACCIWA will conduct extensive fieldwork in SWA to collect high-quality observations, spanning the entire process chain from surface-based natural and anthropogenic emissions to impacts on health, ecosystems, and climate. Combining the resulting benchmark dataset with a wide range of modeling activities will allow (a) assessment of relevant physical, chemical, and biological processes, (b) improvement of the monitoring of climate and atmospheric composition from space, and (c) development of the next generation of weather and climate models capable of representing coupled cloud-aerosol interactions. The latter will ultimately contribute to reduce uncertainties in climate predictions. DACCIWA collaborates closely with operational centers, international programs, policy-makers, and users to actively guide sustainable future planning for West Africa. It is hoped that some of DACCIWA’s scientific findings and technical developments will be applicable to other monsoon regions.

Aspect sensitivity of VHF radar echoes observed in the middle and upper troposphere during the passage of a cut‐off low

The aspect sensitivity of VHF radar signals is related to the enhancement of the echo power and the narrowing of the spectral width at vertical incidence relative to the off-zenith directions. The strong aspect sensitivity is usually associated with stable atmospheric layers. In this paper the aspect sensitivity measured between an altitude of 4 and 15 km during the passage of a tropopause folding linked to a cut-off low is analyzed and discussed. The study mainly uses data acquired during a 2-day period with a three-beam VHF radar with relatively broad beam widths (6.8°). The angular dependence of the echoes is investigated using the power ratio of vertical to oblique beam and the spectral width obtained at vertical incidence. These two diagnostic parameters provided a nearly identical depiction of the stratospheric descent associated with the upper frontal zone. However, a discrepancy is observed between the location of the stratospheric subsidence and the tropopause height offered by the radar and that deduced from the meteorological analysis produced by an operational forecasting numerical model. Owing to the aspect sensitivity of the echoes, an underestimate amounting to 20% was found in the horizontal wind measurement. Finally, a comparison of simultaneous observations of two closely spaced VHF radars (100 km) has shown a good spatial coherence of the aspect sensitivity structures.

The influence of synoptic circulations and local processes on temperature anomalies at three French observatories.

The relative contribution of the synoptic-scale circulations to local and mesoscale processes was quantified in terms of the variability of middle latitude temperature anomalies from 2003 to 2013 using meteorological variables collected from three French observatories and reanalyses. Four weather regimes were defined from sea level pressure anomalies using National Center for Environmental Prediction (NCEP) reanalyses with a K-means algorithm. No correlation was found between daily temperature anomalies and weather regimes, and the variability of temperature anomalies within each regime was large. It was therefore not possible to evaluate the effect of large scales on temperature anomalies by this method. An alternative approach was found with the use of the analogues method: the principle being that for each day of the considered time series, a set of days which had a similar large-scale 500 hPa geopotential height field within a fixed domain were considered. The observed temperature anomalies were then compared to those observed during the analogue days: the closer the two types of series, the greater the mark of the large scale. This method highlights a widely predominant influence of the large-scale atmospheric circulation on the temperature anomalies. It showed a potentially larger influence of the Mediterranean Sea and orographic flow on the two southern observatories. Low-level cloud radiative effects substantially modulated the variability of the daily temperature anomalies.