Understanding the daily cycle of evapotranspiration: a method to quantify the influence of forcings and feedbacks

A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m-2 h-1. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches -30 W m-2 h-1. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variation

Parasitism of Lepidopterous Stem Borers in Cultivated and Natural Habitats

Plant infestation, stem borer density, parasitism, and parasitoid abundance were assessed during two years in two host plants, Zea mays (L.) (Cyperales: Poaceae) and Sorghum bicolor (L.) (Cyperales: Poaceae), in cultivated habitats. The four major host plants (Cyperus spp., Panicum spp., Pennisetum spp., and Sorghum spp.) found in natural habitats were also assessed, and both the cultivated and natural habitat species occurred in four agroecological zones in Kenya. Across habitats, plant infestation (23.2%), stem borer density (2.2 per plant), and larval parasitism (15.0%) were highest in maize in cultivated habitats. Pupal parasitism was not higher than 4.7% in both habitats, and did not vary with locality during each season or with host plant between each season. Cotesia sesamiae (Cameron) and C. flavipes Cameron (Hymenoptera: Braconidae) were the key parasitoids in cultivated habitats (both species accounted for 76.4% of parasitized stem borers in cereal crops), but not in natural habitats (the two Cotesia species accounted for 14.5% of parasitized stem borers in wild host plants). No single parasitoid species exerted high parasitism rates on stem borer populations in wild host plants. Low stem borer densities across seasons in natural habitats indicate that cereal stem borer pests do not necessarily survive the non-cropping season feeding actively in wild host plants. Although natural habitats provided refuges for some parasitoid species, stem borer parasitism was generally low in wild host plants. Overall, because parasitoids contribute little in reducing cereal stem borer pest populations in cultivated habitats, there is need to further enhance their effectiveness in the field to regulate these pests

Diurnal cycle of the West African Monsoon water cycle: regional and seasonal variability.

International audienceThe ground-based Global Positioning System (GPS) receivers deployed in the framework of AMMA provide an unprecedented insight into diurnal variations of precipitable water vapour over West Africa. They display a strong seasonal dynamics and distinct features along latitude. Overall, GPS data point to the significance of an afternoon maximum of PWV during the monsoon season. These data are used together with surface evapo-transpiration from the ALMIP land surface model simulations forced with TRMM 3B42v6 three-hourly precipitation data. The combination of the three datasets yields a comprehensive description of the atmospheric water cycle at a sub-daily timescale. A description of the monthly mean diurnal cycle in the water budget terms is discussed as a function of location and time. A focus is made on the pre- and post-onset periods. The diurnal cycle of the atmospheric processes involved are investigated with the help of high-resolution radiosonde data and the ECMWF model analysis and forecasts

Diurnal cycle of the West African Monsoon water cycle: regional and seasonal variability.

International audienceThe ground-based Global Positioning System (GPS) receivers deployed in the framework of AMMA provide an unprecedented insight into diurnal variations of precipitable water vapour over West Africa. They display a strong seasonal dynamics and distinct features along latitude. Overall, GPS data point to the significance of an afternoon maximum of PWV during the monsoon season. These data are used together with surface evapo-transpiration from the ALMIP land surface model simulations forced with TRMM 3B42v6 three-hourly precipitation data. The combination of the three datasets yields a comprehensive description of the atmospheric water cycle at a sub-daily timescale. A description of the monthly mean diurnal cycle in the water budget terms is discussed as a function of location and time. A focus is made on the pre- and post-onset periods. The diurnal cycle of the atmospheric processes involved are investigated with the help of high-resolution radiosonde data and the ECMWF model analysis and forecasts

Journal of extracellular vesicles

Plasma and other body fluids contain cell-derived extracellular vesicles (EVs), which participate in physiopathological processes and have potential biomedical applications. In order to isolate, concentrate and purify EVs, high-speed centrifugation is often used. We show here, using electron microscopy, receptor-specific gold labelling and flow cytometry, that high-speed centrifugation induces the formation of EV aggregates composed of a mixture of EVs of various phenotypes and morphologies. The presence of aggregates made of EVs of different phenotypes may lead to erroneous interpretation concerning the existence of EVs harbouring surface antigens from different cell origins

Modelling of the Thermodynamical Diurnal Cycle in the Lower Atmosphere : A Joint Evaluation of Four Contrasted Regimes in the Tropics Over Land

International audienceThe diurnal cycle is an important mode of variability in the Tropics that is not correctly predicted by numerical weather prediction models. The African Monsoon Multidisciplinary Analyses program provided for the first time a large dataset to document the diurnal cycle over West Africa. In order to assess the processes and mechanisms that are crucial for the representation of the diurnal cycle, four different regimes that characterize the varying conditions encountered over land along a surface-temperature gradient are selected. A single-column modelling framework is used in order to relate the features of the simulated diurnal cycle to physical processes in these four distinct cases. Particular attention is given to providing realistic initial and boundary conditions at the surface and in the atmosphere, enabling the use of independent data for the evaluation of the simulations. The study focuses on the simulation of the surface energy budget and low-level characteristics and analyzes the balance between cloud/surface/boundary-layer processes at the sub-diurnal time scale. The biases and drawbacks of the simulations are found to change along the temperature gradient but they always involve the representation of clouds. They also explain parts of the bias obtained with the same model when used in a less constrained configuration. Surface-atmosphere-cloud interactions arising at the sub-diurnal time scale are invoked to explain the distinct features of the low-level diurnal cycle observed over West Africa

Understanding the daily cycle of evapotranspiration: a method to quantify the influence of forcings and feedbacks

A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m-2 h-1. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches -30 W m-2 h-1. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variation

Exchanging natural enemies species of lepidopterous cereal stemborers between African regions

Difficulties of identification of natural enemies of cereal stemborers for 'redistribution' in Africa are discussed. Tritrophic level studies on wild and cultivated habitats of borers and beneficial species are needed to judge the impact of a natural enemy species in the cropping system. Areas witli low Pest pressure which are climatically favorable for Pest development may be selected for study. Based on the results of various countrywide surveys to map the relative importance of Sesaniia calnmistis Hampson, Eldam sncc@rim (Walker) and Busseola fuscn (Fuller) in western Africa, recommendations are given for sites for tritrophic level studies. It is hypothesised that because maize is not always present in the field and because of its high susceptibility, natural biological control has to come I'rom wild habitats. This emphasises the importance of the knowledge on the wild host-plant range. Survey results complemented with oviposition and life-table studies in the laboratory showed that, ratlier than being reservoirs for pests, most wild grass species act as trap plants causing mortalities of 100%. A comparison of light trap catches with pupae found on wild hosts and the scarcity of known wild hosts in areas with high Pest pressure suggest gaps in Our knowledge of the range of host plant species. Based on comparison of known natural enemy complexes in East and West Africa, the scelionid egg parasitoid Teleriorrius isis and an East African strain of the braconid larval parasitoid Cotesia sesainine (Cameron) are proposed for redistribution against B. fuscn and S. calaniis fis, respectively. Telenonius isis has never been reported from East Africa whereas C. sesaniiae is common in East and southern Africa and scarce in western Africa, suggesting that C. sesaniiae is probably not adapted to S. culnmistis and B.fiisca in this region