Dedicated Field Experiments Research Articles

Controlled drought stress affects rosemary essential oil composition with minimal impact on biomass yield

Rosemary (Salvia rosmarinus Spenn.), an indigenous medicinal and aromatic plant within the Mediterranean region, yields essential oils renowned for their antimicrobial, antioxidant, anticancer, and anti-inflammatory properties. When subjected to various stress conditions, rosemary intensifies the production of essential oils as a defensive mechanism, as occurs when it is subjected to drought stress, making it a potentially interesting crop for marginal dryland areas of Sicily and other Mediterranean areas. Managing rosemary cultivation through controlled drought stress close to the balsamic period may increase the concentration and quality of essential oils and optimize the use of limited resources like water. We conducted two on-farm field experiments in Sicily, southern Italy, to assess the effect of controlled drought stress on rosemary biomass, essential oil yield, and composition. The hypothesis was that drought could improve the concentration of essential oils and modify their chemical composition by increasing valuable aromatic components without negatively impacting crop biomass production, increasing interest in cultivating this species. The experiments were co-designed with farmers selecting dedicated field experiments, cultivars, and crop management. Drought stress was applied during autumn and spring in correspondence with medium-high temperatures and rainwater scarcity. The irrigation input was reduced for three weeks during the balsamic period for each trail. Under stressed conditions, morphometric traits were measured. At harvest time, essential oils were extracted using microwave-assisted extraction. Under the same farm conditions, the canopy height was significantly higher in ‘Barbeque’ than ‘Tuscan Blue cv. (67.3 cm vs. 43.8 cm) This indicates a genotype influence on stress response. Essential oil yield increased by 30 % in plants of Tuscan blue cv. subjected to stress conditions in one of two farms. Additionally, drought stress altered the aromatic profile of the essential oil, resulting in differing percentages of key compounds between stressed and unstressed plants. Generally, there was a higher α-pinene percentage in unstressed plants and higher bornyl-acetate and β-caryophyllene concentrations in drought-stressed plants. In the Scicli farm, α-pinene decreased in response to water stress, while camphene and myrcene increased. Our results summarized that controlled drought stress had a minimal impact on rosemary growth, increasing the percentages of some terpenes in its essential oils. No severe decrease was observed in the synthesis of essential oils, suggesting that rosemary is suitable for marginal soils in many Mediterranean countries to protect soils and diversify crops in farms.

Future climate change will accelerate maize phenological development and increase yield in the Nemoral climate

Climate change will bring warmer and wetter conditions and more frequent extreme events in the Nemoral climate zone. These changes are expected to affect maize growth and yields. In this study, we applied the AgroC model to assess climate change impact on changes in growing environmental conditions, growing season length, yield and potential yield losses due to multiple abiotic stresses. The model was calibrated and validated using data from dedicated field experiments conducted in Lithuania during four meteorologically contrasting years (2015, 2016, 2017 and 2019). We simulated the climate impacts on rainfed maize for long-term future climate conditions from 2020 to 2100 under the RCP2.6 (low), RCP4.5 (medium) and RCP8.5 (high) emission scenarios. As a result, we found that air temperature, sum of growing degree days and amount of precipitation during the growing season of maize will increase, especially under medium and higher emission scenarios (RCP4.5 and RCP8.5), with significantly positive effect on yields. The simulation results showed that average maize grain yield will increase under RCP2.6 by 69 kg ha−1 per decade, under RCP4.5 by 197 kg ha−1 per decade and under RCP8.5 by 304 kg ha−1 per decade. The future potential maize yield reveals a progressive increase with a surplus of +10.2% under RCP4.5 and +14.4% under RCP8.5, while under RCP2.6 the increase of potential yield during the same period will be statistically not significant. The yield gap under RCP2.6 and RCP4.5 will fluctuate within a rather narrow range and under RCP8.5, it will decrease.

Characterizing CO2 residual trapping in-situ by means of single-well push-pull experiments at Heletz, Israel, pilot injection site – experimental procedures and results of the experiments

Two dedicated field experiments have been carried out at the Heletz, Israel pilot CO2 injection site. The objective has been to quantify the CO2 residual trapping in-situ, based on two distinctly different methods. Both experiments are based on the principle of a combination of hydraulic, thermal and/or tracer tests before and after creating the residually trapped zone of CO2 and using the difference in the responses of these tests to estimate the in-situ residual trapping.In Residual Trapping Experiment I (RTE I), carried out in autumn 2016, the main characterization test before and after the creation of the residually trapped zone were hydraulic withdrawal tests. In this experiment, the residually trapped zone was also created by fluid withdrawal, by first injecting CO2, then withdrawing fluids until CO2 was at residual saturation. The second experiment, Residual Trapping Experiment II (RTE II), was carried out autumn 2017. In this experiment, the residually trapped CO2 zone was created by CO2 injection, followed by the injection of CO2-saturated water, to push away the mobile CO2 and leave the residually trapped CO2 behind. In this test, the main reference test carried out before and after creating the residually trapped zone was injection and recovery of gas partitioning tracer Krypton. This paper presents the experimental procedures and results of these experiments. A hydraulic withdrawal test as a characterization method was robust and gave a clear signal. Given the difficulties in injecting water optimally saturated with CO2, in order not to dissolve the residually trapped CO2 or to create situations with excess mobile gas, withdrawal test may also be a generally preferable hydraulic testing method, in comparison to injection. The limitation of any hydraulic test is that it only gives an averaged value over the test section. At Heletz additional information about CO2 distribution was obtained based on thermal measurements and by monitoring the pressure difference between the two sensors in the bolehole. The latter could be used to estimate the amount of mobile CO2 in the well test section.Tracer experiments with gas partitioning tracers can in principle give more detailed information of CO2 residual distribution in the reservoir than hydraulic tests can, but are also far more complicated to carry out, involving sophisticated and sensitive equipment. In the Heletz case the optimal injection of CO2-saturated water turned out to be difficult to achieve. Creating the zone of residual saturation by means of fluid withdrawal rather than by injecting CO2-saturated water seemed a more robust approach. Monitoring the gas contents in the test interval gave good guidance on the state of the system. Model interpretations of the two experiments to obtain values for CO2 residual saturation are presented in companion papers in this same Special Edition.

Model analysis of CO2 residual trapping from single-well push pull test — Heletz, Residual Trapping Experiment II

Residual or capillary trapping is one of the key trapping mechanisms for CO2 geological storage. At the Heletz, Israel, pilot injection site, two dedicated field experiments have been carried out to characterize it in-situ. This paper presents the model analyses of the second of these tests, the Residual Trapping Experiment II (RTE II). In the experiment hydraulic, tracer and thermal tests before and after the generation of the residually trapped zone are used to quantify residual saturation. The creation of the residually trapped zone is based on injection of CO2-saturated-water following injection of free-phase supercritical CO2. For the modeling, both a radial-symmetric model with homogeneous layer properties and 3D model with stochastically heterogeneous properties were used. Extensive parameter sensitivity studies were carried out and various well-geometry related fluid injection/withdrawal scenarios were considered. In terms of the best estimate for the maximum residual saturation, this experiment, like the previous RTE I experiment, gave the best agreement with a residual gas saturation of 0.1, this value being somewhat lower than the core-measured value of 0.2. Overall, the pressure response provided a very robust signal enabling to distinguish different values of residual saturation as well as the extent of sections where gas blocking for water flow could have occurred. Analysis of the tracer data indicated the presence of phenomena such as gas-blocking water flow, the importance of multi-layer and channelized flow and transport, and the importance of taking into account the processes in the actual injection/production well also.

Ideotype definition to adapt legumes to climate change: A case study for field pea in Northern Italy

One of the key strategies to alleviate negative impacts of climate change on crop production is the development of new cultivars better adapted to the conditions expected in the future. Despite the role of legumes as protein sources, medium- and long-term strategies currently debated mainly focus on agricultural policies and on improved management practices, whereas ideotyping studies using climate projections are scarcely reported. The objective of this study was to define pea ideotypes improved for yield and irrigation water productivity targeting current climate and four future projections centred on 2040, resulting from the combination of two General Circulation Models (HadGEM2 and GISS-ES) and two Representative Concentration Pathways (RCP4.5 and RCP8.5). The STICS model was used, with the default pea parameterization refined using data from two years of dedicated field experiments. Ideotypes were defined by combining STICS and the E-FAST sensitivity analysis method focusing on model parameters representing traits on which breeding programs are ongoing. Results showed that climate change is expected to decrease the productivity of current pea cultivars (up to -12.6%), and that increasing irrigation (to cope with the expected less favourable rainfall distribution) would not avoid yield losses. The proposed ideotypes, characterized by a shorter vegetative phase and by increased tolerance to high temperature, performed better than current varieties, providing higher yields (+4.5%) and reduced water consumption (-20%). For the first time, we demonstrated the suitability of STICS for ideotyping purposes and used a simulation model to define pea breeding strategies targeting future climate conditions.

Growth simulation and yield prediction for perennial jujube fruit tree by integrating age into the WOFOST model

Mathematical models have been widely employed for the simulation of growth dynamics of annual crops, thereby performing yield prediction, but not for fruit tree species such as jujube tree (Zizyphus jujuba). The objectives of this study were to investigate the potential use of a modified WOFOST model for predicting jujube yield by introducing tree age as a key parameter. The model was established using data collected from dedicated field experiments performed in 2016–2018. Simulated growth dynamics of dry weights of leaves, stems, fruits, total biomass and leaf area index (LAI) agreed well with measured values, showing root mean square error (RMSE) values of 0.143, 0.333, 0.366, 0.624 t ha−1 and 0.19, and R2 values of 0.947, 0.976, 0.985, 0.986 and 0.95, respectively. Simulated phenological development stages for emergence, anthesis and maturity were 2, 3 and 3 days earlier than the observed values, respectively. In addition, in order to predict the yields of trees with different ages, the weight of new organs (initial buds and roots) in each growing season was introduced as the initial total dry weight (TDWI), which was calculated as averaged, fitted and optimized values of trees with the same age. The results showed the evolution of the simulated LAI and yields profiled in response to the changes in TDWI. The modelling performance was significantly improved when it considered TDWI integrated with tree age, showing good global (R2≥0.856, RMSE≤0.68 t ha−1) and local accuracies (mean R2≥0.43, RMSE≤0.70 t ha−1). Furthermore, the optimized TDWI exhibited the highest precision, with globally validated R2 of 0.891 and RMSE of 0.591 t ha−1, and local mean R2 of 0.57 and RMSE of 0.66 t ha−1, respectively. The proposed model was not only verified with the confidence to accurately predict yields of jujube, but it can also provide a fundamental strategy for simulating the growth of other fruit trees.

Estimating Crop Nutritional Status Using Smart Apps to Support Nitrogen Fertilization. A Case Study on Paddy Rice.

Accurate nitrogen (N) management is crucial for the economic and environmental sustainability of cropping systems. Different methods have been developed to increase the efficiency of N fertilizations. However, their costs and/or low usability have often prevented their adoption in operational contexts. We developed a diagnostic system to support topdressing N fertilization based on the use of smart apps to derive a N nutritional index (NNI; actual/critical plant N content). The system was tested on paddy rice via dedicated field experiments, where the smart apps PocketLAI and PocketN were used to estimate, respectively, critical (from leaf area index) and actual plant N content. Results highlighted the system’s capability to correctly detect the conditions of N stress (NNI < 1) and N surplus (NNI > 1), thereby effectively supporting topdressing fertilizations. A resource-efficient methodology to derive PocketN calibration curves for different varieties—needed to extend the system to new contexts—was also developed and successfully evaluated on 43 widely grown European varieties. The widespread availability of smartphones and the possibility to integrate NNI and remote sensing technologies to derive variable rate fertilization maps generate new opportunities for supporting N management under real farming conditions.

On the efficient computation of range and Doppler data in noise radar

AbstractIn noise radar, digital signal processing algorithms for implementing the computation of the Cross Ambiguity Function through range correlation and Doppler compensation call for optimized solutions. In fact, to achieve a high coherent processing gain, they often compute a large amount of data beyond the maximum range and/or the maximum radial velocity of interest, adding useless information. A novel, efficient algorithm, called Range Filter Bank, is proposed to implement a scope-tailored computation of range/Doppler data in continuous emission noise radar. Downstream its theoretical analysis, the algorithm has been applied to a real-life case study based on dedicated field experiments, in which good quality kinematic data of a car moving at various speeds have been extracted.

Do farmers in Germany exploit the potential yield and nitrogen benefits from preceding oilseed rape in winter wheat cultivation?

ABSTRACTField experiments show that wheat grown after oilseed rape (OSR) achieves higher yield levels, while the nitrogen (N) application is reduced. However, field experiment data are based on few locations with optimised management. We analysed a large dataset based on farm data to assess the true extent of break crop benefits (BCB) for yield and N fertilisation within German commercial farming.Across all German states and years, average yield of wheat preceded by OSR was 0.56 Mg ha−1 higher than yield of wheat preceded by cereals (7.09 Mg ha−1), although considerable variation between regions was observed. Mean N application across all states to wheat after OSR was 5 kg ha−1 lower than to wheat after cereals. Choice of wheat types for different end uses (bread flour or animal feed) showed higher (0.77 Mg ha−1) or lower (0.44 Mg ha−1) BCB for yield of wheat cultivated after OSR compared with after cereals. The calculated BCB for yield and N fertilisation were lower than expected from dedicated field experiments and fertiliser recommendations. Thus the advantages of OSR as a preceding crop are generally utilised by commercial farmers in Germany but there is room for improvement.

WOFOST-GTC: A new model for the simulation of winter rapeseed production and oil quality

Rapeseed is one of the most important sources of vegetable oils, and its cultivation in Europe is expanding due to the economic incentives to grow energy crops. Given the unique characteristics of this crop, simulation studies targeting yield predictions and scenario analysis should be performed using specific models rather than using generic crop simulators adapted to rapeseed via calibration. This study presents a new model – WOFOST-GTC – which implements a dynamic representation of the rapeseed canopy architecture and includes modelling approaches to simulate oil content and composition. We reduced the number of model parameters to 35, compared to the 97 parameters of the original WOFOST model, from which it derives. WOFOST-GTC was developed using data collected in dedicated field experiments carried out in northern Italy in 2012–2013. The model ability to reproduce the underlying processes was evaluated using data collected in Europe between 1993 and 2013. In particular, dynamics involved with production and oil quality were evaluated on 7 and 18 datasets, respectively. The aboveground biomass and photosynthetic area index at different depths in the canopy were accurately simulated (R2=0.86 and 0.78, respectively). Despite the lower complexity, WOFOST-GTC proved to be as accurate as the original WOFOST model. The simulation of the seed oil content (R2=0.76) and of the oleic (R2=0.95), linoleic (R2=0.88) and α-linolenic acid (R2=0.95) fractions was accurate. Hence, we propose WOFOST-GTC as a suitable simulation model to analyse the rapeseed production and oil quality under different weather and management scenarios.

Evaluation of WARM for different establishment techniques in Jiangsu (China)

WARM is a model for rice simulation accounting for key biotic and a-biotic factors affecting quantitative and qualitative (e.g., amylose content, chalkiness) aspects of production. Although the model is used in different international contexts for yield forecasts (e.g., the EC monitoring and forecasting system) and climate change studies, it was never explicitly evaluated for transplanting, the most widespread rice establishment method especially in tropical and sub-tropical Asia. In this study, WARM was tested for its ability to reproduce nursery growth and transplanting shock, using data on direct sown and transplanted (both manual and mechanical) rice collected in 24 dedicated field experiments performed at eight sites in Jiangsu in 2011, 2012 and 2013. The agreement between measured and simulated aboveground biomass data was satisfactory for both direct sowing and transplanting: average R2 of the linear regression between observed and simulated values was 0.97 for mechanical transplanting and direct sowing, and 0.99 for manual transplanting. RRMSE values ranged from 5.26% to 30.89%, with Nash and Sutcliffe modelling efficiency always higher than 0.78; no notable differences in the performance achieved for calibration and validation datasets were observed. The new transplanting algorithm – derived by extending the Oryza2000 one – allowed WARM to reproduce rice growth and development for direct sown and transplanted datasets (i) with comparable accuracy and (ii) using the same values for the parameters describing morphological and physiological plant traits. This demonstrates the reliability of the proposed transplanting simulation approach and the suitability of the WARM model for simulating rice biomass production even for production contexts where rice is mainly transplanted.

In situ estimates of submesoscale horizontal eddy diffusivity across an ocean front

In the last decade, the rapid advancements in computational power have favored the development of high‐resolution numerical models capable of directly resolving small‐scale structures such as fronts and filaments. Such models have greatly improved our understanding of submesoscale dynamics. At the same time, the small dimensions and short duration of these structures still pose major challenges for small‐scale dedicated field experiments. For this reason, submesoscale studies from in situ observations are still relatively scarce and quantitative estimates of key physical parameters for high‐resolution numerical models, such as horizontal eddy diffusivity, are still lacking. This study presents a novel approach for computing in situ horizontal eddy diffusivity associated with frontal structures by combining cross‐front widths derived from surface thermosalinograph sections with stirring rates estimated from Lagrangian drifter trajectories. The method is applied to the measurements collected across a frontal structure observed in the western part of the Gulf of Lion during the Latex10 campaign (LAgrangian Transport EXperiment, 1–24 September 2010). A total of 76 estimates of eddy diffusivity were obtained for strain rates of 0.70 and 1.21 day−1 and front widths (horizontal scales) ranging between 1 and 4 km. The estimates are log‐normally distributed, with 70% of the values ranging between 0.4 and 5 m2 s−1. Further analysis based on high‐resolution simulations and remote sensed observations, as well as dedicated field experiments will help to assess the robustness of some of the assumptions at the base of the proposed approach, and to extend the results to different ocean regions.

Tropospheric water vapor from solar spectrometry and comparison with Jason microwave radiometer measurements

Atmospheric water vapor is a crucial factor in achieving highest accuracies in the field of space geodesy, particularly in the determination of height. A new instrumental approach to high‐precision determination of tropospheric water vapor has been investigated and realized in the construction of a mobile solar spectrometer. The optical approach as applied to the spectrometer allows the simultaneous measurement of single vibrational‐rotational absorption lines in the wide wavelength range between 728 nm and 915 nm. Dedicated field experiments have been carried out in the frame of an EU project for calibrating the Jason onboard microwave radiometer. A comparison of the spectrometer results with a conventional ground‐based water vapor radiometer and radiosondes revealed a fit on the order of 10 mm for the wet path delay, which corresponds to 1.6 kg/m2 integrated water vapor content (IWV). The comparison with the measurements from the radiometer of Jason reveals an even better agreement for the wet path delay. Long time series are foreseen for a detailed statistical analysis. The mobile solar spectrometer can be considered as a novel portable tool for determining tropospheric water vapor and as an excellent space‐borne radiometer calibration and validation system.

Preface to special section on Global Aerosol System

[1] This special issue brings together papers that were presented in a workshop of experts on the global aerosol system and its impacts on climate. Aerosols originate as urban haze and rural haze, and owing to long range transport of pollutants, becomes widespread aerosol layers across the globe. [2] Atmospheric aerosols play a dual role in the climate system and the hydrologic cycle (see reviews in the work of Ramanathan et al. [2001] and Kaufman et al. [2002]). Their first role is interaction with solar and thermal radiation: by scattering sunlight and reflecting a fraction of it back to space aerosols cool the climate system; by absorbing sunlight in the atmosphere aerosols further cool the surface but warm the atmosphere, changing the temperature and humidity profiles and the conditions for cloud development. [3] Aerosol particles play a second role by affecting the hydrologic cycle: serving as cloud condensation nuclei and ice nuclei, aerosols control the cloud droplet concentration and size. Higher number concentrations of submicron pollution aerosol tend to decrease the cloud droplet size and prevent or delay development of precipitation. Supermicron sea-salt particles can reintroduce precipitation and oppose the effect of pollution aerosol. These processes may cause changes in precipitation patterns to which the human civilization adapted during the last millennium, changes in cloud cover, and possible changes in the frequency of extreme events. [4] No less important is the aerosol effect on health, air quality, and agriculture that brought policy responses in Europe and North America in the last century and is affecting policy decisions in east and southern Asia today. Therefore advancements in aerosol science, together with research on the effect of greenhouse and trace gases and increased ability to predict the impact of human activity on the regional and global aerosol, can be expected to have direct impact on energy use and economic activities in nations around the world. [5] Unraveling these aerosol effects is still difficult because of the high spatial and temporal variability of sources, complex interaction of aerosol with atmospheric trace gases and clouds, and the short lifetime of aerosol of about a week, owing to wet and dry depositions. A single aerosol particle over the Mediterranean Sea can include anthropogenic sulfate and carbonaceous material from Europe, coating natural dust particle from Africa. [6] Today’s aerosol assessments of climate are based on aerosol chemical transport models and climate models that cannot yet resolve the aerosol interaction with clouds with their microscale physical and chemical exchanges. Models make good progress in capturing aerosol evolution, but source characterization is difficult. Accurate assessments of aerosol distribution and composition, the anthropogenic component, and the impacts on radiation and water cycle therefore require orchestrated observations of aerosol, precursor trace gases, clouds and precipitation from satellites, networks of surface-based instruments, laboratory studies, and dedicated field experiments. The observations require sufficient information content, accuracy, and global coverage to permit their integration into and testing of aerosol chemical transport, assimilation, and climate models [Intergovernmental Panel on Climate Change (IPCC), 2001]. [7] In the last decade, research of the global aerosol system was invigorated by an array of new measurements in several temporal and spatial scales, the fruits of which we start to see in this special issue, and the development of new models: [8] 1. Global, daily, and multidaily observations of aerosol, clouds, and their properties derived from satellite measurements of the scattered solar light, utilizing its spectral, angular, and polarization properties. [9] 2. Satellite measurements of the vertical distribution of aerosol, clouds, and precipitation using lidars and radars and thermal infrared and microwave radiometers. [10] 3. Surface networks that continuously measure in situ and remotely (both passive radiometers and active sensors) the aerosol physical and chemical properties, distribution of solar radiation, clouds, and precipitation. [11] 4. Intensive field campaigns that concentrate the measurement power in a given region for a limited period of time and help understand the aerosol sources and aerosol and cloud processes. [12] 5. Laboratory analysis of aerosol particles and measurements of aerosol processes. [13] 6. Development of transport models that start to mimic the aerosol distribution measured from satellite and surface networks. [14] 7. Assimilation of the aerosol models and measurements into climate models to study the effects of aerosol on JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, D10S01, doi:10.1029/2004JD005724, 2005

A satellite view of aerosols in the climate system.

Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.

Modeling stratified wave and current bottom boundary layers on the continental shelf

The Glenn and Grant [1987] continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated. The Reynolds fluxes for sediment mass and fluid momentum are closed using a continuous, time‐invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment‐induced stratification throughout the constant stress region. Glenn and Grant [1987] use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer. For typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations. Within the wave boundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sediment transport throughout the constant stress layer. Sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf. Comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth‐integrated sediment transport that can be 2 orders of magnitude less than, and time‐averaged shear velocities that can be nearly half of, that predicted by the neutral version. Sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm‐driven flows. Sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedicated field experiments to refine further estimates of these parameters.

Seismic Imaging of Mantle Plumes

The mantle plume hypothesis was proposed thirty years ago by Jason Morgan to explain hotspot volcanoes such as Hawaii. A thermal diapir (or plume) rises from the thermal boundary layer at the base of the mantle and produces a chain of volcanoes as a plate moves on top of it. The idea is very attractive, but direct evidence for actual plumes is weak, and many questions remain unanswered. With the great improvement of seismic imagery in the past ten years, new prospects have arisen. Mantle plumes are expected to be rather narrow, and their detection by seismic techniques requires specific developments as well as dedicated field experiments. Regional travel-time tomography has provided good evidence for plumes in the upper mantle beneath a few hotspots (Yellowstone, Massif Central, Iceland). Beneath Hawaii and Iceland, the plume can be detected in the transition zone because it deflects the seismic discontinuities at 410 and 660 km depths. In the lower mantle, plumes are very difficult to detect, so specific methods have been worked out for this purpose. There are hints of a plume beneath the weak Bowie hotspot, as well as intriguing observations for Hawaii. Beneath Iceland, high-resolution tomography has just revealed a wide and meandering plume-like structure extending from the core-mantle boundary up to the surface. Among the many phenomena that seem to take place in the lowermost mantle (or D″), there are also signs there of the presence of plumes. In this article I review the main results obtained so far from these studies and discuss their implications for plume dynamics. Seismic imaging of mantle plumes is still in its infancy but should soon become a turbulent teenager.

Size distribution and scattering phase function of aerosol particles retrieved from sky brightness measurements

Ground‐based measurements of the solar transmission and sky radiance in a horizontal plane through the Sun are taken in several geographical regions and aerosol types: dust in a desert transition zone in Israel, sulfate particles in Eastern and Western Europe, tropical aerosol in Brazil, and mixed continental/maritime aerosol in California. Stratospheric aerosol was introduced after the eruption of Mount Pinatubo in June 1991. Therefore measurements taken before the eruption are used to analyze the properties of tropospheric aerosol; measurements from 1992 are also used to detect the particle size and concentration of stratospheric aerosol. The measurements are used to retrieve the size distribution and the scattering phase function at large scattering angles of the undisturbed aerosol particles. The retrieved properties represent an average on the entire atmospheric column. A comparison between the retrieved phase function for a scattering angle of 120°, with phase function predicted from the retrieved size distribution, is used to test the assumption of particle homogeneity and sphericity in radiative transfer models (Mie theory). The effect was found to be small (20%±15%). For the stratospheric aerosol (sulfates), as expected, the phase function was very well predicted using the Mie theory. A model with a power law size distribution, based on the spectral dependence of the optical thickness, a, cannot estimate accurately the phase function (up to 50% error for λ = 0.87 μm). Before the Pinatubo eruption the ratio between the volumes of sulfate and coarse particles was very well correlated with α. The Pinatubo stratospheric aerosol destroyed this correlation. The aerosol optical properties are compared with analysis of the size, shape, and composition of the individual particles by electron microscopy of in situ samples. The measured volume size distributions before the injection of stratospheric aerosol consistently show two modes, sulfate particles with rm &lt; 0.2 μm and coarse particles with rm &gt; 0.7 μm. The “window” in the tropospheric aerosol in this radius range was used to observe a stable stratospheric aerosol in 1992, with rm ∼ 0.5 μm. A combination of such optical thickness and sky measurements can be used to assess the direct forcing and the climatic impact of aerosol. Systematic inversion for the key aerosol types (sulfates, smoke, dust, and maritime aerosol) of the size distribution and phase function can give the relationship between the aerosol physical and optical properties that can be used to compute the radiative forcing. This forcing can be validated in dedicated field experiments.