Conditional Wavelet Technique Applied to Aircraft Data Measured in the Thermal Internal Boundary Layer During Sea-Breeze Events

We describe a wavelet-based technique to determine the spectral turbulentcontribution to the vertical flux of sensible heat in a position-wavelength representation. This technique combines awavelet transform (Morlet wavelet) with conditional sampling. We apply this methodto aircraft datacollected during a sea-breeze circulation (BEMA97 experiment) with heterogeneousturbulence conditions horizontally and vertically as well. The turbulent fluxes are analysed with the conditional wavelet techniqueas a function of the wavelength and the horizontal distance.The turbulent processes within the thermal internal boundary layer associatedwith the sea breeze are clearly identified. The results exhibit the wavelength bands through which the upward flux (originating fromthe surface) and the downward flux (originating from the top of theboundary layer) are important.     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

Nitrogen compound emission from biomass burning in tropical African savanna FOS/DECAFE 1991 experiment (Lamto,Ivory Coast)

Gaseous nitrogen compounds (NO _x , NO _y , NH₃, N₂O) were measured at ground level in smoke plumes of prescribed savanna fires in Lamto, in the southern Ivory Coast, during the FOS/DECAFE experiment in January 1991. During the flaming phase, the linear regression between [NO _x ] and [CO₂] (differences in concentration between smoke plumes and atmosheric background) results volumic emission ratio [NO _x ]/[CO₂]=1.37×10^–3 with only slight differences between heading and backing fires. Nearly 90% of the nitrogen oxides are emitted as NO. Average emission ratios of other compounds are: 1.91, 0.047, and 0.145×10^–3 for NO _y , NH₃ and N₂O, respectively. The emission ratios obtained during this field experiment are compred with corresponding values measured during former experiments with the same plant species in combustion chambers. An accurate determination of both the biomass actually burned and of the plant nitrogen content, allows an assessment of emission fluxes of N-compounds from Guinean savanna burns. Preliminary results dealing with the influence of fire on biogenic emissions from soils are also reported.     

Influence of landscape and hydrological factors on stream–air temperature relationships at regional scale

Identifying the main controlling factors of the stream temperature (Tw) variability is important to target streams sensitive to climate and other drivers of change. The thermal sensitivity (TS), based on relationship between air temperature (Ta) and Tw, of a given stream can be used for quantifying the streams sensitivity to future climate change. This study aims to compare TS for a wide range of temperate streams located within a large French catchment (110,000 km²) using 4 years of hourly data (2008–2012) and to cluster stations sharing similar thermal variabilities and thereby identify environmental key drivers that modify TS at the regional scale. Two successive classifications were carried out: (a) first based on Ta–Tw relationship metrics including TS and (b) second to establish a link between a selection of environmental variables and clusters of stations. Based on weekly Ta–Tw relationships, the first classification identified four thermal regimes with differing annual Tw in terms of magnitude and amplitudes in comparison with Ta. The second classification, based on classification and regression tree method, succeeded to link each thermal regime to different environmental controlling factors. Streams influenced by both groundwater inflows and shading are the most moderated with the lowest TS and an annual amplitude of Tw around half of the annual amplitude of Ta. Inversely, stations located on large streams with a high distance from source and not (or slightly) influenced by groundwater inflows nor shading showed the highest TS, and so, they are very climate sensitive. These findings have implications for guiding river basin managers and other stakeholders in implementing thermal moderation measures in the context of a warming climate and global change.     

Influence of pit removal methods on river network position

Digital elevation models often contain depressions that result in areas described as having no drainage, referred to as sinks or pits. These depressions disrupt the drainage surface, which disrupt routing of flow over the surface. Most of the attributes that can be extracted from a digital elevation model rely on flow‐routing algorithms to calculate the upslope contributing area. There is little information on the influence of the various algorithms on the position and on the connectivity of the extracted networks. The aim of this study was to assess the effects of pit removal methods, data sources and flow‐routing algorithms on the position of river networks. The results show that all factors and methods have an impact on the position of the extracted networks. The pit removal method combining filling and carving extracted river networks closer to the reference, as well the elevation models with higher resolution. Single‐flow direction methods provided more accurate positioning of river network, in this test area where the drainage is generally well defined. Copyright © 2011 John Wiley & Sons, Ltd.     

The F4 tornado of August 3, 2008, in Northern France: Case study of a tornadic storm in a low CAPE environment

A strong tornado hit seven cities of northern France in the late evening of Sunday, 3 August 2008, causing severe damage along its 19 km path from Pont-sur-Sambre to Boussois. Three people were killed in the collapse of their house and 18 were injured. More than 1000 houses were damaged and several thousand trees were uprooted or fallen down.The authors led a damage survey in the hours that followed the disaster, then investigated this case, in order to determine the characteristics of this tornado precisely and to better understand the conditions that led to its formation. Weather radar analysis shows that the convective cell that gave rise to the tornado took on a fairly pronounced S-shaped structure, with a persistent mesocyclone in the central part of the convective system. The synoptic and mesoscale pattern associated with this severe storm was very dynamic, and characterized by a coupling between a low-level jet and a highly divergent jet-stream. The authors have reconstructed a vertical profile for this case study, in order to describe the tornadic environment precisely. The reconstructed profile reveals two main elements, namely an environment having a very modest vertical instability on one hand, and the presence of intense wind shear, notably in the lowest layers of the atmosphere on the other hand. This conclusion is supported by the analysis of many instability and shear parameters.     

Vulnerability and adaptation of African rural populations to hydro-climate change: experience from fishing communities in the Inner Niger Delta (Mali)

In this paper we examine ways Sahelian floodplain fishers have adapted to the strong environmental variations that have affected the region in the last two decades. We analyse their vulnerability and adaptive capacity in the face of expected changes in rainfall combined with the predicted effects of dam construction. Data from the Inner Niger Delta in Mali were used to show that fishers were highly sensitive to past and recent variations in the hydro-climatic conditions. Moreover, it appears their traditional livelihood strategies, although diversified, sophisticated and well suited to historical conditions, offer a limited set of options to adapt to increased environmental constraints. For fish-dependent households that have adopted a mixed set of activities through farming, the high seasonality and constraints characterizing both their main activities (fishing and farming) does not allow switching between activities. For those households that undertake seasonal fishing migrations, there is little opportunity to modify migration routes or find new settlements sites inside the delta because of the high population density in this area. In sum, although the adoption of diversified and spatially discrete patterns in livelihood activities is often presented as a strategy to reduce vulnerability, such a strategy does not appear sufficient to allow fishers of the delta to successfully face the increasing constraints associated with the changes in hydro-climatic conditions. In such a context, fishing communities will be driven towards more drastic strategies of adaptation and/or coping such as switching to new activities based on agricultural innovations or emigration from the delta. Both strategies present many hazards, particularly in the absence of supportive public policy.     

A simple heat-salt oscillator

A theoretical study is made of a simple mixed-layer model, in the form of a well-mixed constant-depth layer, forced from above by a heat flux k_T(T_A−T) and salinity flux k_S(S_A−S), where T_A and S_A are two reference values and T and S the temperature and salinity of the layer. The layer has a turbulent exchange of heat and salt with underlying water, kept at constant temperature and salinity, which is small in a statically stable case; large in a statically unstable case. If k_T>k_S, self-sustained oscillations may occur. In one cycle, a fast temperature rise, a slower salinity increase, and a final relaxation when the layer adjusts to the conditions of the underlying water, are observed.     

Investigation of the 600-km discontinuity under France through travel-time and amplitude anomalies

In this paper, we give a model of the 600-km discontinuity under France, which explains large travel-time and amplitude anomalies on teleseisms as recorded in the French network. It features a large descent from the Ardennes to the Pyrénées, of which tectonic implications are discussed.     

Relative performance of future altimeter systems and tide gauges in constraining a model of North Sea high-frequency barotropic dynamics

We evaluate in this paper the ability of several altimeter systems, considered separately as well as together with tide gauges, to control the time evolution of a barotropic model of the North Sea shelf. This evaluation is performed in the framework of the particular model errors due to uncertainties in bathymetry. An Ensemble Kalman Filter (EnKF) data assimilation approach is adopted, and observing-systems simulation experiments (OSSEs) are carried out using ensemble spread statistics. The skill criterion for the comparison of observing networks is, therefore, not based on the misfit between two simulations, as done in classic twin experiments, but on the reduction of ensemble variance occurring as a consequence of the assimilation. Future altimeter systems, such as the Wide Swath Ocean Altimeter (WSOA) and satellite constellations, are considered in this work. A single WSOA exhibits, for instance, similar performance as two-nadir satellites in terms of sea-level correction, and is better than three satellites in terms of model velocity control. Generally speaking, the temporal resolution of observations is shown to be of major importance for controlling the model error in these experiments. This result is clearly related to the focus adopted in this study on the specific high-frequency response of the ocean to meteorological forcing. Altimeter systems lack adequate temporal sampling for properly correcting the major part of model error in this context, whereas tide gauges, which provide a much finer time resolution, lead to better global statistical performance. When looking into further detail, tide gauges and altimetry are demonstrated to exhibit an interesting complementary character over the whole shelf, as tide gauge networks make it possible to properly control model error in a ∼100-km coastal band, while high-resolution altimeter systems are more efficient farther from the coast.