Optimal nudging strategies in regional climate modelling: investigation in a Big-Brother experiment over the European and Mediterranean regions

The objective of this work is to gain a general insight into the key mechanisms involved in the impact of nudging on the large scales and the small scales of a regional climate simulation. A “Big Brother experiment” (BBE) approach is used where a “reference atmosphere” is known, unlike when regional climate models are used in practice. The main focus is on the sensitivity to nudging time, but the BBE approach allows to go beyond a pure sensitivity study by providing a reference which model outputs try to approach, defining an optimal nudging time. Elaborating upon previous idealized studies, this work introduces key novel points. The BBE approach to optimal nudging is used with a realistic model, here the weather research and forecasting model over the European and Mediterranean regions. A winter simulation (1 December 1989–28 February 1990) and a summer simulation (1 June 1999–31 August 1999) with a 50 km horizontal mesh grid have been performed with initial and boundary conditions provided by the ERA-interim reanalysis of the European Center for Medium-range Weather Forecast to produce the “reference atmosphere”. The impacts of spectral and indiscriminate nudging are compared all others things being equal and as a function of nudging time. The impact of other numerical parameters, specifically the domain size and update frequency of the large-scale driving fields, on the sensitivity of the optimal nudging time is investigated. The nudged simulations are also compared to non-nudged simulations. Similarity between the reference and the simulations is evaluated for the surface temperature, surface wind and for rainfall, key variables for climate variability analysis and impact studies. These variables are located in the planetary boundary layer, which is not subject to nudging. Regarding the determination of a possible optimal nudging time, the conclusion is not the same for indiscriminate nudging (IN) and spectral nudging and depends on the update frequency of the driving large-scale fields τ _a . For IN, the optimal nudging time is around τ = 3 h for almost all cases. For spectral nudging, the best results are for the smallest value of τ used for the simulations (τ = 1 h) for frequent update of the driving large-scale fields (3 and 6 h). The optimal nudging time is 3 for 12 h interval between two consecutive driving large-scale fields due to time sampling errors. In terms of resemblance to the reference fields, the differences between the simulations performed with IN and spectral nudging are small. A possible reason for this very similar performance is that nudging is active only above the planetary boundary layer where small-scale features are less energetic. As expected from previous studies, the impact of nudging is weaker for a smaller domain size. However the optimal nudging time itself is not sensitive to domain size. The proposed strategy ensures a dynamical consistency between the driving field and the simulated small-scale field but it does not ensure the best “observed” fine scale field because of the possible impact of incorrect driving large-scale field. This type of downscaling provides an upper bound on the skill possible for recent historical past and twenty-first century projections. The optimal nudging strategy with respect to dynamic downscaling could add skill whenever the parent global model has some level of skill.     

Stress models for Tharsis formation, Mars

A critical survey is presented of most stress models proposed for the formation of the tectonic structures in the Tharsis volcano-tectonic province on Mars and provides new constraints for further models. First papers, in the 1970s, attempted to relate the Tharsis formation to asthenospheric movements and lithosphere loading by magma bodies. These processes were then quantified in terms of stress trajectory and magnitude models in elastic lithosphere (e.g. Banerdt et al., J. Geophys. Res. 87(B12), 9723–9733, 1982). Stresses generated by dynamic lithosphere uplift were rapidly dismissed because of the poor agreement between the stress trajectories provided by the elastic models and the structural observations. The preferred stress models involved lithosphere loading, inducing isostatic compensation, and then lithosphere flexure. Some incomsistency with structural interpretation of Viking imagery has been found. In the early 1990s, an attempt to solve this problem resulted in a model involving the existence of a Tharsis-centred brittle crustal cap, deteched from the strong mantle by a weak crustal layer (Tanaka et al., J. Geophys. Res. 96(E1), 15617–15633, 1991). Such a configuration should produce loading stresses akin to those predicted by some combination of the two loading modes. This model has not been quantified yet, however it is expected to reconcile stress trajectories and most structural patterns. Nevertheless, some inconsistencies with observed structures are also expected to remain. Parallel to this approach focused on loading mechanisms, the idea that volcanism and tectonic structures could be related to mantle circulation began to be considered again through numerical convection experiments, whose results have however not been clearly correlated with surface observations. Structural clues to early Tharsis dynamic uplift are reported. These structures have little to do with those predicted by elastic stress modelling of dynamic lithosphere uplift. They denote the existence of unsteady stress trajectories responsible for surface deformations that cannot be readily predicted by elastic models. These structures illustrate that improving current stress models for Tharsis formation shall come from deeper consideration of rock failure criterion and load growth in the lithosphere (e.g. Schultz and Zuber, J. Geophys. Res. 99(E7), 14691–14702, 1994). Improvements should also arise from better understanding rheological layering in the lithosphere and its evolution with time, and from consideration of stress associated to magma emplacement in the crust, which may have produced many tectonic structures before loading stress resulting from magma freezing became significant (Mège and Masson, Planet. Space Sci. 44, 1499–1546, 1996a).     

Results from the Eso Large Program on Transneptunian Objects and Centaurs

BVRI photometry of 107 TNOs and Centaurs establishes the range of spectral gradients to be between –5 to 55%/100 nm (with one exception). A cluster of very red Cubewanos is firmly identified in orbits of low inclination and eccentricity beyond 40 AU from the Sun. Further correlations between surface colours and dynamical parameters (inclination and perihelion distance) are suggested for Cubewanos and scattered disk objects, but lack complete confidence for their reality. Plutinos and Centaurs do not show any clear correlation between surface colours and orbital parameters. We present in this paper 12 spectra obtained in the visible region and nine of them for which we obtained also near infrared spectra up to 2.4 microns. A few other objects have been observed, but the data are still under reduction and analysis. The principal reported results obtained are: (i) a wide range of visible slopes; (ii) evidence for surface variations on 2001 PT₁₃; and (iii) possible detection of few percent of water ice (1999 TC_36}, 2000 EB₁₇₃, 1999 DE₉, 2001 PT₁₃, 2000 QC₂₄₃, 1998 SG₃₅).     

High resolution spectra of galaxies

We present inversion techniques which aim at recovering the composite nature and the kinematics of a stellar population from its high resolution absorption line spectrum. The originality of the combined inversion is its potential to recover both the stellar content and the kinematics simultaneously. These techniques use new synthetic high resolution spectra produced by PéGASE and minimization algorithms. We apply them to mock data representing the bulge and disk population of the inner region of spiral galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

6. Environmental Context

On Earth, the Archaean aeon lasted from 4.0 to 2.5 Ga; it corresponds to a relatively stable period. Compared with today, internal Earth heat production was several times greater resulting in high geothermal flux that induced the genesis of rocks such as komatiites and TTG suites, which are no more generated on Earth since 2.5 Ga. Similarly, the details of plate tectonic modalities (plate size, plate motion rate, plate thickness, tectonic style, irregular crustal growth, etc...) were different of modern plate tectonics. Both atmosphere and ocean compositions have been progressively modified and the greater heat production favoured the development of hydrothermalism and therefore created niches potentially favourable for the development of some forms of life. Catastrophic events such as giant meteorite falls or world-sized glaciations drastically and suddenly changed the environment of Earth surface, thus being able to strongly affect development of life. Even if specialists still debate about the age of the oldest indubitable fossil trace of life, Archaean can be considered as having been extremely favourable for life development and diversification.     

Fluvial sedimentation in a salt‐controlled mini‐basin: stratal patterns and facies assemblages,Sivas Basin,Turkey

The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo‐Miocene basin that contains numerous salt‐walled mini‐basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono‐stratigraphic study of the Emirhan mini‐basin and its 2·6 km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized – playa‐lake, braided stream and lacustrine – each corresponds to a relatively long‐lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape‐fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini‐basin records the remobilization of diapir‐derived detritus and the presence of evaporitic bodies interbedded within the mini‐basin, implying the growth of salt walls expressed at the surface as palaeo‐topographic highs. The mini‐basin also records the signature of a regional change in stratigraphic assemblage, passing from playa‐lake facies to large‐scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini‐basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir‐derived detritus intercalated within the otherwise clastic‐dominated succession.     

Observations of Saturn's outer ring and new satellites during the 1980 edge-on presentation

Observations of Saturn's satellites and external rings during the 1980 edge-on presentation were obtained with a focal coronograph. A faint satellite traveling in the orbit of Dione and leading it by 72° has been detected, together with the two inner satellites already suspected (cf. J. W. Fountain and S. M. Larson, 1978,Icarus36, 92–106). The external ring has been observed on both east and west sides; it may extend up to $\text{?8.3}$ Saturn radii, and appears structured.     

Integrated Onshore‐Offshore Investigation of a Mediterranean Layered Coastal Aquifer

Most of the Mediterranean coastal porous aquifers are intensively exploited. Because of climatic and anthropogenic effects, understanding the physical and geological controls on groundwater distribution and flow dynamics in such aquifers is crucial. This study presents the results of a structural investigation of a system located along the coastline of the Gulf of Lions (NW Mediterranean). A key aspect of this study relies on an onshore‐offshore integrated approach combining outcrops, seismic profiles, and borehole data analysis. This multidisciplinary approach provides constraints on pore‐fluid salinity distribution and stratigraphic organization, which are crucial in assessing the modes of groundwater/seawater exchanges. Onshore, Lower Pliocene deposits dip gently seaward. They are unconformably overlain by Holocene clays in the lagoons. Offshore the Pliocene deposits either outcrop at the seabed or are buried below nonconsolidated sands infilling paleo‐valleys. Beneath the lido, the groundwater salinity distribution consists of salty pore water, overlying fresher pore water. Active circulation of groundwater masses is inferred from the geophysical results. In particular, offshore outcrops and paleo‐valleys may play an important role in salt water intrusion.