Dynamical downscaling of temperature variability over Tunisia: evaluation a 21-year-long simulation performed with the WRF model.

8 pagesInternational audienceThis study evaluates the capabilities of the Weather Research and Forecasting (WRF) model to reproduce the space-time variability of near-surface air temperature over Tunisia. Downscaling is based on two nested domains with a first domain covering the Mediterranean Basin and forced by 21 years of ERA-Interim reanalysis (1991-2011), and a second domain (12 km spatial resolution) centered on Tunisia. Analyses and comparisons are focused on daily average (Tavg), minimum (Tmin) and maximum (Tmax) near-surface air temperatures and are carried out at the annual and seasonal timescales. WRF results are assessed against various climatological products (ERA-Interim, EOBS and a local network of 18 surface weather stations). The model correctly reproduces the spatial patterns of temperature being significantly superimposed with local topographic features. However, it broadly tends to underestimate temperatures especially in winter. Temporal variability of temperature is also properly reproduced by the model although systematic cold biases mostly concerning Tmax, reproduced throughout the whole simulation period, and prevailing during the winter months. Comparisons also suggest that the WRF errors are not rooted in the driving model but could be probably linked to deficiencies in the model parameterizations of diurnal/nocturnal physical processes that largely impact Tmax / Tmin

Spatial analysis of within-vineyard grapevine sensitivity to solar irradiance patterns. A case study using Pinot noir in a complex terrain

Communication orale, texte intégralWe analyze how the patterns of the solar irradiance affect the growth of Pinot noir grapevines at stand-scale over a hilly terrain bounded by a coniferous forest. Results show that the patterns of irradiance deficit are the primary driving force of the growth. Linear trends display good correlation between the total deficit of irradiance, and the developed growth index (R2 up to 0.96 ). This result also suggests the impact of low minimum temperature due to cold air drainage and stagnation. Results lead to propose a model able to capture at stand-scale 90% of the whole spatial variability of the growth. The model accounts for the diffuse component of the deficit of irradiance and the squared temperature anomalies. Combining such models through the use of GIS enables to better understand the driving mechanisms of grapevine growth over complex terrain, towards a better management of the “terroir” potential whithin climate change

Projected impacts of climate change on viticulture over French wine regions using downscaled CMIP6 multi-model data

Climate change is a major challenge for the French wine industry. Climatic conditions in French vineyards have already changed and will continue to evolve impacting viticulture. This study aims to analyse the evolution of agro- and eco-climatic indices based on phenology simulation of French wine-growing regions. This evolution was analysed on a recent-past period (1962–1991 to 1992–2021) using SAFRAN climate data and on a future projected period (1985–2014 to 2041–2070) with two SSP trajectories (SSP2-4.5 and SSP5-8.5). A set of 19 CMIP6 climate models downscaled at 8 km grid resolution over France coupled with three phenological and a water balance model were used. Phenological model parameters and training system characteristics were adapted to each region to match as much as possible current practices. Temperatures during the growing season have increased by +1 °C to +2.1 °C since the second half of the 20th century and could rise to +3.7 °C in regions around the Mediterranean by 2070. The inter-model variance concerning the precipitation is high, a significant change (decrease) in precipitation during the grapevine growing season is observed only for the regions of western France (Oceanic climate) over the period 2040–2071 with the SSP5 trajectory. All simulated phenological stages have shifted toward earlier dates. Their occurrence should be even earlier by 2070 with an average advance of up to 22 days for the mid-veraison of Pinot noir in eastern France. The theoretical maturity date (sugar content) should also be advanced from 19 to 30 days depending on the considered region and SSP. Thermal conditions closer to the photosynthetic optimum should promote onset by the early second half of the 21st century. The increase in both the number of hot days and grapevine water deficit during the period of fruit development should impact grape production in quality and quantity in all wine-growing regions. Spring frost projections show no significant change in risk for the second half of the 21st century, compared to current conditions

Estimation de la ressource forestière par télédétection radar

International audienc

Modelling solar radiation patterns for stand-scale studies in complex terrain

International audienceA spatial model has been developed to simulate stand-scale patterns of irradiance over an hilly terrain bounded by a forest. Conceptually, the model is based on semi-empiric radiation equations and efficient viewshed algorithm further improved to account for sloping terrain and the presence of the forest. Model performance was assess on both radiometry and geometry. Comparison shows good agreement with observations. Model experiments were conducted for cloudy-less condition (i.e. at potential) along whole vegetative period of grapevine stand. Anal- ysis of deficit of irradiance for year 2003 shows that patterns are mainly due to the presence of the forest implying strong spatial gradient with cumulated deficit that reach value up to 2900MJ/m². Trends show more contrasted spatial than temporal behaviours with higher deficit located on the upper part of the stand. If close to forest, deficit is mainly due to direct component drop, low-lying part of the stand have direct component rapidly balanced (5m) by diffuse component. While for upper part the balance is only attained between 20m and 30m. The results demonstrate that the model is well-suited to analyse topoclimate on crop production

Estimation de paramètres de la ressource forestière par télédétection radar à synthèse d'ouverture : apport du couplage d'un modèle architectural de plantes avec un modèle de diffusion électromagnétique

Mapping forest parameters such as aerial biomass is important for sustainable forestmonitoring, and highiight forest ecosystems functioning. To this aim, SAR data offer goodpotentialities due to their use day and night independently of cloud cover. Cross polarized HV lowfrequency (P- and L-band) backscatter showed good sensitivity to forest biomass up to 200-300m3[ha. Most of past studies were realized over “simple” forest medium (homogeneous coniferousplantations over flat terrain) and demonstrated that additionai works are needed to take intoaccount architectural tree properties and topographic effects on forest SAR backscatter. Towardsthe use of various SAR in various forest conditions, a better understanding of the bio-physical linkbetween SAR backscatter and forest parameters is needed by using a theoretical electromagneticmodeiling approach. However, a reajistic and complete canopy architectural description for ailscatterers (trunk, branches, needles or leaves) as a function of growth stage is lacking. An alternative isto deveiop a new approach using the architectural plant model cailed AMAP developed by CIRAD toderive input parameters to theoretical modeis.The first goal of this thesis cailed “direct problem” is to improve the interpretation of waveniatter interaction, using a coupling approach of the vector radative transfer model developed byMIT!CESBIO with the AMAP model. The second goal cailed “inverse problem” is to obtainbiomass map using SAR data, AMAP or driving AMAP by parameters obtained using SAR data orother sources through a GIS. In ail cases, effects of terrain siope must be addressed and removedusing a SAR post-processing chain developed at LCT.The forested area under study is mainly composed of homogeneous forest stands ofAustrian pine plantations located in the central part of Lozère departement (south of France)which offer a large range of topographic situations. Various spaceborne SAR data (ERS, JERS andSIR-C) have been acquired over the test site. A GIS-based methodology was developed towardsthe analysis of radar backscatter for different growth stages and topographics situations. On theother hand, the forest canopy description based on the AMAP model was implemented into a newinterface called AMAP2SAR.Highest sensitivity of corrected backscatter with forest parameters related to abovegroundbiomass was achieved at L-HV (55°) for bole volume up to 150 m3/ha. Sensitivity was foundsimilar to the flat terrain case, after applying specific radiometric corrections using a semiempirical angular correction model. Moreover, modeling resuits indicates the need to take intoaccount the vertical variability within the canopy by using the AMAP model wich was lackingbefore. Else, the AMAP2SAR flexibility providing information on any part of the canopy appear tobe a powerful tool to interpret forest SAR backscatter. Thus, the link between L-HV backscatterand primary branches density and diameter, wich are correlated through allometric equations toforest classical parameters such as trunk density and basal area and thus with bole volume(m3/ha) was explicited. Also, the modelling approach helped us to refine and validate the semiempirical angular correction model.Finaily, forest parameters inversion using SAR data, AMAP, or their coupling through aGIS are presented. Results show the potentialities and limits in using such coupled tools andleaded us to discuss the potentials of these biomass retrieval methods.La possibilité de spatialiser certains paramètres forestiers comme la biomasse sur pied représente un enjeu significatif pour la gestion raisonnée de la ressource forestière, et unemeffleure compréhension du fonctionnement des écosystèmes forestiers. A cette fin, l’imagerie parradar à synthèse d’ouverture (RSO), en plus de son utilisation tout temps, a montré ces dernièresannées de bonnes potentialités par la sensibilité de la rétrodiffusion basses fréquences (P et L) enpolarisation croisée (HV) aux paramètres de la masse ligneuse, jusqu’à des volumes de l’ordre de200-300 m3fha. Ces études, réalisées sur des milieux forestiers simples (essentiellementplantations de résineux) ont démontré dans une certaine mesure que l’architecture de l’arbreainsi que la topographie sont des paramètres importants à prendre en compte, nécessitanttoutefois des études complémentaires. Plus globalement, la généralisation à des conditionsforestières variées, nécessite une meilleure compréhension du lien biophysique entre l’écho radaret les paramètres recherchés, lien perturbé par la topographie, grâce à une approche demodélisation de la diffusion électromagnétique des ondes par le couvert. Toutefois, ce typed’approche bute sur l’absence de description précise du milieu forestier en termes géométrique etarchitectural des diffuseurs élémentaires (feuilles, branches, troncs). Ainsi, les récents modèlesarchitecturaux de croissance des plantes du type AMAP (Atelier de Modélisation de l’Architecturedes Plantes du CIRAD) s’avèrent intéressants à cette fin.Le premier objectif, de type problème direct, vise à affiner la compréhension du lienphysique entre la rétrodiffusion radar et les paramètres forestiers, à l’aide d’une démarcheoriginale de couplage du modèle AMAP avec le modèle de transfert radiatif vectoriel duMIT/CESBIO. Un deuxième objectif, de type problème inverse, est d’évaluer la faisabilitéd’estimation spatialisée de paramètres forestiers à l’aide de données RSO seules ou en synergieavec le modèle AMAP, intégrés dans un SIG. Dans les deux cas, les perturbations dues au reliefsont abordées explicitement, et nécessite la mise en oeuvre et la validation de post-traitementsdéveloppés récemment au LCT.L’étude est menée sur un site forestier “simple” de parcelles monospécifiques équiennes depin noir d’Autriche, localisé en Lozère et situé dans différentes configurations topographiques oùdes données RSO variées (ERS, JERS et surtout SIR-C) ont été acquises. La méthodologie estbasée sur un SIG permettant la mise en correspondance des données expérimentales (RSO, MNT,paramètres forestiers). Elle intègre également le développement et le calage du modèle AMAPpour le pin noir, et d’une nouvelle interface dénommé AMAP2SAR, permettant d’accéder à unecaractérisation architecturale réaliste du couvert forestier.Les résultats en bande L et en particulier en polarisation HV montrent une bonnesensibilité aux paramètres forestiers de la masse ligneuse, pour des volumes de fûts jusqu’à 150m3fha. Cette sensibilité est très similaire à celle observée auparavant en terrain plat, aprèsl’application d’une phase de correction des perturbations radiométriques dûs au relief, impliquantle développement d’un modèle semi-empirique de correction angulaire. Par la suite, lamodélisation de la rétrodiffusion prenant en compte la variabilité verticale dans le couvert grâce àAMAP a montré les faiblesses de l’hypothèse d’homogénéité verticale utilisée jusqu’à maintenantpar les modèles électromagnétiques. Ceci a permis de mieux mettre en évidence le lien de larétrodiffusion de volume dominante pour la configuration L-HV avec la densité et le diamètre desbranches primaires, paramètres bien corrélés allométriquement à des paramètres plus classiquesdu couvert comme la densité et la section des fûts, et donc du volume à l’hectare des fûts. L’impacttopographique a également été modélisé ce qui a permis de valider le modèle semi-empirique decorrection angulaire précité.Finalement, des résultats d’inversion de paramètres forestiers à partir des données radar,du modèle AMAP ou des deux couplés dans un SIG sont présentés. L’ensemble de ces résultatspermet de cerner les limites de ce type d’approche et de discuter des perspectives d’applications etdes travaux à mener à cette fin