The Local Geoid Model of Cameroon: CGM05

This paper deals with the geoid determination in Cameroon by a gravimetric solution. A number of data files were compiled for this work, containing about 62,000 points on land and ocean areas and also including data derived from satellite altimetry. A hybrid global geopotential model (EGM-GGM) supplied the longer wavelength components of this geoid model, CGM05. This global model is obtained by adjusting the GRACE model GGM02C to degree and order 360 using the harmonic coefficients of the model EGM96 beyond the maximal degree 200 of GGM02C. The medium wavelength components were computed using the best gridded residual gravity anomalies, by integration in Stokes’ formula. The digital terrain model GLOBE contributed to its short wavelength components. The residual terrain model (RTM) was applied to first determine a quasi-geoid model. This intermediate surface was converted to the geoid using a grid of simple Bouguer gravity anomalies. The validation of CGM05 is based on comparisons to global and regional geoids. A GPS/levelling geometric geoid computed in a small part of the target area shows that the absolute accuracy of this local geoid model is 14 cm. After a four-parameter fitting to the GPS/levelled reference surface, this absolute accuracy reduced to 11 cm

Accurate gravity anomaly interpolation: a case-study in cameroon, central africa

Many treatments in geodesy and geophysics require regularly gridded gravity anomalies. The gridding of gravity data needs interpolation. For the predicted data to be accurate, the smoothest type ofgravity anomaly should be used along with the most indicated prediction method. This paper presents the comparison of various prediction methods applied on different types of gravity anomalies andconsidering the relative geological complexity of the study area. Many algorithms are tested and the suitability of each type of anomaly and each prediction method discussed in a case-study in Cameroon (Central Africa), using a set of 43,000 gravity data points to determine the must accurate prediction technique

An application of Audiomagnetotelluric prospecting method to determine the dip of the sedimentary-metamorphic contact

From the geometric mean values of apparent resistivity, iso-resistivity contour maps were drawn using audiomagnetotellurics data collected along two profiles traversing the metamorphic and sedimentary formations. On these maps, resistivity discontinuity observed along the contour lines indicated the existence in this area of a contact between two blocks of different lithologies. By drawing tangents along a specific contour line (20 Ωm) which is considered as reference separating the two formations, the dips of down dropping under each station were calculated. The values of dip in the sedimentary formation, which have maximum values of 10 ˚ at a depth of 1400 m and 17 ˚ at a depth of 800 m, I

Programme Fortran 77 d\'ajustement du modèle de géoïde EGM96 sur la surface de référence de points GPS/nivelés.

Les altitudes recherchées lors du nivellement sont mesurées par rapport à une surface de référence : le géoïde. Le nivellement géométrique est un processus long et onéreux, on préfère désormais procéder par nivellement GPS (Global Positioning System). La surface de référence est alors réalisée en ajustant le modèle de géoïde dont on dispose sur la référence obtenue d'un ensemble de points GPS/nivelés. Les modèles de géoïde locaux ne sont pas encore disponibles dans toutes les régions. Un modèle de géoïde global peut alors être utilisé. Cet article présente un programme exécutable sur un environnement Fortran, qui permet d'ajuster le modèle de géoïde global le plus utilisé EGM96 par moindres carrés dans une région, à partir d'une modélisation par régression linéaire. Il fournit en sortie tous les résultats dans un fichier texte. Le programme crée également une grille de géoïde ajusté dans laquelle l'utilisateur peut interpoler l'ondulation ajustée en un nouveau point où il vient de faire des mesures GPS. Les résultats d'un test à partir des données sur le Cameroun montrent les résidus escomptés, avec un écart-type 0,08 m. Ce programme présente ces avantages qu'il utilise la valeur exacte de l'ondulation EGM96 en chaque point du fichier d'entrée de l'utilisateur et a un temps d'exécution optimal quel que soit le nombre de points pris en compte.Heights obtained from levelling are determined above a reference surface: the geoid. Spirit levelling is a timeconsuming high cost process. An alternative leveling process by GPS (Global Positioning System) is now used instead. The reference surface is therefore realised by fitting the available geoid model to the surface obtained from a set GPS/levelled points. However, local geoid models are still not available in many parts of the world. A local geoid can be replaced by a global one to achieve this. The program presented in this paper is executable on a Fortran surrounding and it permits to fit the must widely used global geoid EGM96 in the least squares sense in a region, from a linear regression modelling. The results are presented in an output text file and an output grid where the user can interpolate the adjusted geoid undulation at a point where he made new GPS measurements. An experimental test obtained form data in Cameroon produced reasonable residuals, with a standard deviation of 0.08 m. Some advantages of using this program are that it computes the exact value of the EGM96 geoid undulation at all points of the user's input file and its execution is very fast, regardless the number of input data points. Keywords: Ajustement, nivellement, géoïde, moindres carrés, interpolation; Fitting, levelling, geoid, least squares, interpolation. Journal des Sciences Pour l\'Ingénieur. Vol. 9 2008: pp. 11-2

Crustal thickness beneath Atlas region from gravity, topographic, sediment and seismic data

202009 bcmaVersion of RecordPublishe

Process-oriented assessment of RCA4 regional climate model projections over the Congo Basin under 1.5. C and 2. C global warming levels: influence of regional moisture fluxes

Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin (CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model (RCM) RCA4, driven by eight General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for the 1.5 ∘C and 2 ∘C global warming levels (GWLs), and under the representative concentration pathways (RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons (March–May and September–November), the rain-belt is misplaced southward (northward) in December–February (June–August), reducing the latitudinal extent of rainfall. Moreover, ERA-Interim reanalysis driven RCM simulation and RCM–GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper-tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March–May than in September–November, and larger for global warming of 2.0 ∘C than at 1.5 ∘C. There is an increase of zonal moisture divergence fluxes in upper atmospheric layers (>700hPa) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of 0.5 ∘C will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro-power generation, sanitation and ecosystems

Process-oriented assessment of RCA4 regional climate model projections over the Congo Basin under 1.5. C and 2. C global warming levels: influence of regional moisture fluxes

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin (CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model (RCM) RCA4, driven by eight General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for the 1.5∘C and 2∘C global warming levels (GWLs), and under the representative concentration pathways (RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons (March–May and September–November), the rain-belt is misplaced southward (northward) in December–February (June–August), reducing the latitudinal extent of rainfall. Moreover, ERA-Interim reanalysis driven RCM simulation and RCM–GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper-tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March–May than in September–November, and larger for global warming of 2.0∘C than at 1.5∘C. There is an increase of zonal moisture divergence fluxes in upper atmospheric layers (>700hPa) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of 0.5∘C will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro-power generation, sanitation and ecosystems