Assessing the performance of WRF model in predicting high-impact weather conditions over Central and Western Africa: an ensemble-based approach

Natural Hazards - For numerical weather prediction over a particular region, it is important to know the best combination of physical parameterizations available in the considered modelling frame...     

Flood forecasting based on an artificial neural network scheme

Natural Hazards - Nowadays, floods have become the widest global environmental and economic hazard in many countries, causing huge loss of lives and materials damages. It is, therefore, necessary...     

Are squall lines detected by NCEP-NCAR reanalyses?

Summary The aim of the paper is to show that NCEP-NCAR reanalyses do display modifications in the atmosphere at the occurrence of squall lines (SLs). 135 SLs were detected at Dakar (Senegal) during the summers of 1981–95. A compositing method was applied to temperature, specific humidity and zonal wind component at grid-point 15° N; 17.5° W close to Dakar, for the 135 SLs’ occurrences. The slight modifications of these three parameters as computed here proved to be statistically significant.     

Tourmalinization related to Late Proterozoic-Early Palaeozoic lode gold mineralization in the Bin Yauri area, Nigeria

Three types of tourmaline occurrence have been identified in the area of Bin Yauri, Nigeria, mesothermal lode-gold mineralization. These are: (1) stratabound tourmalinites in pelitic metasediments, (2) tourmaline in a hydrothermal alteration assemblage within hornfelsed wall rocks, (3) tourmaline in auriferous quartz and quartz-carbonate veins. Although the tourmaline occurrences are all within or close to the contact aureole of a granodiorite intrusion, geochemical characteristics of the tourmalines are broadly similar and reflect a common metasedimentary source. Two stages of tourmalinization are envisaged. The earlier (ca. 1100 Ma) involved syngenetic-diagenetic formation of tourmalinites, while the later (ca. 500 Ma) involved epigenetic (hydrothermal) tourmaline-gold mineralization, possibly derived by dehydration and devolatilization of metasedimentary sequences containing tourmaline-rich rocks or tourmalinites. Electron microprobe analyses indicate that the tourmalines are intermediate members of the schorl-dravite solid solution series. Plots involving FeO, MgO, and/or Al₂O₃ from these analyses are used to constrain the sources and processes of tourmalinization. Two metallogenic implications are derived from this study. One is that, although the tourmalinites are barren of gold and base metal mineralization, their occurrences nevertheless encourages exploration for syngenetic-exhalative massive sulphide deposits in the region. The other implication applies to the potential use of tourmaline in deciphering the physico-chemical conditions of gold-mineralizing fluids in the Bin Yauri area.     

The origin and significance of non-aqueous CO2 fluid inclusions in the auriferous veins of Bin Yauri,northwestern Nigeria

Non-aqueous CO₂ and CO₂-rich fluid inclusions are found in the vein quartz hosting mesothermal gold-sulphide mineralization at Bin Yauri, northwestern Nigeria. Although mineralizing fluids responsible for gold mineralization are thought to be CO₂-rich, the occurrence of predominantly pure to nearly pure CO₂ inclusions is nevertheless unusual for a hydrothermal fluid system. Many studies of similar CO₂-rich fluid inclusions, mainly in metamorphic rocks, proposed preferential loss (leakage) of H₂O from H₂O-CO₂ inclusions after entrapment. In this study however, it is proposed that phase separation (fluid immiscibility) of low salinity CO₂-rich hydrothermal fluids during deposition of the gold mineralization led to the loss of the H₂O phase and selective entrapment of the CO₂. The loss of H₂O to the wallrocks resulted in increasing oxidizing effects. There is evidence to suggest that the original CO₂-rich fluid was intrinsically oxidized, or perhaps in equilibrium with oxidizing conditions in the source rocks. The source of the implicated fluid is thought to be subducted metasediments, subjected to dehydration and devolatilization reactions along a transcurrent Anka fault/shear system, which has been described as a Pan-African (450–750 Ma) crustal suture.     

The Challenges of Modeling Climate Variability and Change in West Africa

This paper overviews observations and examines modeling issues associated with the mean state, climate variability and climate change in West Africa. The Tropical Rain Measuring Mission (TRMM) satellite allows for the first time estimates of Unconditional, Convective and Stratiform rain rates in West Africa. The 1998 estimated TRMM rates are compared to long-term observed rain rates and a merged rain data set (CMAP) during 1998. Further, the TRMM estimates are compared to the simulated rain rates from the Community Climate Model Version 3.6. The TRMM Precipitation Radar rain estimates are generally lower than either the long-term observations or the CMAP rates during 1998. Moreover, the TRMM rain estimates show a significant fraction of the total rain (convective + stratiform) is characterized as stratiform rain (30–40%). The CCM3 simulates primarily convective rain and negligible amounts of non-convective rain for West Africa. Furthermore, the TRMM high-resolution rain patterns strongly imply that rain in West Africa occurs on mesoscales in association with mesoscale convective systems (squall lines, mesoscale convective complexes and non-squall tropical clusters). We demonstrate this by briefly examining two mesoscale convective systems during May 1998 with METEOSAT data. Regional climate models may offer the best solution to understanding climate change in West Africa because of their ability to capture mesoscale systems and better their representation of orographic features. Adequate boundary conditions from Global Climate Models are still necessary for regional climate model simulations to successfully reproduce mean climate conditions and provide understanding with respect to future climate change. Observations in West Africa should be maintained or increased for monitoring climate variability and possibility of climate change in West Africa, proper initialization of numerical weather prediction models and the validation of climate models.