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...     

Evaluation of probabilistic precipitation forecast determined from WRF forecasted amounts

This paper examines the connection between the probability of precipitation and forecast amounts from Weather Research and Forecasting (WRF) model runs over Central and West Africa. A one season period (June–September 2010) was used to investigate the quantitative precipitation forecast–probability relationship. The predictive capability of this relationship was then tested on an independent sample of data (June–September 2011); 2010 and 2011 were wet and dry years, respectively. The results show that rainfall is less likely to occur in those areas where the model indicates no precipitation than it is elsewhere in the domain. Rainfall is more likely to occur in those regions where precipitation is predicted, especially where the predicted precipitation amounts are largest. The probabilities of rainfall forecasts based on this relationship are found to possess skill as measured by relative operating characteristic curves, reliability diagrams, and Brier skill scores. Skillful forecasts from the technique exist throughout 24-h periods for which WRF output was available. The results suggest that this forecasting tool might assist forecasters throughout the season in a wide variety of weather events and not only in areas of difficult-to-forecast convective systems.     

Mineralogy and geochemical behaviour during weathering of greenstone belt under tropical dry conditions in the extreme North Cameroon (Central Africa)

Mineralogy, major, trace and rare earth elements of a weathering profile developed on tertiary greenstone belt in the extreme North Cameroon are reported. The aim of which was to investigate mineralogical evolution and element mobilization and redistribution during weathering under dry tropical climate. The weathering profile consists of four main horizons: (1) a spheroidal weathering zone constituted by a corestone–shell complex, (2) a C horizon, (3) a Bw horizon and an Ah horizon. The results indicate that nontronite, a Fe-rich smectite, is the exclusive clay mineral formed in the exfoliated shells and the C horizon. It is associated with kaolinite in the upper horizons. The coexistence of these two clay minerals induced a decrease of CEC and pH which becomes neutral. The weathering index (WI) values reveal that weathering becomes more and more intensive from the corestone up to Bw horizon, which is the most weathered horizon in the weathering profile. Mass balance calculations, using Th as immobile element, indicate that Ti is quite mobile and that Al and Fe are relatively enriched at the bottom and strongly leached at the top of the profile. Alkalis and alkaline earth elements are strong leached through out the profile, except Ca which displays similar trend as Al and Fe. The same goes for LILE (Cs, Sr), TTE (Cr, Co, Ni) and HSFE (Y, Nb, Hf). In opposite, REE are depleted at the bottom and enriched in the upper horizons, with more enrichment for LREE than for HREE. It appears that weathering of greenstone belt causes a fractionation of HREE and induces a concentration of LREEs. Ce and Eu anomalies display opposite behaviour.     

Validation of general circulation climate models and projectionsof temperature and rainfall changes in Cameroon and someof its neighbouring areas

Summary  Four coupled atmosphere-ocean general circulation models were examined for the ability of their control runs to simulate present climate given present forcings. The area of study is mainly Cameroon and some of its surrounding areas (0–25° E, 5° S-30° N). These models are from the UK Meteorological Office Hadley Centre (HadCM2), the German Max-Planck-Institut für Meteorologie (ECHAM4), the Canadian Centre for Climate Modelling and Analysis (CGCM1) and the Australian Commonwealth Science and Industrial Research Organisation (CSIRO-Mk2). The ability of the models to reproduce the observed spatial and temporal patterns was studied. ECHAM4 and HadCM2 were found to reproduce the spatial pattern well, with a correlation of more than 90%. They also simulated the main annual features of both temperature and rainfall. The CSIRO-Mk2 model was slightly less successful and the CGCM1 had the worst results for the area, especially as concern rainfall. In view of these results, ECHAM4 and HADCM2 were used to evaluate projected changes in rainfall and temperature resulting from increased concentration of greenhouse gases in the atmosphere for the 30 year period 2040 to 2070. Received February 15, 1999/Revised March 10, 2000     

Sensitivity of the simulated African monsoon of summers 1993 and 1999 to convective parameterization schemes in RegCM3

In this study, the International Center for Theoretical Physics Regional Climate Model version 3 (RegCM3) was used to investigate the sensitivity of the simulation of the West African monsoon using four different cumulus and closures parameterization schemes of Anthes Kuo (AK), Grell and Fristish Chappell (GFC), Grell and Arakawa Schubert (GAS), and MIT-Emmanuel (EM) while maintaining other physical packages unchanged. The contrasting monsoon years of 1993 and 1999, which were dry and wet years, respectively, were simulated. The model was integrated from a period of 5 months, starting from May 1 to September 30 of each year using the European Centre for Medium-Range-Weather Forecast (ECMWF) Reanalysis data (ERA40) as input boundary conditions. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds Sea Surface Temperature interpolated to 6 h was used as the lower boundary forcing. The results show that in West Africa, monsoon precipitations are sensitive to the choice of cumulus parameterization and closure schemes. None of the schemes is able to simulate the monsoon rainfall accurately, and furthermore, there is little difference in behavior among schemes between dry and wet years. The spatial features of precipitation are not identical among schemes, although they all show a northward shift of the rain bands, giving a very wet Sahel and dry Guinean Coast. The GFC and EM schemes are able to capture the diurnal cycle of precipitation and the zonal averages of stratiform rain fractions as observed in the Tropical Rainfall Measuring Mission (TRMM), although they overestimated rainfall amounts. The most important deficiencies, however, cannot be attributed to the schemes. In particular, the northward shift of both the rain band and the AEJ in RegCM3 is the result of unrealistic soil moisture resulting from the way albedo is parameterized, leading to an excessive northward penetration of monsoon flow. A sensitivity study showed that an adjustment of initial albedo values over the Sahel improved the simulation, and in particular the position of rain bands and of the AEJ.     

On convection and static stability during the AMMA SOP3 campaign

Using radiosonde dataset from 15 weather stations over West Africa, this paper investigates the contribution of the couple convection-static stability in the framework of the African monsoon multidisciplinary analyses Special Observing Period 3 (AMMA SOP3) experiment. Within this 31-day period, the boundary layer-winds depictions have revealed the West African monsoon’s (WAM) depth (around 1500 m) is not thick enough to trigger intense convection. However, the midlevel winds distribution (700–600 hPa) has shown the average African easterly jet core strength (15 m s^?1) is sufficient to allow the development of African easterly waves (AEWs) necessary for squall lines activities. In return, in the upper levels (200–100 hPa), the speed (below 18 m s^?1) of the mean Tropical easterly jet (TEJ) core cannot favor midlevel updrafts. The free tropospheric humidity (FTH) depiction has indicated convective events are more likely in the western Sahel where the highest FTH (FTH >50 %) are recorded. The static stability analysis has testified that convection is stronger in the semi-arid (SA) area during night time (0000 GMT). However, convective activities are inhibited in the wet equatorial (WE) region due to mean low-level stability. We used METEOSAT Second Generation (MSG) infrared (IR10.8) imagery of the 8th September 2006 to confirm that result. Furthermore, a maximum midtropospheric static stability combined with maximum relative humidity (RH) was found on the fringe of the Saharan air layer’s (SAL) top (altitude around 5.3 km) in the WE region.     

Extending Service Life of Household Water Filters by Mixing Metallic Iron with Sand

The use of metallic iron filters (Fe⁰ filters) has been discussed as a promising low‐cost option for safe drinking water production at household level. Filter clogging due to the volumetric expansive nature of iron corrosion has been identified as the major problem of Fe⁰ filters. Mixing Fe⁰ and sand (yielding Fe⁰/sand filters) has been proposed as a tool to extent filter service life. However, no systematic discussion rationalizing Fe⁰/sand mixtures is yet available. This communication theoretically discussed suitable Fe⁰/sand proportions for efficient filters. Results suggested that Fe⁰/sand filters should not contain more that 50 vol% Fe⁰ (25 wt% when Fe⁰ is mixed with quartz). The actual Fe⁰ percentage in a filter will depend on its intrinsic reactivity.     

A penalty mechanism for discontinuous Galerkin processing of Darcy velocity

The Darcy velocity plays an important role in the flow in porous media, particularly when a miscible displacement is concerned. One major requirement when approximating this velocity is the continuity of its normal component. The discontinuous Galerkin methods, by nature, are not well designed for this challenge, since approximations are performed in space of totally discontinuous polynomials.We propose in such context a penalty approach, in order to enhance the continuity of the normal component of the Darcy velocity. The resulting formulation is shown to be stable whatever the origin of the pressure but requires the inversion of a global matrix. We then propose two modifications leading to the inversion of only local matrices. Error estimates are furnished and the analysis of the penalty parameter vis-a-vis the computed pressure is addressed. We show that the proposed reconstructions have better performance compared to the simple local differentiation of the computed pressure. Numerical tests are provided to illustrate the theoretical results.