scholarly journals Comments on “Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics”

2009 ◽  
pp. 100802101247096
Author(s):  
Thomas Spengler ◽  
Joseph Egger
Keyword(s):  
West Africa ◽  
Dry Season

scholarly journals Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin)

2013 ◽  
Vol 17 (12) ◽  
pp. 5079-5096 ◽  
Author(s):  
A. Richard ◽  
S. Galle ◽  
M. Descloitres ◽  
J.-M. Cohard ◽  
J.-P. Vandervaere ◽  
...  
Keyword(s):  
West Africa ◽  
Transpiration Rate ◽  
Riparian Forest ◽  
Dry Season ◽  
Groundwater Table ◽  
Actual Evapotranspiration ◽  
Virtual Experiment ◽  
Deep Groundwater ◽  
Riparian Area ◽  
Study Results

Abstract. Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from a permanent and deep groundwater table to the atmosphere even during the dry season. It should be the case for riparian forests too, as these streambank forests are key landscape elements in Sudanian West Africa. The interplay of riparian forest and groundwater in the local hydrodynamics was investigated, by quantifying their contribution to the water balance. Field observations from a comprehensively instrumented hillslope in northern Benin were used. Particular attention was paid to measurements of actual evapotranspiration, soil water and deep groundwater levels. A vertical 2-D hydrological modelling approach using the Hydrus software was used as a testing tool to understand the interactions between the riparian area and the groundwater. The model was calibrated and evaluated using a multi-criteria approach (reference simulation). A virtual experiment, including three other simulations, was designed (no forest, no groundwater, neither forest nor groundwater). The model correctly simulated the hydrodynamics of the hillslope regarding vadose zone dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics. The virtual experiment showed that the riparian forest transpiration depleted the deep groundwater table level and disconnected it from the river, which is consistent with the observations. The riparian forest and the deep groundwater table actually form an interacting transpiration system: the high transpiration rate in the riparian area was shown to be due to the existence of the water table, supplied by downslope lateral water flows within the hillslope soil layer. The simulated riparian transpiration rate was practically steady all year long, around 7.6 mm d−1. This rate lies within high-end values of similar study results. The riparian forest as simulated here contributes to 37% of the annual hillslope transpiration, and reaches 57% in the dry season, whereas it only covers 5% of the hillslope area.

scholarly journals The impact of air–sea coupling and ocean biases on the seasonal cycle of southern West African precipitation

2019 ◽  
Vol 53 (11) ◽  
pp. 7027-7044
Author(s):  
Caroline M. Wainwright ◽  
Linda C. Hirons ◽  
Nicholas P. Klingaman ◽  
Richard P. Allan ◽  
Emily Black ◽  
...  
Keyword(s):  
West Africa ◽  
Atlantic Ocean ◽  
Crop Yields ◽  
Coupled Model ◽  
Dry Season ◽  
Global Ocean ◽  
Cmip5 Models ◽  
Rainfall Regime ◽  
Climate Projections ◽  
The Impact

Abstract The biannual seasonal rainfall regime over the southern part of West Africa is characterised by two wet seasons, separated by the ‘Little Dry Season’ in July–August. Lower rainfall totals during this intervening dry season may be detrimental for crop yields over a region with a dense population that depends on agricultural output. Coupled Model Intercomparison Project Phase 5 (CMIP5) models do not correctly capture this seasonal regime, and instead generate a single wet season, peaking at the observed timing of the Little Dry Season. Hence, the realism of future climate projections over this region is questionable. Here, the representation of the Little Dry Season in coupled model simulations is investigated, to elucidate factors leading to this misrepresentation. The Global Ocean Mixed Layer configuration of the Met Office Unified Model is particularly useful for exploring this misrepresentation, as it enables separating the effects of coupled model ocean biases in different ocean basins while maintaining air–sea coupling. Atlantic Ocean SST biases cause the incorrect seasonal regime over southern West Africa. Upper level descent in August reduces ascent along the coastline, which is associated with the observed reduction in rainfall during the Little Dry Season. When coupled model Atlantic Ocean biases are introduced, ascent over the coastline is deeper and rainfall totals are higher during July–August. Hence, this study indicates detrimental impacts introduced by Atlantic Ocean biases, and highlights an area of model development required for production of meaningful climate change projections over the West Africa region.

scholarly journals Prediction of Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

2009 ◽  
Vol 24 (4) ◽  
pp. 1064-1084 ◽  
Author(s):  
Peter Knippertz ◽  
Andreas H. Fink
Keyword(s):  
West Africa ◽  
Local Population ◽  
Temporal Correlation ◽  
Global Precipitation Climatology Project ◽  
Precipitation Anomaly ◽  
Dry Season ◽  
Precipitation Event ◽  
Boreal Winter ◽  
False Alarms ◽  
Global Precipitation

Abstract Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally results in significant impacts on the local population. The dynamics and predictability of this phenomenon have been studied very little. Here, a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.5°–15°N, 10°W–10°E. The analysis is based upon Global Precipitation Climatology Project (GPCP) merged satellite–gauge pentad rainfall estimates and 5-day 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) precipitation forecasts, and covers the 23 dry seasons (November–February) during 1979/80–2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than +200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. The results give evidence for a preconditioning by another disturbance about 1 week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified, the model shows moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. The results should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings.

A note on Eulophonotus myrmeleon Fldr. (Lepidoptera, Cossidae), a stem borer of cocoa in West Africa

1963 ◽  
Vol 54 (1) ◽  
pp. 1-3 ◽  
Author(s):  
P. F. Entwistle
Keyword(s):  
West Africa ◽  
Pupal Stage ◽  
Stem Borer ◽  
Dry Season ◽  
Larval Period ◽  
Larval Migration ◽  
Wood Particles ◽  
Adult Lives ◽  
Injurious Effects

The biology of Eulophonotus myrmeleon Fldr., the larva of which bores in the woody stems and branches of cocoa throughout West Africa, was studied in Ghana and Nigeria. In the laboratory the adult lives for a maximum of four days during which about 500 eggs are laid. The larval period is not less than three months and the pupal stage lasts three weeks. Larvae desert galleries in drying wood, and the excavation of new galleries in fresh wood is begun beneath the protection of a tent of wood particles bound together with silk. Populations are seldom large enough for their activities to be serious in Ghana, but the severe dry season in Western Nigeria causes considerable die-back in cocoa, creating conditions favouring larval migration and thus increasing the injurious effects of E. myrmeleon.

Differences in Plant and Soil Water Relations in and Around a Forest Gap in West Africa during the Dry Season may Influence Seedling Establishment and Survival

1996 ◽  
Vol 84 (1) ◽  
pp. 83 ◽  
Author(s):  
E. M. Veenendaal ◽  
M. D. Swaine ◽  
V. K. Agyeman ◽  
D. Blay ◽  
I. K. Abebrese ◽  
...  
Keyword(s):  
West Africa ◽  
Soil Water ◽  
Water Relations ◽  
Seedling Establishment ◽  
Dry Season ◽  
Forest Gap

scholarly journals Observation of the Diurnal Cycle in the Low Troposphere of West Africa

2008 ◽  
Vol 136 (9) ◽  
pp. 3477-3500 ◽  
Author(s):  
Marie Lothon ◽  
Frédérique Saïd ◽  
Fabienne Lohou ◽  
Bernard Campistron
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
West Africa ◽  
Planetary Boundary Layer ◽  
Diurnal Cycle ◽  
Dry Season ◽  
Nocturnal Jet ◽  
The Impact ◽  
Made In

Abstract The authors give an overview of the diurnal cycle of the low troposphere during 2006 at two different sites, Niamey (Niger) and Nangatchori (Benin). This study is partly based on the first observations of UHF wind profilers ever made in West Africa in the context of the African Monsoon Multidisciplinary Analysis (AMMA) project. Also used are the radiosoundings made in Niamey and ground station observations at Nangatchori, which allow for the study of the impact of the dynamics on the water vapor cycle and the turbulence observed at the ground. Profiler measurements revealed a very consistent year-round nocturnal low-level jet maximal around 0500 UTC and centered at 400-m above the ground, with wind speed around 15 m s−1. This jet comes either from the northeast during the dry season or from the southwest during the wet season, in relation with the position of the intertropical discontinuity. The radiosoundings made in Niamey highlight both the role of the nocturnal jet in bringing water vapor from the south during the night when the intertropical discontinuity has reached the vicinity of the considered area at the end of the dry season and the role of the daytime planetary boundary layer in mixing this water vapor within a larger depth of the troposphere. The planetary boundary layer processes play a large role in the diurnal cycle of the position of the intertropical discontinuity itself. The observations of turbulence made at the ground in Nangatchori showed that the best signature of the nocturnal jet close to surface can be seen in the turbulent kinetic energy and skewness of the air vertical velocity, rather than on the mean wind itself. They reveal the downward transport of momentum from the jet core aloft to the surface.

scholarly journals Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

2008 ◽  
Vol 136 (9) ◽  
pp. 3579-3596 ◽  
Author(s):  
Peter Knippertz ◽  
Andreas H. Fink
Keyword(s):  
West Africa ◽  
Land Surface ◽  
Dry Season ◽  
Dynamical Analysis ◽  
Socioeconomic Factor ◽  
Precipitation Event ◽  
Operational Forecasts ◽  
Warm Anomaly ◽  
Upper Level ◽  
Strong Control

Abstract Precipitation is a major socioeconomic factor in the Guineo-Soudanian zone of tropical West Africa with its distinct summer rainy season from May to October. Albeit rare, precipitation during the dry season can have substantial impacts on the local hydrology and human activities reaching from the rotting of harvests to improved grazing conditions. This study provides an observationally based synoptic and dynamical analysis of an abundant rainfall event during the dry season of 2003/04 that affected the countries of Nigeria, Benin, Togo, and Ghana. The results point to a forcing of the rainfalls from the extratropics in the following ways: 1) Upper-level clouds and moisture to the east of a weak, quasi-stationary extratropical disturbance enhance the greenhouse effect over the Sahel and the adjacent Sahara, and thereby cause a net-column warm anomaly and falling surface pressure. 2) One day before the precipitation event, negative pressure tendencies are further enhanced through warm advection and subsidence associated with the penetration of a more intense upper-trough into Algeria. 3) The resulting northward shift and intensification of the weak wintertime heat low allows low-level moist southerlies from the Gulf of Guinea to penetrate into the Soudanian zone. 4) Finally, daytime heating of the land surface and convective dynamics initiate heavy rainfalls. Operational forecasts of this event were promising, pointing to a strong control by the comparatively well-predicted extratropical upper-level circulation.

Sign in / Sign up

Export Citation Format

Share Document