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

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.

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Comparison of GPCP Monthly and Daily Precipitation Estimates with High-Latitude Gauge Observations

Journal of Applied Meteorology and Climatology ◽

10.1175/2009jamc2147.1 ◽

2009 ◽

Vol 48 (9) ◽

pp. 1843-1857 ◽

Cited By ~ 57

Author(s):

David T. Bolvin ◽

Robert F. Adler ◽

George J. Huffman ◽

Eric J. Nelkin ◽

Jani P. Poutiainen

Keyword(s):

High Latitude ◽

Global Precipitation Climatology Project ◽

Precipitation Event ◽

Finnish Meteorological Institute ◽

Monthly Scale ◽

Precipitation Measurement ◽

Precipitation Estimates ◽

Wide Variability ◽

Global Precipitation Measurement ◽

Global Precipitation

Abstract Monthly and daily products of the Global Precipitation Climatology Project (GPCP) are evaluated through a comparison with Finnish Meteorological Institute (FMI) gauge observations for the period January 1995–December 2007 to assess the quality of the GPCP estimates at high latitudes. At the monthly scale both the final GPCP combination satellite–gauge (SG) product is evaluated, along with the satellite-only multisatellite (MS) product. The GPCP daily product is scaled to sum to the monthly product, so it implicitly contains monthly-scale gauge influence, although it contains no daily gauge information. As expected, the monthly SG product agrees well with the FMI observations because of the inclusion of limited gauge information. Over the entire analysis period the SG estimates are biased low by 6% when the same wind-loss adjustment is applied to the FMI gauges as is used in the SG analysis. The interannual anomaly correlation is about 0.9. The satellite-only MS product has a lesser, but still reasonably good, interannual correlation (∼0.6) while retaining a similar bias due to the use of a climatological bias adjustment. These results indicate the value of using even a few gauges in the analysis and provide an estimate of the correlation error to be expected in the SG analysis over ocean and remote land areas where gauges are absent. The daily GPCP precipitation estimates compare reasonably well at the 1° latitude × 2° longitude scale with the FMI gauge observations in the summer with a correlation of 0.55, but less so in the winter with a correlation of 0.45. Correlations increase somewhat when larger areas and multiday periods are analyzed. The day-to-day occurrence of precipitation is captured fairly well by the GPCP estimates, but the corresponding precipitation event amounts tend to show wide variability. The results of this study indicate that the GPCP monthly and daily fields are useful for meteorological and hydrological studies but that there is significant room for improvement of satellite retrievals and analysis techniques in this region. It is hoped that the research here provides a framework for future high-latitude assessment efforts such as those that will be necessary for the upcoming satellite-based Global Precipitation Measurement (GPM) mission.

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

Monthly Weather Review ◽

10.1175/2008mwr2295.1 ◽

2008 ◽

Vol 136 (9) ◽

pp. 3579-3596 ◽

Cited By ~ 32

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.

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On the Importance of Gauge-Undercatch Correction Factors and Their Impacts on the Global Precipitation Estimates

10.20944/preprints202106.0179.v1 ◽

2021 ◽

Author(s):

Mohammad Reza Ehsani ◽

Ali Behrangi

Keyword(s):

Surface Air Temperature ◽

Global Precipitation Climatology Project ◽

Boreal Summer ◽

Boreal Winter ◽

Correction Factors ◽

Dynamic Correction ◽

Near Surface ◽

Annual Variations ◽

Precipitation Climatology ◽

Global Precipitation

Precipitation gauges are critical for measuring precipitation rates at regional and global scales and are often used to calibrate precipitation rates estimated from other instruments such as satellites. However, precipitation measured at the gauges is affected by gauge-undercatch that is often larger for solid precipitation. In the present work, two popular gauge-undercatch correction factors are assessed: one utilizes a dynamic correction model and is used in the Global Precipitation Climatology Centre (GPCC) Monitoring product and the other one employs a fixed climatology and is used in the Global Precipitation Climatology Project (GPCP) product. How much the choice of correction factors can impact the total estimate of precipitation was quantified over land at seasonal, annual, regional, and global scales. The correction factors are also compared as a function of the environmental variables used in their development, among those are near-surface air temperature, relative humidity, wind speed, elevation, and precipitation intensity. Results show that correction factors can increase the annual precipitation rate based on the gauges by ~9.5 % over the global land (excluding Antarctica), although this amount can vary from ~6.3% (in boreal summer) to more than 10% (in boreal winter), depending on the season and the method used for gauge-undercatch correction. Annual variations of correction factors can also be large, so the use of the fixed climatology correction factors requires caution. Given their magnitudes and differences, selection of appropriate correction factors can have important implications in refining the water and energy budget calculations.

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Dynamics and Predictability of a Heavy Dry-Season Precipitation Event over West Africa—Sensitivity Experiments with a Global Model

Monthly Weather Review ◽

10.1175/2008mwr2622.1 ◽

2009 ◽

Vol 137 (1) ◽

pp. 189-206 ◽

Cited By ~ 12

Author(s):

Florian Meier ◽

Peter Knippertz

Keyword(s):

West Africa ◽

North Atlantic ◽

Initial Conditions ◽

Local Population ◽

Global Model ◽

Precipitation Event ◽

Sensitivity Experiments ◽

The North ◽

The North Atlantic ◽

Upper Level

Abstract In January 2002 the Cape Verde region in tropical West Africa was hit by an exceptionally heavy precipitation event. Rain rates of up to 116 mm (48 h)−1 caused harmful impacts on the local population. The rainfall was triggered by a series of two upper-level disturbances penetrating from the extratropics to the West African coast. This study investigates the dynamics and predictability of this event on the basis of simulations with the global model Global Model Europe (GME) of the German Weather Service [i.e., Deutscher Wetterdienst (DWD)] initialized by the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data. Free forecasts satisfactorily reproduce the upper-level disturbances and the precipitation up to a lead time of 7 days. Several sensitivity experiments are conducted to unveil the reasons for this comparably high predictability and to identify dynamical precursors. The relevance of the upper-level wave structure in the extratropics is examined by modifications of the initial conditions using a quasigeostrophic potential vorticity (PV) inversion technique. While a reservoir of high PV over the North Atlantic and a PV ridge over Europe are found to be crucial for the upper-level wave amplification and the rainfall over West Africa, latent heating over the North Atlantic affects the event rather little in contrast to previous case studies. Surface properties like orography and sea surface temperature anomalies modify the precipitation quantity, but appear not to be essential for the occurrence of the extreme event on the simulated time scale.

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Values and Preferences for Domestic Water Use: A Study from the Transboundary River Basin of Mékrou (West Africa)

Water ◽

10.3390/w10091232 ◽

2018 ◽

Vol 10 (9) ◽

pp. 1232

Author(s):

Vasileios Markantonis ◽

Celine Dondeynaz ◽

Dionysis Latinopoulos ◽

Kostas Bithas ◽

Ioannis Trichakis ◽

...

Keyword(s):

West Africa ◽

Willingness To Pay ◽

River Basin ◽

Citizen Participation ◽

Human Life ◽

Local Population ◽

Western World ◽

African Countries ◽

Domestic Water ◽

Water Provision

Water is indispensable for human life and sufficient domestic use is considered as a regularity in the western world. The conditions are substantially different in African countries where poverty and lack of life-supporting services prevail. The provision of domestic water is an essential problem, which requires action. The lack of sufficient funding for the development of infrastructure supports claims for citizen participation in related costs. However, can citizens pay and to what extend for sufficient water provision? The present study investigates a household’s willingness to pay for domestic water in the transboundary Mékrou River Basin in West Africa (Burkina Faso, Benin and Niger) and explores the payment for domestic water provision to poverty. The paper uses the results of a household survey that was undertaken in the Mekrou basin including a representative sample from all three countries. Based on this survey the paper presents basic socio-economic characteristics of the local population as well as qualitative water provision and management attributes. In the core of the econometric analysis the paper presents the results of the survey’s Contingent Valuation (CV) scenario estimating the households’ willingness to pay (WTP) for a domestic water provision. The households of the Mekrou basin are willing to pay 2.81 euro per month in average for a domestic water provision network but this is strongly related with the wealth of households. This finding although it may support the “user pays principle”, it also raises serious questions over the provision of water to poor households.

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

Hydrology and Earth System Sciences ◽

10.5194/hess-17-5079-2013 ◽

2013 ◽

Vol 17 (12) ◽

pp. 5079-5096 ◽

Cited By ~ 18

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.

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