African Monsoon Multidisciplinary Analysis Project Research Articles

Evaluation of the ISBA-TRIP continental hydrologic system over the Niger basin using in situ and satellite derived datasets

Abstract. During the 1970s and 1980s, West Africa has faced extreme climate variations with extended drought conditions. Of particular importance is the Niger basin, since it traverses a large part of the Sahel and is thus a critical source of water for an ever-increasing local population in this semi arid region. However, the understanding of the hydrological processes over this basin is currently limited by the lack of spatially distributed surface water and discharge measurements. The purpose of this study is to evaluate the ability of the ISBA-TRIP continental hydrologic system to represent key processes related to the hydrological cycle of the Niger basin. ISBA-TRIP is currently used within a coupled global climate model, so that the scheme must represent the first order processes which are critical for representing the water cycle while retaining a limited number of parameters and a simple representation of the physics. To this end, the scheme uses first-order approximations to account explicitly for the surface river routing, the floodplain dynamics, and the water storage using a deep aquifer reservoir. In the current study, simulations are done at a 0.5 by 0.5° spatial resolution over the 2002–2007 period (in order to take advantage of the recent satellite record and data from the African Monsoon Multidisciplinary Analyses project, AMMA). Four configurations of the model are compared to evaluate the separate impacts of the flooding scheme and the aquifer on the water cycle. Moreover, the model is forced by two different rainfall datasets to consider the sensitivity of the model to rainfall input uncertainties. The model is evaluated using in situ discharge measurements as well as satellite derived flood extent, total continental water storage changes and river height changes. The basic analysis of in situ discharges confirms the impact of the inner delta area, known as a significant flooded area, on the discharge, characterized by a strong reduction of the streamflow after the delta compared to the streamflow before the delta. In the simulations, the flooding scheme leads to a non-negligible increase of evaporation over large flooded areas, which decreases the Niger river flow by 15% to 50% in the locations situated after the inner delta as a function of the input rainfall dataset used as forcing. This improves the simulation of the river discharge downstream of the delta, confirming the need for coupling the land surface scheme with the flood model. The deep aquifer reservoir improves Niger low flows and the recession law during the dry season. The comparison with 3 satellite products from the Gravity Recovery and Climated Experiment (GRACE) shows a non negligible contribution of the deeper soil layers to the total storage (34% for groundwater and aquifer). The simulations also show a non negligible sensitivity of the simulations to rain uncertainties especially concerning the discharge. Finally, sensitivity tests show that a good parameterization of routing is required to optimize simulation errors. Indeed, the modification of certain key parameters which can be observed from space (notably river height and flooded zones height changes and extent) has an impact on the model dynamics, thus it is suggested that improving the model input parameters using future developments in remote sensing technologies such as the joint CNES-NASA satellite project SWOT (Surface Water Ocean Topography), which will provide water heights and extentat land surface with an unprecedented 50–100 m resolution and precision.

Influence of the Mediterranean Sea on the West African monsoon: Intraseasonal variability in numerical simulations

The influence of May to September sea surface temperature (SST) anomalies in the Mediterranean Sea on the West African monsoon is investigated, analyzing the outputs of numerical sensitivity experiments performed using three atmospheric general circulation models (Action de Recherche Petite Échelle Grande Échelle, European/Hamburg, and University of California, Los Angeles) in the framework of the African Monsoon Multidisciplinary Analysis project. The precipitation and atmospheric dynamics response to the SST forcing is explored, in terms of intraseasonal variability, evaluating the results from the individual models and from the multimodel mean. A positive precipitation response to warmer than average conditions in the Mediterranean Sea is found in the Sudano‐Sahelian belt in August–September. The proposed dynamic mechanism underlying the Mediterranean action on the West African monsoon is based on the modifications produced by the SST forcing in the moisture content in the lower troposphere. A warmer eastern Mediterranean in August–September feeds the lower troposphere with additional moisture, with a consequent reinforcement of northerly moisture transport toward the Sahel. Furthermore, warmer SST is linked to a strengthening of the Saharan heat low and to an enhancement of the moist static energy meridional gradient over West Africa, favoring the northward displacement of the monsoonal front.

Mixing state of aerosols and direct observation of carbonaceous and marine coatings on African dust by individual particle analysis

The mixing state of aerosols collected at M'Bour, Senegal, during the Special Observing Period conducted in January–February 2006 (SOP‐0) of the African Monsoon Multidisciplinary Analysis project (AMMA), was studied by individual particle analysis. The sampling location on the Atlantic coast is particularly adapted for studying the mixing state of tropospheric aerosols since it is (1) located on the path of Saharan dust plumes transported westward over the northern tropical Atlantic, (2) influenced by biomass burning events particularly frequent from December to March, and (3) strongly influenced by anthropogenic emissions from polluted African cities. Particle size, morphology, and chemical composition were determined for 12,672 particles using scanning electron microscopy (automated SEM‐EDX). Complementary analyses were performed using transmission electron microscopy combined with electron energy loss spectrometry (TEM‐EELS) and Raman microspectrometry. Mineral dust and carbonaceous and marine compounds were predominantly found externally mixed, i.e., not present together in the same particles. Binary internally mixed particles, i.e., dust/carbonaceous, carbonaceous/marine, and dust/marine mixtures, accounted for a significant fraction of analyzed particles (from 10.5% to 46.5%). Western Sahara was identified as the main source of mineral dust. Two major types of carbonaceous particles were identified: “tar balls” probably coming from biomass burning emissions and soot from anthropogenic emissions. Regarding binary internally mixed particles, marine and carbonaceous compounds generally formed a coating on mineral dust particles. The carbonaceous coating observed at the particle scale on African dust was evidenced by the combined use of elemental and molecular microanalysis techniques, with the identification of an amorphous rather than crystallized carbon structure.

The ECMWF re‐analysis for the AMMA observational campaign

AbstractDuring the 2006 African Monsoon Multidisciplinary Analysis (AMMA) field experiment, an unprecedented number of soundings were performed in West Africa. However, due to technical problems many of these soundings did not reach the Global Telecommunication System and therefore they could not be included in the operational numerical weather prediction (NWP) analyses. This issue, together with the realization that there was a significant bias in the radiosonde humidity, led to the conclusion that a re‐analysis effort was necessary. This re‐analysis was performed at the European Centre for Medium‐Range Weather Forecasts (ECMWF) spanning the wet monsoon season of 2006 from May–September. The key features of the ECMWF AMMA re‐analysis are presented, including the use of a newer model version with improved physics, all the AMMA radiosonde data available from the AMMA database and a new radiosonde humidity bias‐correction scheme. Data‐impact experiments show that there is a benefit from these observations, but also highlight large model physics biases over the Sahel that cause a short‐lived impact of the observations on the model forecast. The AMMA re‐analysis is compared with independent observations to investigate the biases in the different parts of the physics. In the framework of the AMMA project, a hybrid dataset was developed to provide a best estimate of the different terms of the water cycle. This hybrid dataset is used to evaluate the improvement achieved from the use of extra AMMA observations and of a radiosonde humidity bias‐correction scheme in the water cycle of the West African monsoon. Finally, future model developments that offer promising improvements in the water cycle are discussed. Copyright © 2010 Royal Meteorological Society

Soil moisture active and passive microwave products: intercomparison and evaluation over a Sahelian site

Abstract. This paper presents a comparison and an evaluation of five soil moisture products based on satellite-based passive and active microwave measurements. Products are evaluated for 2005–2006 against ground measurements obtained from the soil moisture network deployed in Mali (Sahel) in the framework of the African Monsoon Multidisciplinary Analysis project. It is shown that the accuracy of the soil moisture products is sensitive to the retrieval approach as well as to the sensor type (active or passive) and to the signal frequency (from 5.6 GHz to 18.8 GHz). The spatial patterns of surface soil moisture are compared between the different products at meso-scale (14.5° N–17.5° N and 2° W–1° W). A general good consistency between the different satellite soil moisture products is shown in terms of meso-scale spatial distribution, in particular after convective rainfall occurrences. Comparison to ground measurement shows that although soil moisture products obtained from satellite generally over-estimate soil moisture values during the dry season, most of them capture soil moisture temporal variations in good agreement with ground station measurements.

Wind profiler analysis of the African Easterly Jet in relation with the boundary layer and the Saharan heat‐low

AbstractWind profiler measurements in Niger and Benin during the African Monsoon Multidisciplinary Analysis project are used to study the intraseasonal variability of the low and mid troposphere at several time‐scales. We focus on the African Easterly Jet (AEJ) and its interaction with the Saharan Heat‐Low (HL) and the planetary boundary layer (PBL).We find a pronounced diurnal cycle of the AEJ, characterised by a decrease of wind speed during the afternoon, reaching a minimum at 1800 UTC of about 15–20% of the daily average during the pre‐onset period. This decrease is out of phase with the HL intensity, but in phase with the daytime turbulent mixing associated with the PBL. The interaction between the PBL and mid‐troposphere is likely responsible for this daily decrease of the AEJ.During the transition periods (dry to wet or wet to dry), the HL seems to govern the AEJ; however, slightly before the monsoon onset, it has no direct influence on the jet. During that time, we find smaller AEJ wind speed for deeper PBL, as found at the diurnal time‐scale. This is consistent with the still large surface heating at that time, which favours deep PBL growth, with a top inversion often higher than the shear layer between the monsoon and the easterlies. After the monsoon onset, deep convection, African Easterly Waves (AEWs) and dry intrusions make the synoptic environment complex and blur the interaction between AEJ and PBL. We still find weaker AEJ for deeper PBL, but likely without a direct connection between them. Copyright © 2009 Royal Meteorological Society

The AMMA-CATCH Gourma observatory site in Mali: Relating climatic variations to changes in vegetation, surface hydrology, fluxes and natural resources

The Gourma site in Mali is one of the three instrumented meso-scale sites deployed in West-Africa as part of the African Monsoon Multi-disciplinary Analysis (AMMA) project. Located both in the Sahelian zone sensu stricto, and in the Saharo–Sahelian transition zone, the Gourma meso-scale window is the northernmost site of the AMMA-CATCH observatory reached by the West African Monsoon. The experimental strategy includes deployment of a variety of instruments, from local to meso-scale, dedicated to monitoring and documentation of the major variables characterizing the climate forcing, and the spatio-temporal variability of surface processes and state variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. This paper describes the Gourma site, its associated instrumental network and the research activities that have been carried out since 1984. In the AMMA project, emphasis is put on the relations between climate, vegetation and surface fluxes. However, the Gourma site is also important for development and validation of satellite products, mainly due to the existence of large and relatively homogeneous surfaces. The social dimension of the water resource uses and governance is also briefly analyzed, relying on field enquiry and interviews. The climate of the Gourma region is semi-arid, daytime air temperatures are always high and annual rainfall amounts exhibit strong inter-annual and seasonal variations. Measurements sites organized along a north–south transect reveal sharp gradients in surface albedo, net radiation, vegetation production, and distribution of plant functional types. However, at any point along the gradient, surface energy budget, soil moisture and vegetation growth contrast between two main types of soil surfaces and hydrologic systems. On the one hand, sandy soils with high water infiltration rates and limited run-off support almost continuous herbaceous vegetation with scattered woody plants. On the other hand, water infiltration is poor on shallow soils, and vegetation is sparse and discontinuous, with more concentrated run-off that ends in pools or low lands within structured endorheic watersheds. Land surface in the Gourma is characterized by rapid response to climate variability, strong intra-seasonal, seasonal and inter-annual variations in vegetation growth, soil moisture and energy balance. Despite the multi-decadal drought, which still persists, ponds and lakes have increased, the grass cover has largely recovered, and there are signs of increased tree cover at least in the low lands.

Seasonal evolution of the West African heat low: a climatological perspective

The West African heat low (WAHL), a region of high surface temperatures and low surface pressures, is a key element of the West African monsoon system. In this study, we propose a method to detect the WAHL in order to monitor its climatological seasonal displacement over West Africa during the period 1979–2001, using the European Centre for Medium-range Weather Forecast (ECMWF) ERA-40 reanalyses. The low-level atmospheric thickness (LLAT), a variable defined as the difference of geopotential heights at 700 and 925 hPa, is used to detect the dilatation of these levels generated by an increase of the temperature. We define grid points with 10% highest values of the LLAT as the WAHL. We show that our method reliably positions the WAHL over areas of high surface temperatures and low surface pressures, and that it is effective at detecting heat lows. In the course of the year, the climatological WAHL is shown to migrate north-westward from a position south of the Darfur mountains in the winter (November–March) to a location over the Sahara, between the Hoggar and the Atlas mountains, during the summer (June–September). The temperature tendency equation is used to investigate the processes controlling the displacement of the WAHL, and more particularly the heating at low levels. The specific period of the onset of the WAHL in its summer location over the Sahara (referred to as the Saharan heat low -SHL- onset) is also analysed during the 1984–2001 period, using complementary brightness temperature data from the European Union-funded Cloud Archive User Service (CLAUS). The climatological onset of the SHL occurs around 20 June, i.e. just before the climatological monsoon onset date. The present study suggests that the onset of the WAHL occurs approximately 5 days before the monsoon onset for the 1984–2001 period. This is confirmed independently by comparing the SHL onset date and the monsoon onset date for the 1984–2001 period. The seasonal evolution of the WAHL for the year 2006 (the year of the African Monsoon Multidisciplinary Analysis project Special Observation Period) is analysed and compared with the climatological results. The operational ECMWF analyses were used for that purpose. Except in April, the spatial distribution of the WAHL remains relatively unchanged and agrees with the climatology. The onset of the SHL in 2006 occurs on 18 June, which is close to the climatological date, in spite of the delay in the onset of the rainy season in Sahel.

Size distribution, shape, and composition of mineral dust aerosols collected during the African Monsoon Multidisciplinary Analysis Special Observation Period 0: Dust and Biomass‐Burning Experiment field campaign in Niger, January 2006

Dust samples were collected onboard the UK community BAe‐146 research aircraft of the Facility for Airborne Atmospheric Measurements (FAAM) operated over Niger during the winter Special Observation Period of the African Monsoon Multidisciplinary Analysis project (AMMA SOP0/DABEX). Particle size, morphology, and composition were assessed using single‐particle analysis by analytical scanning and transmission electron microscopy. The aerosol was found to be composed of externally mixed mineral dust and biomass burning particles. Mineral dust consists mainly of aluminosilicates in the form of illite and kaolinite and quartz, accounting for up to 80% of the aerosol number. Fe‐rich particles (iron oxides) represented 4% of the particle number in the submicron fraction. Diatoms were found on all the samples, suggesting that emissions from the Bodélé depression were also contributing to the aerosol load. Satellite images confirm that the Bodélé source was active during the period of investigation. Biomass burning aerosols accounted for about 15% of the particle number of 0.1–0.6 μm diameter and were composed almost exclusively of particles containing potassium and sulfur. Soot particles were very rare. The aspect ratio AR is a measure of particle elongation. The upper limit of the AR value distribution is 5 and the median is 1.7, which suggests that mineral dust particles could be described as ellipsoids whose major axis never exceeds 1.9 × Dp (the spherical geometric diameter). This is consistent with other published values for mineral dust, including the recent Aerosol Robotic Network retrieval results of Dubovik et al. (2006).

Overview of observations from the RADAGAST experiment in Niamey, Niger: Meteorology and thermodynamic variables

An overview is presented of the meteorological and thermodynamic data obtained during the Radiative Atmospheric Divergence using Atmospheric Radiation Measurement (ARM) Mobile Facility, Geostationary Earth Radiation Budget (GERB) data, and African Monsoon Multidisciplinary Analysis (AMMA) stations (RADAGAST) experiment in Niamey, Niger, in 2006. RADAGAST combined data from the ARM Program Mobile Facility (AMF) at Niamey airport with broadband satellite data from the GERB instrument on Meteosat‐8. The experiment was conducted in collaboration with the AMMA project. The focus in this paper is on the variations through the year of key surface and atmospheric variables. The seasonal advance and retreat of the Intertropical Front and the seasonal changes in near‐surface variables and precipitation in 2006 are discussed and contrasted with the behavior in 2005 and with long‐term averages. Observations from the AMF at Niamey airport are used to document the evolution of near‐surface variables and of the atmosphere above the site. There are large seasonal changes in these variables, from the arid and dusty conditions typical of the dry season to the much moister and more cloudy wet season accompanying the arrival and intensification of the West African monsoon. Back trajectories show the origin of the air sampled at Niamey and profiles for selected case studies from rawinsondes and from a micropulse lidar at the AMF site reveal details of typical atmospheric structures. Radiative fluxes and divergences are discussed in the second part of this overview, and the subsequent papers in this special section explore other aspects of the measurements and of the associated modeling.

Comparison of ground‐based GPS precipitable water vapour to independent observations and NWP model reanalyses over Africa

AbstractThis study aims at assessing the consistency between different precipitable water vapour (PWV) datasets over Africa (between 35°N and 10°S). This region is characterized by large spatial and temporal variability of humidity but also by the scarcity of its operational observing network, limiting our knowledge of the hydrological cycle. We intercompare data from observing techniques such as ground‐based Global Positioning System (GPS), radiosondes, AERONET sun photometers and SSM/I, as well as reanalyses from European Centre for Medium‐Range Weather Forecasts (ERA‐40) and National Center for Environmental Prediction (NCEP2). The GPS data, especially, are a new source of PWV observation in this region. PWV estimates from nine ground‐based GPS receivers of the international GPS network data are used as a reference dataset to which the others are compared. Good agreement is found between observational techniques, though dry biases of 12–14% are evidenced in radiosonde data at three sites. Reasonable agreement is found between the observational datasets and ERA‐40 (NCEP2) reanalyses with maximum bias ⩽9% (14%) and standard deviation ⩽17% (20%). Since GPS data were not assimilated in the ERA‐40 and NCEP2 reanalyses, they allow for a fully independent validation of the reanalyses. They highlight limitations in the reanalyses, especially at time‐scales from sub‐daily to periods of a few days. This work also demonstrates the high potential of GPS PWV estimates over Africa for the analysis of the hydrological cycle, at time‐scales ranging between sub‐diurnal to seasonal. Such observations can help studying atmospheric processes targeted by the African Monsoon Multidisciplinary Analysis project. Copyright © 2007 Royal Meteorological Society

Using coarse remote sensing radar observations to control the trajectory of a simple Sahelian land surface model

In the Sahel, land surface processes are significantly interacting with climate dynamics. In this paper, we present an original method to control a simple Sahelian land surface model coupled to a radiative transfer model (RTM) on the basis of ERS wind scatterometer (WSC) observations. In a first step, a sensitivity study is implemented to identify those parameters of the land surface model that can be estimated through the assimilation of WSC data. The assimilation scheme relies on evolution strategies (ES) algorithm that aims at solving the parameter evaluation problem. These algorithms are particularly well suited for complex (nonlinear) inverse problems. The assimilation scheme is applied to several study sites located in the Sahelian mesoscale site of the African Monsoon Multidisciplinary Analysis Project (Gourma region, Mali). The results are compared with ground observations of herbaceous mass. After the WSC data assimilation, the simulated herbaceous mass curves compare well with observations [187 kilogram of dry matter per hectare (kg DM/ha) of average error]. The simulated water fluxes exhibit a behaviour in agreement with ground measurements performed over similar ecosystems during the Hapex Sahel experiment. The accuracy of estimated herbaceous mass and water fluxes resulting from uncertainties on climatic forcing variable is evaluated using a stochastic approach. The average error on the herbaceous mass values mainly depends on the rainfall estimate accuracy and ranges from 139 to 268 kg DM/ha that compares well with a previous study based on the sole inversion of the radiative transfer model. Finally, this study underlines the need for a multispectral assimilation approach to get a better constraint on water fluxes estimation.