Intertropical Discontinuity Research Articles

Analysis of Water Vapor over Nigeria Using Radiosonde and Satellite Data

AbstractThe Satellite Application Facility on Climate Monitoring (CM-SAF) focuses on retrieving geophysical parameters from satellite data by using inversion schemes that are based on radiative transfer theory. In this study, the focus is on the daily mean vertically integrated water vapor [i.e., total precipitable water vapor (TPW)] and the monthly mean layered vertically integrated water vapor [i.e., low-level precipitable water vapor (LPW), midlevel precipitable water vapor (MPW), and upper-level precipitable water vapor] obtained from CM-SAF Germany for the 2004–08 period and for 20 radiosonde stations distributed over Nigeria. The mean annual cycle shows a structure with two maxima, in May and September, respectively, for all low- and midlevel water vapor in the coastal and Guinea savannah regions, whereas at the midland and Sahelian regions, an almost constant maximum lasting between May and September occurs. The two-maxima structure manifested by TPW, LPW, and MPW at the coastal and Guinea savannah regions and the single maximum exhibited by them at the midland and the Sahelian regions are in synchronism with the movement of the intertropical discontinuity. In addition, comparisons have been made between the precipitable water from CM-SAF (CM-SAF PWV) and precipitable water from radiosonde (RS-PWV). Comparisons of the products at the seasonal scale show that CM-SAF PWV at all levels is larger than that of RS-PWV. In addition, a seasonal intercomparison between them shows that CM-SAF PWV and RS-PWV agree significantly at all of the stations that were investigated.

Palynological investigation of haze dust in Ayetoro-Itele Ota, Southwest Nigeria

The biological contents of the March 2010 hazy dust in Nigeria deserve verification due to the attendant panic it brought to the public. 3 g of dust deposited on the car bonnet of one of the authors was treated chemo-palynologically and microscopic study of the residue was carried out. The study reveals that the dust was rich in pollen grains, fungal spores and hyphae. No pteridophyte spore was recovered. High proportion of diatoms almost doubling that of the sporomorphs was recorded. Savanna and derived savanna pollen grains and the Sahara desert freshwater diatoms recovered indicate that the dust was still the harmattan borne by the Northeast Trade winds though coming heavily and lately in March instead of the traditional January. This indicates a delay in the northward movement of the Inter Tropical Discontinuity (ITD) with the consequential abnormal hazy atmosphere. Some typical Sudan and Sahel vegetations’ pollen were not recovered inspite of the dust crossing over them, perhaps due to being dropped along the way or March not being their flowering periods. Pollen of Vitex cf. doniana dominates the palynoflora along with Isoberlinia doka which might be candidate culprits for clinical conditions in the experimental area. Key words: Saharan dust, palynoflora, diatoms, intertropical discontinuity.

Inferring change points and nonlinear trends in multivariate time series: Application to West African monsoon onset timings estimation

[1] Time series in statistical climatology are classically represented by additive models. For example, a seasonal part and a linear trend are often included as components of the sum. Less frequently, hidden elements (e.g., to represent the impact of volcanic forcing on temperatures) can be integrated. Depending on the complexity and the interactions among the different components, the statistical inference challenge can quickly become difficult, especially in a multivariate context where the timings and contributions of hidden signals are unknown. In this article we focus on the statistical problem of decomposing multivariate time series that may contain both nonlinear trends and change points (discontinuities), the change points being assumed to occur simultaneously in time for all variables in the multivariate analysis. The motivation for such a study comes from the statistical analysis of the West African monsoon (WAM) phenomenon for which unknown preonset and onset dates occur each year. The impacts of such onsets can be statistically viewed as yearly change points that affect, almost synchronously, trends in observed time series such as daily Outgoing Longwave Radiation and the Intertropical Discontinuity. Our proposed model corresponds to a multivariate additive model with nonlinear trends and possible yearly discontinuities, modeling the onsets. An inference scheme based on a nonlinear Kalman filtering approach is proposed. It enables to identify the different parts hidden in the original multivariate vector. Our inference strategy is tested on simulated data and applied to the analysis of the WAM phenomenon during the period 1979–2008. Our extracted onset dates are then compared to the ones obtained from past studies.

Rain Days and Their Predictability in South-western Region of Nigeria

This study was able to determine the rainy days of the first two months in the rainy season and the last two months to the end of rainy season including their annual totals in South-western Nigeria. To be able generate models for their prediction, the study used composite effect of the rainfall - engendering factor of the sea surface temperature of the tropical Atlantic Ocean, Land-sea thermal contrast between Southwestern Nigeria and the tropical Atlantic Ocean, surface location of Inter-tropical Discontinuity and the land surface temperature in Southwestern Nigeria. The specific locations over which rainfall data were collected to represent Southwestern Nigeria include Ikeja, Benin City, Ibadan and Ilorin. Stepwise multiple regression analysis was used to construct the model. The analysis shows that the hypothesized rain-engendering factors are adequate in predicting rainy days in Southwestern Nigeria. Out of the four rainfall engendering factors, two of them (sea surface temperature and land-sea thermal contrast) dominate the explanatory factors in the model, constituting 87% of total explanatory factors found significant in the models generated. The study also show that all of the areas of the tropical Atlantic, right from the Gulf of Guinea, through the St. Helena and Ascension Island, up to the Benguela current region have significant bearing to the inter-annual variability in the rainy days in Southwestern Nigeria. It was also noted that the direction of relationship between sea surface temperature, land-sea thermal contrast and rainy days in Southwestern Nigeria is not simple and straightforward, but the sea surface temperature anomalies experience changes in location, extent and time of the year.

Regional‐scale convection patterns during strong and weak phases of the Saharan heat low

AbstractThe West African heat low, which is defined as a thermal depression generally below 700 hPa over West Africa, is stationary over the Sahara during the summer season and is often referred to as the Saharan heat low (SHL). This SHL displays synoptic and intra‐seasonal pulsations of its intensity on time scales under 60 days.In this study, we analyse the regional‐scale pattern of low troposphere dynamics and convective activity associated with weak and strong phases of the SHL during the summer, using 18 years (1984–2001) of concomitant European Centre for Medium‐range Weather Forecasts re‐analysis data and satellite observations of brightness temperature provided by the European Union‐funded Cloud Archive User Service. Strong SHL phases, defined by an increase of low‐level atmospheric thickness, are accompanied by (1) a positive anomaly of temperature in the low layers, (2) a strengthening of the monsoon flow, (3) both a negative anomaly of the divergence flux and an enhancement of the moisture advection over the central Sahel, (4) an intensification of the African easterly jet and (5) enhanced upward vertical motions located south of the inter‐tropical discontinuity (ITD). These conditions appear to be favourable for the occurrence of convection over the central Sahel. The strongest, most widespread convective activity over the central and eastern Sahel occurs on the day when the SHL intensity is the strongest. In contrast, a significant decrease of the convective activity was detected over Senegal and to the West of the Jos Plateau.Opposite patterns are found during weak SHL phases, with a negative anomaly of convection over the central and eastern Sahel connected with anomalous subsidence south of the ITD and a negative moisture advection anomaly. Conversely, convection is seen to be enhanced along the Atlantic coast and particularly over Senegal. Copyright © 2010 Royal Meteorological Society

An Assessment of the Influence of the Inter‐Tropical Discontinuity on Inter‐Annual Rainfall Characteristics in Nigeria

AbstractThis study assessed the influence of the Inter‐Tropical Discontinuity (ITD) on inter‐annual rainfall characteristics in Nigeria between 1970 and 2000. This involved determining the strength and direction of the relationships between the total annual rainfalls in the extreme southern and northern parts of the country and the total annual rainfalls during the period of the Little Dry Season in southwestern Nigeria, and the surface locations of the ITD over Nigeria and some other factors that may impact ITD characteristics (such as the pressure differences between Azores, Libyan and St. Helena anticyclones and the sea surface temperature of the Gulf of Guinea). Results indicate that although the surface location of the ITD significantly accounts for rainfall inter‐annual variability in Nigeria, it does so in the northern part of the country only. Pressure differences between the various anticyclones were observed to be another significant factor influencing inter‐annual rainfall variability in the north. However, the influence of the sea surface temperature of the Gulf of Guinea on the rainfall characteristics in the northern region is ill‐defined. It was also noted that the only factor influencing inter‐annual variability in the Little Dry Season rainfall in the southwest and the total annual rainfalls in the south is the sea surface temperature of the Gulf of Guinea. The results obtained indicate that the total annual rainfalls in the north have significant positive relationships with the surface location of the ITD but significant negative relationships with the pressure differences between the Azores, Libyan and St. Helena anticyclones. The Little Dry Season rainfalls and total annual rainfalls in the south have significant positive relationships with the sea surface temperature of the Gulf of Guinea.

Diurnal cycle of the intertropical discontinuity over West Africa analysed by remote sensing and mesoscale modelling

AbstractThe diurnal cycle of the Intertropical discontinuity (ITD), i.e. the interface at the ground between moist monsoon air and dry Harmattan air, is an important factor in the West African monsoon system. During the whole of 2006, high‐resolution ground‐based remote‐sensing measurements were performed in the area of Djougou, Benin, which made it possible to observe the ITD and the associated sharp gradient of temperature and humidity in detail. In order to extend the point measurements to a three‐dimensional view of the ITD and to enhance the knowledge of the related processes, the mesoscale atmospheric model Meso‐NH was run for a case study (84 h period) in April 2006. In addition Meteosat infrared observations were used to determine the ITD position and its movement. From these observations a northward propagation of the moist air front (ITD) of 8–12 m s−1 was calculated. The propagation speed of the front evaluated from the satellite observations was replicated by the model and the time of the front arrival over Djougou was simulated with a maximum error of 1 hour. With respect to the ground‐based observations, the model was able to reproduce the dynamics and thermodynamics of the front as well as the diurnal cycle of the planetary boundary layer over the study area. This agreement suggests the use of the model to further describe processes in the lower atmosphere at high resolution. Copyright © 2009 Royal Meteorological Society

Vertical structure of aerosols and water vapor over West Africa during the African monsoon dry season

Abstract. We present observations of tropospheric aerosol and water vapor transport over West Africa and the associated meteorological conditions during the AMMA SOP-0 dry season experiment, which was conducted in West Africa in January–February 2006. This study combines data from ultra-light aircraft (ULA)-based lidar, airborne in-situ aerosol and gas measurements, standard meteorological measurements, satellite-based aerosol measurements, airmass trajectories, and radiosonde measurements. At Niamey (13.5° N, 2.2° E) the prevailing surface wind (i.e. Harmattan) was from the northeast bringing dry dusty air from the Sahara desert. High concentrations of mineral dust aerosol were typically observed from the surface to 1.5 or 2 km associated with the Saharan airmasses. At higher altitudes the prevailing wind veered to the south or southeast bringing relatively warm and humid airmasses from the biomass burning regions to the Sahel (<10° N). These elevated layers had high concentrations of biomass burning aerosol and were typically observed between altitudes of 2–5 km. Meteorological analyses show these airmasses were advected upwards over the biomass burning regions through ascent in Inter-Tropical Discontinuity (ITD) zone. Aerosol vertical profiles obtained from the space-based lidar CALIOP onboard CALIPSO during January 2007 also showed the presence of dust particles (particle depolarization (δ)~30%, lidar Ångström exponent (LAE)<0, aerosol backscatter to extinction ratio (BER): 0.026~0.028 sr−1) at low levels (<1.5 km) and biomass burning smoke aerosol (δ<10%, LAE: 0.6~1.1, BER: 0.015~0.018 sr−1) between 2 and 5 km. CALIOP data indicated that these distinct continental dust and biomass burning aerosol layers likely mixed as they advected further south over the tropical Atlantic Ocean, as indicated an intermediate values of δ (10~17%), LAE (0.16~0.18) and BER (0.0021~0.0022 sr−1).

Estimate of Sahelian dust emissions in the intertropical discontinuity region of the West African Monsoon

A three‐dimensional mesoscale numerical simulation has been performed to investigate the dust emissions over the Sahel associated with strong near‐surface winds in the region of the West African Inter Tropical Discontinuity (ITD) during the summer, when the ITD is located over Niger and Mali around 18°N. The study focuses on the period from 2 to 12 July 2006, in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. The comparison with observations suggests that the model can be used reliably to analyze and quantify the dust emissions associated with the strong near‐surface winds blowing over the Sahelian dust sources during the period of interest. The daily mean values of dust load related to the strong winds on both side of the ITD, as estimated from the simulation within the model domain (2°W–16°E, 12–28°N), are in excess of 2 Tg on some of the days of the 2–12 July 2006 period. In the present case, the dust load associated with the strong winds south of the ITD accounts for between one third and two thirds of the total load mobilized in the ITD region over the entire domain on a given day. It is simulated to range between 0.5 and 0.8 Tg on average. This study suggests that emissions driven by strong surface winds occurring on both sides of the ITD while lying across the Sahel may contribute significantly to the total dust load over West and North Africa observed annually.

On the late northward propagation of the West African monsoon in summer 2006 in the region of Niger/Mali

This paper investigates the fine‐scale dynamical processes at the origin of the late northward migration of the monsoon flow in summer 2006 in the region of Niger and Mali (onset on 3 July 2006 compared to the climatological onset date, 24 June). Compared to a 28‐year climatology, 2006 NCEP‐2 reanalyses show evidence of an anomalous pattern during 10 days between 25 June and 3 July 2006, characterized by the African Easterly Jet (AEJ) blowing from the northeast along a narrow northeast/southwest band located over the Hoggar and Air mountains associated with an unusually strong northeasterly harmattan in the lee of the mountains. Using data collected during the African Monsoon Multidisciplinary Analysis (AMMA) experiment and mesoscale numerical simulations, this study shows evidence of interaction between the AEJ and the orography supported by the reduced gravity shallow water theory which explains the enhancement of the harmattan downstream of the Hoggar and Air mountains in summer 2006. The enhanced harmattan contributes to move southward the intertropical discontinuity (ITD) defined as the interface between the cool moist southwesterly monsoon flow and the warm dry harmattan. Finally, an interaction between the ITD and African Easterly waves contributes to propagate the ITD southward retreat about 1500 km to the west of the Hoggar and Air mountains.

Dry cyclogenesis and dust mobilization in the intertropical discontinuity of the West African Monsoon: A case study

Three‐dimensional mesoscale numerical simulations were performed over Niger in order to investigate dry cyclogenesis in the West African intertropical discontinuity (ITD) during the summer, when it is located over the Sahel. The implications of dry cyclogenesis on dust emission and transport over West Africa are also addressed using the model results, together with spaceborne observations from the Spinning Enhanced Visible and Infra‐Red Imager (SEVIRI) and the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP). The study focuses on the case of 7–8 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. Model results show the formation of three dry cyclones in the ITD during a 24‐h period. Simulations are used to investigate the formation and the development of one of these cyclones over Niger in the lee of the Hoggar‐Aïr Mountains. They show the development of the vortex to be associated with (1) strong horizontal shear and low‐level convergence existing along the monsoon shearline and (2) enhanced northeasterly winds associated with orographic blocking of air masses from the Mediterranean Sea. The dry cyclone was apparent between 0700 and 1300 UTC in the simulation, and it was approximately 400 km wide and 1500 m deep. Potential vorticity in the center of vortex reached nearly 6 PVU at the end of the cyclogenesis period (1000 UTC). The role of the orography on cyclogenesis along the ITD was evaluated through model simulations without orography. The comparison of the characteristics of the vortex in the simulations with and without orography suggests that the orography plays a secondary but still important role in the formation of the cyclone. Orography and related flow splitting tend to create low‐level jets in the lee of the Hoggar and Aïr mountains which, in turn, create conditions favorable for the onset of a better defined and more intense vortex in the ITD region. Moreover, orography blocking appears to favor the occurrence of a longer‐lived cyclone. Furthermore, model results suggested that strong surface winds (∼11 m s−1) enhanced by the intensification of the vortex led to the emission of dust mass fluxes as large as 3 μg m−2 s−1. The mobilized dust was mixed upward to a height of 4–5 km to be made available for long‐range transport. This study suggests that the occurrence of dry vortices in the ITD region may contribute significantly to the total dust activity over West Africa during summer. The distribution of dust over the Sahara‐Sahel may be affected over areas and at time scales much larger than those associated with the cyclone itself.

Rainfall regime across the Sahel band in the Gourma region, Mali

Summary The Sahel is characterized by low and highly variable rainfall, which strongly affects the hydrology and the climate of the region and creates severe constraints for agriculture and water management. This study provides the first characterization of the rainfall regime for the Gourma region located in Mali, Central Sahel (14.5–17.5°N and 2–1°S). The rainfall regime is described using two datasets: the daily long term raingauge records covering the period 1950–2007, and the high frequency raingauge records collected under the African Monsoon Multidisciplinary Analysis (AMMA) project between 2005 and 2008. The first rainfall dataset was used to analyse the interannual variability and the spatial distribution of the precipitation. The second dataset is used to analyse the diurnal cycle of precipitation and the nature of the rainfall. This study is complementary to previous analyses conducted in Sahelian areas located further south, where the influence of the continental Sahara heat low is expected to be less pronounced in summer. Rainfall regimes in the Gourma region present a succession of wet (1950–1969) and dry decades (1970–2007). The decrease of summer cumulative rainfall is explained by a reduction in the number of the rainy days in southern Gourma, and a decrease in both the number of rainy days and the daily rainfall in northern and central Gourma. This meridional difference may be related to the relative distances of the zones from the intertropical discontinuity, which is closer to the northern stations. The length of the rainy season has varied since the 1950s with two episodes of shorter rainy seasons: during the drought of the 1980s and also since 2000. However, this second episode is characterized by an increase in the daily rainfall, which suggests an intensification of rainfall events in the more recent years. High-frequency data reveal that a large fraction of the rainfall is produced by intense rain events mostly occurring in late evenings and early mornings during the core of the rainy season (July–September). Conversely, rainfall amounts are less around noon, and this mid-day damping is more pronounced in northern Gourma. All these characteristics have strong implications for agriculture and water resources management.

The impact of a mesoscale convective system cold pool on the northward propagation of the intertropical discontinuity over West Africa

AbstractThe interaction between a mesoscale convection system cold pool and the intertropical discontinuity (ITD) is investigated on two consecutive days (5 and 6 June 2006), in the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period (SOP 1a). Satellite imagery and surface meteorological observations are used to track the movement of the ITD prior to and after its interaction with the cold pool. The impact of the northward propagating cold pool on the Sahelo‐Saharan planetary boundary layer (PBL) thermodynamics and aerosol vertical distribution is analyzed by means of airborne and ground‐based measurements over the two‐day period. Complementary European Centre for Medium‐range Weather Forecasts (ECMWF) analyses are also used. The influence of the cold pool persists as a coherent dynamic and thermodynamic structure over these two study days, influencing a front at least 1000 km long and approximately 200 km wide, spreading across southern Mali. Dropsonde measurements revealed that the cold pool was bringing moisture quite far north over the northern Sahel and to the southern fringes of the Sahara, as well as cooling the desert surface. As a result, the vertical development of the Sahelo‐Saharan PBL was found to be delayed on 6 June compared with 5 June. Further north, over the part of the Sahara not impacted by the propagating cold pool, the airborne measurements suggest that the large amounts of dust in the upper part of the Saharan aerosol layer, as observed by lidar, may also have had an impact on the Saharan PBL, by reducing the insolation in this region. The airborne observations were also used to assess the quality of the ECMWF analyses at 1200 UTC on 5 and 6 June, as well as + 36 h, + 24 h and + 12 h forecasts valid at 1200 UTC on 6 June. A quantitative improvement was seen in the 6 June forecast valid at 1200 UTC once information relevant to the propagation of the ITD was assimilated, i.e. for the + 24 h and + 12 h forecasts. The analyses were able to reproduce the northward surge of moisture associated with the cold pool, suggesting that ECMWF analyses may be used to analyze the role of propagating density currents on the so‐called monsoon onset mechanism. Copyright © 2009 Royal Meteorological Society

Multi-platform observations of a springtime case of Bodélé and Sudan dust emission, transport and scavenging over West Africa

The structure of the Saharan air layer over Niger and Benin during a major springtime dust event from the Bodele region and Sudan is investigated using airborne lidar and dropsonde measurements. Aircraft operations were conducted on 13 and 14 June 2006, within the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. Complementary ground-based and satellite observations, as well as European Centre for Medium-range Weather Forecasts analyses are used to investigate the regional aspects of emission, transport and deposition in the period from 9 to 14 June 2006, to provide a framework for the interpretation of the airborne measurements. The study details the transport patterns of dust from eastern Saharan sources towards the southwest in the springtime, and highlights the role of the intertropical discontinuity and the Darfur mountains in injecting the aerosols from Bodele and Sudan, respectively, over the monsoon flow, and in the African easterly jet (AEJ) region. It also illustrates the impact of the daily variability of the emissions in the source regions on the dust load advected westward by the AEJ and observed over Benin. The impact of a mesoscale convective system (MCS) on the dust load and vertical distribution observed over Benin and southern Niger was also investigated, nearly 12 hours after its passage over Benin. The only discernible impact on the dust distribution is observed to be associated with widespread subsidence in the wake of the MCS, over northern Benin and Niger. Wet scavenging related to convective or stratiform rain could not be observed, as processed air masses were replaced by fresh dust transported in the upper Saharan air layer by the AEJ. Over southern Benin, the dust distribution appeared to be mostly controlled by processes affecting the planetary boundary layer upstream, i.e. over Nigeria or Chad. Because springtime dust from remote eastern sources (such as observed in this case) is mainly transported into the AEJ, it could impact on the radiation budget in the AEJ region, thereby possibly modifying the West African weather at the synoptic scale.

West African Monsoon observed with ground‐based GPS receivers during African Monsoon Multidisciplinary Analysis (AMMA)

A ground‐based GPS network has been established over West Africa in the framework of African Monsoon Multidisciplinary Analysis (AMMA) in tight cooperation between French and African institutes. The experimental setup is described and preliminary highlights are given for different applications using these data. Precipitable water vapor (PWV) estimates from GPS are used for evaluating numerical weather prediction (NWP) models and radiosonde humidity data. Systematic tendency errors in model forecasts are evidenced. Correlated biases in NWP model analyses and radiosonde data are evidenced also, which emphasize the importance of radiosonde humidity data in this region. PWV and precipitation are tightly correlated at seasonal and intraseasonal timescales. Almost no precipitation occurs when PWV is smaller than 30 kg m−2. This limit in PWV also coincides well with the location of the intertropical discontinuity. Five distinct phases in the monsoon season are determined from the GPS PWV, which correspond either to transition or stationary periods of the West African Monsoon system. They may serve as a basis for characterizing interannual variability. Significant oscillations in PWV are observed with 10‐ to 15‐day and 15‐ to 20‐day periods, which suggest a strong impact of atmospheric circulation on moisture and precipitation. The presence of a diurnal cycle oscillation in PWV with marked seasonal evolutions is found. This oscillation involves namely different phasing of moisture fluxes in different layers implying the low‐level jet, the return flow, and the African Easterly Jet. The broad range of timescales observed with the GPS systems shows a high potential for investigating many atmospheric processes of the West African Monsoon.

Uplift of Saharan dust south of the intertropical discontinuity

In situ observations from a flight made during the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation (GERBILS) field campaign (June 2007) show significant dust uplift into the monsoon flow immediately south of the intertropical discontinuity in the western Sahara. Dust loadings were highest in the moist monsoon air and the observations are consistent with dust uplift by the nocturnal monsoon winds. There is some evidence that cold pools within the monsoon flow contributed to the dust uplift: regions of elevated dust, water vapor, and ozone within the monsoon air are consistent with precipitation cooling and moistening air from upper levels and the resultant dusty cold pools propagating northward. However, only southward propagating cold pool outflows could be observed in satellite imagery. Using European Centre for Medium‐Range Weather Forecasts analyses and satellite data, it is shown that the asymmetry in the seasonal dust cycle is closely related to the downdraft convective available potential energy (DCAPE) from convective storms. There is both more dust and more DCAPE during monsoon onset than during retreat. The larger DCAPE values during monsoon onset, as well as the stronger nocturnal monsoon flow and the stronger heat trough circulation, are expected to contribute to the higher dust loadings at this time. Both the monsoon flow and cold pool outflows within it result in dust uplift in the western Sahara during the monsoon onset, which is when the maximum dust uplift occurs. For dust modeling, this shows the importance of accurately modeling not only the monsoon flow itself but also deep convection and cold pools.

Dust emissions in the West African heat trough the role of the diurnal cycle and of extratropical disturbances

The summertime West African heat trough (HT) is one of the most active dust sources in the world. A detailed case study during May/June 2006 based upon analyses from the European Centre for Medium-Range Weather Forecasts and a new Meteosat dust product illustrates two important mechanisms of dust emissions in this region: (1) The dry continental-scale HT circulation exhibits a strong diurnal cycle characterized by nocturnal low-level jets and downward mixing of momentum to the surface during the build-up of the planetary boundary layer in the morning. This leads to strong gusty surface winds and dust emission, mostly along the northern side of the HT, but also within the southerly monsoon flow. Transports lead to an accumulation of dust near the axis of the HT. (2) Triggered by a lee cyclogenesis south of the Atlas Mountains, the Intertropical Discontinuity that separates dry Saharan and moist monsoonal air shifts northward and allows deep moist convection to penetrate into the Sahara. The evaporation of precipitation in the dry desert air also generates strong gusty winds and dust emissions. This study helps to clarify the physical mechanisms for a previously discovered relation between the annual cycles of dustiness on one hand and near-surface convergence and gustiness on the other hand.

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

AbstractThe 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.

Dust emissions over the Sahel associated with the West African monsoon intertropical discontinuity region: A representative case‐study

AbstractNear‐dawn airborne lidar and dropsonde observations acquired on 7 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1, were used to investigate dust mobilization, lifting and transport in the intertropical discontinuity (ITD) region over western Niger. Atmospheric reflectivity data from the LEANDRE 2 lidar system enabled us to analyse the structure of dust plumes in the context of wind and thermodynamic information provided by the WIND lidar system and dropsondes. Dust mobilization was mainly observed in two locations: (a) within the monsoon flow as the result of the passage of a density current originating from a mesoscale convective system over southwest Niger, and (b) at the leading edge of the monsoon flow where the near‐surface winds and turbulence were strong, because the monsoon flow was behaving as an intrusive density current. The circulation in the head of the monsoon density current lifted the mobilized dust towards the wake, along an isentropic surface. Behind and away from the leading edge, some of the mobilized dust was observed to mix across the monsoon–harmattan interface, due to the existence of mechanical shear above the monsoon layer. The dust thus becomes available for long‐range transport by the harmattan. Because dust sources are widespread over the Sahel and presumably active on many days when the ITD is located in this region during summer, dust emissions associated with the described mechanism may influence the radiation budget over West Africa. Copyright © 2008 Royal Meteorological Society

Boundary layer observations in West Africa using a novel microwave radiometer

Boundary layer measurements in Nangatchori, Benin were performed over the period of one full year (2006) using a novel ground-based microwave profiler and additional remote-sensing instruments. In this paper, the diurnal cycle of the ITD (Inter-Tropical Discontinuity) in the transition period between dry and wet season during the month of April is described in detail. Dry air masses from the north (Sahel) and moist air from the south (tropical Atlantic Ocean) cause very sharp temperature and humidity gradients in the low troposphere over West Africa. Continuous observations of these phenomena in terms of temperature and humidity profiles have been achieved for the first time with a high temporal resolution of less than 15 minutes. Especially the ability of the microwave radiometer to provide temperature profiles with high vertical resolution through multi-angle measurements gives detailed boundary layer information in conjunction with meteorological tower observations. Together with additional lidar ceilometer observations of aerosol backscatter the change of air masses can be seen very well. The high data availability of > 85 % allows a statistical analysis of the full month of April in which Nangatchori comes increasingly under the influence of tropical air. Thus the data set is well suited for an improved process understanding, model evaluation in a data sparse area, and possibly together with additional observations the development of improved boundary layer parameterizations for atmospheric models.