scholarly journals Seasonal Ensemble Predictions of West African Monsoon Precipitation in the ECMWF System 3 with a Focus on the AMMA Special Observing Period in 2006

2010 ◽  
Vol 25 (2) ◽  
pp. 768-788 ◽  
Author(s):  
Adrian M. Tompkins ◽  
Laura Feudale
Keyword(s):  
West Africa ◽  
Monsoon Rainfall ◽  
Shortwave Radiation ◽  
Gulf Of Guinea ◽  
Monsoon Precipitation ◽  
Seasonal Forecasts ◽  
The West ◽  
African Monsoon ◽  
The North ◽  
Sst Bias

Abstract The West Africa monsoon precipitation of the ECMWF operational Seasonal Forecast System (SYS3) is evaluated at a lead time of 2–4 months in a 49-yr hindcast dataset, with special attention paid to the African Monsoon Multidisciplinary Analysis (AMMA) special observation period during 2006. In both the climatology and the year 2006 the SYS3 reproduces the progression of the West Africa monsoon but with a number of differences, most notably a southerly shift of the precipitation in the main monsoon months of July and August and the lack of preonset rainfall suppression and sudden onset jump. The model skill at predicting summer monsoon rainfall anomalies has increased in recent years indicating improvements in the ocean analysis since the 1990s. Examination of other model fields shows a widespread warm sea surface temperature (SST) bias exceeding 1.5 K in the Gulf of Guinea throughout the monsoon months in addition to a cold bias in the North Atlantic, which would both tend to enhance rainfall over the Gulf of Guinea coast at the expense of the monsoon rainfall over the Sahel. Seasonal forecasts were repeated for 2006 using the same release of the atmospheric forecast model forced by observed SSTs, and the monsoon rainfall reverts to its observed position, indicating the importance of the SST biases. A lack of stratocumulus off the west coast of Africa in SYS3 was hypothesized as a possible cause of the systematic rain and SST biases. Two more sets of ensembles were thus conducted with atmospheric model upgrades designed to tackle radiation, deep convection, and turbulence deficiencies. While these enhancements improve the simulation of stratocumulus significantly, it is found that the improvement in the warm SST bias is limited in scope to the southern cold tongue region. In contrast, the changes to the representation of convection cause an increase in surface downwelling shortwave radiation that, combined with latent heat flux changes associated with the wind stress field, increases the SST warm bias on and to the north of the equator. Thus, while the precipitation shortfall in the Sahel is reduced with the new physics, the overestimated rainfall of SYS3 in the coastal region is further enhanced, degrading the model systematic errors overall in the West Africa region. Finally, the difference in the systematic biases between the coupled and uncoupled systems was noted to be an impediment to the development of seamless forecasting systems.

scholarly journals On What Scale Can We Predict the Agronomic Onset of the West African Monsoon?

2016 ◽  
Vol 144 (4) ◽  
pp. 1571-1589 ◽  
Author(s):  
Rory G. J. Fitzpatrick ◽  
Caroline L. Bain ◽  
Peter Knippertz ◽  
John H. Marsham ◽  
Douglas J. Parker
Keyword(s):  
West Africa ◽  
Noise Analysis ◽  
West African Monsoon ◽  
West African ◽  
New Method ◽  
The West ◽  
African Monsoon ◽  
Spatial Consistency ◽  
Local Stakeholders ◽  
Local Rainfall

Abstract Accurate prediction of the commencement of local rainfall over West Africa can provide vital information for local stakeholders and regional planners. However, in comparison with analysis of the regional onset of the West African monsoon, the spatial variability of the local monsoon onset has not been extensively explored. One of the main reasons behind the lack of local onset forecast analysis is the spatial noisiness of local rainfall. A new method that evaluates the spatial scale at which local onsets are coherent across West Africa is presented. This new method can be thought of as analogous to a regional signal against local noise analysis of onset. This method highlights regions where local onsets exhibit a quantifiable degree of spatial consistency (denoted local onset regions or LORs). It is found that local onsets exhibit a useful amount of spatial agreement, with LORs apparent across the entire studied domain; this is in contrast to previously found results. Identifying local onset regions and understanding their variability can provide important insight into the spatial limit of monsoon predictability. While local onset regions can be found over West Africa, their size is much smaller than the scale found for seasonal rainfall homogeneity. A potential use of local onset regions is presented that shows the link between the annual intertropical front progression and local agronomic onset.

The Role of Intraseasonal Variability in the Nature of Asian Monsoon Precipitation

2007 ◽  
Vol 20 (17) ◽  
pp. 4402-4424 ◽  
Author(s):  
Carlos D. Hoyos ◽  
Peter J. Webster
Keyword(s):  
Indian Ocean ◽  
Time Scales ◽  
Temporal Variability ◽  
Asian Monsoon ◽  
Monsoon Rainfall ◽  
Mountain Range ◽  
Monsoon Precipitation ◽  
The West ◽  
The Mean ◽  
Rainfall Estimates

Abstract The structure of the mean precipitation of the south Asian monsoon is spatially complex. Embedded in a broad precipitation maximum extending eastward from 70°E to the northwest tropical Pacific Ocean are strong local maxima to the west of the Western Ghats mountain range of India, in Cambodia extending into the eastern China Sea, and over the eastern tropical Indian Ocean and the Bay of Bengal (BoB), where the strongest large-scale global maximum in precipitation is located. In general, the maximum precipitation occurs over the oceans and not over the land regions. Distinct temporal variability also exists with time scales ranging from days to decades. Neither the spatial nor temporal variability of the monsoon can be explained simply as the response to the cross-equatorial pressure gradient force between the continental regions of Asia and the oceans of the Southern Hemisphere, as suggested in classical descriptions of the monsoon. Monthly (1979–2005) and daily (1997–present) rainfall estimates from the Global Precipitation Climatology Project (GPCP), 3-hourly (1998–present) rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) estimates of sea surface temperature (SST), reanalysis products, and satellite-determined outgoing longwave radiation (OLR) data were used as the basis of a detailed diagnostic study to explore the physical basis of the spatial and temporal nature of monsoon precipitation. Propagation characteristics of the monsoon intraseasonal oscillations (MISOs) and biweekly signals from the South China Sea, coupled with local and regional effects of orography and land–atmosphere feedbacks are found to modulate and determine the locations of the mean precipitation patterns. Long-term variability is found to be associated with remote climate forcing from phenomena such as El Niño–Southern Oscillation (ENSO), but with an impact that changes interdecadally, producing incoherent responses of regional rainfall. A proportion of the interannual modulation of monsoon rainfall is found to be the direct result of the cumulative effect of rainfall variability on intraseasonal (25–80 day) time scales over the Indian Ocean. MISOs are shown to be the main modulator of weather events and encompass most synoptic activity. Composite analysis shows that the cyclonic system associated with the northward propagation of a MISO event from the equatorial Indian Ocean tends to drive moist air toward the Burma mountain range and, in so doing, enhances rainfall considerably in the northeast corner of the bay, explaining much of the observed summer maximum oriented parallel to the mountains. Similar interplay occurs to the west of the Ghats. While orography does not seem to play a defining role in MISO evolution in any part of the basin, it directly influences the cumulative MISO-associated rainfall, thus defining the observed mean seasonal pattern. This is an important conclusion since it suggests that in order for the climate models to reproduce the observed seasonal monsoon rainfall structure, MISO activity needs to be well simulated and sharp mountain ranges well represented.

The Royal Navy and West Africa, 1843-1857

2008 ◽  
pp. 133-168
Author(s):  
Mark C. Hunter
Keyword(s):  
West Africa ◽  
Free Trade ◽  
West African ◽  
Economic Interest ◽  
Royal Navy ◽  
The West ◽  
The North ◽  
West African Coast ◽  
African Coast ◽  
American Diplomacy

This chapter analyses the British naval policies concerning West Africa between 1843 and 1857. During this period, Britain sought to encourage legitimate commerce and curtail slavery for its own economic interest, while domestically America feared the British domination of the West African coast. As such, suspicion and mistrust was rife between the two nations, and is in great detail via the abolitionist activity in the North of England; the actions of free traders and slavers; Royal Navy operations; the competition for trade between Britain and France; Commodore Charles Hotham’s slavery suppressing naval strategy; British free trade treaties; and the naval methods of enforcing British goals. It concludes in 1857, with British interests torn between strategic naval aims and domestic pressures, and British and American diplomacy still tense over West African policies.

scholarly journals Coupled Model Simulations of the West African Monsoon System: Twentieth- and Twenty-First-Century Simulations

2006 ◽  
Vol 19 (15) ◽  
pp. 3681-3703 ◽  
Author(s):  
Kerry H. Cook ◽  
Edward K. Vizy
Keyword(s):  
Twentieth Century ◽  
West African Monsoon ◽  
West African ◽  
First Century ◽  
Gulf Of Guinea ◽  
Physical Processes ◽  
The West ◽  
African Monsoon ◽  
The Third ◽  
Twenty First Century

Abstract The ability of coupled GCMs to correctly simulate the climatology and a prominent mode of variability of the West African monsoon is evaluated, and the results are used to make informed decisions about which models may be producing more reliable projections of future climate in this region. The integrations were made available by the Program for Climate Model Diagnosis and Intercomparison for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The evaluation emphasizes the circulation characteristics that support the precipitation climatology, and the physical processes of a “rainfall dipole” variability mode that is often associated with dry conditions in the Sahel when SSTs in the Gulf of Guinea are anomalously warm. Based on the quality of their twentieth-century simulations over West Africa in summer, three GCMs are chosen for analysis of the twenty-first century integrations under various assumptions about future greenhouse gas increases. Each of these models behaves differently in the twenty-first-century simulations. One model simulates severe drying across the Sahel in the later part of the twenty-first century, while another projects quite wet conditions throughout the twenty-first century. In the third model, warming in the Gulf of Guinea leads to more modest drying in the Sahel due to a doubling of the number of anomalously dry years by the end of the century. An evaluation of the physical processes that cause these climate changes, in the context of the understanding about how the system works in the twentieth century, suggests that the third model provides the most reasonable projection of the twenty-first-century climate.

scholarly journals Seasonal forecasts for regional onset of the West African monsoon

2012 ◽  
Vol 40 (11-12) ◽  
pp. 3047-3070 ◽  
Author(s):  
Michael Vellinga ◽  
Alberto Arribas ◽  
Richard Graham
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Seasonal Forecasts ◽  
The West ◽  
African Monsoon

scholarly journals Rainfall retrievals over West Africa using SEVIRI: evaluation with TRMM-PR and monitoring of the daylight time monsoon progression

2010 ◽  
Vol 7 (4) ◽  
pp. 6351-6380 ◽  
Author(s):  
E. L. A. Wolters ◽  
B. J. J. M. van den Hurk ◽  
R. A. Roebeling
Keyword(s):  
West Africa ◽  
Near Infrared ◽  
Rain Rate ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Cloud Particle ◽  
African Monsoon ◽  
Trmm Pr ◽  
Rain Rates

Abstract. This paper describes the application of the KNMI cloud physical properties – precipitation properties (CPP-PP) algorithm over West Africa. The algorithm combines condensed water path (CWP), cloud phase (CPH), cloud particle effective radius (re), and cloud-top temperature (CTT) information, retrieved from visible, near-infrared and infrared observations of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat-9 to estimate precipitation occurrence and intensity. It is investigated whether the CPP-PP algorithm is capable of retrieving rain occurrence and intensity over West Africa with a sufficient accuracy, using tropical monsoon measurement mission precipitation radar (TRMM-PR) and a small number of rain gauge observations as reference. As a second goal, it is assessed whether SEVIRI is capable of monitoring both the seasonal and synoptical evolution of the West African monsoon (WAM). It is shown that the SEVIRI-detected rainfall area agrees well with TRMM-PR, having a correlation coefficient of 0.86, with the areal extent of rainfall by SEVIRI being ~10% larger than TRMM-PR. The mean retrieved rain rate from CPP-PP is about 8% higher than from TRMM-PR. The frequency distributions of rain rate reveal that the median rain rates of CPP-PP and TRMM-PR are similar. However, rain rates >7 mm h−1 are retrieved more frequently by SEVIRI than by TRMM-PR, which is partly explained by known biases in TRMM-PR. Finally, it is illustrated that both the seasonal and synoptical time scale of the WAM can be well detected from SEVIRI daytime observations. It was found that the daytime westward MCS travel speed fluctuates between 50 and 60 km h−1. Furthermore, the ratio of MCS precipitation to the total precipitation was estimated to be about 27%. Our results indicate that rainfall retrievals from SEVIRI can be used to monitor the West African monsoon.

scholarly journals Aerosol influences on low-level clouds in the West African monsoon

2019 ◽  
Author(s):  
Jonathan W. Taylor ◽  
Sophie L. Haslett ◽  
Keith Bower ◽  
Michael Flynn ◽  
Ian Crawford ◽  
...  
Keyword(s):  
West Africa ◽  
Southern Hemisphere ◽  
Biomass Burning ◽  
West African Monsoon ◽  
Cloud Microphysics ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Low Level ◽  
Local Emissions

Abstract. Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of regional, in situ measurements of cloud microphysics, taken during June – July 2016, as part of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign, assessing spatial and temporal variation in the properties of these clouds. LLC developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Additional sea breeze clouds developed near the coast in the late morning, reaching a maximum extent around 12:00 LST. Regional variation in LLC cover was largely determined by the modulation of the cool maritime inflow by the local orography, with peaks on the upwind side of hills and minima on the leeward sides. In the broad-scale cloud field, no lasting impacts related to anthropogenic aerosol were observed downwind of major population centres. The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities, but were not statistically significant across the region. The majority of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the southern hemisphere. Consequently, all clouds measured (inland and offshore) had significantly higher CDNC and lower effective radius than clouds over the remote south Atlantic from literature. A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated CDNC by 13–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the southern hemisphere is likely to dampen any effect of these increased local emissions on cloud-aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African monsoon.

scholarly journals Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene

2017 ◽  
Vol 30 (19) ◽  
pp. 7621-7642 ◽  
Author(s):  
Marco Gaetani ◽  
Gabriele Messori ◽  
Qiong Zhang ◽  
Cyrille Flamant ◽  
Francesco S. R. Pausata
Keyword(s):  
West Africa ◽  
Radiative Forcing ◽  
Numerical Models ◽  
Substantial Reduction ◽  
West African Monsoon ◽  
West African ◽  
Heat Fluxes ◽  
Dust Concentration ◽  
The West ◽  
African Monsoon

Abstract Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (~6.5°) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.

Marriage among !Kung Bushmen

Africa ◽  
1959 ◽  
Vol 29 (4) ◽  
pp. 335-365 ◽  
Author(s):  
Lorna Marshall
Keyword(s):  
West Africa ◽  
The West ◽  
The North ◽  
South West ◽  
South West Africa ◽  
Opening Paragraph

Opening ParagraphBecause within the area we indicate by shading on the map the !Kung Bushmen intermarry among themselves, by custom and preference, members of the Harvard Peabody Smithsonian Kalahari Expeditions needed a convenient way of referring to that area as a unit and arbitrarily called it the region of Nyae Nyae.Nyae Nyae is a corruption of the !Kung name //Nua!ei. The name Nyae Nyae refers strictly to a group of pans in South West Africa (S.W.A.) centred approximately at Gautscha Pan at about 19° 48′ 30″ S, 20° 34′ 36″ E. We extend the application of the name to an area around the pans of about 10,000 square miles, lying for the most part in S.W.A. but reaching some miles over the border of the Bechuanaland Protectorate (B.P.). There are no strictly conceived boundaries around the area. We can only vaguely define it by saying that it does not include Karakuwise to the west or Chadum to the north. It does not, we think, reach eastward much farther than Kai Kai, or southward much beyond Blaubush Pan (40 or 50 miles south of Gam).

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