The interactions between mesoscale convective systems (MCSs) and the synoptic scale environment over West Africa are crucial for determining the nature and variability of the weather and climate of the region. In addition to their important role in influencing West African rainfall, the mesoscale and synoptic scale weather systems (African easterly waves, AEWs) also have a key role in mobilizing and transporting dust. General understanding of the scale interactions including their impact on dust is poor and has until now been severely hindered by lack of useful observations. The related overarching aims of this research are: (i) To improve our knowledge and understanding of the interactions between MCSs and the synoptic environment, and (ii) to improve knowledge and understanding of the role MCSs and AEWs play in mobilizing and transporting dust.
Intellectual Merit Central to addressing both aims are the special observations that were made as part of the African Monsoon Multidisciplinary Analysis (AMMA), a special observing campaign over West Africa in summer 2006. Of particular importance to this research are the observations from the MIT Doppler radar. These observations will be used by the PIs to highlight the nature and variability of the intensity and structure of the MCSs. A particularly novel aspect of this work is the estimation of the 4-dimensional diabatic heating fields associated with the MCS passages. The impact these heating fields have on mesoscale potential vorticity structures will be investigated by imposing diabatic heating fields (guided by the observations) in an idealized version of the WRF regional model. A case study will be carried out that explores the multi-scale aspects of the PV field associated with the passage of an AEW and embedded MCSs during the summer of 2006 - something that has not been possible previously. The MIT radar data will also be combined with several ARM (Atmospheric Radiation Measurements) observations, including the 95 Ghz vertically-pointing Doppler radar, to explore in detail how the dust is mobilized in association with the passage of the MCSs. This analysis will be compared to the nature of the synoptic environment, discussed above, and will help to shed light on the relative contributions of Sahelian and Saharan dust to the total dust amounts observed over West Africa (and the tropical Atlantic).
Broader impacts The research has broad impacts that include: - Improving knowledge and understanding of the key weather systems in the West African Monsoon. - Educating and training three graduate students who will develop their understanding of weather and climate as well as their skills in data analysis and modeling. - Contributing to knowledge of how the variability of West African weather climate impacts downstream tropical cyclogenesis, through providing detailed analysis of the weather systems before they reach the tropical Atlantic and head for the U.S.
" has been broadly concerned with the surface sediment in the African continent north of the equator: with its transfer into the atmosphere by gusty winds, with its subsequent transport across the Atlantic Ocean to the western hemisphere, with its color varying geographically from white to yellow to red, with its magnetic properties that vary with both latitude and rainfall, and with the idea that dust raised in the Bodélé Depression of Chad could provide surface sediment (Belterra clay) in the Amazon basin of South America by long-range transport in the atmosphere. The last topic also led to an investigation linking drought and flood in the Amazon basin to variability in the hurricane season in the tropical Atlantic Ocean. These several topics all have relevance to the Earth’s climate, and are now discussed in turn in somewhat greater detail. The transfer of the fine component of sediment, otherwise known as dust, into the atmosphere in Africa has been studied with a radar in Niger, with satellite imagery, and with a continental-scale lightning location network. These studies have shown that cold air emanating from storms can routinely move over distances of a few hundred kilometers and occasionally over more than a thousand kilometers, and continually stir up dust. This same dust subsequently lofted in the strong thermals over strongly heated desert surface can accumulate in the westward moving air currents over Africa to convey the dust across the Atlantic Ocean to Barbados in the Caribbean Sea. Exceptional northward extensions of the West African monsoon that occur only a few times during a typical wet season have been shown to contribute substantially to the transport of dust out of the African continent. In northern hemisphere winter, dust is transported from Africa in the northeasterly Harmattan wind to the South American continent, and much attention has focused in recent years on the contribution of the Bodélé Depression in Chad (the so-called ‘dustiest place on Earth’) to this transport. The analysis of lead (Pb) isotopes from samples in the Bodélé Depression and samples of the Belterra clay in Brazil, the predominant surface material in the Amazon basin, has shown that dust from the Bodélé Depression cannot be the dominant contributor to surface sediment in the Amazon basin. The analysis suggests that the predominant source for Amazon sediment lies within the continent of South America. The collection of Belterra clay samples in the Amazon basin renewed our earlier interest in the droughts and floods there, as recorded by the river gauge in Manaus harbor (in Amazonas State). This led to a separate study showing that exceptional drought coincided with superlative hurricane activity in the Atlantic Ocean, and in contrast, exceptional flood accompanied greatly reduced hurricane activity. This behavior can be attributed to the variable large-scale subsidence of air over the Amazon basin in response to upwelling by hurricane-related convection in the tropical Atlantic Ocean. The color of the African surface sediment is highly variable, and is commonly white to pale yellow in the Sahara Desert, but transitions to a bright orange/red in the Sahel region, south of the Sahara. The white color of the Sahara is primarily due to quartz (Si O2) and the orange/red color to two common iron oxides: hematite (Fe2O3) and goethite (FeO(OH)). Laterite, a common geological material throughout the West African Sahel, is abundant in both of these minerals. Only small amounts of these constituents (<1% by weight) can have profound effects on albedo, a measure of the reflection of sunlight back to space from the sediment surface. The usual explanation for the diminished albedo in the Sahel (relative to the ‘white’ Sahara) is the effect of increasing vegetation, but our studies show that the mineral coloring agents dominate the geographical albedo variation down to latitudes in West Africa of ~12 N. A large collection of sediment samples over the African continent has been carried out to investigate geographical variations in hematite and goethite. The same collection of sediment samples has been used to explore magnetic properties, with one property of particular interest—the magnetic susceptibility. Consistent with other studies in other geographical locations, the magnetic susceptibility varies systematically with the climatological rainfall, with larger values in the Sahel (where rainfall is moderate) and smaller values in the Sahara Desert (and the Western Desert of Egypt), where rainfall is infrequent. The magnetic susceptibility is controlled by the ferrimagnetic iron oxides magnetite (Fe3O4) and maghemite (γ-Fe2O3), whose abundances are physically linked with the presence and action of water.
