AMMA-UK
African Monsoon Multidisciplinary Analyses – UK: AMMA-UK
Summary of the Research Programme – 5 October 2004
1. Introduction
- Africa has experienced demonstrable climatic changes in the past 3 decades, leading to severe effects on water resources, agriculture and health. However, our predictive methods for this region remain unreliable: ‘The diversity of African climates, high rainfall variability, and a very sparse observational network make predictions of future climate change very difficult at the subregional and local level’[1].
- Tropical Africa is a primary global source for biogenic precursors of key greenhouse forcing agents, yet there have never been detailed observations of the tropospheric composition in this region.
- The convective processes controlling tropical African climate and atmospheric composition are poorly observed and predicted worldwide: Africa is a natural laboratory for tropical continental climate.
An international programme, AMMA (African Monsoon Multidisciplinary Analyses), has been instigated with the aim of obtaining and using high quality interdisciplinary observations to make a step-change in our understanding of the West African Monsoon (WAM) system. This is an opportunity to address problems in our understanding of the African environment and its global interactions which is unlikely to be repeated for many decades to come. As part of the international AMMA programme we will undertake a substantial effort, to measure and explain the physical and chemical processes which determine the local climate and its global impacts. At the heart of AMMA-UK is the study of the interaction of the land surface with the atmosphere over the highly variable land surface types and soil moisture patterns of West Africa. This land-atmosphere interaction is critical to the monsoon state, the continental water and energy cycles and the global atmospheric composition. In order to observe these processes, an interdisciplinary approach is necessary, linking long-term measurements of the seasonal climatic changes in the land surface and atmosphere with short term, intensive measurements of the coupled system. A suite of atmospheric tracers will be used to explain the dynamical properties of the system, and accurate modelling of the dynamics will be used to explain the emission and export of trace gases and particles.
- Aims of AMMA-UK
We have two over-arching aims:
(1) To improve our scientific understanding of the WAM and its interaction with the physical, chemical and biological environment regionally and globally.
(2) To ensure that this multidisciplinary research addresses the needs of prediction and decision making.
There are several specific goals for this research programme. We here organise the research into five work packages (WPs) around common themes. Many of these goals will require collaborative work between staff attached to different WPs. Detailed objectives of each work package are specified in section 5 and summarised on p. 11. Reference is also made to Observational Work Packages (OWPs: section 6).
WP1: Land surface and atmosphere interactions.We aim for the first time to quantify the seasonal energy cycle responsible for the WAM, using detailed in-situ observations, remote sensing, and surface modelling.
WP2: WAM microclimate and applications.We aim to quantify the microclimate of the region in the sub-canopy layer in order to downscale global model predictions and earth observation products to the scales and parameters required for disease prediction.
WP3: Convection and WAM dynamics. To explain the basic dynamics and transport properties of the WAM system, including the diurnal cycle of the boundary layer control on the monsoon, and the response to shallow and deep cumulus convection.
WP4: Tropospheric composition. We aim to quantify the role of the WAM system in the emission of biogenic species and their impact on the global atmosphere.
WP5: The tropical tropopause layer (TTL). To quantify the impact of deep convective transport on the TTL.
3. Science of the WAM
3.1 Basic climate
The migration of the ITCZ associated with the annual monsoons[2] controls the bulk of the rainfall over tropical Africa. The WAM consists of southwesterly winds north of the equator, which bring humidity and rainfall into the continent, as far north as the southern Sahara and as far east as Sudan and Ethiopia. This produces a strong meridional gradient in rainfall (~1 mm / km) stretching from the tropical forest of the Guinea coast to the Sahara desert in the north. There is significant variability in the rainfall on a spectrum of timescales including, most notoriously the persistent drought which has afflicted the Sahel since the 1970s[3]. The long-term effects of the reduced rainfall have included agricultural failure, population migration and severe health issues. Evidence suggests that this decadal variability is due to a combination of remote forcing, primarily through SST patterns[4], and internal dynamics of the land-atmosphere system[5],[6].
The existence of the WAM arises as a responseis due to the meridional gradients in surface fluxes, and hence boundary layer entropy, from the Atlantic Ocean in the Gulf of Guinea northwards across the continent. There is a strong control on the WAM by SSTs in the Gulf of Guinea[7]. However, Zheng et al[8] illustrated the importance of internal feedbacks between rainfall and soil moisture for explaining the persistence of the anomalies through the wet season. From observational analyses, similar feedbacks have been found to operate at much smaller scales[9], producing local rainfall gradients of tens of mm / km over a season. The diseases meningitis and malaria are closely linked to thesesuch local-scale rainfall patterns and are widespread in the Sahel.
The continental gradients in net radiation depend on the distribution of surface albedo and temperature. These exhibit strong seasonality, linked to the timing of the wet season and rapid growth of the vegetation layer. The partition of radiation between sensible and latent heat is also critical: evaporation is strongly moisture-limited in the region. Observations from HAPEX-Sahel[10] show the very strong impact on surface fluxes of rain in the preceding days on surface fluxes, as the soil surface dries out. Further south, the vegetation is denser with less exposed soil, and fluxes depend on soil moisture throughout the root zone so that the ‘memory’ of antecedent rainfall is longer in this region and there is the potential for a hydrological feedback from one year to the next[11]. To test this hypothesis, and to explore the influence of boundary layer entropy gradients on the WAM, land surface observations over the full range of West African conditions are essential for several annual cycles. These observations are central to this proposal (WP1 and OWP1).
Sultan and Janicot[12] recently identified intraseasonal variability in the rainfall, on time scales of 15 and 30-60 days. The causes of such variability are not well understood, but there is evidence of a link to global tropical dynamics,[13] and land surface processes[14]. Such intraseasonal variability is critical to agriculture and to the occurrence of disease outbreaks, and forecasting of this phenomenon is of paramount importance.
The low level southwesterly WAM winds are capped by the African Easterly Jet (AEJ), with a peak of about 12-15m/s at around 600 hPa and 15N2. The AEJ’s easterly shear with height is an important factor in the existence of Mesoscale Convective Systems (MCSs), which travel westwards and deliver much of the rainfall in the Sahel. The AEJ also supports synoptic activity in the form of African Easterly Waves (AEWs), which modulate rainfall over the continent and are the primary source of precursor vortices for hurricanes in the Atlantic. The structure of AEWs is still not well defined due to the lack of routine observations:[15] northern vortices in particular have never been well sampled. New results based on Meteosat imagery[16] have shown that in the northern Sahel there is a high degree of coupling between near-surface soil moisture and the synoptic rainfall forcing. Similarly, MCS events force AEW development through PV modification[17], while AEWs modulate the environment in which MCSs develop and persist[18]. There are few case studies of AEWs and those that exist[19] are based on model analyses and very sparse upper air data. We wish to measure AEW structure at good resolution, in order to generate and test reliable model simulations of these events, and thereby begin to explain the role of land surface feedbacks and MCS forcing in their dynamics (WP3.2, 3.4).
3.2 Basic WAM dynamics and associated transport processes
The WAM region is an archetype for the effects of dry and moist tropical convection on convective transport. The system is controlled by dry convection over the Sahara and cumulonimbus convection in the ITCZ further south[20]. In between these extremes, the monsoon region represents a transition zone: Fig. 1 outlines the basic climatic zones. The system is characterized by a monsoon layer at lower levels in which the air is connected to the land surface by diurnally-varying dry convection and shallow cumulus. Above the monsoon layer is the Saharan Air Layer (SAL), which appears to have a rather adiabatic structure (closely linked to the temperature of the Saharan boundary layer) and extends up to about 550 hPa. The importance of the SAL in the climate of west Africa and the Atlantic has been stressed by numerous authors[21], and the export of dust in this layer has an influence over the whole of the tropical Atlantic, and beyond.
Intermittent MCSs (return periods of a few days at Niamey) transport chemical species from the mixed layer and the troposphere high into the Tropical Tropopause Layer (TTL), with detrainment likely at significant levels of changing stability, notably the top of the SAL[22]. Such deep convection can transport relatively short-lived compounds to the upper troposphere, which otherwise would not normally reach these altitudes. Once in the upper troposphere the lifetimes of many of these trace constituents are much longer and they can be transported long distances, impacting the composition of the TTL on regional and global scales. Downdraughts in convective systems also carry midlevel air to the surface rapidly.
MCS events in the Sahel occur intermittently, followed by periods of more regular evolution. Thus we can divide the monsoon transport processes into: (i) deep convective transport; (ii) near-adiabatic advection in the free troposphere, which is particularly important at night when the CBL has collapsed[23],[24]; and (iii) daytime mixing in the CBL and shallow cumulus. The diurnal cycle of the CBL implies an important distinction between (ii) and (iii). At night, chemical species at low levels may be advected meridionally by the monsoon circulation, with significant ‘upgliding’ and ‘downgliding’ due to the baroclinicity (as in the filamentation of equivalent potential temperature between 11N and 15N, 700-800 hPa in Fig. 2). There is good evidence that local inhomogeneities in soil moisture and convective events break this advection into low-level mesoscale flows (e.g. in Fig 2 at A and C, below 800 hPa) rather than a simple large scale monsoon circulation. In contrast, during the day the low level air is mixed vertically in the dry convective mixed layer, and by shallow cumulus, with horizontal advection playing a much weaker role[25]. In this daytime regime, the coupling of the soil moisture with the CBL activity is strong, and local, on scales down to 10km or smaller (see Fig. 3) and we anticipate associated changes in chemical emissions. Fig. 4 shows how high the sensitivity of the emissions can be under rapid changes in surface and CBL temperature.
In this project, we will measure the close environment of MCSs, and observe their upper level outflow, to assess their capacity for rapid uplift of different species. Over subsequent days we will observe the slower response of the system over several daily cycles, in the monsoon dynamics, the land surface recovery, and the emission and transport and transformation of biogenic species. We will use the chemical tracers to help explain the monsoon transport processes, and use land surface observations to help explain the chemical sources.
3.3 Atmospheric composition of the WAM and its global consequences
Systematic observations of the atmospheric chemistry over West Africa have never before been made. In the summer monsoon season, the region is believed to be responsible for peak global biogenic emissions, well removed spatially from anthropogenic emissions and temporally distinct from the winter period of biomass burning emissions. Anthropogenic fossil fuel and biofuel emissions are small and biomass burning, although dominant in the dry season, is negligible during the monsoon. For these reasons the WAM provides a much ‘cleaner’ signal in the study of biogenic emissions and their regional effects on tropospheric chemical composition than would be afforded in other locations and times. This understanding is important since isoprene emissions are the largest source of reactive carbon in the atmosphere, completely dwarfing emissions of anthropogenic VOCs (volatile organic compounds). Furthermore, natural emissions are expected to change with a changing climate; higher temperatures and rainfall will likely increase soil NOX emissions, while higher carbon dioxide and temperatures will likely increase isoprene emissions (Fig. 4).
The biogenic VOCs react rapidly with OH and O3, but their oxidation also leads to HOX precursors and to the generation of secondary organic aerosols. NOX affects the HOX budget and the reaction of NO with HO2 and organic peroxy radicals (RO2) leads to the production of O3. VOCs and NOX are thus key components in determining the oxidizing capacity and radiative properties of the atmosphere. However the degree to which the natural emissions of VOCs and NOX from the WAM region impact the global atmospheric composition depends on the export of these and their degradation products to other regions. Since many of these products can be removed via aqueous and heterogeneous processing within clouds, the interaction between the monsoon dynamics and chemistry is again a critical factor. Little particulate characterisation has been carried out in the West African region; however, comparison with the LBA experiments in Amazonia indicates that secondary organic particulate matter may be the dominant component of submicron particles in the monsoon layer. These are likely to be relatively hydrophobic, which affects their ambient size, scattering properties and cloud nucleating ability. In contrast, dust will be a significant source of particulate in the SAL aloft. This is in contrast to the dry season when the dust laden SAL is entrained into lower level air containing carbon rich absorbing particles produced by biomass burning which are effective CCN[26]. The latter will be studied during the winter DABEX experiment[27]. The two seasons will therefore have distinctly different particulate which will be investigated in this project.
For these reasons, one component of this project will focus on the emission, processing, export and impact of biogenic species. The interactions of the land surface and monsoon dynamics with the chemistry are critical to this analysis. The strong meridional gradients of the vegetation types and soil moisture of West Africa means that there are strong gradients in emissions, and small changes in seasonal, interannual or synoptic climate will have large effects on the emissions from West Africa. Through our understanding of land surface processes in the region, and systematic observation of the coupled land-atmosphere system, via surface observations (OWP1,2, and the AERONET data) and aircraft observations (OWP4), we aim to quantify the processes which link land surface variability and atmospheric composition.
3.4 WAM and the global Tropical Tropopause Layer
Transport of chemical species from the troposphere into the stratosphere is one of the key questions facing global climate chemistry. In particular, the movement of trace gases within tropical cumulonimbus systems to and beyond the tropopause has global importance in the budgets of halogenated species (especially short-lived species). During the peak of the WAM a range of biogenic species are likely to be uplifted in convective events, but there have been few observations to measure this process. A recently funded EU Integrated Project led by Pyle and Harris - SCOUT - is coordinating European activity to make systematic observations of the TTL dynamics and chemistry, and there are already strong links between the SCOUT and AMMA observational strategies.
Our perception of the tropical tropopause region has changed substantially over the last few years. It is now recognised that penetration of tropical convection to the cold point is relatively rare, particularly over the oceans[28] and therefore that convective transport plays only a minor role in transport across the cold-point tropopause. The majority of convection apparently penetrates only to around 13km. In the region between 13km and the cold point (at about 17km) the frequency of convective penetration gradually reduces with height. It is this region between the level of maximum convective outflow and the cold point that is now often called the TTL. We need to measure the outflow from MCSs within the TTL, and relate it to the tropospheric environment and inflow to the storms. With a radiosounding campaign of unprecedented temporal and spatial resolution (OWP3 and OWP5), we plan to resolve the structures of TTL response to MCS events. Such observations will be a necessary input to modelling studies within AMMA and for our collaborators within SCOUT.