JET2000 Project

The aims of JET2000 were to provide detailed synoptic observations of the African Easterly Jet (AEJ) and African Easterly Waves (AEWs) over tropical West Africa. One focus of these observations was accurate measurements of the boundary layer state as it varies across the baroclinic zone (the jet) and as it varies in time.

Through combining the observations taken using the C-130 aircraft with different modelling studies we aimed to advance the understanding of the AEJ and AEWs. We also aimed to ascertain the impact of including extra observations in the data sparse West African region on operational analyses and numerical weather prediction (NWP) forecasts, both locally over West Africa and downstream over the tropical Atlantic.

Outcomes

JET2000 conducted a successful airborne field campaign over about 7 days in August 2000, which is described by Thorncroft et al. (2003). With the resulting observational data at its heart, the project had a number of significant outcomes which were much broader than the original aims of the project, and have had a significant legacy.

1. Monsoon structure and physics

Analysis of our observations has led to a new, quantitative dynamical and thermodynamical description of the African Easterly Jet system (Parker et al. 2005a). Significantly, this view moves beyond an ‘airmass’ explanation (in which regions of air are separated by a discrete ‘front’): now we are able to describe the AEJ system as a set of interacting convective regimes, between the Inter-tropical Convergence Zone (ITCZ) and the dry convection of the Sahara. Intrusion of Saharan air above the monsoon layer controls the cloud types and therefore the dynamics and transport properties of the system. This new interpretation has been very influential in steering other research activities and priorities for weather and climate model development.

Our observations with the aircraft pushed into the southern part of the Sahara desert, and found some remarkable structures in the very dry atmosphere of this region. The Intertropical discontinuity (ITD) representing the transition between the monsoon and the Saharan air was found to be a very distinct, fine-scale feature. These first observations were an important stimulus to more comprehensive studies of the Saharan atmosphere in the years to follow (e.g. Flamant et al. 2007).

2. Diurnal cycle of the monsoon

Our results highlighted the importance of the diurnal cycle of convection in the monsoon circulation (Parker et al. 2005b). By day, the monsoon winds are suppressed by dry convective turbulence, and the monsoon flow is relatively weak. After sunset, the monsoon accelerates towards the “heat low” over the Sahara. The monsoon can be viewed as a nocturnal wind system, and these are the processes which need to be considered for accurate prediction of the monsoon and its rainfall. This diurnal cycle also tells us about the role of the monsoon system in the transport of atmospheric constituents into the global atmosphere.

3. Land-atmosphere interaction

We found, for the first time, very significant atmospheric response to the soil moisture on the land surface (Taylor et al. 2003). By day, the atmospheric state over the Sahel maps very closely onto the underlying patterns of soil moisture, with cool, humid air above a wet surface and hot, dry air above a dry surface. These patterns are distinct on very small scales, down to a few kilometres, and have a strong influence on the local climates on these scales. Through these physical processes there can be feedbacks in rainfall on timescales of a few days.

These first results have subsequently been investigated in much more detail, leading to conclusions regarding rainfall feedbacks globally.

4. Evaluation of forecast models, and impact of observations on models

From the observations we found, somewhat to our surprise, that the analysis fields produced by numerical weather prediction (NWP) models have useful skill for the tropospheric winds and thermodynamics over West Africa (Thorncroft et al 2003). The AEJ was analysed by models accurately, to within a degree or so in latitude, and a couple of m/s in strength. Therefore we can have confidence in climatological analyses of these parameters, obtained from NWP analyses. However, the NWP model forecasts showed a different story, with a systematic error in model forecasts for the region (very similar error for both the Met Office and ECMWF models), apparently associated with error in the thermodynamics of the boundary layer of the analysis. These results pointed to fundamental problems in the representation of physical processes in the models. Later work, in the Cascade project, has shown how these boundary layer errors can be understood in terms of problems in the way the models represent the diurnal cycle of deep convective cloud, and the “cold pools” of air which are generated by the storms.

Our observation that forecasts have systematic errors, while analyses are not too bad, raises the question of how the observing network, known to be pretty sparse in this part of the world, is managing to generate good forecasts through data assimilation. Further ‘Data denial’ experiments (also known as OSSEs) have explored the relative impacts of different datasets on the generation of a good analysis from poor forecasts. While all data sources influence the analysis, these tests indicated the importance of radiosonde observations relative to other forms of data (Tompkins et al. 2005).

These operational NWP results have had a big impact on the international research community, feeding into the strategy for long-term observations and monitoring over the continent through the CLIVAR/VACS process and through research-led observations in the AMMA programme.

5. Impact on other programmes of research

The results of JET2000 have done much to shape the AMMA-EU European Framework 6 project, which was awarded €11.7 M funding. The key partners in JET2000 (U. Leeds, SUNY at Albany, CEH Wallingford, ECMWF, Met Office) are central partners in the international AMMA programme.

The JET2000 data has been used independently by colleagues at: the Met Office, ECMWF, Meteo-France (2000 was designated a special year for intercomparison studies in France), UEA, and the University of Cologne.

The observational data were supplied to all the local African Meteorological  Services on CD-ROM. African PhD students have also accessed the data via the BADC archive.

6. Weather forecasting

In JET2000 we had to plan research flights in very complex atmospheric conditions. We learned that there were no references or textbooks describing West African weather and forecasting (nor any for the tropics). In the event, the research scientist Mariane Diop-Kane, who is also an experienced weather forecaster from Dakar, worked in the Met Office in Bracknell to make bespoke forecasts for the experiment. We talked about the need for a “Forecasters’ Handbook” for the region: this book was finally published in 2017:

Meteorology of Tropical West Africa: The Forecasters' Handbook
Douglas J. Parker, Mariane Diop-Kane (Editors)
ISBN: 978-1-118-39130-3
496 pages
April 2017, Wiley-Blackwell

Figure showing the new perspective on the AEJ system which emerged from JET2000. This figure has been widely reproduced.

Papers

Thorncroft CD; Parker DJ; Burton RR; Diop M; Ayers JH; Barjat H; Devereau S; Diongue A; Dumelow R; Kindred DR; Price NM; Saloum M; Tayor CM; Tompkins AM (2003) The JET2000 project - Aircraft observations of the African easterly jet and African easterly waves, B AM METEOROL SOC, 84, pp.337-351. doi: 10.1175/BAMS-84-3-337

Taylor CM; Ellis RJ; Parker DJ; Burton RR; Thorncroft CD (2003) Linking boundary-layer variability with convection: A case-study from JET2000, Q J ROY METEOR SOC, 129, pp.2233-2253. doi: 10.1256/qj.02.134

Parker DJ; Thorncroft CD; Burton RR; Diongue-Niang A (2005a) Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment, Q J ROY METEOR SOC, 131, pp.1461-1482. doi: 10.1256/qj.03.189

Parker DJ; Burton RR; Diongue-Niang A; Ellis RJ; Felton M; Taylor CM; Thorncroft CD; Bessemoulin P; Tompkins AM (2005b) The diurnal cycle of the West African monsoon circulation, Q J ROY METEOR SOC, 131, pp.2839-2860. doi: 10.1256/qj.04.52

Taylor CM; Parker DJ; Lloyd CR; Thorncroft CD (2005) Observations of synoptic-scale land surface variability and its coupling with the atmosphere, Q J ROY METEOR SOC, 131, pp.913-937. doi: 10.1256/qj.04.119

Tompkins AM; Diongue-Niang A; Parker DJ; Thorncroft CD (2005) The African easterly jet in the ECMWF Integrated Forecast System: 4D-Var analysis, Q J ROY METEOR SOC, 131, pp.2861-2885. doi: 10.1256/qj.04.136

Flamant C; Chaboureau JP; Parker DJ; Taylor CA; Cammas JP; Bock O; Timouk F; Pelon J (2007) Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter-tropical discontinuity region of the West African Monsoon, Q J ROY METEOR SOC, 133, pp.1175-1189. doi: 10.1002/qj.97

Grant

NER/A/S/1999/00092