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 FENNEC

University of Oxford University of Leeds UCL Imperial College London University of Reading

Fennec - The Saharan Climate System: A NERC consortium proposal

Summary

The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense solar heating develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. Temperatures in the high 40s, with uplift of dry air through more than 6000m of the atmosphere, are routine in what is thought to be the deepest such layer on the planet. This large zone is also where the thickest layer of dust anywhere in the Earth's atmosphere is found. The atmospheric aerosol loading and thermodynamics over the Sahara are unique, and have major impacts on the climate of the whole North African sector, Europe and the Atlantic. The large low pressure system drives the West African Monsoon and the dry, dusty air layers are closely related to the tropical cyclones which form over the Atlantic Ocean. Likewise, the dusty air strongly influences atmospheric heating, a process which is poorly understood.

Weather and climate prediction models show significant systematic errors over the Sahara desert manifested as differences in radiation reaching and leaving the surface, surface temperature, winds, and in representation of the boundary layer. Potential improvements in these models are currently limited due to the lack of observations in the central Saharan region with which models can be confronted. Current datasets are limited to some stations on the desert margins, and data from a handful of research flights in 2006/7. Fundamental uncertainties exist in the source areas, amount and properties of dust, the surface reflectivity, and the variability of the boundary layer structure (including diurnal cycle) that can only be addressed by a large scale, multi-platform, extended duration observational campaign in the Saharan Heat Low (SHL) region. This proposal will make key SHL measurements during 2011 to quantify the physical processes controlling the Saharan climate system. The consortium will use the data in combination with operational numerical models to analyse aerosol, radiative, land surface, thermodynamic and dynamic processes over the Sahara, and will, for the first time, both evaluate and attribute errors in weather and climate models for this region.

This project forms a consortium of leading experts in the climate system of the Sahara, the use of earth observation to examine radiative fluxes and dust, the use of aircraft and ground based observations to provide key data sets for use in models, and in modelling dust uplift, synoptic scale processes and climate in this region.

Fennec will deliver:

  • A new and unique dataset for the central Sahara and surrounding regions including ground, air and satellite observations, targeted to improve the representation of this area, and its influences on a wider scale, in NWP and climate models;
  • A novel and comprehensive characterisation of the thermodynamic, dynamic and compositional structure of the troposphere over the Sahara desert and surrounding regions;
  • A quantitative evaluation of how the limited representation of these processes in weather and climate models leads to errors, and how these errors can be reduced;
  • A description of the mechanisms of dust emission from the key source regions at a range of scales and an assessment of the associated radiative forcing.

In addition to our scientific objectives, Fennec has some strategic objectives for the enhancement in UK capabilities in earth system science:

  • to develop state of the art instrumentation suitable for remote monitoring in arid tropical environments;
  • to develop operational protocols for combined turbulent and radiative flux measurements in convective boundary layers with the FAAM BAe146 aircraft.
Surface temperature climatology in June over the Saharan heat low region as simulated by ECHAM5 Surface temperature climatology in June over the Saharan heat low region as simulated by HadGEM1

Surface temperature climatology in June over the Saharan heat low region as simulated by ECHAM5 (top) and HadGEM1 (bottom). The location and the strength of the temperature maximum (heat low) is quite different in the two models.

Sea level pressure climatology in June over the Saharan heat low region as simulated by ECHAM5 Sea level pressure climatology in June over the Saharan heat low region as simulated by HadGEM1

Sea level pressure climatology in June over the Saharan heat low region as simulated by ECHAM5 (top) and HadGEM1 (bottom). The location and the strength of the heat low is quite different in the two models - this impacts on circulation.