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Volume 16, issue 5 | Copyright
Atmos. Chem. Phys., 16, 3563-3575, 2016
https://doi.org/10.5194/acp-16-3563-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 17 Mar 2016

Research article | 17 Mar 2016

The contrasting roles of water and dust in controlling daily variations in radiative heating of the summertime Saharan heat low

John H. Marsham1,2, Douglas J. Parker2, Martin C. Todd3, Jamie R. Banks4, Helen E. Brindley4, Luis Garcia-Carreras2, Alexander J. Roberts2, and Claire L. Ryder5 John H. Marsham et al.
  • 1National Centre for Atmospheric Science (NCAS), Leeds, UK
  • 2School of Earth and Environment, University of Leeds, Leeds, UK
  • 3Department of Geography, University of Sussex, Brighton, UK
  • 4Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College, London, UK
  • 5Department of Meteorology, University of Reading, Reading, UK

Abstract. The summertime Sahara heat low (SHL) is a key component of the West African monsoon (WAM) system. Considerable uncertainty remains over the relative roles of water vapour and dust aerosols in controlling the radiation budget over the Sahara and therefore our ability to explain variability and trends in the SHL, and in turn, the WAM. Here, new observations from Fennec supersite-1 in the central Sahara during June 2011 and June 2012, together with satellite retrievals from GERB, are used to quantify how total column water vapour (TCWV) and dust aerosols (from aerosol optical depth, AOD) control day-to-day variations in energy balance in both observations and ECWMF reanalyses (ERA-I). The data show that the earth-atmosphere system is radiatively heated in June 2011 and 2012. Although the empirical analysis of observational data cannot completely disentangle the roles of water vapour, clouds and dust, the analysis demonstrates that TCWV provides a far stronger control on TOA net radiation, and so the net heating of the earth-atmosphere system, than AOD does. In contrast, variations in dust provide a much stronger control on surface heating, but the decreased surface heating associated with dust is largely compensated by increased atmospheric heating, and so dust control on net TOA radiation is weak. Dust and TCWV are both important for direct atmospheric heating. ERA-I, which assimilated radiosondes from the Fennec campaign, captures the control of TOA net flux by TCWV, with a positive correlation (r = 0.6) between observed and modelled TOA net radiation, despite the use of a monthly dust climatology in ERA-I that cannot capture the daily variations in dustiness. Variations in surface net radiation, and so the vertical profile of radiative heating, are not captured in ERA-I, since it does not capture variations in dust. Results show that ventilation of the SHL by cool moist air leads to a radiative warming, stabilising the SHL with respect to such perturbations. It is known that models struggle to capture the advective moistening of the SHL, especially that associated with mesoscale convective systems. Our results show that the typical model errors in Saharan water vapour will lead to substantial errors in the modelled TOA energy balance (tens of W m−2), which will lead to errors in both the SHL and the WAM.

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The roles of water, clouds and airborne dust in controlling the heating of the Sahara are uncertain, which has major implications for the West African monsoon. Observations from the Fennec project, with satellite data, show that total atmospheric water content provides a far stronger control on total radiative heating than dust does, but dust provides the stronger control on surface heating. Therefore major heating errors in global models are likely due to known errors in water transport.
The roles of water, clouds and airborne dust in controlling the heating of the Sahara are...
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