Publication Abstracts

Miller et al. 2009

Miller, R.L., A. Slingo, J.C. Barnard, and E. Kassianov, 2009: Seasonal contrasts in the surface energy balance of the Sahel. J. Geophys. Res., 114, D00E05, doi:10.1029/2008JD010521.

Over much of the world, heating of the surface by sunlight is balanced predominately by evaporative cooling. However, at the ARM Mobile Facility (AMF) in Niamey, Niger, evaporation makes a significant contribution to the surface energy balance only at the height of the rainy season, when precipitation has replenished the reservoir of soil moisture. The AMF was placed at Niamey from late 2005 to early 2007 to provide measurements of surface fluxes in coordination with geostationary satellite retrievals of radiative fluxes at the top of the atmosphere, as part of the RADAGAST experiment to calculate atmospheric radiative divergence. We use observations at the Mobile Facility to investigate how the surface adjusts to radiative forcing throughout the year. The surface response to solar heating varies with changes in atmospheric water vapor associated with the seasonal reversal of the West African monsoon, which modulates the greenhouse effect and the ability of the surface to radiate thermal energy directly to space. During the dry season, sunlight is balanced mainly by longwave radiation and the turbulent flux of sensible heat. The ability of longwave radiation to cool the surface drops after the onset of southwesterly surface winds at Niamey, when moist, oceanic air flows onshore, increasing local column moisture and atmospheric opacity. Following the onset of southwesterly flow, evaporation remains limited by the supply of moisture from precipitation. By the height of the rainy season, however, suffcient precipitation has accumulated that evaporation is controlled by incident sunlight, and radiative forcing of the surface is balanced comparably by the latent, sensible, and longwave fluxes. Evaporation increases with the leaf area index, suggesting that plants are a significant source of atmospheric moisture, and may tap moisture stored beneath the surface that accumulated during a previous rainy season. Surface radiative forcing during a dust aerosol outbreak is balanced comparably by net surface longwave and the sensible heat flux during the dry season, with the sensible flux increasing in importance with the onset of the summer monsoon winds. Measurements of surface fluxes by the AMF indicate broader features of the West African monsoon circulation, and should be used to evaluate model simulations of the Sahel climate.

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BibTeX Citation

  author={Miller, R. L. and Slingo, A. and Barnard, J. C. and Kassianov, E.},
  title={Seasonal contrasts in the surface energy balance of the Sahel},
  journal={J. Geophys. Res.},

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RIS Citation

ID  - mi03410s
AU  - Miller, R. L.
AU  - Slingo, A.
AU  - Barnard, J. C.
AU  - Kassianov, E.
PY  - 2009
TI  - Seasonal contrasts in the surface energy balance of the Sahel
JA  - J. Geophys. Res.
VL  - 114
SP  - D00E05
DO  - 10.1029/2008JD010521
ER  -

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