Summary: Can desert dust explain the outgoing longwave radiation anomaly over
the Sahara during July 2003?
Jim M. Haywood,1
Richard P. Allan,2
and Nicolas Clerbaux3
Received 14 July 2004; revised 21 October 2004; accepted 2 December 2004; published 9 March 2005.
 Measurements of the top-of-the-atmosphere outgoing longwave radiation (OLR) for
July 2003 from Meteosat-7 are used to assess the performance of the numerical weather
prediction version of the Met Office Unified Model. A significant difference is found over
desert regions of northern Africa where the model emits too much OLR by up to 35 WmÀ2
the monthly mean. By cloud-screening the data we find an error of up to 50 WmÀ2
with cloud-free areas, which suggests an error in the model surface temperature, surface
emissivity, or atmospheric transmission. By building up a physical model of the radiative
properties of mineral dust based on in situ, and surface-based and satellite remote sensing