The Diurnal Cycle of the West African Monsoon and its Relationship to African Easterly Waves

The West African Monsoon is often illustrated in literature by the mean state of the circulation showing a broad southerly moist monsoon inflow, with ascent over the Saharan heat low and subsidence over the Gulf of Guinea. In reality, this circulation has particularly strong diurnal variability.

During the day strong solar heating of the surface leads to a deep and turbulent boundary layer which inhibits large scale horizontal atmospheric motions. At night, boundary layer turbulence subsides and synoptic scale pressure gradients are the dominant influence on the circulation. This leads to a ‘surge' of moist monsoon inflow at night time and more localised mesoscale circulations in the day. These daily changes lead to significant differences in weather conditions at the ground and a strong diurnal component to the timing of convective rain events.

In this talk we will examine the impact of these diurnal variations on the large scale and local scale weather and take a general look at how past and present field campaigns have advanced our understanding of the diurnal cycle of West Africa.

Variability of the diurnal cycle will be discussed in relation to the passage of African easterly waves and evidence from the AMMA field campaign, such as the nocturnal tethered balloon experiments which took place in Mali in 2005 and the SODARs deployed in Niger in 2006, will be used to show how these large scale waves can act as weather modulators.

Finally, we will discuss the operational applications of this science and how we might use model forecasts of African Easterly Waves to improve our weather and seasonal predictions of rainfall over West Africa.