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Ann. Geophys., 26, 1355-1363, 2008
www.ann-geophys.net/26/1355/2008/
© European Geosciences Union 2008
Sensitivity of African easterly waves to boundary layer conditions
A. Lenouo1 and F. Mkankam Kamga2
1Department of Physics, Faculty of Science, University of Douala, P.O. Box 24157 Douala, Cameroon
2LEMAP, Department of Physics, Faculty of Science, University of Yaoundé I, Cameroon
Abstract. A linearized version of the quasi-geostrophic model (QGM) with an
explicit Ekman layer and observed static stability parameter and
profile of the African easterly jet (AEJ), is used to study the
instability properties of the environment of the West African
wave disturbances. It is found that the growth rate, the
propagation velocity and the structure of the African easterly
waves (AEW) can be well simulated. Two different lower boundary
conditions are applied. One assumes a lack of vertical gradient of
perturbation stream function and the other assumes zero wind
perturbation at the surface. The first case gives more realistic
results since in the absence of horizontal diffusion, growth rate,
phase speed and period have values of 0.5 day−1,
10.83 m s−1 and 3.1 day, respectively. The zero wind
perturbation at the surface case leads to values of these
parameters that are 50 percent lower. The analysis of the
sensitivity to diffusion shows that the magnitude of the growth
rate decreases with this parameter. Modelled total relative
vorticity has its low level maximum around 900 hPa under no-slip,
and 700 hPa under free slip condition.
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