Impact of Boundary-Layer Processes on Near-Surface Turbulence Within the West African Monsoon |
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Authors: | Fabienne Lohou Frédérique Saïd Marie Lothon Pierre Durand Dominique Serça |
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Institution: | (1) University of Cologne, Cologne, Germany;(2) Institute for Geophysics and Meteorology, University of Cologne, Cologne, Germany |
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Abstract: | High frequency measurements of near-surface meteorological data acquired in north Benin during the 2006 West African monsoon
seasonal cycle, in the context of the African Monsoon Multidisciplinary Analysis (AMMA) experiment, offer insight into the
characteristics of surface turbulence in relation to planetary boundary-layer (PBL) processes. A wide range of conditions
is encountered at the lower and upper limits of the PBL: (i) from water-stressed to well-fed vegetation, and (ii) from small
to large humidity and temperature jumps at the PBL top inversion, due to the Saharan air layer overlying the monsoonal flow.
As a result, buoyant convection at the surface and entrainment at the PBL top play very different roles according to the considered
scalar. We show that, when the boundary-layer height reaches the shear level between the monsoonal and Harmattan flows, the
temperature source and humidity sink at the boundary-layer top are sufficient to allow the entrainment to affect the entire
boundary layer down to the surface. This situation occurs mainly during the drying and moistening periods of the monsoon cycle
and affects the humidity statistics in particular. In this case, the humidity turbulent characteristics at the surface are
no longer driven solely by buoyant convection, but also by entrainment at the boundary-layer top. Consequently, the Monin–Obukhov
similarity theory appears to fail for the parameterisation of humidity-related moments. |
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