Abstract:
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At the top of the planetary boundary layer, the entrainment of air, which incorporates
dry and warm air from the free troposphere into the boundary layer, is a key process
for exchanges with the free troposphere since it controls the growth of the boundary layer.
Here, we focus on the semi-arid boundary layer where the entrainment process is analyzed
using aircraft observations collected during the African Monsoon Multidisciplinary Analysis
experiment and large-eddy simulations. The role of the entrainment is specifically enhanced
in this region where very large gradients at the planetary boundary-layer top can be found
due to the presence of the moist, cold monsoon flow on which the dry, warm Harmattan flow
is superimposed. A first large-eddy simulation is designed based on aircraft observations of 5
June 2006 during the transition period between dry conditions and the activemonsoon phase.
The simulation reproduces the boundary-layer development and dynamics observed on this
day. From this specific case, sensitivity tests are carried out to cover a range of conditions
observed during seven other flightsmade in the same transition period in order to describe the
entrainment processes in detail. The combination of large-eddy simulations and observations allows us to test the parametrization of entrainment in a mixed-layer model with zero-order
and first-order approximations for the entrainment zone. The latter representation of the
entrainment zone gives a better fit with the conditions encountered in the Sahelian boundary
layer during the transition period because large entrainment thicknesses are observed. The
sensitivity study also provides an opportunity to highlight the contribution of shear stress
and scalar jumps at the top of the boundary layer in the entrainment process, and to test a
relevant parametrization published in the recent literature for a mixed-layer model. |