Observations and Large-Eddy Simulations of Entrainment in the Sheared Sahelian Boundary Layer |
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Authors: | Guylaine Canut Fleur Couvreux Marie Lothon David Pino Frédérique Saïd |
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Institution: | 1.Laboratoire d’Aérologie,Université de Toulouse, CNRS UMR 5560,Toulouse,France;2.CNRM-GAME,Météo-France and CNRS,Toulouse,France;3.Applied Physics Department,BarceloneTech (UPC) and Institute for Space Studies of Catalonia (IEEC-UPC),Barcelona,Spain |
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Abstract: | 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 active
monsoon 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 flights made 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. |
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