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Communication incl. Poster: BibTeX citation key:  Canut
Canut, G., Couvreux, F., Lothon, M., Lohou, F. & Saïd, F. 2009. The entrainment process between the Monsoon flow and the Saharan Air Layer in the West African Monsoon system. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: roussot 2009-11-04 19:45:35
Categories: Atmospheric processes
Keywords: Atmospheric Boundary Layer, Clouds - Convection
Creators: Canut, Couvreux, Lohou, Lothon, Saïd
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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This study uses aircraft observations from the AMMA experiment and Large Eddy Simulations (LES) to investigate the role of the planetary boundary layer (PBL) and turbulence in the interaction between the moist south-westerly monsoon flow and the overlying dry north-easterly flow. At the top of the PBL, entrainment, which incorporates dry and warm air from the free troposphere into the PBL, is a key process. The LES, by resolving the dry tongues (coherent structures of entrainment) in the upper part of the PBL, allows us to better understand the entrainment process and its relation with the shear at the interface between the two opposite flows and with the mean PBL structure.
A first LES simulation is designed based on observations of 5 June 2006, a case under strong wind conditions during the transition period from dry conditions to the active monsoon phase. This LES is able to reproduce the boundary-layer development observed this day. From this specific case, sensitivity tests are carried out to cover a range of conditions observed during 7 other flights made in the same transition period. These LES allow us to further describe the entrainment zone.A previous analysis of 15 flights before and after the monsoon onset revealed several results to investigate. Among them: (i) a rapid growing of the PBL that cannot be entirely explained by the entrainment nor by the large-scale vertical velocity, (ii) a large spectrum of entrainment rate around the typical value of 0.2, (iii) larger entrainment close to the intertropical discontinuity, and smaller entrainment in the active phase.
The combination of LES and observations allows us to further analyse those results and to tackle with some unresolved issues on entrainment, linked with the difficulty to estimate it with observations and to parameterize it in models.
Added by: roussot