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Keywords:

  • entrainment;
  • monsoon;
  • Harmattan;
  • atmospheric turbulent fluxes

Abstract

During the African Monsoon Multidisciplinary Analysis (AMMA) field campaign, the ATR research aircraft made observations of fluxes and thermodynamics during three 15-day periods, which allowed the seasonal evolution of the atmospheric boundary-layer (ABL) characteristics to be monitored before and after the monsoon onset. As expected, temperature and humidity showed a contrast between dry warm conditions and moister cooler conditions from one period to the other. Most of the time, the wind blew from the west (northwesterly to southwesterly) in the ABL and from the east in the free troposphere. Following rainfall events occuring in July and August, surface sensible heat flux decreased and evaporation increased while the momentum flux remained large in the entire boundary layer, whatever the period. The aim of this paper is to characterize turbulence in terms of fluxes and length-scales for ABLs that exhibit particular characteristics relative to (i) entrainment at the top, (ii) wind rotation at the interface between the monsoon and the Saharan air layer and (iii) seasonal variability. In spite of the poorer accuracy of the turbulent flux estimations at the top of the ABL, the flux profiles were observed to increase or decrease linearly with altitude which enabled accurate estimates of entrainment flux ratios. It was found that the moisture flux distribution in the ABL was governed by top-down processes during the driest period and a mixture of top-down and bottom-up processes during the monsoon period. Significant differences in turbulence length-scales were also highlighted. Copyright © 2009 Royal Meteorological Society