Modelling of the Diurnal Cycle in the Lower Atmosphere over Land: a Joint Evaluation of Four Contrasted Regimes

The diurnal cycle is an important mode of variability in the Tropics. The African Monsoon Multiscale Analysis (AMMA) field campaign that took place in West Africa with an intense period of observations in the summer 2006 provided, for the first time, a large set of observations to document the low-level diurnal cycles in West Africa. Four different regimes are selected in order to characterize the varying conditions encountered along a meridional transect from heavily cloudy conditions (to the South of the InterTropical Convergence Zone, ITCZ), a wet convective regime (within the ITCZ), a drier convective regime (to the North of the ITCZ) up to arid conditions (Gounou et al., BLM, 2012), sampling the different conditions encountered in this region. Those data are used here to evaluate the representation of the diurnal cycles of the boundary layer by different models. First, two Numerical Weather Prediction models, ARPEGE from Meteo-France and ECMWF, are evaluated in this area. A modeling framework is then developed for single-column modeling in order to relate the observed features of the modeled diurnal cycles to the different parameterizations. A particular attention is given to provide the best estimate of initial conditions at the surface and in the atmosphere, and of boundary conditions in the form of large-scale advection. This framework allows to reproduce the different regimes. The analysis then focuses on the simulation of the surface energy budget and of the boundary-layer characteristics. The bias and drawbacks of the model depend on the regime and always involve the representation of clouds. Eventually, different sensitivity tests are carried out in order to analyze the importance of the coupling with the surface and the role of the large-scale advection. It highlights the importance of surface-atmosphere-cloud interactions taking place at sub-diurnal time scale to explain the different features of the low-levels over West Africa.