Spatial variability in airborne surface flux measurements during HABEX-Sahel

The spatial variability of sensible and latent heat flux measured by aircraft over a 90-km × 75-km area, near the Central Supersites of HAPEX-Sahel (Hydrologic and Atmospheric Pilot Experiment, Sahel) is discussed. The data from six flights are presented four of which were obtained during the rainy season; the others being obtained at the beginning of the dry season.A basic difference in the behaviour of the latent heat transfer is revealed when the measurements under dry and wet conditions are compared: the latent heat flux is far more heterogeneous under drying conditions so that the estimation accuracy is reduced if the same integration length is used. It was found that under dry conditions the contribution of low frequency eddies is more important than that of local turbulence: the surface moisture is probably less homogeneous than in the wet period, but it is mostly the interaction between the marine and continental air masses linked to the closeness of the inter-tropical convergence zone that seems to drive the transfers.Two-dimensional fields of fluxes are constructed to study their spatial variability according to the hydrological conditions. These fields are systematically compared with those of the mean parameters likely to drive the transfers. Some characteristics, common to several fields, are revealed that can be considered specific to the climate in this region, at this time of year: an east-west gradient of albedo, a south-north gradient of the sensible heat flux, surface temperature and air temperature and a north-south gradient of specific humidity. However, the aerodynamic formula that relates flux to mean parameters, fails at a 25-km × 25-km scale but gives good results at a larger scale (90 km × 75 km): at this scale, the Dalton number is around 2.5 × 10−3