SPATIAL VARIABILITY IN AIRBORNE SURFACE FLUX MEASUREMENTS DURING HABEX-SAHEL

The spatial variability of sensible and latent heat flux measured by a ircraft over a 90-km x 75-km area, near the Central Supersites of HAPE X-Sahel (Hydrologic and Atmospheric Pilot Experiment, Sahel) is discus sed. The data from six Flights are presented four of which were obtain ed 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 cond itions are compared: the latent heat flux is far more heterogeneous un der drying conditions so that the estimation accuracy is reduced if th e same integration length is used. It was found that under dry conditi ons the contribution of low frequency eddies is more important than th at of local turbulence: the surface moisture is probably less homogene ous than in the wet period, but it is mostly the interaction between t he marine and continental air masses linked to the closeness of the in ter-tropical convergence zone that seems to drive the transfers. Two-d imensional fields of fluxes are constructed to study their spatial var iability according to the hydrological conditions. These fields are sy stematically compared with those of the mean parameters likely to driv e the transfers. Some characteristics, common to several fields, are r evealed 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 flu. surface temperature and air tempera ture and a north-south gradient of specific humidity. However, the aer odynamic formula that relates flux to mean parameters, fails at a 25-k m x 25-km scale but fives good results at a larger scale (90 km x 75 k m): at this scale, the Dalton number is around 2.5 x 10(-3).