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This paper discusses the potential of radar signal to characterise the bare surface roughness in arid or semi-arid regions. The used microwave sensor is the SAR of ERS. Ground truth measurements were acquired over different arid sites in the South of Tunisia. An empirical approach is proposed to derive the surface roughness from SAR measurements. The relationships with two different kinds of roughness have been studied: the geometric roughness, which is characterised by a rather new parameter called Zs, and the classical aerodynamic roughness Z 0 .

Méthodes et observatoires Résumé Les observatoires de recherche sur l'environnement (ORE) ont pour mission de produire pour la recherche des observations systématiques et durables, à des fins de surveillance de l'évolution de l'environnement du globe. Cet article présente brièvement quatre ORE qui concernent la météorologie, l'hydrologie et le changement climatique.

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<p>In the semi-arid Sahel region, wet deposition can represent more than half of the total annual deposition and are associated to different rainfall types, from stratiform precipitation to convective systems. Surface parameters such as temperature, wind speed, wind direction as well as rainfall rate can be used to distinguish these situations. We investigate the behaviour of dust wet deposition at the event-scale based on a multiannual (2007 to 2016) monitoring of wet deposition fluxes, PM10 concentration, precipitation and meteorological parameters in two Sahelian stations Banizoumbou (Niger, 13.54°N, 2.66 E) and Cinzana (Mali, 13.28°N, 5.93°W) of the INDAAF network. Rainfall events have been classified into three types: (i) stratiform, convective associated with (ii) weak precipitation or (iii) intense precipitation. This classification is based on selected criteria regarding evolutions of surface temperature, of wind speed and direction before and after the rainfall onset as well as on the event rainfall rate. Based on an interpretation of hundreds of single events, almost 25% of wet deposition events are associated with non-convective situation, more than 40% with atmospheric convective situation and weak precipitation, and more than 35% events with atmospheric convective situation combined with intense precipitation. This exhaustive work over a long-time period of measurements illustrates the predominance of convective situations regarding wet deposition in the two Sahelian stations. Washout ratios (WR) have been computed from PM10 concentrations, precipitation and deposition fluxes for each kind of events when data were concomitant. The dependency of WR to precipitation amount is shown to differ depending on the rain types. For instance, the decreasing dependency of WR with the precipitation amount of non-convective events has been quantified and could be explained by a dilution effect of the deposition. On the contrary, no clear dependency of WR with the precipitation has been observed for atmospheric convective conditions associated with intense rainfall rate.</p>