Global-scale surface roughness effects at L-band as estimated from SMOS observations
Résumé
The Soil Moisture and Ocean Salinity (SMOS) mission is the first satellite dedicated to providing global surface soil moisture products. SMOS operates at L-band (1.4 GHz) and, at this frequency, the signal not only depends on soil moisture and vegetation optical depth but is also significantly affected by surface effects and, in particular, by the soil roughness. However, when dense vegetation is present, the L-band signal is poorly sensitive to the surface effects. First, by using multiple regressions between soil moisture (SM) and brightness temperature (TB) at different incidence angles and polarizations, the SMOS sensitivity to the surface effects was evaluated. A global-scale map of SMOS sensitivity to the surface effects was computed and showed that, for 87% of the land surface, the SMOS observations were sensitive to these effects, while very low sensitivity to the surface effects was estimated over 13% of the land surfaces. For instance, over broadleaf evergreen forest (mainly the Amazon and Congo forests), SMOS was sensitive to the surface effects over only half of the pixels considered. In a second step, in L-MEB (L-band Microwave Emission of the Biosphere), the forward emission model of the SMOS algorithm, the vegetation and roughness effects were combined in a single parameter, referred to as TR in this study. By inverting L-MEB, SM and TR were retrieved at global scale from the SMOS Level 3 (13) TB observations during 2011. Assuming a linear relationship between TR and the Leaf Area Index (LAI) obtained from MODIS data, the effects of roughness (H-r) and vegetation were decoupled and a global map of soil roughness effects was estimated. It was found that the spatial pattern of the H-r values could be related to the main vegetation types. Higher values of roughness (H-r = 0.32-0.39) were obtained for forests (broadleaf evergreen, deciduous and mixed coniferous) while lower values (H-r = 0.14-0.16) were obtained for deserts, shrubs and bare soils. Intermediate values (H-r = 0.20-0.23) were obtained over grasslands, tundra and cultivated land. Over vegetation biomes composed of forests and wooded grasslands, the H-r values were mainly correlated to the vegetation density (r similar to 0.55). For deserts, shrubs and bare soils, the H-r values were mainly correlated to the topography slopes (r similar to 0.53). The global maps presented in this study could lead to improved retrievals of soil moisture and vegetation optical depth for present and future microwave remote sensing missions such as SMOS and Soil Moisture Active Passive (SMAP).