Global Climatologies of Vegetation Aerodynamic Roughness for Momentum: A Fusion of MODIS and ICESat-2 Observations

Abstract

This paper presents global climatology fields of vegetation aerodynamic roughness for momentum based on 17 years of MODIS data. The approach combines MODIS-derived leaf-area index and ICESat-2 canopy heights with a previously developed representation of the roughness sublayer to generate data products for both the vegetation roughness length for momentum (z0m) and the zero-plane displacement height (d0). The principal products consist of a mean seasonal cycle of the two roughness parameters at 500-m spatial resolution based on a nominal 8-day time step. The second, derivative data set removes seasonality while maintaining spatial information by assigning temporal averages of the underlying 8-day data to each 500-m pixel. A third data set includes global spatial and temporal averages for a range of MODIS land cover types, analogous to the look-up tables employed in land surface modeling. Comparison to field data indicates that satellite-derived maps of vegetation height produce more favorable roughness results than generic land class-aggregated values published in the literature, although land cover classification mismatches between field and satellite footprints make validation challenging. The resulting roughness length and displacement-height fields possess internal consistency, with small differences overall as compared to static, cover-type dependent estimates, making them a realistic alternative for incorporation into regional and global-scale earth science models that benefit from improved representation of land-atmosphere interactions.