Surface Roughness parameters at a Tropical Semi-Arid Site: 4 years of Atmosphere Boundary Layer observations in the Deccan Plateau region at Shadnagar, India

Abstract

Accurate inputs of surface roughness parameters are vital for estimating exchange of fluxes between surface and atmosphere, dispersion and transport of pollutants, wind erosion, and wind energy assessment studies, especially over heterogeneous and undulating surfaces. The surface roughness parameters are derived using observations from a 32m meteorological tower during 2016 to 2019. The site is located in the Deccan Plateau region at Shadnagar, India (17°01'59.7"N, 78°11'14.2"E) and experiences a tropical semi-arid climate. We employed multi-level low-frequency measurements (1-min) with a logarithmic wind profile method and single-level high-frequency measurements (20 Hz) with six alternative methods. The zero-plane displacement height (\(d\)) is observed to be 15.8 ± 0.37 m using the former approach, and 15.7 ± 1.7 m using the latter. Similarly, the aerodynamic roughness length for momentum (\({z}_{0}\)) is observed to be 0.1 ± 0.06 m and 0.27 ± 0.2 m, respectively. The \(d\) and \({z}_{0}\) vary within the range 12.8 to 19.4 m and 0.05 to 0.42 m respectively, across eight wind-direction sectors. Influence of seasonal changes in vegetation due to shedding of leaves is evident; \(d\) and \({z}_{0}\) vary by 3 to 6% and 11 to 50%, respectively. Vertically extrapolated winds compared favourably against 100 m ERA5 reanalysis winds over the western sector, suggesting the optimal parameterization within ERA5 for cropland and shrub land covers which dominate these sectors. Our findings indicate that canopy height-based rule-of-thumb guidelines may not suitably estimate roughness parameters for the heterogeneous native land cover, dominated by shrubs and sparsely distributed trees, of the region.