Roughness Length Variability over Heterogeneous Surfaces

Monin-Obukhov similarity theory and the empirical formulae of Businger et al. (1971) and Dyer (1974) are used to calculate roughness lengths and surfacelayer heat fluxes from multilevel observations of wind, temperature, and humidity measured at three locations in the Weather Information Network Display System at Cape Canaveral Air Force Station, Florida. Relationships between roughness length and surface-layer wind speed and direction in varying thermal stability conditions are analyzed during two four-day periods: a diurnally-cycling coastal wind circulation regime on 1-4 June 2008 and the passage of Tropical Storm Fay on 18-21 August 2008. Spatial and temporal variations in roughness lengths for a period of one year are compared to landscape features near the three observation platforms using shadow analysis of satellite photographs. Wind speeds during the coastal wind event remained below 10 ms-1, and roughness lengths calculated from observations below 60 m corresponded to surface roughness elements within about 300 m. At the same height in the tropical storm case, for wind speeds exceeding 20 ms-1, evidence is presented that indicates roughness lengths are related to surface features up to 1.5 km upstream.