Variability of refractivity in the surface layer

Abstract

The author and members of the Boundary Layer Studies Group collected atmospheric surface layer profile properties affecting RF propagation during the Roughness and Evaporation Duct experiment off the windward coast of Oahu. We measured temperature, humidity and pressure profiles from the surface and up to 100 m by multi-level buoy-mounted sensors and a rawinsonde attached to a kite flown from a small vessel. We obtained the profiles concurrently with S-, X- and Ku-Band propagation measurements along a 26-km path. Using existing surface-layer bulk models, profiles of the gradient of modified refractivity were computed from the buoy data at one level and compared with the actual values obtained from the kite and the buoy. The bulk estimates did not agree well with the buoy data within the lowest 5 m. The kite and buoy data did not show the strong gradients just above the surface that were expected from theory. This same effect was noted when the kite experiment was repeated over much colder water near San Diego, CA. A refractive model was not able to forecast the variability of measured RF propagation characteristics when driven with the merged kite and buoy data. Using bulk estimates did result in some forecast skill.