Estimating the refractive index structure parameter (Cn2) over the ocean using bulk methods

Infrared scintillation measurements were obtained along a 7-km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter cb were computed from the buoy data and compared with scintillation-derived C-n(2) values. The bulk C-n(2) estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk C-n(2) estimates became increasingly higher than the scintillation values as the air-sea temperature difference increased. This disagreement may be due to enhanced wave-induced mixing of the lower atmosphere that decreases the vertical temperature and humidity gradients in stable conditions from the assumed Monin-Obukhov similarity (MOS) theory forms, resulting in bulk C-n(2) values that are too high. The bulk C-n(2) estimates decrease rapidly when the absolute air-sea temperature difference approaches small positive values. These predicted decreases in C-n(2) were nor observed in either the path-averaged scintillation measurements or in single-point turbulence measurements, indicating that bulk models for estimating scalar structure parameters based on mean air-sea scaler differences are not valid when the mean air-sea difference approaches zero. The authors believe that the most promising means toward improving the bulk C-n(2) model is to obtain a better understanding of the MOS functions over the ocean for a wide stability range, and particularly of the role of ocean waves in modifying near-surface vertical gradients and turbulence characteristics.