Correction method for a shipborne ultrasonic anemometer in measuring the refractive index structure constant in a marine environment

Abstract

The atmospheric refractive index structure constant (C n 2) was measured in a coastal region using a shipborne three-dimensional ultrasonic anemometer and a micro-thermometer. The results showed that the C n 2 measurements from both instruments were generally consistent but deviated in some cases, especially as the wind speed increased. Careful examination of the data revealed that this was due to noise introduced by wind-induced high-frequency oscillations of the extended arm carrying the ultrasonic probes, which also resulted in higher energy levels in the high-frequency region of the temperature spectrum. To address these issues, an attempt was made to filter the temperature data using a first-order low-pass filtering (LPF) algorithm. For our case, the corrected temperature spectrum reflects a reasonable distribution of turbulent energy at different frequencies, making it obey the −5/3 power law of Kolmogorov turbulence within the inertial subrange. In addition, the C n 2 values estimated from the corrected temperature data showed excellent agreement with the micro-thermometer measurements. The findings of this study provide experience and guidance for the measurement of optical turbulence in the marine environment.