Effects of water vapour on the structure parameter of the refractive index for near-infrared radiation

The refractive index of air (n) mainly depends on temperature and water vapour content. For near-infrared radiation, temperature is the main determining factor. To determine the structure parameter of temperature (C-T(2)) from the structure parameter of the refractive index (C-n(2)), the influence of water vapour content on n needs to be taken into account as a correction. Three levels of approximation are presented. The first involves the standard deviations of T and q (sigma(T) and sigma(q)) as well as the correlation coefficient between T and q (R-Tq). The second approximation involves R-Tq and the Bowen ratio (beta), and the last uses only the Bowen ratio. The latter is the classical Bowen ratio correction. Evaluation of the validity of the assumptions used in the derivation reveals that a large error may be introduced (for situations with R-Tq<0.8) by replacing σ(T)/σ(q) with the Bowen ratio. Consequently, estimating C-T(2) from C-n(2) using the Bowen ratio can contain errors of 5-40% for \beta\<1. Adding R-Tq as extra information reduces the errors only marginally. For |β|>1, the correction is small, and all three approximations give errors of less than 1% in C-T(2). When C-T(2) is used to compute the sensible heat flux, the influence of the quality of the correction for water vapour fluctuations on the measured energy balance is small: for small \beta\, the correction is large, but the absolute value of the heat flux is small, whereas for large \beta\, the correction is insignificant.