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

Abstract

The refractive index of air (n) mainly depends on temperature andwater vapour content. For near-infrared radiation, temperature isthe main determining factor. To determinethe structure parameter of temperature ($C_T^2$) fromthe structure parameter of the refractive index($C_n^2$), the influence ofwater vapour content on n needs to be taken into account as acorrection. Three levels of approximation are presented. The firstinvolves the standard deviations of T and q (σ_T andσ_q) as well as the correlation coefficient betweenT and q (R_Tq). The second approximation involvesR_Tq and the Bowen ratio (β), 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 derivationreveals that a large error may be introduced (for situations withR_Tq < 0.8) by replacing σ_T/σ_qwith the Bowen ratio.Consequently, estimating $C_T^2$ from $C_n^2$ using the Bowen ratiocan contain errors of 5–40% for |β| < 1. Adding R_Tqas extra information reduces the errors only marginally. For |β| > 1,the correction is small, and all three approximations give errors of lessthan 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 forwater vapour fluctuations on the measured energy balance is small: forsmall |β|, the correction is large, but the absolute value ofthe heat flux is small, whereas for large |β|, the correctionis insignificant.