An improved vertical correction method for the inter-comparison of Integrated Water Vapour measurements

Integrated Water Vapour (IWV) measurements from similar or different techniques are often inter-compared for calibration and validation purposes. Results are traditionally interpreted is terms of bias (difference of the means), standard deviation of the differences, and slope and offset parameters of a linear regression between the IWV measurements of the tested instrument with respect to the reference instrument. When the two instruments are located at different elevations, a correction must be applied to account for the contribution of atmosphere between the sites. Therefore, empirical formulations are often used. In this work it is shown that the widely-used model based on a standard, exponential, profile for water vapour density cannot properly correct the contribution of the atmospheric layer on the bias, slope, and offset parameters simultaneously. For example, correcting the bias degrades the slope and offset parameters, and vice-versa, with this model. An alternative method is proposed to derive an empirical model from real profiles observed by radiosondes. The method is developed for the special case of a tropical mountainous area with high IWV contents and strong diurnal and seasonal variations. Its application is illustrated with two examples, i) GPS to GPS comparisons and ii) GPS to satellite microwave radiometer comparisons.