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Abstract. A semiempirical method for estimating the error and optimum number of sampled levels in precipitable water vapour (PWV) determinations from atmospheric radiosoundings is proposed. Two terms have been considered: the uncertainties in the measurements and the sampling error. Also, the uncertainty has been separated in the variance and covariance components. The sampling and covariance components have been modelled from an empirical dataset of 205 high-vertical-resolution radiosounding profiles, equipped with Vaisala RS80 and RS92 sondes at four different locations: Güímar (GUI) in Tenerife, at sea level, and the astronomical observatory at Roque de los Muchachos (ORM, 2300 m a.s.l.) on La Palma (both on the Canary Islands, Spain), Lindenberg (LIN) in continental Germany, and Ny-Ålesund (NYA) in the Svalbard Islands, within the Arctic Circle. The balloons at the ORM were launched during intensive and unique site-testing runs carried out in 1990 and 1995, while the data for the other sites were obtained from radiosounding stations operating for a period of 1 year (2013)(2014). The PWV values ranged between ∼ 0.9 and ∼ 41 mm. The method sub-samples the profile for error minimization. The result is the minimum error and the optimum number of levels.The results obtained in the four sites studied showed that the ORM is the driest of the four locations and the one with the fastest vertical decay of PWV. The exponential autocorrelation pressure lags ranged from 175 hPa (ORM) to 500 hPa (LIN). The results show a coherent behaviour with no biases as a function of the profile. The final error is roughly proportional to PWV whereas the optimum number of levels (N 0 ) is the reverse. The value of N 0 is less than 400 for 77 % of the profiles and the absolute errors are always < 0.6 mm. The median relative error is 2.0 ± 0.7 % and the 90th percentile P 90 = 4.6 %. Therefore, whereas a radiosounding samples at least N 0 uniform vertical levels, depending on the water vapour content and distribution of the atmosphere, the error in the PWV estimate is likely to stay below ≈ 3 %, even for dry conditions.

Abstract. An unbiased method to estimate the error and the optimum number of sampled levels in Precipitable Water Vapour (PWV) determinations from atmospheric radiosoundings is proposed. Two components have been considered, the uncertainties in the measures and the sampling error. The sampling component has been modelled from an empirical dataset of 64 high vertical resolution radiosounding profiles equipped with sondes Vaisala RS80 and RS92. The balloons were launched at the astronomical Roque de los Muchachos Observatory (ORM, ~2200 masl), during intensive and unique site testing runs carried out in 1990 and 1995, and from the neighbour operational station of Güímar, in Tenerife (TFE, ~105 masl) in 2013–2014. The PWV values ranged between ~0.9 mm and ~41 mm. The method takes into account the dependence on the number of samples measured, after sub-sampling the profile for error minimization, and was tested by comparison with a dataset of 42 extremely low resolution profiles only sampling the standard levels (~15 levels). The results show that errors are larger for the wettest atmosphere conditions. On the other hand, drier conditions requires a larger optimum number of samples. The optimum number of samples N0 is less than 200 for PWV ≥ 10 mm. For drier conditions, as in astronomical sites, N0 grows up to ~550 levels. This result may be important forPWV determinations in astronomical observatories. The absolute errors are always < 0.6 mm, with a median relative error of 2.4 &amp;pm; 0.8 % and extreme value of 7.9 % in the driest condition (PWV = 0.89 mm). These errors reduce the uncertainties previously reported in the literature. Nevertheless, errors grow up to 30 % in poorly sampled profiles (the number of samples being less than N0) for dry atmospheres. Alternative equations for direct error estimation, specifically for PWV from radiosoundings equipped with Vaisala RS80 and RS92 sensors, are also provided.

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