Abstract
Abstract. Precise quantification of climate change depends on long time series of meteorological variables. Such time series should be as homogeneous as possible but some changes of measurement conditions cannot be prevented. At German climate reference stations, parallel measurements are used to analyze the effects of changes in measurement systems for example for the transition from manual to automatic instruments. These parallel measurements aim to identify measurement uncertainties and to analyze the comparability of measurement systems to investigate the homogeneity. In this study, we investigate daily sunshine duration. Traditionally, manual measurements of daily sunshine duration are taken with Campbell-Stokes sunshine recorders. For automatic measurements the SONIe or SCAPP instrument is used. The different measurement principles (glass sphere and photodiode) cause systematic differences between the observations. During summer, values for manual observations are larger especially in case of frequent alternations between sunny and cloudy conditions. Furthermore, the standard deviation of the differences between the two measurement systems is larger during summer because of the greater day length. To adjust the automatic measurements a linear regression model is suggested based on parallel measurements from 13 climate reference stations in Germany. To validate the regression coefficients, a leave-one-out cross validation was performed (by leaving out data of individual stations). The regression coefficients (derived from different sets of stations) are similar, thereby indicating a robust data set for the estimation of the linear model. With this method we want to prevent breaks in long time series of daily sunshine duration caused by the transition from manual to automatic instruments.
Highlights
Parallel measurements can be used to quantify the impact of changes in the measurement systems
At German climate reference stations, parallel measurements are used to analyze the effects of changes in measurement systems for example for the transition from manual to automatic instruments
We investigate daily sunshine duration
Summary
Parallel measurements can be used to quantify the impact of changes in the measurement systems. The first results on the comparison of temperature measurements are summarized in Kaspar et al (2016) In contrast to these results the effect of changing the measurement instrument for daily sunshine duration has a larger influence on the homogeneity. The Campbell-Stokes sunshine recorder is a reference instrument to measure daily sunshine duration and is used worldwide since 1880 (Sanchez-Lorenzo et al, 2013). The intensity of solar radiation is not strong enough to reach the intensity to burn a line in the paper card of the Campbell-Stokes but the threshold of the automatic instruments is reached. The automatic measurements are performed with the SONIe (Brocken, Frankfurt, Hohenpeißenberg, Lindenberg, Potsdam, Schleswig, Görlitz, Hamburg, Aachen, Fichtelberg, Konstanz) or SCAPP (Helgoland, Aachen-Orsbach) instrument (available at Climate Data Center, 2019). Helgoland Schleswig Hamburg-Fuhlsbüttel Potsdam Lindenberg Brocken Görlitz Aachen Aachen-Orsbach Fichtelberg Frankfurt Main (airport) Konstanz Hohenpeißenberg
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