Comparison of manual and automatic daily sunshine duration measurements at German climate reference stations

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.

Development and Validation of an Autonomous System for Measurement of Sunshine Duration

“…Furthermore, the observed differences were characterized by E max of 2.3 h and E ABS of 0.378 h per day. The behavior of the observed differences, based on the absolute maximum and average errors, were strongly influenced by the high cloudiness index, which induces the Campbell-Stokes heliograph to measure only solar radiation levels above 280 W m −2 [12,37,38]. The degree of correlation between the data is shown in Figure 15.…”

“…The digital sensors used to determine the SD use, in short, most of the shadow formation method, or method of contrast, to measure global and diffuse components of solar radiation. These sensors employ silicon-based devices to identify variations higher than 120 W m −2 between the components of solar radiation [5,11,12]. The devices that use the contrast method, unlike the IRSR and the pyrheliometer, do not need additional adjustment after being positioned on a horizontal base [5].…”

Analyzing the impact of automatization using parallel daily mean temperature series including breakpoint detection and homogenization

“…Figure 1 shows Frankfurt (airport) as one example of a German climate reference station. The geographical position of the stations and the time period of available parallel measurements are summarized in Table 1 (see Hannak et al, 2019). In Kaspar et al (2016) more information about the instruments and characteristics of climate reference stations can be found.…”

“…Another reason for the annual cycle in the differences of daily maximum temperature in Germany is the different screen characteristics (e.g., shelter size and ventilation) between the modern and the historical screen. Parallel measurements of daily sunshine duration are analyzed in the study of Hannak et al (2019) with the result of significant differences between manual and automatic daily sunshine duration measurements. To homogenize the daily sunshine duration data, a regression model (as introduced in their study) can be used to adapt the automatic measurements.…”

“…Parallel measurements of daily sunshine duration are analyzed in the study of Hannak et al . (2019) with the result of significant differences between manual and automatic daily sunshine duration measurements. To homogenize the daily sunshine duration data, a regression model (as introduced in their study) can be used to adapt the automatic measurements.…”