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Theor Appl Climatol (2012) 110:539547 DOI 10.1007/s00704-012-0685-z
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A century of apparent atmospheric transmission over Davos, Switzerland
D. Lachat & C. Wehrli
Received: 15 October 2011 /Accepted: 1 May 2012 /Published online: 3 July 2012 # Springer-Verlag 2012
Abstract Pyrheliometer measurements have been carried out at the Physikalisches-Meteorologisches Observatorium Davos from 1909 to present which results in the longest stationary direct irradiance record worldwide. A variety of different instruments and measurement scales were homogenized and adjusted to the World Radiometric Reference. Following the pyrheliometric ratioing technique, a timeseries of apparent atmospheric transmission was calculated and analyzed for possible trends in column transparency over Davos. In order to assess the trend found in the transmission record, the long-term annual cycle in apparent atmospheric transmission was used to deseasonalize the daily transmission data. The trends of early brightening, dimming and brightening were localized and found to be small but highly significant.
1 Introduction
Solar radiation and atmospheric transmission have been of increasing interest in climate research in the past decades. In this context, recent studies have observed decadal trends in solar radiation received at the Earth's surface. They claim a
decrease of solar radiation of 69 W/m from the 1950s to the end of the 1980s followed by an ongoing recovery from the 1990s to present. These periods are referred to as Global Dimming and Global Brightening, respectively. Trends in cloud coverage and atmospheric transmission are prime focal points in the quest for possible causes of these changes. The observed trends in surface solar radiation are in line with trends in atmospheric transmission determined from pyrheliometer measurements (Wild et al. 2005).
Most of the observational pyrheliometer records begin measurements around 1950. The previously longest stationary surface solar radiation record was taken at Stockholm, beginning 1923. There, a significant trend of Global Dimming and Global Brightening was found (Wild 2009).
In recent years, Ohvril et al. (2009) have reconstructed a composite timeseries using pyrheliometer data of six different stations in Eastern Europe covering together the period 19062007. There, the trend found in column transparency calculated from direct irradiance reflects the characteristics of the Global Dimming and Global Brightening found in global radiation records.
However, the pyrheliometer record of Davos, Switzerland, was examined by Hoyt and Frhlich (1983)...