Calibrated digital images of Campbell–Stokes recorder card archives for direct solar irradiance studies

[摘要] A systematic, semi-automatic method for imaging the cards from the widely used Campbell–Stokessunshine recorder is described. We show how the application of inexpensive commercial equipmentand practices can simply and robustly build an archive of high-quality card images and manipulatethem into a form suitable for easy further analysis. Rectified and registered digital images areproduced, with the card's midday marker in the middle of the longest side, and with a temporalscaling of 150 pixels per hour. The method improves on previous, mostly manual, analyses bysimplifying and automating steps into a process capable of handling thousands of cards ina practical timescale.A prototype method of extraction of data from this archive is then testedby comparison with records from a co-located pyrheliometer at a resolution of the order ofminutes.The comparison demonstrates that the Campbell–Stokes recorder archive contains a timeseries of downwelling solar-irradiance-related data with similar characteristics to that ofbenchmark pyrheliometer data from the Baseline Surface Radiation Network.A universal transferfunction for card burn to direct downwelling short-wave radiation is still some way off and is thesubject of ongoing research.Until such time as a universal transfer function is available,specific functions for extracting data in particular circumstances offer a useful way forward.The new image-capture method offers a practical way to exploit the worldwide sets of long-termCampbell–Stokes recorder data to create a time series of solar irradiance and atmospheric aerosolloading metrics reaching back over 100 yr from the present day.

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