23 Dec 2020

23 Dec 2020

Review status: this preprint is currently under review for the journal AMT.

Cloud height measurement by a network of all-sky-imagers

Niklas Benedikt Blum1,2, Bijan Nouri1, Stefan Wilbert1, Thomas Schmidt2, Ontje Lünsdorf2, Jonas Stührenberg2, Detlev Heinemann2, Andreas Kazantzidis3, and Robert Pitz-Paal4 Niklas Benedikt Blum et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Solarforschung, Paseo de Almería, 73, 2, E-04001 Almeria, Spain
  • 2Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Vernetzte Energiesysteme, Carl-von-Ossietzky-Straße 15, 26129 Oldenburg, Germany
  • 3Laboratory of Atmospheric Physics, Department of Physics, University of Patras, 26500 Patras, Greece
  • 4DLR, Institut für Solarforschung, Linder Höhe, 51147 Köln, Germany

Abstract. Cloud base height (CBH) is an important parameter for many applications such as aviation, climatology or solar irradiance nowcasting (forecasting for the next seconds to hours ahead). The latter application is of increasing importance to operate distribution grids as well as photovoltaic power plants, energy storage systems and flexible consumers.

To nowcast solar irradiance, systems based on all-sky-imagers (ASIs), cameras monitoring the entire sky dome above their point of installation, have been demonstrated. Accurate knowledge of CBH is required to nowcast the spatial distribution of solar irradiance around the ASI's location at a resolution down to 5 m. Two ASIs located at a distance of usually less than 6 km can be combined into an ASI-pair to measure CBH. However, the accuracy of such systems is limited. We present and validate a method to measure CBH using a network of ASIs to enhance accuracy. To the best of our knowledge, this is the first method to measure CBH by a network of ASIs which is demonstrated experimentally.

In this study, the deviations of 42 ASI-pairs are studied in comparison to a ceilometer and characterized by camera distance. The ASI-pairs are formed from seven ASIs and feature camera distances of 0.8...5.7 km. Each of the 21 ASI-tuples formed from seven ASIs yields two independent ASI-pairs as the ASI used as main and auxiliary camera respectively is swapped. Deviations found are compiled into conditional probabilities telling how probable it is to receive a certain reading of CBH from an ASI-pair given that true CBH takes on some specific value. Based on such statistical knowledge, in the inference the likeliest actual CBH is estimated from the readings of all 42 ASI-pairs.

Based on the validation results, ASI-pairs with small camera distance (especially if < 1.2 km) are accurate for low clouds (CBH < 4 km). In contrast, ASI-pairs with camera distance of more than 3 km provide smaller deviations for greater CBH. No ASI-pair provides most accurate measurements under all conditions. The presented network of ASIs at different distances proves that, under all cloud conditions, the measurements of CBH are more accurate than using a single ASI-pair.

Niklas Benedikt Blum et al.

Status: open (extended)
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Niklas Benedikt Blum et al.

Niklas Benedikt Blum et al.


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Short summary
Cloud base height (CBH) is important e.g. to forecast solar irradiance and with it photovoltaic production. All-sky-imagers (ASIs), cameras monitoring the sky above their point of installation, can provide such forecasts and also measure CBH. We present a network of ASIs to measure CBH. The network provides numerous readings of CBH simultaneously. We combine these by a statistical procedure. Validation attests significantly higher accuracy of the combination compared to two ASIs alone.