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

Abstract

Cloud base height (CBH) is an important parameter for many applications such as aviation, climatology or solarirradiance nowcasting (forecasting for the next seconds to hours ahead). The latter application is of increasing importance tooperate 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 theirpoint of installation, have been demonstrated. Accurate knowledge of CBH is required to nowcast the spatial distribution of5solar irradiance around the ASI’s location at a resolution down to5 m. Two ASIs located at a distance of usually less than6 kmcan be combined into an ASI-pair to measure CBH. However, the accuracy of such systems is limited. We present and validatea method to measure CBH using a network of ASIs to enhance accuracy. To the best of our knowledge, this is the first methodto 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.10The ASI-pairs are formed from seven ASIs and feature camera distances of0.8...5.7 km. Each of the 21 ASI-tuples formedfrom 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 CBHfrom an ASI-pair given that true CBH takes on some specific value. Based on such statistical knowledge, in the inference thelikeliest actual CBH is estimated from the readings of all 42 ASI-pairs.15Based 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 than3 kmprovide smaller deviations for greater CBH.No ASI-pair provides most accurate measurements under all conditions. The presented network of ASIs at different distancesproves that, under all cloud conditions, the measurements of CBH are more accurate than using a single ASI-pair.