Evaluation of cloud base height provided by ceilometers and a proposal for a visibility-based quantitative definition for aviation

Abstract. Ceilometers are established sensors for deriving cloud base height (CBH) from a laser backscatter signal. However, various types of ceilometers provide different CBHs for the same cloud situation due to the application of manufacturer-specific, proprietary algorithms and, above all, the lack of a generally valid quantitative definition. This is particularly critical for air traffic control in low cloud situations. In the framework of the AutoMETAR project by Deutscher Wetterdienst (DWD) and in collaboration with the Universities of Leicester and Hamburg, the multiphase "Ceilometer campaign Hansestadt Hamburg" (CircaHH) was carried out. Across three measurement campaigns conducted between September 2016 and May 2019, primarily during the fall-spring seasons when low clouds are more prevalent, photographs of the 300 m tall "Hamburg Weather Mast" were taken for subsequent image analysis. Since the mast is increasingly obscured from the top downwards if the CBH descends, the contrast ratio of its alternating red and white segments was used to calculate the extinction coefficient profile. Several independent methods were analyzed for their suitability and accuracy to derive CBH from these extinction profiles. Our results suggest that the slant optical range (SOR) using a threshold value of 1000 m is an appropriate quantitative CBH definition for aviation, providing pilots with more useful information for an oblique downward or upward view. This SOR definition was also applied to extinction coefficient profiles derived from backscatter signals of different ceilometers using our own implementation of the Klett inversion method (KIM). This cross-check reveals that CBH differences due to various ceilometer backscatter input data for the KIM are significantly smaller than CBH differences associated with different ceilometer algorithms. Despite the complex experimental setup, our image analysis can serve as a new reference method for evaluating vertical profiles of the ceilometer backscatter signal for low clouds and can be used as a quality check for CBHs from any ceilometer firmware. Our KIM, combined with the SOR definition, has the potential to improve automated airport weather reports for CBH and cloud amount like routine Meteorological Aerodrome Report (METAR) and local routine report (MET REPORT).