A Threshold-Based Method for Cloud Base Height Detection using Ceilometers: Application to Long-term observations in deriving Cloud Vertical Structure

Abstract. Ceilometers are widely used for cloud base height (CBH) detection, primarily through proprietary manufacturer algorithms. Although these algorithms are routinely employed at airports and meteorological stations, their suitability for climatological applications is often limited under complex atmospheric conditions. In this study, we propose an improved threshold-based detection (TBD) method for CBH retrieval that is applicable to both calibrated and non-calibrated ceilometer return signals. The detected cloud layers show good agreement with collocated observations from space-borne active sensors. A comparative analysis between the manufacturer’s algorithm and the proposed TBD approach demonstrates significant improvement in CBH separation. The method is further shown to be adaptable to ceilometers of different makes operating under diverse environmental conditions. The TBD approach is applied to long-term observations (April 2020 – October 2025) from a CL51 ceilometer deployed in the coastal urban environment of Kolkata, eastern India, to investigate cloud processes and characterize cloud vertical structure. A parameter termed normalized cloud occurrence is estimated for single-, double-, triple-, and all-layer cloud cases to qualitatively examine cloud vertical distribution. The close similarity between the occurrence patterns of single-layer and all-layer clouds indicates the dominance of single-layer clouds over the study region, while multi-layer cloud occurrences provide additional insight into cloud vertical structure. Seasonal and diurnal analyses reveal the persistent presence of low-level clouds (< 2 km) throughout the day across all seasons. The CBH of low-level clouds gradually increases after 09:00 local time, peaks during 12:00–15:00, and subsequently decreases, likely driven by solar-heating-induced convection. Such convection facilitates vertical cloud development up to 8–12 km, depending on the season, except during winter. Additionally, a persistent elevated cloud layer near 4 km is observed, likely associated with temperature variations around the 0 °C isotherm. The derived cloud vertical structure has important implications for understanding cloud radiative forcing and improving atmospheric model predictions.