Behavior of Cloud Base Height over Sumatra Mountains Region from Ceilometer Observations

Abstract

Accurate knowledge of cloud properties is essential for reducing uncertainties in weather forecasts and improving climate prediction models. This study investigates the characteristics of cloud base height (CBH) using high-resolution ceilometer data collected at Kototabang, West Sumatra, Indonesia (0.202°S, 100.317°E), situated at 864.5 m above sea level. Analyzing data recorded over 18 years (April 2002 – December 2021), we found that clouds in Kototabang are predominantly characterized by low CBH (below 2 km), which accounts for 71.3 % of total cloud occurrences, followed by medium CBH (2–6 km) at 25.4 %. Most clouds in Kototabang consist of single-layer clouds (57.78 %), while two-layer and three-layer clouds are observed at 8.41 % and 0.56 %, respectively. Furthermore, CBH tends to decrease as the number of cloud layers increases, with the distance between cloud layers being relatively small, typically less than 0.5 km. The dominance of low CBH in Kototabang is consistent with the frequent occurrence of intense convective clouds in this region, as reported in previous studies. This pattern is also evident from cloud cover data derived from the fifth-generation ECMWF reanalysis (ERA5), which shows high cloud cover (HCC) during the wet season and low HCC during the dry season. Additionally, the mean CBH of the lowest cloud layer varies with weather conditions, reaching 1.80 km during non-rainy condition and decreasing to 1.3 km during rainfall events. CBH in Kototabang exhibits distinct seasonal and diurnal variations. The monthly cloud occurrence follows a bimodal pattern, closely aligning with the monthly average rainfall recorded by an optical rain gauge, which peaks in November and April. During the wet season (April and November), low-CBH clouds (<2 km) are more dominant, whereas clouds with medium and high CBH (2–6 km) are more frequently observed during the dry season (May–July). The prevalence of low CBH in the wet season indicates the presence of thick convective clouds that develop rapidly, as corroborated by ERA5 cloud cover data. A clear diurnal pattern of CBH is also observed. In the morning (06:00–12:00 LST), when rainfall occurrence is minimal, CBH is generally higher. However, a noticeable decline in CBH is observed after 13:00 LST, coinciding with the increasing dominance of convective clouds over the Sumatran landmass. ERA5 data also support this diurnal variation. The observed CBH variations in Kototabang are a genuine atmospheric phenomenon rather than an artefact of ceilometer measurement limitations. Separating CBH data between rainy and non-rainy periods confirms the persistence of these seasonal and diurnal variations.