Temporal and vertical distributions of the occurrence of cirrus clouds over a coastal station in the Indian monsoon region

Abstract

Knowledge of the spatiotemporal coverage of cirrus clouds is vital in quantifying the radiation budget of the Earth–atmosphere system. In this paper, we present the diurnal and vertical distributions of the occurrence of cirrus clouds during different seasons as well as the interannual variation in the occurrence of cirrus over Kattankulathur (12.82∘ N, 80.04∘ E) on the west coast of the Bay of Bengal. Long-term (2016–2018) continuous micropulse lidar (MPL) observations demonstrate laminar and descending cirrus clouds that occur either as single or multiple layers. The single-layer cirrus occurrence shows a diurnal pattern with frequent occurrence in the late evening (∼ 30 %–40 %), whereas multilayer cirrus clouds occur in the early morning (∼ 10 %–20 %). For the diurnal pattern in single-layer cirrus cloud occurrences, convective processes dominate during the pre-monsoon, southwest (SW) monsoon, and northeast (NE) monsoon seasons, while the freeze-drying process is favorable during the winter season. However, both convective and freeze-drying processes are dominant in the diurnal pattern of the multilayer cirrus occurrences. The occurrence reaches a maximum (∼ 40 %) during the SW and NE monsoon seasons, and it shows a minimum (∼ 25 %) during the winter season. The vertical distributions indicate that the maximum occurrence is confined within the tropical tropopause layer (TTL) during all seasons. Cirrus cloud rarely occurs above the tropopause; however, it frequently occurs below the TTL during all seasons. The vertical extent of the occurrence has a broader altitudinal coverage (∼ 8–17 km) during December–March and June–September, while the altitudinal coverage is narrower during April–May (∼ 10–17 km) and October–November (∼ 9–15 km). Cirrus cloud occurrence also exhibits interannual variations, with higher occurrence during 2016 compared with 2017 and 2018, in association with the El Niño–Southern Oscillation (ENSO).