Seasonal distribution of cirrus cloud characteristics and their rapid descent from polarization lidar measurements at the west coast of India

Abstract

Cirrus clouds are highly reflective clouds that play an important role in the atmospheric radiative balance and contribute to climate change. We present the seasonal distribution of cirrus cloud occurrence, characteristics and evolution over a coastal station (Goa, 15.4°N, 73.8°E, 40 m above MSL) in India using ground-based micro pulse lidar and radiosonde observations from July 2013 to June 2015. The frequency of cirrus occurrence was approximately 44%, 33%, 17% and 6% in winter (JF), premonsoon (MAM), southwest monsoon (JJAS) and postmonsoon season (OND), respectively. Thicker cirrus was more frequent in September, with an average value of > 1.7 ± 0.89 km. The frequency of cirrus descent occurs almost similarly in monsoon and postmonsoon (42% and 43% respectively). Two extreme cirrus descending cases were observed during the monsoon: (1) on 22 August 2014 with a displacement of 9.6 km from the initial height of 14 km with a descending rate and geometrical thickness of 0.4 ms−1 and 1.5 km, respectively, and (2) on 14 September 2013 with a displacement of 7.8 km with descending rate and thickness of 0.2 ms−1 and 2.3 km respectively. Relatively high humidity (> 60%) and low (< − 20 °C) ambient temperature caused thick cirrus formation in September 2013. However, during August 2014 event, relatively thinner cirrus was observed, and the same is attributed to lower humidity conditions (45 to 60%). The wind shear associated with the tropical easterly jet (TEJ) plays a major role in cirrus displacement by modifying their vertical velocity. The positive and negative vertical velocity fields cause cirrus to have a descending shape. The gradual descending pattern of cirrus is moving parallel to the vertical velocity contour, while the rapid descending in 2014 was mainly due to the occurrence of cirrus perpendicular to the velocity contour. Overall, this study highlights the descending nature of cirrus clouds over a coastal station in India, which can impact surface radiative characteristics