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 re ective clouds that play an important role in the atmospheric radiative balance and hence contribute to climate change. We present seasonal distribution of cirrus cloud occurrence, their characteristics and evolution over a coastal station (Goa, 15.4 o N, 73.8 o E, 40m above MSL) in India, using ground-based Micro Pulse lidar and Vaisala radiosonde observations from July 2013 to June 2015.The frequency of cirrus occurrence was observed around 44%, 33%, 17% and 6% in winter (JF), premonsoon (MAM), south west monsoon (JJAS) and post-monsoon (OND) respectively. Thicker cirrus were more frequent in September with an average value of > 1.7 ± 0.89 Km. The frequency of cirrus descent occurs almost similar in monsoon and post-monsoon (42% and 43% respectively) months. 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; (2) on 14 September 2013 with a displacement of 7.8 km with descending rate and thickness of 0.2 ms − 1 and 2.3km respectively. Relatively high humidity (> 60%) and low (<-20 o C) ambient temperature caused thick cirrus formation in September 2013. However, during August 2014 event, relatively thinner (optically) 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) appears to play a major role in cirrus displacement by modifying their vertical velocity. The positive and negative vertical velocity elds cause cirrus to 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 represents the descending nature of cirrus clouds and their impact on the surface radiative characteristics over a coastal station in India.