Cloud fraction retrieval and its variability during daytime from ground-based sky imagery over a tropical station in India

Nikumbh, Akshaya and Padmakumari, B and Sunil, Sneha (2019) Cloud fraction retrieval and its variability during daytime from ground-based sky imagery over a tropical station in India. In: JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS, 190 . pp. 74-83.

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Abstract

Total Sky Imager (TSI) is operated at a high altitude station in the Western Ghats of the Indian sub-continent from June 2012 to June 2013. The TSI image content is classified as clear, thin and thick clouds using red blue ratio (RBR) threshold technique and then cloud fraction (CF) is retrieved. An algorithm is proposed to improve the accuracy of CF retrieval, by adapting the methodologies used in earlier studies and new developments are suggested in sections of the validation of clear sky image and the aerosol correction factor. Time series of thin, thick and total CF are constructed for each day at 5 min interval. These measurements at high temporal resolution are first of their kind over a tropical station in India. Both thick and thin CFs showed high temporal variability. The transition from one season to another season also showed distinct CF variability. Monthly mean CF showed a mono model distribution in the winter months (December and January) and a bimodal distribution in the pre-monsoon months (March, April, May) including February. During the monsoon season (JJAS) about 90% CF is observed throughout the day. The post-monsoon months (October, November) show a bimodal distribution with one peak in the morning hours and another peak in the afternoon hours. Unlike thin CF, total and thick CFs showed almost similar variability. The comparison of CF retrieved from MODIS (Terra and Aqua) and TSI showed a good correlation for overcast sky conditions, while partial cloudy skies showed less correlation due to the effect of satellite viewing angles. Such continuous measurements of CF at high temporal resolution are essential for cloud radiative forcing studies.

Item Type:	Journal Article
Publication:	JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Additional Information:	copyright for this article belongs to PERGAMON-ELSEVIER SCIENCE LTD
Keywords:	Sky imager; Cloud fraction; Aerosol correction factor; RBR threshold; High altitude station; Retrieval algorithm
Department/Centre:	Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Date Deposited:	21 Aug 2019 05:35
Last Modified:	21 Aug 2019 05:35
URI:	http://eprints.iisc.ac.in/id/eprint/63287

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