Diurnal Variability of Cloudiness Over the Indian Subcontinent Using ISRO Satellite Kalpana1

Abstract:

Clouds play an important role in modulating Earth’s radiation budget by cooling in the shortwave and warming in the longwave. While our understanding about the global distribution of cloudiness or cloud fraction (CF) has advanced in the recent years due to GEWEX activities, comprehensive understanding of the diurnal pattern of cloudiness is still lacking. In this work, we examined ten years (2008-2017) of ISRO geostationary satellite Kalpana1 CF data, which are available at 30 mins interval. First we evaluated Kalpana1 CF at 1030 am IST (Indian standard time) against the traditional GEWEX dataset (MISR and MODIS onboard Terra). We observed that Kalpana1 derived cloudiness matches more closely with CF from new MISR resolution-corrected cloud product in comparison to MODIS and old MISR cloud products. Mean seasonal CF values over Indian subcontinent calculated from Kalpana1 and resolution-corrected new MISR data are 0.22 and 0.31 (DJF), 0.33 and 0.29 (MAM), 0.42 and 0.39 (JJA), 0.35 and 0.32 (SON) respectively. Kalpana1 cloudiness is observed to be highest during 2330 IST over most parts of the Arabian Sea (AS) during (JJA), but over the southern Arabian Sea it peaks at 1730-2030 IST. Over the Bay of Bengal (BoB), cloudiness peaks at 1730-2030 IST in the northern part, while it peaks at 0530 IST in the southern part. Late afternoon-early evening peak is prominent over most of the Indian landmass. It is very interesting to observe that the diurnal pattern of cloudiness changes seasonally too. In DJF, LTM (local time of maximum) CF occurs over most of the Indian landmass around 9 am in the morning and in north-east India it occurs much early around 0630 am, while over oceanic regions, it varies from late afternoon to midnight. In MAM, LTM CF over most parts of the AS and BoB, and as well as southern part of Indian landmass is 0430-0530 pm, while over northern part of Indian subcontinent it peaks up at around 10 am. AS, BoB and Indian landmass have their peak CF occurrences in between noon (12 am) and afternoon (15 pm) during SON. Our analysis suggests that the Kalpana1 cloud dataset has great potential for use in understanding diurnal as well as seasonal variation of cloudiness and improvement in climate models over the Indian subcontinent in addition to its traditional use in weather models as assimilated product.