Guwahati/Mumbai: Southwest Monsoon has withdrawn from the entire East and Northeast India, the Regional Meteorological Centre, Guwahati announced today. The withdrawal line of Southwest Monsoon now passes through 20.0°N/96.0°East, 17.0°N/91.5°E, Machilipatnam, Kurnool, Badami, Vengurla and 16.0°N/70.0°E, it added.
Meanwhile, scientists of the Mumbai-based Indian Institute of Geomagnetism (IIG), today said that cloud parameters using a ground-based airglow imager indicated a large shift in monsoon rainfall pattern. Estimation of cloud speed and wind pattern over Kolhapur in Maharashtra, during the months of March to May, from the year 2016 to 2020 revealed a change in pre-monsoon cloud fraction and direction of its propagation indicating a large shift in monsoon rainfall pattern.
The scientists employed all-sky imager data (ASI), (usually utilised for upper atmospheric studies) for cloud tracing in the low latitude station Kolhapur (16.8° N, 74.2° E). Generally, the all-sky imager is used to observe night airglow, however, the scientists used it to trace the cloud. They used cloudy data from the nighttime for this research. The utilisation of waste data or cloudy sky data which is considered irrelevant for airglow study, for this investigation was a novel feature of this work.
ASI has very high spatial resolution and this helped the scientists compare the data with INSAT data of 10 km resolution. Using the data collected over the period of 2016 to 2020 during March, April and May months of airglow monitoring, the researchers calculated the cloud motion vector, cloud coverage percentage and direction of cloud movement. The slowest speed of 10±3 m s–1 was observed in 2017, while in other years it was over 15±3 m s–1. The clouds were found to move in a south-westerly direction during the time period under consideration.
The Kolhapur location being closer to the Arabian Sea, the winds as well as the cloud motion noted in the data were observed in the South-West direction.
The scientists noted that the direction of the cloud propagation is turning in the Southward direction as the year progresses. “This is more evident in the March and April month data. This may have a linkage to large climatic changes,” they said. Using this analysis, the researchers showed that there is a shift occurring in the monsoon pattern, which may be imprinted in the rainfall behaviour as well.
It is known that clouds can scatter the incoming solar radiation in the atmosphere and act as a blanket to the earth’s outgoing long-wave radiation. Clouds have a significant impact on the Earth’s climate which by direct and indirect processes is non-linear in nature. The effect is modulated by the space-time distribution and their height, thickness, size distribution and so on. Satellite sensors detect the cloud motion and under the assumption that clouds move with winds, derive the Cloud Motion Vector, ‘CMV’. Values of CMV are very useful in understanding the synoptic-scale atmospheric dynamics and circulations.
– global bihari bureau
