Rain-producing cloud cover decreasing over India, says Pune IMD’s study
Mumbai city news: Climate scientists have said urbanisation, greenhouse gas emissions and aerosols could have reduced low cloud covermumbai Updated: May 29, 2017 15:53 IST
Low cloud cover, which is largely responsible for a good monsoon, has declined in most parts of the country over 50 years, a first-of-its-kind study by the India Meteorological Department (IMD), Pune, revealed. The study showed that India has, on an average, lost one rainy day during the period. Climate experts said as the monsoon contributes to 70% of the country’s annual rainfall, the decrease in low cloud cover and loss of rainy days is a cause for concern.
“There are three types of clouds – low, medium and high. Rain-producing clouds are low clouds,” said AK Jaswal, lead author. “Decreasing trend signifies that cloud formation is not taking place like before, and less cloud means less rainfall.”
The study, however, states the reason for this is uncertain. Climate scientists have said urbanisation, greenhouse gas emissions and aerosols could have possibly reduced low cloud cover.
Analysing data between 1961 and 2010 from 215 ground-based weather stations, the three-member team found an average of 1.22% decline in low cloud cover per decade during monsoons. Low clouds during monsoon decreased to 37% in 2010 as compared to 46.3% in 1960, with the average low cloud cover steadily declining from the mid-1980s.
On an average, the annual low cloud cover decreased to 22% in 2010 from 26.4% in 1960. The study showed the decline started from the late 1970s, with the highest in 1960 and lowest in 1989 at 19.9%. However, there was an increase in low cloud cover in the Indo-Gangetic plains and northeast India.
Researchers said the central region of the country and the west coast has the most consistent evidence for decreasing low cloud cover between 1961 and 2010 – in the range of 4% per decade to 6% per decade. The Indo-Gangetic plains, in contrast, showed a significant increase in the range of 4% per decade to 6%.
“Because of the profound influence of clouds on both the water balance and global radiation budget, even small variations can alter the climate response. Since monsoon season alone contributes to 70% of annual rainfall, the significant decrease in LCC (low cloud cover) as well as NRD (number of rainy days) in monsoon season during 1961-2010 obtained in this study is a cause of worry,” stated the study.
Scientists, who were not part of the study, said the findings are “important”, but need to be supplemented with the study of clouds via satellites, and that a more comprehensive study using a combination of observations and models is needed to ascertain any causal link between the low cloud cover and monsoon.
“This is comprehensive work based on the ground-based observation of low clouds in India. The steady decline of low cloud cover from 42% to 36% is surprising, and may have been compensated by an increase in mid-level or high-level clouds,” said J Srinivasan, honorary professor, Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru.
Srinivasan said, “The decline is primarily in western and southern India, but not in the Indo-Gangetic plain, which indicates that urbanisation and particulate pollution could be one of the factors.”
Stating that changes in cloud cover could owing due to greenhouse gas feedback or aerosols, or both, professor Sagnik Dey, Centre for Atmospheric Science at the Indian Institute of Technology – Delhi, said all types of clouds need to be investigated..
“In India, we usually have stratiform and convective rain. Any change in low cloud cover will impact stratiform rain directly and convective rain indirectly through the dynamic and microphysical changes in cumulus (cloud) life cycles,” said Dey.
The US-based National Oceanic and Atmospheric Administration define stratiform rain as being relatively continuous and uniform in intensity. Convective rain – it is more intense and of shorter duration – takes place when the earth’s surface heats causing air to rise rapidly. The air then cooks and moisture condenses into clouds resulting in precipitation/rain.
Along with low cloud cover, the study showed that there has been on an average one less rainy day across the country over the past 50 years. Meteorologically, rain above 2.5mm per day is considered as one rainy day.
“Even though there is a significant decreasing trend (95% confidence level) in the number of rainy days over five decades, there is no significant trend that overall rainfall across the country is also decreasing. This goes on to show that whenever there is cloud formation, it is accompanied by extreme rainfall over less number of days,” said Jaswal.
Adding, “Intense rain results in more surface run-off since water will not percolate and recharged ground water. Hence it becomes important to store rain water in the form of rainwater harvesting, building ponds and creating catchment areas even in forests that can recharge water bodies.”