In a first, effects of land moisture on monsoon studied
Researchers analyse monsoon data from 1980 to 2014mumbai Updated: Oct 07, 2017 19:07 IST
There could be an answer as to why rain forecasts sometimes go awry.
Researchers led by the Indian Institute of Technology – Bombay (IITB) have found that land-based moisture is as important a determinant as moisture from the sea that influences monsoon and widespread extreme rain events over central India — from Gujarat to Assam — that also includes Mumbai.
This is the first time that the role of land moisture on monsoon has been studied that will aid in forecasting rain accurately. Conventionally, atmospheric processes such as moisture transported from oceans and large scale wind circulations have been associated with monsoon over India.
A four-member team, based on data between 1980 and 2014, studied the role of moisture transport from both land and ocean in active and break periods during the monsoon which has implications for Kharif crops. A monsoon active period is when there is more than above average rain for a minimum of three consecutive days; break period is when there is less rain for three consecutive days.
“During monsoon, the most important aspect for agriculture and water management is forecasting rain for the next 15 or 20 days,” said Subimal Ghosh, associate professor, civil engineering department, IITB.
At present, the India Meteorological Department predicts monsoon active and break period through an initiative called Extended Range Forecast. “But it works well only for some active and break periods, and is not very good on the regional scale. And therefore climate models need to factor in the role of moisture from both ocean and land for accurate forecasts,” said Ghosh.
Researchers said monsoon active and break period in India are characterised by two things – east-west and north-south asymmetry.
Describing their east-west asymmetry finding as “important and interesting”, the study found that the Ganga basin produces a lot of moisture during the monsoon due to evapotranspiration from dense vegetation and agricultural land. As a result, during the active period this moisture along with winds gets dissipated only over the Central Indian region resulting in high rainfall. However, during the break period, high speed winds transport the moisture that gets evaporated from the Ganga Basin leading to rainfall over northeast India.
In contrast, the north-south asymmetry means when the Central Indian landmass receives good rain, the Indian Ocean does not – and vice-versa. “We found that the Central Indian region and Indian Ocean are always competing for moisture for precipitation. And this moisture comes from ocean and not land,” said Ghosh.
A multi-institute study led by Pune-based Indian Institute of Tropical Meteorology published earlier this week found that widespread extreme rain events in Central India have recorded a three-fold increase between 1950 and 2015 largely due to moisture incursion from the Arabian Sea.
The eight-member team found that 36% of the total moisture during extreme rain events originates from the Arabian Sea compared to the Bay of Bengal (26%) as previously thought, and central Indian Ocean (9%).
“Fortunately, the IMD predicted heavy rains in Mumbai on August 29 almost seven days ahead of time. The fact that the moisture comes from the Northern Arabian Sea should allow us to make longer term predictions more reliably,” said professor Raghu Murtugudde, University of Maryland.
He added, “Information flow of the predictions to decision makers must be streamlined - closing down schools, transportation, protecting hospitals and critical infrastructures have to be made very efficient and effective.”
The importance of moisture
A four-member team led by the Indian Institute of Technology – Bombay (IITB) studied the role of moisture transport from both land and ocean in active and break periods during the monsoon which is important for cultivation of Kharif crops.
Land-based moisture is a result of evapotranspiration that occurs when transpiration takes place from vegetation and evaporation takes places from water bodies.
Based on 34 years data, the team found that climate models need to factor in both moisture incursion from both ocean and land before making rainfall predictions – important for agriculture and water management.
The study assumes significance in the backdrop of India Meteorological Department forecast of monsoon active period for Maharashtra at the start of the season. Following prediction of heavy rain during June and July, farmers completed sowing in June. But after a brief spell, it didn’t rain because it was a break period, causing huge financial losses.
The study, published in Scientific Reports (Nature Publishing Group) on Friday, is performed by professors Amey Pathak and Subimal Ghosh, IIT-Bombay, professor Praveen Kumar of University of Illinois, Urbana-Champaign and professor Raghu Murtugudde of University of Maryland.
Police cases against IMD
Mid-July, a farmer from Beed district filed a police complaint against Colaba and Pune weather stations of the India Meteorological Department (IMD) for their ‘misleading’ rain forecast. This was the first time such a complaint was filed against IMD accusing the agency of cheating farmers and alleged connivance between IMD officials and seed and fertiliser companies for a ‘faulty’ forecast.