The monsoon is here; soon, most places in the country would start receiving rainfall. Every year, its forecast is the subject of public discussion. The Meteorological Department issues two long-range forecasts: one in April, and the other towards the end of May or start of June. In the first, it gives the forecast of the quantity of rainfall the country as a whole is likely to receive during the four-month monsoon season. The second forecast is slightly more specific as it gives the likely distribution of rainfall over four broad regions and also in individual months.
Still, these forecasts are general in nature; they don’t tell us, for example, how much rain New Delhi would get in the season. Long-range forecast for an extended period of time is possible for large areas like countries or regions but is extremely difficult for small areas like a town or city due to the complexity of the controlling factors.
Our group’s recent work is aimed at understanding the factors that control the rain formation during the monsoon through analysis of oxygen isotope tracer. We have tried to evolve a mathematical model for Bengaluru rains based on isotope composition of rainwater. The model inputs are wind speed, sea surface temperature and the relative humidity over the ocean.
Bengaluru was a good location for us to do this study: It is equidistant from both the eastern and western Indian coast, but almost the entire rain it gets is from monsoon winds coming from the Arabian Sea, which move from west to east. This simple feature makes this place amenable for mathematical modelling.
Monsoon rains depend on the nature of moisture formation over the ocean and the land during the season. The air moving over the oceans collects the moisture generated as a result of evaporation from the warm seas. During the monsoon, the wind flows from the oceans towards the land carrying this moisture with it. Evaporation from water bodies and the vegetation adds more moisture to the air mass. These moisture-laden winds, when uplifted by convection, cool to saturation and produce precipitation. Stronger winds are able to take moisture deeper inside.
By analysis of satellite-based observations available from the Tropical Rainfall Measurement Mission Project of Goddard Space Flight Center at NASA, we find that monsoon winds can give rise to rains even over the ocean. We calculated that in a normal monsoon year, the monsoon winds reaching Bengaluru drop about 5 per cent of their moisture content over the ocean itself, followed by 20 per cent drop over the land. These ‘rainouts’ influence the isotope content of the vapours reaching the land.
Ocean water contains two isotopes of oxygen, namely, O-16 and O-18. Being heavier, O-18 precipitates more. The more the ‘rainout’, the more is the loss of O-18. Therefore, by calculating the ratio of O-16 to O-18 in rainwater in Bengaluru, we can get an idea of how strong the monsoon winds were and how much ‘rainout’ happened on the way. We developed a model based on this idea which provides estimates of ‘rainout’. We find good agreement of our model estimate with the ones calculated from the satellite data. This shows that our model provides a reasonable scenario of rain formation.
Understanding of monsoon rain formation based on valid mathematical basis is the first step in the quest for rainfall prediction.
Prosenjit Ghosh & team
Centre for Earth Sciences, Indian Institute of Science, Bangalore