What is it about?
This study explains, for the first time, the progression of the monsoon rains over India. Although the Indian monsoon has been known about for a thousand years, and monitored scientifically since the early days of weather forecasting, the monsoon onset has remained unexplained. Onset happens first in the extreme south, over Kerala, around 1 June, but moves relatively slowly toward the northwest of the country over a period of about 6 weeks, reaching the far northwest around 15 July. This direction of movement has always been a puzzle, because it progresses against the direction of the mean winds in the lower troposphere. From analysis of many years of data from balloon-borne soundings over India, we have been able to show that the progression of the monsoon onset is controlled by the retreat of a layer of dry air at around 3-5 km altitude. The dry air, which suppresses rainfall in the pre-monsoon season, is progressively humidified by smaller, non-precipitating clouds, until it becomes moist enough to support rain.
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Why is it important?
This conceptual understanding is important for several reasons: in particular it will help forecasters to understand the weather features which need to be monitored in monsoon onset forecasting, and it offers an explanation of why weather and climate models have such large errors for the monsoon. Models struggle in representing small cumulus clouds, and we suggest that these clouds are essential in establishing suitable conditions for monsoon rain. Our research aims to make improvements in weather and climate predictions for India, which will assist farmers and regional planners.
Perspectives
I enjoyed working on this result. Having lots of experience of the African monsoon system, I had assumed that people knew everything there is to know about the Indian monsoon, but when I came to look at it, some simple textbook explanations were absent or didn't make sense. When we looked at the measurements, the physical mechanisms seemed obvious. This is perhaps an example where it's good for scientists to come into a new area (albeit armed with plenty of specialist scientific skills) and look at the simplest questions with fresh eyes. This is a slightly strange paper, in that we looked at the observations and used them to explain conceptually how the Indian monsoon system works. Having written the paper, the result looks obvious to us and others, but we didn't, or couldn't, construct tests to establish the significance of the result: in philosophical terms, are we making a scientific statement which in principle could be disproved? The essence of the argument is that the role of clouds in moistening the dry air, to make rainfall possible further downwind (to the south) could be disproved, if it were found that these clouds don't actually change the likelihood of rainfall downwind, and are passive.
Douglas Parker
University of Leeds
Read the Original
This page is a summary of: The interaction of moist convection and mid-level dry air in the advance of the onset of the Indian monsoon, Quarterly Journal of the Royal Meteorological Society, May 2016, Wiley,
DOI: 10.1002/qj.2815.
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