From pre-monsoon showers, cloud cover to moisture levels — local land and atmospheric conditions play a crucial role in determining the intensity and nature of heatwaves in the Indo-Gangetic Plains, a new study led by IIT Bombay researchers has found. At a time when India is reeling under intensive heatwaves, the study said that examination of such ‘local precursors’ – besides meteorological systems – can play a crucial role in providing early and location-specific warnings to mitigate impact of heatwaves.
In India, heatwave events typically occur during the pre-monsoon months. Amongst several meteorological factors, heatwaves are closely linked with anti-cyclonic flow over a region. Spanning hundreds of kilometers, anti-cyclones are a large-scale high-pressure system in the atmosphere which leads to an influx of warm, dry air into the Indo-Gangetic Plains region — creating favourable conditions for a heatwave.
However, a new study by researchers at IITBombay show that while anticyclones are critical, it is the local land and atmospheric conditions within the region that play a key role in determining the nature of the heatwave. The findings have come to light in a study titled — ‘Local Land-Atmosphere Interactions Precondition Moist and Dry Heatwaves Under Large-Scale Subsidence Over the Indo-Gangetic Plains’ — authored by Manali Saha, Vishal Dixit and Lanka Karthikeyan of IIT Bombay’s Centre of Studies in Resources Engineering and Centre for Climate Studies.
How was the research conducted and what it reveals?
The research entailed analysis of 10 major pre-monsoon heatwave episodes since 2010, using a heat-budget approach and data from ERA5 atmospheric reanalysis which is a repository of detailed global weather records from 1940 onwards.
For the study, the research team compared heatwave areas with nearby non-heatwave areas, which fell under similar large-scale conditions of an overarching anticyclone.
The analysis showed that even under the same anticyclone zone, two heat waves can behave differently in terms of feel, with local land and atmospheric conditions like humidity and soil moisture serving as the central role.
Of varying intensities each, the 10 heatwaves in the research’s data set were categorised into moist and dry heatwaves depending on the levels of humidity.
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Moist heatwaves, the study revealed, followed localised events like pre-monsoon showers which added extra moisture in the atmosphere a few days prior to the heatwave. This moisture, in turn, led to moist soil and increased evaporation, resulting in formation of clouds with low night-time clouds trapping heat.
“These nighttime low-level clouds act as heat traps by not allowing the outgoing radiation to escape. Thus, the interaction between pre-monsoon showers, increased humidity, and nighttime low-level cloud formation creates a complex atmospheric system that can contribute to the development and persistence of these conditions for four to five days to facilitate the formation of these events,” read the study published in the journal, Geophysical Research Letters.
Meanwhile, despite being under the influence of the same anticyclonic system, dry heatwaves were seen developing in areas with lack of moisture advection, dry soils and clear skies, as the moist heatwave.
According to the researchers, while humid and moist heatwaves are more dangerous as sweating and thereby, natural cooling of the human body, become ineffective, dry heatwaves have higher mortality rates as they affect larger regions for longer periods.
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How can the findings aid forecasting?
With the study determining the role of localised factors, Manali Saha – the lead author of the study – said that recognising the distinction between the two heatwaves can play a crucial role in improving early warning systems. “Our results suggest that monitoring local land and atmospheric conditions becomes more important for predicting when and where heatwaves will develop within the region. This could help shift forecasts from broad regional warnings to more location-specific early warnings,” said Saha, a PhD scholar at IIT Bombay.
For perspective, currently, the India Meteorological Department (IMD) issues heatwave warnings and heat index on the basis of surface level temperatures with no specifications of moist or dry heatwaves. Further, the weather bureau issues no independent bulletin for feels-like temperatures.
To bridge the lacunae, the IIT Bombay research has identified a set of local precursors ranging from patterns of pre-monsoon rainfall to humidity near the surface and cloud formation. According to Prof. Karthikeyan Lanka, Associate Professor and member of the research team, close monitoring of the local precursors can aid in determining which region is likely to experience heatwaves. Eyeing to bridge the gap, the research team has now charted plans to develop its heatwave early warning system. Prof. Vishal Dixit, a climate scientist at IIT Bombay said, “The idea is to build a machine-learning–based decision support system that can assist operational agencies in improving heatwave early warning and location-specific forecasts across India.”
