Rare interaction of western disturbance and low-pressure systems could bring the extreme events
The western disturbance over Afghanistan and Pakistan. Screengrab: Windy.com The interaction of a western disturbance with a monsoon low pressure system (LPS) could bring extremely heavy rainfall, possibly even cloud bursts, flash floods and landslides, to large areas in North West India from July 8 onwards.
A western disturbance is an extra tropical storm in the upper layers of the atmosphere that is carried towards India by the subtropical jet stream, a band of fast flowing winds that circulates the Earth.
A low pressure system, on the other hand, is an area of low atmospheric pressure that generally forms over seas and oceans and causes rainfall.
Such an interaction is rare as WDs generally do not form during the monsoon months. However, in recent years, the impacts of global warming have increased the chances of such occurrences as WDs have become more common during summer and monsoon months.
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The India Meteorological Department (IMD) indicated the presence of a WD over Afghanistan and Pakistan in a press release on July 6. A low pressure system, which enhances monsoon rainfall, is forming in the Bay of Bengal.
The interaction of a western disturbance with a monsoon low pressure system. Screengrab: earthnullschool
The IMD has also warned of isolated very heavy rainfall over Uttarakhand and east Rajasthan over the next five days and over west Uttar Pradesh (UP) on July 6, but the impact could be more widespread.
“The monsoon trough, which provides a pathway to the propagation of LPSs, is currently south of its normal position. So this LPS is likely going to travel towards northwestern parts of India where this interaction is expected to occur between July 8 and 11,” Akshay Deoras, research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, United Kingdom, told Down To Earth (DTE).
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“We’ve been waiting for a few weeks to see if this sort of interaction might occur since both ingredients (WDs and monsoon) have been in place for a while,” Kieran Hunt, Research Fellow in Tropical and Himalayan Meteorology, University of Reading, UK, told DTE.
“Whenever you have that combination of a quick advance of the monsoon across northern India and a delayed northern withdrawal of the subtropical jet, there is a risk of WD-monsoon interactions dumping heavy precipitation over North West India,” Hunt added.
This will trigger heavy rainfall over northern parts of Rajasthan, Haryana, Punjab, Chandigarh, Jammu and Kashmir, and adjoining western parts of Himachal Pradesh. Some impact is expected in western Uttarakhand, Delhi-National Capital Region and western parts of UP in this period, according to Deoras.
The heavy rainfall could trigger flash floods, landslides and travel disruptions in the mountainous states of Jammu and Kashmir, Himachal Pradesh and Uttarakhand.
An interaction between a WD and a monsoon low pressure system had triggered the catastrophic Uttarakhand floods in June 2013, which killed over 5,000 people and destroyed a lot of infrastructure in the hills.
“While these interactions are most frequent during post-monsoon months such as October, they have been reported during June and July,” said Deoras.
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“According to our database, WDs in July are about 10 per cent as common than during their peak in December / January. This ratio falls even lower — only a few per cent, depending on how you define them — if we start looking only at strong or active WDs,” said Hunt.
The observed frequency of such interactions during July is around half of that in June, as per Deoras.
Hunt highlighted the challenges of forecasting such interactions. “Firstly, these are rare events, so we don’t have a lot of experience with how they are handled by forecast models. For example, 2013 was ten years ago, a lifetime in weather model development,” Hunt explained.
“Secondly, by their nature, they involve the interaction of large masses of moist air with the mountains of the Western Himalaya. The processes involved are not necessarily well-represented in weather forecast models because the mountains tend to be a lot smaller than the model grid squares,” he added.
The interaction that is going to occur in the next few days has been consistently predicted by leading numerical weather prediction models since the end of June, so there is high confidence in the forecast, according to Deoras.
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“There are some clues we can use to estimate the severity of this particular interaction. The monsoon is fully developed over NW India, so there is an abundance of moist air there,” Hunt said.
However, the advancing WD appears to be quite weak and quite far north, as is common outside of the winter months, Hunt added. “So I certainly don’t think it’ll be as severe and widespread as the Uttarakhand event in 2013,” he said.
There is a likelihood of some of the rainfall events being cloud bursts, especially in the hilly regions, because of the deep atmospheric instability already in place due to the monsoon, according to Hunt.
“Crucially, though, there is now quite good evidence that climate change is causing the subtropical jet to stay around North India much later in the year, so we are seeing a significantly increasing trend in WD frequency in May and June,” concluded Hunt.
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