New Delhi:
The extreme rainfall that has ravaged Himachal Pradesh, Punjab, Haryana and Delhi since Saturday will likely subside by Tuesday, although there will be rain over Uttarakhand, Uttar Pradesh and eastern Rajasthan, the India Meteorological Department said on Monday. Extremely heavy rainfall is also likely over northeastern India and adjoining areas of eastern India in the next three days, the weather office said.
In the past two days, an interaction between a western disturbance and the monsoon trough led to extremely heavy rainfall over Himachal Pradesh, Uttarakhand, Punjab and Haryana, causing landslides, flooding and widespread damage, HT reported on Monday. The unusual interaction of a western disturbance with the monsoon trough can be devastating, as witnessed during the 2013 Uttarakhand floods.
The good news is that a fresh western disturbance is not expected immediately although experts warn that Himalayan states need to be prepared for the eventuality, in part due to climate change which has also led to an increase in the moisture holding capacity of the atmosphere that can supercharge rainfall.
India has already seen one western disturbances this year till July 10. Most western disturbances occur in winter in the northern hemisphere (December to March in India), but this year has seen around 20 of them between March and May, perhaps one reason for the milder summer in north and north-west India. Western disturbances during the monsoon are rare (but not unknown). The last time a western disturbance interacted with a monsoon trough in any significant way in India was in June 2013, and it caused the Kedarnath floods.
The western disturbance has moved eastward and with the monsoon trough situated along the Indo-Gangetic plains, the weather office has forecast light or moderate rainfall with isolated heavy showers over Uttarakhand, eastern Rajasthan and Uttar Pradesh for much of this week.
“Those areas need to prepare,” said DS Pai, senior scientist at IMD. “The western disturbance is moving eastwards now, so Uttar Pradesh and adjoining areas can expect good rain.”
Western disturbances typically move to northern latitudes during the south-west monsoon b, said Mahesh Palawat, vice president of climate and meteorology at Skymet Weather Service, a private forecaster.
“We can expect heavy to very heavy rainfall over Himachal Pradesh for one more day and over Uttarakhand for the next two to three days because the monsoon trough is over the Indo-Gangetic plains,” Palawat said. “There is also a cyclonic circulation over northwest Rajasthan and adjoining parts of southern Haryana. So, rainfall will continue over north India.”
IMD defines heavy rain as rainfall of 64.5-115.5 mm in a 24 hour period; very heavy as rainfall between 115.6 mm and 204.4 mm in 24 hours; and extremely heavy as rainfall in excess of 204.5 mm in the same time period. By that measure most northern states have received very heavy rainfall over the weekend, with some receiving extremely heavy rainfall.
Extreme rainfall need low level inflow over a region that helps air to ascend; plenty of moisture around the region for deeper cloud development; and divergence in the upper layers, which helps the weather system to further deepen, explained meteorologist M Rajeevan, former secretary at the earth science ministry.
“In this case, all three have occurred together. Over the region, there were easterlies in the lower level with incursion and transport of moisture. Moisture is also transported from the Arabian Sea. Also, some northwesterlies are coming from Pakistan, which are dry winds,” Rajeevan said. “The meeting of moist easterlies and dry northwesterlies is deadly for heavy rains, when supported by upper-level divergence. These things happened in the (2013) Uttarakhand floods. The only difference this time was that it rained continuously for three days, with the trough moving very slowly.”
In June 2013, a devastating flood inundated large parts of Uttarakhand and killed over 6,000 people. Many of them were pilgrims visiting the state. According to meteorologists, the flooding was caused by extreme rainfall due to interaction of a western disturbance and the monsoon winds.
The intense rainfall indicates that climate change is impacting the south-west monsoon, Rajeevan said. “It rains fewer hours, but when it rains, it rains very heavily.”
Scientists are seeing a clear climate shift in monsoon patterns. “We are seeing long deficit rainfall periods and short spells of heavy rains in a few days. Some regions are getting a month’s rainfall in a day or a few days’ time,” said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology. “Due to global warming, there’s more moisture available, and since the moisture holding capacity of air increases with temperature, there’s more water for a heavy downpour.”
In a changing climate, upland areas such as the Himalayan foothills or the Western Ghats are particularly susceptible to heavy rains and landslides, Koll said. “Due to global warming, there’s extra moisture available, and the hills stop this moisture flow and lift it, which falls as heavy rains. Some of the regions over India where extreme rains have increased are such places where the rains happen due to orographic lifting.” Orography is the study of the topographical relief of mountains.
Flash floods due to cloudbursts and extreme rainfall are difficult to predict, experts said. “We should monitor these events closely and identify areas prone to flash floods. With a radar, the maximum lead time that we could get is about three hours before such an event,” Koll said. “We should also check land use changes and development activities that might have aggravated these flash floods.”
IMD issued an orange category warning for the Himalayan foothills and the Indo-Gangetic plains covering Uttar Pradesh, Uttarakhand, Bihar and Madhya Pradesh. on Tuesday and Wednesday,
A red category warning tell local authorities to take action to prevent disasters associated with heavy rain, and an orange warning indicates authorities should be prepared to act swiftly.
The weather office has recommended that farmers should drain out excess water from fields to avoid water stagnation in Himachal Pradesh, Uttarakhand, Punjab, Haryana, Uttar Pradesh, Rajasthan, sub-Himalayan West Bengal, Sikkim, Bihar, Madhya Pradesh, Gujarat, Konkan, Goa, Tamil Nadu, Kerala, coastal Karnataka, Andhra Pradesh and the northeastern states. Transplanting of rice in Arunachal Pradesh, Meghalaya, Uttarakhand, Himachal Pradesh and adjoining districts of Punjab can be deferred. The sowing of maize, soybean and vegetables in Uttarakhand can be also deferred, it said.
