The India Meteorological Department (IMD) has flagged a confluence of the monsoon trough and an active western disturbance that is set to intensify rainfall over Uttarakhand, Himachal Pradesh, and Jammu & Kashmir, raising risks of flash floods, landslides, and swollen rivers.

• The IMD has also stated that while there is no increasing trend in traditional cloudbursts, instances of “mini-cloudbursts” are rising.

Meteorological Interaction Behind Heavy Rains

• Systems involved:
  • The monsoon trough is running south of its normal position.
  • Western disturbance in the form of cyclonic circulation over north Pakistan and adjoining Punjab.
  • Additional circulations over Rajasthan and Madhya Pradesh are pulling in moisture from the Arabian Sea and Bay of Bengal.
• Effect of interaction: Enhances wind shifts, moisture build-up, and instability, leading to widespread heavy to extremely heavy rainfall in the Himalayan foothills.

Monsoon Trough

• Definition: An elongated low-pressure area stretching from the heat low over Pakistan to the Bay of Bengal.
• Nature: A semi-permanent feature of monsoon circulation.
• Geographical Orientation:
• • Influenced by the east–west alignment of the Himalayas.
  • Affected by the north–south orientation of the Khasi-Jaintia Hills.
• Oscillation: The eastern part of the trough shifts northwards or southwards periodically.
• Impact of Southward Shift: Leads to an active or vigorous monsoon over most parts of India.
• Impact of Northward Shift: Causes break monsoon conditions over central and peninsular India, while producing heavy rainfall along the Himalayan foothills and sometimes leading to Brahmaputra floods.

Western Disturbances (WD)

• Definition: Low-pressure systems, also called Extra-Tropical Cyclones or Mid-Latitude Cyclones, originate in the Mediterranean region and move eastwards across Central Asia.
• Occurrence: Most common in winter months (November–April), with peak frequency in January–February. 
• Characteristics
  • Frontal Systems: WDs are associated with the merging of polar and subtropical air masses, resulting in the formation of a frontal boundary. The interaction between these air masses leads to cloud formation and precipitation.
  • Rain & Snowfall: Causes rainfall in plains and snowfall in higher Himalayan altitudes, vital for water resources.
  • Temperature Fluctuations: Passage of WDs causes warm air (ahead) to be replaced by cold air (behind), leading to noticeable temperature drops.
• Impact on Indian Weather
  • Winter Precipitation: The Main source of winter rainfall in northwestern states (J&K, Himachal Pradesh, Punjab, Haryana, Rajasthan). Essential for Rabi crops.
  • Snowfall in the Himalayas: Heavy snowfall replenishes rivers and ensures steady summer water flow.
  • Disruptions: Can cause road blockages, power outages, and transport difficulties in hilly regions due to heavy snow.

What is Cloudburst? 

• Definition (IMD): A cloudburst is 10 cm or more of rainfall within an hour over an area of 20–30 sq. km.
• Mini Cloudburst: Defined as 5 cm rainfall per hour.
• Nature: Extremely localised events that remain impossible to forecast with current technology.
• Recent examples: Cloud Burst in Dehradun 2022, Amarnath cave, 2022.

Characteristics of Cloudbursts

• High-Intensity Rainfall: Defined by rainfall exceeding 100 mm in an hour, often leading to flash floods.
  • The 2010 Leh cloudburst unleashed torrential rain in a short span, overwhelming local drainage.
• Localised Nature: Impact is limited to small areas, usually less than 20–30 sq. km.
  • The 2022 Amarnath Yatra cloudburst caused severe flooding in a restricted zone.
• Short Duration: Events last only a few minutes to an hour, but the rainfall intensity causes severe destruction.
  • The 2021 Uttarakhand cloudburst lasted ~30 minutes and triggered flash floods.
• Sudden River Surge & Dam Risk: Trigger a rapid rise in river water levels, increasing pressure on dams, which can sometimes result in breaches and devastating floods.
• Occurrence in Hilly Regions: Most frequent in mountainous terrains like the Himalayas due to the orographic effect(uplift of moist air by mountains).
• Lack of Predictability: Extremely sudden and localised, making them difficult to forecast with current technology.

Mechanism of Cloudbursts

• Moisture-Laden Winds: Warm, humid winds from oceans or monsoon currents move inland and rise when they encounter mountains (orographic lift).
• Rapid Uplift of Air: In hilly regions, the steep terrain forces the moist air upward very quickly.
• Cooling & Condensation: As the air rises, it cools rapidly, causing condensation of water vapour into dense clouds.
• Cloud Saturation: When clouds become heavily saturated with moisture, they cannot hold the water any longer.
• Sudden Downpour: Instead of gradual rainfall, the saturated cloud releases a massive amount of water in a short time(100 mm or more within an hour).

Observed Trends of Cloudbursts

• Historical Data: A 2018 IITM Pune study recorded only 28 cloudburst events between 1969–2015.
• Forecasting Limitation: Satellite imagery can detect intense clouding or heavy rainfall, but cannot predict the exact occurrence of a cloudburst.
• Terrain Sensitivity: Even 2–5 cm of rainfall can trigger landslides in fragile terrains, highlighting the severe impact of such downpours.

Consequences and Risks

• Flash floods and landslides in Uttarakhand and Himachal Pradesh, already vulnerable due to saturated soil.
• The Chenab River overflow in Jammu led to landslides, killing 30 people and injuring several.
• Swollen rivers pose risks to downstream towns and cities.
• Disruption to pilgrimage and tourist routes, road blockages, and infrastructure damage are likely.

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