Why cloudburst forecast in India still remains elusive

• Cloudbursts are violent and voluminous amounts of rain pouring down in a short duration over a small area have been reported since the mid-19th century.
• Yet, the characteristics of these events remain elusive, and our efforts in monitoring and forecasting them are at an embryonic stage.

Cloudbursts

• Associated with cumulonimbus clouds that cause thunderstorms
• Occasionally due to monsoon wind surges and other weather phenomena.

Characteristics

• Do not indicate clouds exploding.
• Defined by the amount of rainfall.
• As per IMD, it amounts to 100 mm of rain in an hour.
• Occur over a small geographical region of 20 to 30 sq. km.
• Strong monsoon wind surges along the coast can also result in cloudbursts.
• Eg. Mumbai (2005) and Chennai (2015).

Mechanism

• Often occur during the monsoon season, when the southwesterly monsoon winds bring in copious amounts of moisture inland.
• Moist air that converges over land gets lifted as they encounter the hills.
• Reaches an altitude and gets saturated
• Water starts condensing out of the air forming clouds
• Orographic lifting together with a strong moisture convergence can lead to intense cumulonimbus clouds taking in huge volumes of moisture.
• A single-cell cloud may last for an hour and dump all the rain in the last 20 to 30 minutes
• Some merge to form multi-cell storms and last for several hours.

Cloudburst prone areas

• Occur mostly over the rugged terrains over the Himalayas, the Western Ghats, and northeastern hill States of India.
• Rain on the fragile steep slopes trigger landslides, debris flows, and flash floods.
• Causes large-scale destruction and loss of people and property.
• Reported from the NE States and Western Ghats States during the current monsoon season (2022).

Challenges in detecting cloudbursts

• Resolution of the precipitation radars of satellites can be much smaller than the area of individual cloudburst events.
• So, they go undetected.
• Weather forecast models also face a similar challenge in simulating the clouds at a high resolution.
• Skillful forecasting of rainfall in hilly regions remains challenging due to:
• uncertainties in the interaction between the moisture convergence and the hilly terrain
• the cloud microphysics
• the heating-cooling mechanisms at different atmospheric levels.
• Multiple doppler weather radars effective to monitor moving cloud droplets.
• Expensive affair
• Installing them across the country may not be practically feasible.

Measures

• Mapping the cloudburst-prone regions using automatic rain gauges.
• If cloudburst-prone regions are co-located with landslide-prone regions, these locations can be designated as hazardous.
• People should be moved
• Construction and mining in nearby regions should be restricted

Climate change and global warming

• Climate change is projected to increase the frequency and intensity of cloudbursts worldwide.
• As the air gets warmer, it can hold more moisture and for a longer time.
• A 1-degree Celsius rise in temperature may correspond to a 7-10% increase in moisture and rainfall.
• This increase in rainfall amount does not spread moderately throughout the season.
• As the moisture holding capacity of air increases, it results in prolonged dry periods intermittent with short spells of extreme rains.
• More deeper cumulonimbus clouds form and the chances of cloudbursts also increase.