Why Forecasting Cloudbursts is a challenge?

Context:

Cloudbursts — 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 is at an embryonic stage.

Relevance:

GS-I: Geography (Physical Geography, Climatology, Important Geophysical phenomena), GS-III: Environment and Ecology (Climate Change and its effects), GS-III: Disaster Management

Dimensions of the Article:

1. What is a Cloudburst?
2. Why do cloudbursts happen only in the mountains and hilly areas?
3. Why does cloudburst cause so many deaths?

What is a Cloudburst?

• Cloudbursts are sudden and extreme rainfall events over a limited area in a short span of time. There is no universal definition of a cloudburst.
• The India Meteorological Department (IMD) defines a cloudburst as any event where 100 millimetres of rainfall have fallen in a span of an hour over a region that is 20-30 square kilometres in area. By this definition, 5 cm of rainfall in half an hour would also be classified as a cloudburst.

Characteristics

• In India, cloudbursts often occur during the monsoon season, when the southwesterly monsoon winds bring in copious amounts of moisture inland.
• The moist air that converges over land gets lifted as they encounter the hills.
• The moist air reaches an altitude and gets saturated, and the water starts condensing out of the air forming clouds.
• This is how clouds usually form, but such an orographic lifting together with a strong moisture convergence can lead to intense cumulonimbus clouds taking in huge volumes of moisture that is dumped during cloudbursts.
• Tall cumulonimbus clouds can develop in about half an hour as the moisture updraft happens rapidly, at a pace of 60 to 120 km/hr.
• A single-cell cloud may last for an hour and dump all the rain in the last 20 to 30 minutes, while some of these clouds merge to form multi-cell storms and last for several hours.

How do Cloudbursts occur?

• A cloudburst occurs when moisture-carrying air moves up a hilly terrain, forming a vertical column of clouds known as ‘cumulonimbus’ clouds.
• Such clouds usually cause rain, thunder and lightning. This upward motion of the clouds is known as an ‘orographic lift’.
• These unstable clouds cause an intense rainstorm over a small area after becoming heavy enough and locked in the ridges and valleys between the hills.
• The energy necessary for the cloudburst comes from the upward motion of air. Cloudbursts mostly occur at elevations between 1,000-2,500 metres above sea level.
• The moisture is usually provided by a low-pressure system (usually associated with cyclonic storms in the ocean) over the Gangetic plains associated with low level winds flowing in from the east.
• Sometimes winds flowing in from the north west also aid the occurrence of cloudbursts. The many factors that have to come together to make a cloudburst event happen make them highly unlikely.

Why do cloudbursts happen only in the mountains and hilly areas?

• Cloudbursts do happen in plains as well, but there is a greater probability of them occurring in mountainous zones; it has to do with the terrain.
• Cloudbursts happen when saturated clouds are unable to produce rain because of the upward movement of very warm current of air.
• Raindrops, instead of dropping down, are carried upwards by the air current.
• New drops are formed and existing raindrops gain in size. After a point, the raindrops become too heavy for the cloud to hold on to, and they drop down together in a quick flash.
• Hilly terrains aid in heated air currents rising vertically upwards, thereby, increasing the probability of a cloudburst situation.
• In addition, as pointed out earlier, cloudbursts get counted only when they result in largescale destruction of life and property, which happens mainly in mountainous regions.

-Source: The Hindu