The characteristics of cloud burst events remain elusive, and our efforts in monitoring and forecasting them is at an embryonic stage.

Cyclones can be predicted about one week in advance. However, cloudburst forecasts still remain elusive.

What is Cloudbursts?

• A cloudburst is a localised but intense rainfall activity.
• Short spells of very heavy rainfall over a small geographical area can cause widespread destruction, especially in hilly regions where this phenomenon is the most common.
• Not all instances of very heavy rainfall, however, are cloudbursts.
• A cloudburst has a very specific definition: Rainfall of 10 cm or more in an hour over a roughly 10 km x 10-km area is classified as a cloudburst event.
• By this definition, 5 cm of rainfall in a half-hour period over the same area would also be categorized as a cloudburst.

Which clouds do burst?

• Cloudburst events are often associated with cumulonimbus clouds that cause thunderstorms and occasionally due to monsoon wind surges and other weather phenomena.
• Cumulonimbus clouds can grow up to 12-15 km in height through the entire troposphere (occasionally up to 21 km) and can hold huge amounts of water.
• 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.
• However, cloudbursts are not defined based on cloud characteristics and do not indicate clouds exploding. Cloudbursts are defined by the amount of rainfall.

How is it different from normal rainfall?

• According to the India Meteorological Department (IMD), 100 mm of rain in an hour is called a cloudburst.
• Usually, cloudbursts occur over a small geographical region of 20 to 30 sq. km.

When do they occur?

• 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.

How common are cloudbursts?

• Cloudbursts are not uncommon events, particularly during the monsoon months.
• Most of these happen in the Himalayan states where the local topology, wind systems, and temperature gradients between the lower and upper atmosphere facilitate the occurrence of such events.
• However, not every event that is described as a cloudburst is actually, by definition, a cloudburst.
• That is because these events are highly localized.
• They take place in very small areas which are often devoid of rainfall measuring instruments.

Climate change and cloudbursts: How are they related?

• 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. We call this the Clausius Clapeyron relationship.
• 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 get 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.
• Deeper cumulonimbus clouds will form and the chances of cloudbursts also increase.

Why are they so destructive?

• The consequences of these events, however, are not confined to small areas.
• Because of the nature of terrain, the heavy rainfall events often trigger landslides and flash floods, causing extensive destruction downstream.
• This is the reason why every sudden downpour that leads to destruction of life and property in the hilly areas gets described as a “cloudburst”, irrespective of whether the amount of rainfall meets the defining criteria.
• At the same time, it is also possible that actual cloudburst events in remote locations aren’t recorded.

Detecting cloudbursts

• Satellites are extensively useful in detecting large-scale monsoon weather systems.
• However the resolution of the precipitation radars of these satellites can be much smaller than the area of individual cloudburst events, and hence they go undetected.
• Weather forecast models also face a similar challenge in simulating the clouds at a high resolution.
• The skillful forecasting of rainfall in hilly regions remains challenging due to the uncertainties in the interaction between the moisture convergence and the hilly terrain.
• There also involves the cloud microphysics, and the heating-cooling mechanisms at different atmospheric levels.
• Multiple radars can be a quick measure for providing warnings, but radars are an expensive affair, and installing them across the country may not be practically feasible.

Solutions to cloudbursts forecast

• Multiple doppler weather radars can be used to monitor moving cloud droplets and help to provide nowcasts (forecasts for the next three hours).
• A long-term measure would be 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.

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