On May 28, a day of extreme heavy rainfall, the automatic weather station (AWS) installed by CUSAT at Kalamassery, Kochi, recorded something extraordinary. Between 9.05 a.m. and 10.05 a.m., the graph showed a steep and consistent surge. In the hour, rainfall of 103 mm (10.3 cm) was recorded.
Near to CUSAT's in Kalamassery was Indian Meteorological Department's AWS. As a kind of confirmation, this station too recorded near similar reading (100 mm) in the same hour.
If the IMD's definition is accepted, this crushingly heavy one-hour rain over Kalamassery on May 28, was the first recorded cloudburst in Kerala. Here's the IMD's definition: "If 10 cm rainfall is received at a station in one hour, the rain event is termed as cloudburst. CUSAT's Atmospheric Radar Research Centre has confirmed the cloudburst event but, curiously, not the IMD.
Have cloudbursts occurred before?
It is hard to say for a combination of two reasons. One, shortage of data. There are just three or four rain gauge stations in each district. Two, this limited reach of the rain stations is compounded by the highly restricted nature of cloudburst events. Such events can be highly elusive for the existing rain stations.
A cloudburst happens only in the core area (200-500 sq kms) of a massive cloud formation spread over some one lakh sq kms. It is within this large formation, some 10,000-50,000 sq kms inside the large formation, that cumulonimbus clouds or multistoreyed clouds 12-14 kms high that cause extreme heavy rain are formed. And the clouds with cloudburst potential, or the core clouds that are insanely taller (higher than even 14 kms), occur 200-500 sq kms within the cumulonimbus formation. In short, it is only a minute portion of a massive cloud formation that produces cloudburst. Clouds outside the core produce relatively less intense rains.
The limited rain gauge stations in Kerala, therefore, miss cloudbursts more than they detect them. CUSAT's AWS at Kalamassery just got lucky.
Are cloudburst-like events a monsoon phenomena?
Scientists say that such 'short spell intense showers' are caused by towering cumulonimbus clouds. Such clouds are a feature of summer rains, not the monsoon. The rainfall on May 28 is also termed 'summer rains' as the IMD, at that point, had not found all the conditions that would satisfy the onset of the monsoon.
It is usual for these pre-monsoon clouds to vehemently dump their load over land in 15-20 minutes. But if this usually quarter-hour devilish spell prolonged for an hour on May 28, it was the craftwork of two low-pressure events on the eastern side.
One was the Remal cyclone over the Bay of Bengal, and the other was a cyclonic formation off the coast of Tamil Nadu. These formations aggressively pulled the dense moisture-laden westerly winds toward the east. And the westerly that was sucked to the east dragged along with it to the Kerala coast deep rain-bearing clouds that were formed over south Arabian sea.
What are mini or mesoscale cloudbursts?
During monsoon, rainfall of 5-10 cms within two hours can also have a huge impact. Even a rainfall intensity of five or nine centimetres within an hour, even if it falls outside the definition of cloudburst, can be destructive.
Reason why scientists at CUSAT's Advanced Centre for Scientific Research have called them 'mesoscale cloudburst (MsCB) or mini cloudburst.
The impact of a mini cloudburst depends on the nature or the water retention capacity of the soil and its gradient and also its natural ability to drain out the excess rain water. Such mini cloudbursts is seen to have caused immense damage in Kerala's hilly regions where unscientific construction have left the land in these areas highly unstable, in many instances causing landslides. And in midlands, near rivers that are full during monsoon, they have caused floods.
How does IMD categorise rains?
Cloudburst is intense rain (100 mm and above) in an hour, and is an outlier. Otherwise, the IMD differentiates rain into various categories. Ver light rain (0.1-2.4 mm in 24 hours). Light rain (2.5-15.5 mm in 24 hours). Moderate rain (15.6-64.4 mm in 24 hours). Heavy rain (64.5-115.5 mm in 24 hours). Very heavy rain (115.6-205 mm in 24 hours). Extremely heavy rain (205 mm and above).