A MAJOR meteorological puzzle is what kind of atmospheric system it was that off-loaded an unprecedented amount of rain - nearly a metre of water column - over the city of Mumbai in about 24 hours beginning 8-30 a.m. on July 26. The meteorological station at Santacruz in North Mumbai recorded 94.4 cm of rainfall, the highest ever in the history of the city, of which 88.5 cm (nearly 94 per cent) was received in 12 hours from 11-30 a.m. In fact, the first four hours itself saw 13 to 15 cm/hr of rain. The heavy precipitation, unlike a cloudburst which is short-lived, was sustained for over 10 hours.
Intense rainfall (above 20 cm/day) on the West Coast is not uncommon during the monsoon. Such extraordinarily intense rainfall too is not unknown in the region. Dharampur, which is only about 150 km north of Mumbai and about 40 km from coastal Daman, received 98.7 cm of rain on July 2, 1941. According to an analysis, "Intense Rainfall Events Over the West Coast of India", by P.A. Francis of the Indian Institute of Science (IISc), Bangalore - using data of 100 intense rainfall days during 1951-87 - the probability of getting maximum rainfall is high around 15oN latitude and near the Mumbai region (19oN).
According to Francis' study, the last two weeks of July are one of the two periods during the monsoon when the rainfall intensity peaks in the region. The rainfall of July 26 over Mumbai was, in fact, preceded by heavy rainfall along Goa and the Karnataka coast, pointing to strong convective activity over east Arabian Sea which moved northward. But the highest rainfall that Mumbai city ever received in a day was recorded 31 years ago - 57.76 cm.
More unusual is the fact that the spell was squeezed both in time and in space. On the same day, the meteorological station at Colaba in South Mumbai recorded only 7.34 cm of rain. Given the extremely localised nature of the intense rainfall, the rain-bearing cloud system and the atmospheric dynamics that anchored and sustained it over Mumbai for 12-15 hours must have had a spatial extent of 20 km to 30 km only.
Moreover, the rain was accompanied by constant severe thunderstorm activity and strong squalls, indicating a strong continuous convergence of off-shore winds (westerlies). Monsoon winds normally do not have such a characteristic. But such events are also not unknown.
According to D.R. Sikka, former head of the Indian Institute of Tropical Meteorology (IITM), Pune, there have been eight to ten events in the past where heavy rain was accompanied by thunderstorm activity. The movement of a low pressure in the atmosphere (depression) from the Bay of Bengal inland - which is known to strengthen the westerlies from the Arabian Sea - would have contributed to this strong off-shore convergence, thunderstorm activity as well as advection of moisture, points out Sikka. By July 29, the depression had moved over Gujarat causing heavy rain there.
Another interesting feature is the depth of the cloud system that caused the event. According to measurements by the weather radar in Mumbai, which can only measure the depth and extent of cloud cover and cannot measure the precipitation rate or winds, the localised cloud system had a height of 15 km. Although monsoon clouds are usually 6 km to 8 km deep, such cloud heights are not uncommon during intense rainfall events (table 1). In fact, detection of a 15-km-deep cloud system should itself have been a clue to an impending heavy rainfall and probably even sufficient to be on the alert.
According to Francis' analysis, during the very active spells, a large fraction of the West Coast is covered by clouds with depths more than 10 km and such deep cloud systems are more frequent near 15oN and 19oN, the regions which have the highest probability of intense rainfall. But usually clouds having small spatial extent (less than 50 km) tend to be smaller in depth (6 km to 8 km). Here it was a case of deep cloud with a small spatial dimension.
Satellite images of July 26-27 seem to indicate that there were two deep cloud systems, one from the east and one from the west, says G.S. Bhat of the IISc. The two merged around July 26 and the cloud system continued to remain deep during the heavy precipitation. This would suggest that there was continuous convergence of winds and convection of moisture into the system. What caused this sustenance at a very local scale is not known. Of course, the satellite images cannot reveal this because they show only the top of cloud formations, which extend over much larger areas, across thousands of square kilometres.
Intense rainfall events during monsoon in the peninsular region are often found to be associated with the northward motion of the Tropical Convergence Zone (TCZ). The TCZ is a band of deep moist convection and convergence in the lower troposphere, resulting in the formation of deep cloud systems.
The TCZ forms near the equator and girdles the equatorial region. During the monsoon, it moves from the equatorial Indian Ocean and sits over the monsoon region. The mean position of the TCZ during the monsoon is about 15oN, from where it oscillates for two to six weeks.
In satellite pictures, prominent cloud bands characteristic of the TCZ straddling the monsoon region can be seen. The TCZ belt actually represents organised convection on a large scale and its location is often linked to intense rainfall events along the west coast. Francis' analysis shows that 75 per cent of intense rainfall events are associated with TCZ (table 2). In fact, Francis says that if northward propagation of TCZ could be predicted, it may be possible to predict intense rainfall events.
Off-shore convection is the next important cause of heavy rainfall, which is associated with 23 per cent of the events. Rainfall associated with off-shore convection is typically confined to a small region, say over 100 km. But still this is larger than the highly localised system that seems to have caused the Mumbai rains. Mid-tropospheric cyclones (MTC) were found to be linked to 14 of the events. The maximum cyclonic activity was found to occur at heights of 20 km-30 km. Francis found that MTCs were associated with very heavy rainfall, over 30 cm/day, particularly over Gujarat and Maharashtra (north of Mumbai).
The last class of atmospheric system associated with intense rainfall is off-shore vortex. This is usually caused by the mountain orography present along the coastline. The orography results in a high pressure gradient from the coast to the hills, preventing the westerlies from the sea to be able to go inland. They end up circulating close to the shore, sometimes leading to the formation of vortices off-shore. However, only one event was found to be linked to an off-shore vortex. In fact, a combination of different basic mechanisms could, in general, be responsible for intense rainfall events.
Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF), which comes under the Department of Science and Technology (DST), believes that an off-shore vortex was possibly responsible for the Mumbai event. However, other scientists strongly discount such a hypothesis because there is no evidence of that either from satellite images or from the limited terrestrial data that is available from the India Meteorological Department (IMD).
Moreover, off-shore vortices are short-lived (a couple of hours) and as they dissipate new ones form causing the rainfall to be more widespread than was seen in Mumbai. If it was indeed an off-shore vortex, there should be an explanation for its long life of over 10 hours. Moreover, even after the intense rainfall phase was over, strong westerlies have continued from the Arabian Sea. This, points out Sikka, is indicative of some other factors, as yet unclear, at work causing strong convergence and its sustenance.
But whatever the unique rain causing system was - perhaps some unusual process on the mesoscale (across 20 km to 200 km) - it was embedded in the larger synoptic scale meteorological process on which the normal observational meteorology and associated weather forecasts were made.
Hence the question: What was the peculiar mesoscale process that unleashed the torrential rain over Mumbai? It could have been a confluence of various possible causes that Francis identified. As Sulochana Gadgil of the IISc points out, during July 25-26, the TCZ band was located just near Mumbai, thereby indicating the presence of strong convection.
But as of now, since the event was unprecedented, there are only speculations and hand-waving arguments. Only a combined analysis of localised data of winds, cloud motion vectors (derived from high resolution satellite images) and precipitation rates (perhaps at intervals of 15 minutes given the extraordinarily high intensity at which it poured) would throw some light on the underlying causative mechanism.
COMMents
COMMents
SHARE