A Kalboishakhi storm swept across northern Bangladesh on the evening of 27 April.
The animation shows a Kalboishakhi storm in northern Bangladesh, that occurred on 27 April. These storms are named after Boishakh — the Bengali name for the first month of the Bangla year.
Two storms can be seen. The first travelling eastwards parallel to the Himalayas (08:00–15:00 UTC). This storm is seen to die away in northern Bangladesh. The low level outflow from this storm initiated the dramatic convective development of the second storm, at around 15:00 UTC, as the moist air was lifted into the cooler air above.
Sixteen people were killed by high winds and lightning related to the storm in Bangladesh. Kalboishakhi storms are common in April, in advance of the monsoon season, and are caused by very warm, moist air at the surface meeting colder air flowing from the mountains to the north east.
Although the second storm over Bangladesh was more severe than the first, both storms, in Nepal and Bangladesh, caused loss of life and damage to property.
More information and detailed analysis of the feature can be found in the In Depth section.
by Ian Mills and Mark Higgins (EUMETSAT)
At this time of year, prior to the monsoon, the sea surface temperature in Bangladesh is about 30 °C and the air over the land is very warm and moist.
This process can be explained using a thermodynamic diagram.
Radiosonde for Dhaka from 00:00 UTC on 27 April.
Temperatures in the area reached about 41 °C while the dew point of the airmass was around 21 °C.
The construction in red on the thermodynamic diagram shows that convective cloud would form with a 5000 ft cloud base given these temperatures. However, the speed with which the second storm developed suggests that an additional trigger was required to initiate convection.
This trigger was provided by the low level outflow from the first storm causing surface convergence and so providing the initial uplift to trigger the convection. Once lifted the air would continue to rise (along the curved red line in the thermodynamic diagram) until eventually it encountered air at the same temperature. This point was reached at about 130 hPa or about 50000 ft. The amount of energy available would have produced overshooting tops and, so, the actual cloud top would have been several thousand feet higher than this.
Terra MODIS True Colour
Terra MODIS, 28 April, 05:00 UTC
Full Resolution (PNG, 44 KB)
This MODIS true colour image is taken from April 28, the day after the main event. The arc of cloud over the over the Bay of Bengal is the outflow boundary generated by the storms of 27. Here, the outflow boundary has travelled over 800km from its origin
Meteosat-7 Infrared & Visible
See the Meteosat-7 24-hour mixed infrared and visible loop showing the progression of the outflow boundary. Infrared is used from 27 April 17:00 UTC - 28 April 01:00 UTC, visible on 28 April from 01:30 - 10:30 UTC & then infrared again from 11:00 - 17:00 UTC. Remnants of the arc cloud can be tracked for some 1200km from the outflow origin (green arrow on the video).
Kalboishakhis — Bangladesh's deadly storms (Aljazeera)
Storm claims one life‚ injures 13 in Rajbiraj (The Himalayan Times)
Thunderstorm squall line sweeps over Uttar Pradesh