Bangladesh and East India Tornadoes
Background Information
Jonathan D. Finch
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Main Page
Elevated Mixed Layer
Convective Outlooks
Special Cases for the United States
Historical Tornado Tracks for East India and Bangladesh
Meteorological Charts for Historical Tornado Cases for Bengal
Assessing Instability on the Front Range Without Upper Air Data
Latitudinal Comparison of the Geostrophic Wind Approximation
Potential Temperature and Mixing Ratio--Contributions to CAPE on Elevated Terrain
Tornados are no stranger to East India and Bangladesh. Ashraf M. Dewan (Ashraf's email) and I have documented 84 tornados for this region by searching the Pakistan and Bangladesh Observer newspapers back to 1955, and through literature and internet searches. The resulting climatology of tornados for the Bengal region is being published and is copyrighted. Almost all of these tornados (map showing locations of all tornados that killed at least 30 people) occur (julian dates of tornados that killed at least 30 people) from late-March to mid-May and usually move to the southeast or south-southeast. Nineteen out of 25 (80%) of the tornados that killed 100 or more people, and 32 out of 43 (75%) of the tornados that killed 30 or more people, occurred during the first 20 days of April. Sixteen of the 25 (64%) tornados that killed 100 or more people occurred from April 8 to April 18. Most of the tornadoes occurred in a relatively small area of central, south central and southeast Bangladesh, or an area about the size of the state of New Hampshire. The details of each tornado including dates, locations, number of people killed, direction of movement, path width, path length are also provided. The references can be found here.
Although East India and Bangladesh are positioned far to the south(21-26N), the regional topography of south Asia makes Bengal more susceptible to violent thunderstorms. The mid-upper tropospheric flow splits around the Tibetan Plateau, allowing the westerlies to be stronger than the zonal average at 24N. Occasionally in the spring, strong westerlies roar around the Plateau, with a 500mb maximum from New Delhi to Lucknow to Patna to northern and central Bangladesh. Mid-upper level cold pools are steered around the plateau across north India. North India is extremely dry in the winter and spring and a deep mixed layer from the sfc to above 600mb develops south of the Ganges river valley. Daily low-level downslope flow off the mountains of Afghanistan and Pakistan leads to desert conditions during the spring with surface high temperatures ranging from 95-104F on the higher terrain of central India(1500-3000ft) from Bhopal to Ranchi, to 105-120F at low elevations just west of the surface dryline, along with negligible rainfall. When mid-upper level cold pools move across north India, extreme lapse rates are the result. A lee surface low is a daily occurrence on the eastern edge of the 1500-3000ft escarpment around 23-25N and 87E. On some days this low is further south near Jamshedpur while on other days the low is near the base of the Himalayas. North of the low, easterly flow transports moisture westward along the Ganges river sometimes as far west as New Delhi. An excellent example of this is March 20, 1968 when 20 people were killed by a hailstorm in northern India. To the east of this low pressure area, the low-level flow is out of the south starting in March, bringing moisture laden air from the Bay of Bengal. The sea surface temperatures in the Bay of Bengal remain in the 80-85F range year-round. The southerly flow generally accelerates from March into May. This moisture underruns high lapse rates, leading to explosive instability with classic loaded-gun type soundings. Occasionally the mid-level flow becomes strong with 700mb winds from 40 to 50 kts and 500mb winds from 40 to 70kts. Most tornado events in the Bengal region occur with at 50-100kt jet-level winds, 40kt west to west-northwest winds at 700mb, 40-60kt west to west northwest winds at 500mb, along with south to south-southwest flow in the boundary layer and south or east surface winds. There are a few notable exceptions when disastrous tornados occurred with very weak mid-level flow (4-11-64 and 4-13-70). Although the sfc winds increase from March into May, the upper level winds decrease during this time, resulting in a maximum in vertical wind shear in early to mid-April. Surface based capes generally range from 4000-8000j/kg on tornado days and can reach as high as 9000j/kg. Intense thunderstorm activity is almost a nightly occurrence on the Khasi Hills near and around Cherrapunji, and across northeast Bangladesh as the low-level jet impinges on the higher terrain, often resulting in an outflow boundary across northern or central BD. The surface dryline is usually positioned in far east India or western Bangladesh in the spring, and often stretches from northeast to southwest from Rajshahi, BD to Nabadwip, IN to Jhargram, IN to Baripada, IN. Occasionally the dryline pushes east of Calcutta. The sea breeze front along the coast of India from near Balasore to Bhubaneswar is also an important player. A simplified version of the typical, large scale, severe weather pattern for the Bengal region can be found here.
The first documented tornado in the Indian region occurred near Calcutta on April 8, 1838 as documented by (Floyd, 1838).
Tornados are not as common in northwest India and Pakistan, but there are a few documented cases. On March 17, 1978 a tornado killed 28 people in New Delhi. On March 10, 1975 10 people were killed near Ludhiana in extreme northwest India. On March 28, 2001 a tornado killed 3 people near Bhalwal, Pakistan. Azmat Hayat of the Pakistan Meteorological Department has provided an excellent synoptic overview of this event. Some of the data shown on the latter map were generously furnished by the Pakistan Meteorological Department free of charge. The 500mb chart for 00 UTC March 28, 2001, shows how the flow splits around the Tibetan Plateau.
I have prepared for some of the catastrophic tornado events. Unfortunately, upper air data is very sparse or non-existent across Iraq, Afghanistan, Pakistan for all years, and the former Soviet Union after 1990. Even though India has an extensive upper air network, the quality of the thermodynamic data is low compared to China or Saudi Arabia. More specifically, the geopotential heights are often off by 30-100m, making analysis very difficult. Thankfully the wind data over India is good quality. Iran has several upper air sites with decent thermodynamic data, but the wind data is usually missing.
Bangladesh has quite a few surface synoptic observatories and one metar station(Kurmitola in the northern suburbs of Dhaka). The thermodynamic data is usually good. However, I have noticed inconsistencies in the wind data for these stations. Namely, the wind speeds in the synoptic observations seem to be too weak in many instances compared to the metar data at Kurmitola. Oftentimes, the wind speeds at Kurmitola are 2 to 3 times stronger than at the synoptic station in Dhaka, only a few miles away. For example on May 13, 1996, the windspeeds at Kurmitola varied from 16 to 22 kts in the afternoon, while Dhaka only reported 5 to 10 kt winds.
