Indian Journal of Radio & Space Physics

Vol. 35, August 2006, pp. 270-279

Supercell storm at Kolkata, India and neighbourhoodAnalysis of

thermodynamic conditions, evolution, structure & movement

Vivek Sinha

Aerodrome Meteorological Office, Netaji Subhas Chander Bose International Airport, Kolkata 700 052, India

Devendra Pradhan

Cyclone Detection Radar, New Secretariat Building, 1 K S Ray Road, Kolkata 700 001, India

(Email: pradhan_dev1960@yahoo.com)

Received 5 August 2005; revised 13 March 2006; accepted 7 April 2006

On 12th March 2003 a very severe thunderstorm traversed across the Gangetic West Bengal in India and adjoining areas

of Bangladesh. Documentation and analysis of the thermodynamic condition of the atmosphere and evolution, structure and

movement of the storm as tracked by the Doppler radar is presented in this paper. One particular cell of the system lasted for

over 12 h. Based on the internal structure, reflectivity, duration and weather pattern on the ground, it has been concluded

that the particular cell was a supercell.

Keywords: Bulk Richardson number, Convective inhibition energy, Storm relative helicity, Surface rainfall intensity,

Vertically integrated liquid, Weak echo region, Vertical profiles of radar reflectivity

PACS No: 92.60.Wc; 84.40.Xb

1 Introduction

During the pre-monsoon season of March, April

and May, Gangetic West Bengal and surrounding

areas are affected with severe thunderstorm called

Norwesters. On 12 March 2003, a series of severe

thunderstorm occurred over Kolkata (22.3ºN-88.5ºE)

and neighborhood, causing widespread destruction.

As per the local Newspaper report, 11 people lost

their lives and several well-built structures including

the portico and a portion of the perimeter wall of the

international airport of Kolkata collapsed under its

impact. This was the first thunderstorm of the

Norwester season of the year 2003 and possibly the

severest in that year. The severity of the storm was

more pronounced in the northern part of the station,

which was affected twice in quick succession, once at

0820 hrs UTC when the wind speed rose to 74 km/h

and again at about 1100 hrs UTC with wind speed

reaching 82 km/h. The storm which originated

between Dhanbad district in Jharkhand and Bankura

district in West Bengal, produced hailstorm, which

left over 200 people injured in Bankura alone. The

environmental condition was so explosive that the

convective activity, which started at 0800 hrs UTC,

continued well past 2000 hrs UTC and in the process

generated a series of violent thunderstorms. Initially

the system was located in the 270º-300º sector with

almost a clear sky in the 090º-270º sectors, but with

the passage of time the instability grew both

eastwards and southwards.

The present paper is a documentation and

diagnostic study of the evolution and structure of the

storm based on the Doppler weather radar (DWR)

data at Kolkata, which successfully tracked the storm

from 250 km NW of the station, till its dissipation in

the east of the station over Bangladesh. Apart from

radar data, the relevant data of a set of surrounding

stations and overall synoptic situation of the region

has also been taken into account. Analysis of synoptic

and radar data indicates that it was, indeed a very

severe thunderstorm and at least one cell could be

classified as a supercell.

2 Methodology and data To study the process of initiation, apart from

prevailing synoptic condition over the region, the

RS/RW observation of Kolkata has been used to

calculate the convective parameters and shear values

SINHA & PRADHAN: ANALYSIS OF SUPERCELL STORM OVER KOLKATA

271

of the atmosphere. To study the internal structure of

the storm, both the primary and secondary products

generated by the DWR have been taken into account.

The DWR at Kolkata is a S-band radar (2875 MHz)

operating at a wavelength of 10 cm. The peak pulse

power of the radar is 750 kW and maximum

unambiguous range of 400 km; however, during the

present tracking the range was kept at 250 km. The

base data generated by the radar is reflectivity, radial

velocity and spectrum width and derived products are

generated as per the product definition file (PDF).

3 Initiation of thunderstorm

The storm was first spotted by the DWR at Kolkata

at 0800 hrs UTC, at a distance of 150 km WNW of

Kolkata near Bankura. As per the synoptic condition,

a trough of low pressure was prevailing over an area

covering Chattisgarh, Jharkhand, and north Orissa

that helped in initiating deep convection.

4 Sequence of weather

The facility of continuous weather watch and

autographic recording of element is available at

Kolkata airport (42809), which is about 12 km NNE

of the DWR site and at Alipore (42807), which is

around 3 km south of DWR. Till 0750 hrs UTC, weak

(< 5 kts) easterly wind was recorded at the airport with

isolated stratocumulus and cumulonimbus cloud and

widespread altocumulus clouds. The first spell of

heavy thunderstorm and gusty wind was reported at

0820 hrs UTC and the second spell at 1100 hrs UTC,

although feeble rain and thunderstorm (RA/TS)

continued incessantly for 14 h till 2200 hrs UTC.

Once the thunderstorm formation started, the wind

remained north-westerly between 280º and 320º at the

Airport and northerly to north-westerly at Alipore.

Another airport Ranchi (VERC) that is around

240 km WNW of DWR, reported 8 kts of ENEly

wind with low and medium cloud at 0900 hrs UTC

and RA/TS with weak SEly wind at 1200 hrs UTC.

As per the Dines PT anemograph, the first spell of

downdraft at the Airport was recorded at 0845 hrs

UTC in which the wind speed rose to 54 kts and

lasted for 15 min. The second spell of gustiness

started at 1030 hrs UTC and a peak speed of 72 kts

was recorded at 1105 hrs UTC. The gustiness

continued till 1200 hrs UTC, after which it subsided.

Compared to this the Alipore anemograph recorded

a peak gustiness of 22 kts only during the same

period. The shift in wind was clearly revealed by the

anemograph at the Airport. At 0840 hrs UTC just

before the first spell of downburst, the wind direction

changed to north-westerly from north-easterly, only to

become north-easterly at the time of downdraft, and

the same sequence was repeated at the time of second

spell at 1110 hrs UTC. This strong directional shear

might be an indicator of rotational motion within the

downdraft. The damage at the ground fully supports

this fact, particularly the damage at the Airport, as the

iron and steel structure was found twisted and

mingled after it collapsed. It is important to mention

that the Alipore anemograph did not record any such

directional shear. The general observation that the

pressure falls up to 5 mb within one km radius of a

tornado does not hold in this case, as a steeper fall of

5 mb was recorded at Alipore, which was farther

away from the main track, compared to 2 mb fall at

the Airport. Besides, there was a lag of 20 min

between the lowest pressure recorded at Alipore

(recorded at 1040 hrs UTC) and Airport (recorded at

1100 hrs UTC), which again indicates that the storm

has taken a relatively northern course.

Starostin1 has reported that a squall line has a

tendency to dissipate when it approaches the position,

which was previously occupied by another stronger

cell. In effect, the squall line has a tendency to

displace the region of hailstorm evolution. In the

present case the dissipation of supercell between 0918

and 1018 hrs UTC and its subsequent regeneration in

a newer area, supports this observation.

5 Results and Discussion

5.1 Thermodynamic and shear environment

The actual sounding of 0000 hrs UTC of Kolkata

(42809) has been shown in Fig. 1, and various

stability parameters based on the actual sounding and

modified sounding based on 0700 hrs UTC surface

observation has been presented in Table 1.

Together with convective instability, environ-

mental shear plays the most crucial part in generation

and sustenance of severe thunderstorm. Storm relative

helicity (SRH) is an estimate of the potential of the

thunderstorm to acquire rotational updraft in the given

shear environment. In the present case, the 0-3 km

SRH value of 95 is too low, because supercell storm

generally occurs in a SRH value of 150 and above 2.

The low SRH value is due to the fact that the shear in

wind direction is much less than the shear in wind

speeds. Between ground and 6 km height, where the

direction varies from 180º to 260º, the change in

INDIAN J RADIO &SPACE PHYS, AUGUST 2006

-10 0 10 20 30 40

TEMPERATURE, OC Fig. I-The 0000 hrs UTC T-0 gram and hodogrfph of Kolkata

E : &

10 g (3 iii I

Table 1-The value of various convective indices based on the 0000 hrs UTC RSIRW observed at Kolkata

[The second row contains the value of same indices after they were modified with the 0700 hrs UTC surface conditions.]

Station CAPE CIN BRN EHI SRH (0-3 km)

WBZ VGP LFC

Kolkata 21 15 168 130 1.1 95 625 mb 0.325 2384 Modified . 3704 28 23 1 2.5 93 625 mb 0.482 1821

CAPE: Convective available potential energy, CIN: Convective inhibition, BRN: Bulk Richardson Number, EHI: Energy helicity indices. SRH: Storm relative helicity, WBZ: Wet bulb zero, VGP: Vorticity generating parameter, LFC: Level of free convection.

--

speed is from 5 kts to 55 k t ~ , which is very The end result of this complex process is the net pronounced. This type of situation leads to the increase in vertical vorticity and if this process generation of large-scale cross-wise vorticity, with generates large enough values of vertical vorticiry, a small values of stream-wise vorticity, and this is mesocyclone develops. When the wind shear inside reflected in small values of SRH. The horizontal the cell has considerable vertical extension, the vorticity thus generated has a tendency to tilt in updraft and downdraft are so organized that they do vertical, in the updraft region of the thunderstorm. not interfere with each other, instead they com-

SINHA & PRADHAN: ANALYSIS OF SUPERCELL STORM OVER KOLKATA

273

plement the respective process. This particular feature

distinguishes the supercell storm from ordinary storm.

Studies by Wessman and Klemp3 and others have

indicated that a shear magnitude of 20 m/s in the

lower 6 km creates a condition favourable for

organized updraft and downdraft and this in turn

favours the sustenance of supercell, but this cannot be

treated as a threshold value. In general, the deeper the

environmental shear, more efficient is the dynamic

process that controls the supercell. However, for the

entire process, the value of shear should be taken

together with the instability and not in isolation. In the

0000 hrs UTC sounding the CAPE value was 2115

J/kg, which again is not a very significant value for

formation of supercell thunderstorm. Thus if we take

the 0000 hrs UTC shear or convective parameters, it

did not suggest a storm of this magnitude, but when

the sounding is modified with afternoon values, the

picture changes completely. The modified CAPE

value of 3704 J/kg is sufficient to support supercell

storms. As the amount of shear in the environment

increases, with a given amount of available convec-

tive buoyant energy (i.e., instability), there is a range

of shear values within which long-lived convective

storms are likely to occur4. This range of shear values

appears to depend on the amount of instability, with

long-lived storms occurring in an environment of

greater values of shears, which is generally associated

with larger instability. Therefore, unlike relatively

weak wind shear together with high CAPE value that

favours a multi-cell storm, a moderate value of CAPE

does support supercell thunderstorm in certain

circumstances.

In case of Leon province of north-western Spain,

the value of CAPE has never been found to exceed

2000 J/kg, not even for hailstorm5. The mean value

for CAPE has been 132 J/kg for all days, while for

hailstorm days it had a mean value of 365 J/kg.

Although this is much lower than the value generally

found in mid-latitudes, there is no evidence of any

linear relationship between very high CAPE value and

severity of thunderstorm. In central USA major

tornados have typically been associated with

moderate to high CAPE value6 between 1500 and

3500 J/kg and as the value increases beyond 3500

J/kg, the frequency of super storm decreases

significantly. In another case study7 conducted by the

authors, it was found that a very high CAPE value of

5119 J/kg was associated with a relatively weak

thunderstorm; while a value of 3361 J/kg was

associated with a severe line squall, both occurring at

an interval of one day on 1 and 3 June 2004.

The LFC height in 0000 hrs UTC sounding was

2384 m, which is considerably high for initiation, but

in the modified sounding it became 1821 m, which is

an ideal value. The low LFC height implies large

CAPE in lower levels and this in turn implies

increased potential for low-level acceleration. In

general, LFC below 2000 m is found to be conducive

for tornadic thunderstorm.

The CIN value for the 0000 hrs UTC sounding was

168 J/kg, which is somewhat higher for severe

thunderstorm. In fact model studies have suggested that

large low-level stability, which is reflected in a large

value of CIN acts against generation of intense surface

vorticity and a CIN value larger than 150-200 J/kg

extending through the lowest 2-3 km is not likely to

produce a severe thunderstorm. Most of the supercell

thunderstorms producing tornados are generally found

to be associated with a CIN value lying between 50 and

100 J/kg. However, in the modified sounding, the CIN

value was 28 J/kg and this again was a favourable

value, as Rasmussen and Blanchard8 have found that

75% of tornadoes in classic supercell environments had

CIN < 21 J/kg and 60 % of tornadoes in non-supercell

environments had values greater than this. Yet another

parameter, energy helicity index (EHI) was also

calculated, which combines CAPE and SRH into one

index. In 0000 hrs UTC sounding the EHI value was

1.1, which represents potential for supercell but not for

tornado. In the modified sounding the value doubled to

2.5, which represented a condition favourable for

meso-cyclone induced super-cellular tornadoes.

The wet bulb zero (WBZ) in the present case was

at 625 mb (3,711 m), which again indicated moderate

possibility of hail. In general, WBZ heights from

1,520 m, to 3,660 m above ground level (AGL) are

associated with hail at the ground. The potential for

large hail is usually high for WBZ heights of 2130 m

to 3050 m AGL, and possible hail size decreases if

WBZ is below 1830 m or above 3350 m, AGL.

To begin with the 0000 hrs UTC sounding of

Kolkata was not exactly indicative of a very severe

thunderstorm activity, but when the sounding is

modified with 0700 hrs UTC temperature, dew point

and wind velocity, the situation changes significantly

and all the convective parameters indicated towards

severity. It can therefore be concluded that the

situation in the afternoon has become fully conducive

for supercell formation.

INDIAN J RADIO & SPACE PHYS, AUGUST 2006

274

5.2 Reflectivity

The radar reflectivity and radial velocity clearly

indicate the severity of the storm. The maximum

reflectivity picture (MAX_Z) for different stages of

storm between 0818 hrs UTC and 1248 hrs UTC has

been given in Fig. 2. Unlike the convectional PPI and

RHI pictures, the MAX_Z picture provides a three-

dimensional view of the thunderstorm. The maximum

reflectivity in a vertical column presented in dBz, the

location of the cloud, horizontal and vertical extents

are displayed by MAX_Z picture. In the 0818 hrs

UTC picture, one cluster of cell in its dissipating state

can be clearly seen over the station moving across the

northern part of Kolkata city. Although the cells were

disorganized and reflectivity varied between 35 and

50 dBz, it generated strong gusty wind ≈74 km/h

ahead of it, causing the first round of destruction in

the area. In the same picture, a very strong cell

located over Bankura, about 170 km WNW of

Kolkata can also be seen. From its very initial stage

this particular cell was remarkable in its reflectivity

and vertical extension. Unlike the other cells, the

reflectivity of this cell was around 60 dBz and vertical

extension exceeded 16 km. The boundaries were very

sharply defined, outside which reflectivity dropped to

36-40 dBz. The sharp boundary and high reflectivity

was an indication of its predominant hail composition.

The movement of this cell has been tracked in Fig.

3 between 0818 hrs UTC and 1348 hrs UTC. Moving

with an average speed of 40 km/h, the initial

movement of the cell was ESE, but subsequently it

took an easterly course. As it came within 90 km of

the station, the cell started mutilating and gust front

from the parent cell generated daughter cells in SSW

sector. Between 0918 and 1018 hrs UTC there was

further mutilation and few more cells developed in

SSW and SW sector. During this period echo-top

collapsed, its height reduced to 8 km; simultaneously

the downdraft started ahead of it and the gustiness of

the wind increased to 82 km/h. This caused the

second round of destruction, in which the perimeter

wall and a part of portico of Kolkata airport collapsed.

Thus during 0848-1018 hrs UTC, as the system

travelled between Bankura and Kolkata, this parti-

cular cell behaved like a supercell and there was very

little change in the features of the cell. Significantly,

as it moved east of the station, the cell again

intensified and this is evident in the sector picture

between 1148 and 1348 hrs UTC. As the cell regained

strength, the echo top again attained a height of 14 km

and it remained so till the cell went out of 250 km

range of DWR at around 1418 hrs UTC.

The cell caused further destruction in Bangladesh

as it traversed across the country. The fact that there

was no depletion in severity of the storm even after

sunset, suggests that it was the dynamic process rather

than the gust front or buoyancy, which sustained the

storm. In case when severe thunderstorm occurs at

night, it is usually associated with nocturnal low-level

jet (LLJ) that sets in after the cessation of solar

insolation. This LLJ provides the required conver-

gence, advection and lifting to the thunderstorm. The

1200 hrs UTC RS/RW sounding of Kolkata on that

day did report 50-95 knot wind between 6 km and 11

km. On comparison with 0000 hrs UTC wind pattern,

it is found that the core of high wind had descended

considerably and we believe that this has increased

the low-level shear and made the storm scale environ-

ment more favourable for sustenance of supercell

thunderstorm. The regeneration of cell east of the

station can be partly attributed to incursion of warm,

moist air in the system, as the storm enters the region

of southerly flow having maritime origin. During its

entire duration the core reflectivity of this cell

remained in excess of 55 dBz and even when

mutilation of cell occurred in SW quadrant (sector

picture: 1018 hrs UTC) the parent cell retained its

reflectivity.

5.3 Vertical profile of radar reflectivity

The environmental wind shear together with

updraft strength in a convective cell determines the

vertical distribution of reflectivity. The echo at low,

middle and upper levels tends to be vertically aligned

when the shear is relatively weak, or when the updraft

is strong. Using ground-based radar Yuter and Houze9

have suggested that updraft and high reflectivity tend

to coincide at high level, although the correlation is

weak. As the cell attains a mature stage, the liquid

water content is evenly distributed along the entire

column and this gives a uniform distribution of

reflectivity. Zisper and Lutz10

have reported that both

in tropical and mid-latitude meso-cyclonic system,

vertical profile of reflectivity exhibits a gradual

decrease in reflectivity with height above freezing

level, but no such gradient is observed in the present

case. This together with near absence of weak echo

region (WER) or bounded weak echo region (BWER)

in the cell can be attributed to the very strong strength

of updraft. As mentioned earlier, the vertical

distribution of reflectivity has remained uniform for

SINHA & PRADHAN: ANALYSIS OF SUPERCELL STORM OVER KOLKATA

Fig. 2--Maximum reflectivity picture at different stages of thunderstorm evolution [DNB: Dhanbad, PRL: Purulia, BNK: Bankura, KOL: Kolkafa, BPD: Baripada, BST: Basherhat, RSH: Rajshahi, JSR: Jassore, KLN: Khulna]

INDIAN J RADIO & *ACE PHYS, AUGUST 2006

Fig. +Structure and movement nf supercell [The cell regained its strength ns it crossed India and approached KLN (Khulnaf, Bangladesh]

the cell between QSrC8 and 0948 hrs UTC, which can be taken as mature phase of the cell. However, between 1048 and h108 hrs UTC, the collapse of echo-top can be clearly seen (Fig. 2) and this, in turn coincides with the peak gust recarded at 1105 hrs UTC at Airport obseruatory.

: 5.4 Surface rainfall intensity (Sm The SRI picture generated by the DWR at 0918

and 11 1 8 hrs UTC has been given in Fig. 4. In the 0918 hrs UTC picture, the cell located at NNE of the station gives a rainfall potengial of 52-56 mm/h, while the actual rainfall at Airpart between 0850 and 0905 hrs UTC was 16 mm, which comes out to be 54 mm/h. Similarly, during the second spell, the surface

rainfall inte&ity picture of 11 18 hrs UTC gave, an estimate of 60 m d , while the realized rain was 35 mm in 20 min, which is 105 mmlh. Therefore, the actual rain was much higher than the RADAR estimated rain. This discrepan~y can be attributed to the fact that the radar estimate was for a height of 1.5 km AGL. Besides, the A and h coefficients in Marshall-Palmer equation v - q from season to season and location to location.

-

5.5 Vertically integrated liquid Vertically integrated liquid [VIL) is u derived

product from Doppler reflectivity data. It converts reflectivity data into liquid water content value based on distribution of drop size and a coefficient based on

SINHA L PRADHAN: ANALYSIS OF SUPERCELI: STORM OVER KOLKATA

0918hrs UTC Intensity scale

Fig. 4-Estimated surface rainfall intensity at the time af passage of thunders "

It- r 1018 hrs UTC 1048 hrs UTC

--

1218 hrs UTC c 1218 hrs u l-c

)rrn through Kolkata

Fig. +The VIL profile of supercell at different stages [At 1218 hrs UTC the original cell has crossed BST (Basherhat) while a new cell has formed around MDF (Medinapore) in West Bengal]

reflectivity factor. Since reflectivity increases may indicate presence of luge drop size, like hail. exponentially with the drop size, a high reflectivity The VIL profile of the supercell thunderstorm cell has value will generate higher V&. Thus high VIL value be-en shown in Fig. 5 as sector pictures in zoomed

INDIAN J RADIO & SPACE PHYS, AUGUST 2006

Fig. &The hook shape echo of supercell at the time of its maximum intensity over Bankura

mode. In the 0848 hrs UTC picture when the cell was located over Bankura, the classical hook shape of the cell is distinctly visible (Fig. 6). The VIL value is around 23.3 mm on a scale of 0.1-25 mm. Such a high value clearly indicated the presence of large sized hail and this was confirmed by ground truth, as around 250 people were injured in Bankura district itself due to hailstorm. The 1218 hrs UTC picture shows the cell as it has moved east of the station. Once again the cell has regained strength and acquired hook shape. At the same time another strong cell has evolved over Midnapur at a distance of about 200 krn west of the station, which too can be seen in the next 1218 hrs UTC picture. Within 30 min this cell splits up and the stronger of the two cells attains a hook shape in its left flank.

Supercell thunderstorm displays an abrupt decrease in VIL during the collapsing phase of the storm, when echo top collapses and downdraft increases with

corresponding decrease in the updraft. This is again clearly depicted in the 1018 and 1048 hrs UTC pictures as the supercell collapses over the station. However, this interpretation comes with a rider that the VIL is airmass dependent. Convection in colder air-mass may produce large sized hail even with low VIL value, while the convection in warmer air-mass may not produce significant hail activity even with large VIL. Another limitation of VIL interpretation is that, in effect it is the integration of reflectivity in the entire column of the cell and as such storm having greater height will give higher value of VIL than a shorter cell; and this may be a reason for a consistently high value of VIL in the present case.

Conclusions The features and sequence of weather events do

~dicate the presence of supercell within the thunder- , qrm. The entire duration of activity exceeded 12 h

SINHA & PRADHAN: ANALYSIS OF SUPERCELL STORM OVER KOLKATA

279

and during this period the storm underwent a series of

generation and regeneration. This indicates the

complementary role played by the updraft and

downdraft. This is unlike the ordinary thunderstorm,

where the two have an adversary role to play. This is

possibly the first documented study of a supercell

over Indian region using DWR and the interpretation

of Doppler signature has been found to be in good

agreement with the ground observations. However, as

more and more case study of severe thunderstorm

shall be undertaken, the maturity in signature

interpretation will also increase.

This study also underlines the usefulness of DWR

as a tool for both diagnostic and prognostic study.

Whenever radar data at shorter interval is available,

the evolution of meso-scale convective system can be

studied using a very useful method suggested by

Starostin11

. The method called Moving Reference

method, is particularly useful for tracing the

movement of squall line and this can be used in future

studies.

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