Flash Floods in Sindh; An evidence of climate variability in the region

 

Azmat Hayat Khan*

 

Following a severe drought, most parts of Sindh experienced a prolonged siege of moderate to heavy rainfall. This included very heavy downpour on July 26 in Lower Sindh, which caused flash flooding in SE Sindh and meteropolitin City of Karachi and paralysed life

700 hpa chart of june 28, 2003 pouring cool airmass from north.

 
activities in the area. This rainfall event also led to loss of life and property in the area.

Antecedent Conditions

The winter of 2003 saw the end of an El Nino cycle that persisted for the four months prior to the onset of the heavy rains in Pakistan that began in February. The above normal latent heat release due to El Nino shifted the ITCZ to the equator. At the end of this El Nino cycle, the strong ridge over the northern latitudes of country broke down, allowing the westerly jet to bring weather systems across the country in a zonal pattern, bringing soaking rains through much of February to April. The heavy spring rains, which were still in the realm of normalcy, caused soil moisture levels to reach saturation levels by the end of May.

 

As June began, a series of strong synoptic storms brought heavy rain to the northern half of country, caused by a clash of unseasonably cool air coming south from Russia and a strong inflow of moisture brought by the low-level jet stream from the Arabian Sea. Several of these storms in June contained scattered rainfall events of over 2+ inches.

 

Main features during the event

 

By the beginning of July, a trough had developed in the northeast Arabian Sea, which continued the inflow of warm moist air over Sindh and adjoining areas. The westerly jet stream was consistently much farther south than normal, which enhanced the convergence generated by the clash of air masses over southern latitudes of the country. Record, heavy rainfall was enhanced by a variety of factors during the month of July.

 

        The strong persistent jet stream created a quasi-stationary boundary that established an overall cyclonic circulation which provided a favorable environment for developing weather systems.

        A delay in the onset of the SW Monsoon season over subcontinent provided a longer period of an environment favorable to convective precipitation. Monsoon season was nearly one week delayed over central parts of India.

 

 

Perhaps the most remarkable aspect behind the recent flash floods was the frequency and intensity of the Mesoscale Convective Complexities that provided much of the rainfall during July. MCCs are quasi-circular thunderstorm complexes covering an area greater than 50,000 square km and lasting in duration over six hours (Anderson, C. and Arritt, W., 1998). The complexes initiate in the afternoon and reach their peak overnight. The persistent trough in central India and southeastern parts of country created an area of development of MCCs. The low-level jet induced lift and moisture advection into and ahead of the developing system, which helped to propagate the storms. Without this favorable atmospheric environment throughout the month of July, any developing storms would have tended to develop into squall lines with linear rain bands, which had much less rainfall and were of shorter duration.

Moisture feeding at 850hpa level over gangatic plains and southern parts of Pakistan on 25th July, 2003.

 

HRPT cloud height analysis depicting that weather system had developed to a great depth.

 
The typical flash flood environment had abundant moisture through a great depth of the atmosphere. Low values of vertical wind shear were present resulting to moisture convergence over the area. Flash flooding was mainly produced by "train effect" storms. A number of multi-cell cluster storms were present which favoured the development of train effect storms over the area.

 

The upper level winds are the "steering current" for convective storms. It is the jet stream that powers the upper level winds. Jet streaks within the jet stream cause air, which is closer to the surface of the earth, to rise due to a vacuum effect the jet streaks create. As a jet streak enters into a trough, it can energize the trough causing the low pressure to deepen and heights to fall.

 

 

 

 


Significant chart of 23th July, 2002

 

A totally different development arises when the easterly jet moves farther north than usual because the monsoonal wind rising over the southern slopes of the Himalayas brings heavy rains and local floods. The weather over the central and southern parts of subcontinents, however, becomes suddenly drier and remains so for as long as the abnormal shift lasts. The opposite shift has also been observed, with mid-latitude upper air flowing along the south face of the Himalayas and bringing relatively drier conditions to sub-mountainous areas of Pakistan.

Storm precipitation intensities and damages caused.

Heavy downpour during the last weak of July 2003 was recorded at a number of places in southeast Sindh causing damages to loss of life and property. First spell of heavy rains hit the area on 8th July and Larkana; a city in central parts of Sindh received 209mm of rainfall in 24 hrs. 50 to 80 mm of rainfall was also received in other localities of Sindh during the period. Again the system hit lower Sindh and coastal areas of country on 17th July, 2003 and yielded moderate to heavy rainfall over the area.

The system reached its climax on 25th July and most of lower Sindh including metropolitan city of Karachi received heavy rainfall. The system remained active for three days and caused flash flooding. In Karachi, rainfall received was 200% to 300% above than normal at different areas. Heaviest rainfall has been reported by Chhor and Thatta where more than 400mm of rainfall was recorded during the spell. Detail of rainfall received in Sindh region during the month is appended below.

STATION

TOTAL Rainfall (mm)

Normal

Karachi

264.7

85.5

Karachi (Masroor)

253.3

66.4

Karachi (Faisal)

256

67

Karachi (IMG)

213.3

67

Karachi (Garden)

183.5

85

Karachi (Baldia Town)

98.6

85

Hyderabad

204

56.7

Chhor

403.6

79

Badin

294.4

70.5

Jacobabad

97.9

36.8

Rohri

40.3

25.7

Larkana

222.3

 

Nawabshah

281.1

51.8

Thatha

435

71

Diplo

156

 

Mithi

156

 

Chachro

110

 

Nagar Parker

97

 

Keti Bander

61

 

Shah Bander

30

 

Mir Pur Pathoro

154

 

Sajawal

60

 

Jati

60

 

Hundreds of villages of lower Sindh were affected by flash floods. Detail of damages as reported by Relief Commissionorate, Sindh is appended below.

Damages to infrastructure

Losses of Human lives

Animal deaths

Damages to kharif crops

Pacca Houses

Katcha houses

Fully

Partially

Fully

Partially

Deaths

Injured

2000

3000

30,000

40,000

178

235

9455

         Paddy 90%

         Sugarcane 80%

         Cotton 80%

         Beatle Leaf 80%

Conclusion

Increased frequency of such weather extremes during last decade signals climate variability in the region. The quantitative research of few aspects suggests that Pakistan experience a Wet winter with decreasing intensity of La Nina (Sea Surface temperatures in the Pacific Ocean becoming near normal) where as suppressive rainfall activity during monsoon season is observed during strong El Nino event.

Moreover, preliminary analysis using the current El Nino Southern Oscillation-related (ENSO) sea-surface temperature anomalies in the tropical Pacific show a response that replicates the observed precipitation and temperature anomalies to a noticeable extent.

This suggests that the current long-lived ENSO event affected atmospheric features associated with the extreme weather events.

* Senior Meteorologist, R & D Div, Pakistan Meteorological Department, Islamabad. E-Mail: pmd@pmd.gov.pk