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HIGH IMPACT WEATHER AFFECTING ZIMBABWE. By Tavonga Gilbert Diwah. History. High-impact weather and climate related events that have severe consequences for local populations in terms of livelihood, food, water security and health impacts in Zimbabwe include: extreme rainfall events,
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HIGH IMPACT WEATHER AFFECTING ZIMBABWE By Tavonga Gilbert Diwah
History • High-impact weather and climate related events that have severe consequences for local populations in terms of livelihood, food, water security and health impacts in Zimbabwe include: • extreme rainfall events, • late onset or late withdrawal of the rainy seasons, • tropical cyclones, • prolonged drought periods and extensive mid season dry spells.
Extreme Rainfall events • Causes • tropical cyclones, • convective systems and • middle level stratiform clouds. • Convective rainfall is one of the most damaging meteorological phenomena as it can result in massive amounts of rainfall occurring in short periods of time leading to flash floods. Improvements in satellite data over recent years, notably the increased resolution of images, increased range of satellite derived products and shorter time intervals at which the data is received, has improved the monitoring of the development and movement of convective systems over relatively small areas.
Extreme Rainfall events cont. • The Severe Weather Forecasting Demonstration Project (SWFDP) currently being rolled out across the SADC region encouraged member countries to make use satellite products in forecasting severe weather systems affecting their individual countries. Undoubtedly, much of the severe weather experienced in Zimbabwe is associated convective systems and rainfall derived thereof. • Most of the severe weather events reported in Zimbabwe during the past season were heavy rainfall events.
Extensive Breaks, Late onset or early withdrawal of the rainy seasons • Diagnostic examination of ECMWF composite synoptic-scale features, indicates that lower northerly and upper easterly flow is enhanced during wet spells, corresponding with a tropical low over Zambia and an anticyclone off the south-east coast of Africa. • Dry spells exhibit an increase in mid-latitude cyclones off the south-west coast of Africa and tropical cyclones near Madagascar
Extensive Breaks, Late onset or early withdrawal of the rainy seasons cont. • . Mid-tropospheric troughs are located over the east and west coasts of Africa near 25°S and serve to increase anticyclonicvorticity over Zimbabwe in dry spells. Cyclonic vorticity and low geopotential heights occur on two or three days in the wet spell, coincident with intense cumulus convection and high rain rates. The eddy covariances of zonal wind and specific humidity are dominated by eastward fluxes in dry spells. The eddy fluxes are shifted 15-20° longitude eastward in the wet spells.
Case Study: Analysis of the 2002/03 Rainfall Season. • The 2002/03 rainfall season came against the backdrop of a weak El-Nino event in the eastern equatorial Pacific, which only petered off during early 2003. As a result, the 2002/03-rainfall season was largely characterized by prolonged dry spells and patchy rains in some parts of the country. However, the latter part of the second half of the season (January to March) saw the development of various rain-bearing weather systems which brought a lot of rainfall leading to flooding in Mashonaland Central, southern sections of Manicaland and Masvingo provinces. • The average cumulative rainfall that occurred across the country since the beginning of the season to March 26, 2003 was 593.8 mm. This is below the national long-term mean of 662.3 mm by 68.5 mm but higher than that of the 2001/02 season.
Temporal Distribution of rainfall The effective rainfall season started in the last 11 days of October for most parts of the country with the exception of most of Mashonaland Central and the central districts of Mashonaland East, which remained dry until the beginning of November. The last 20 days of November were generally dry across the whole country. .
Temporal Distribution of rainfall • Dry spells were also evident in the first and third dekads of December, the first, second and third dekads of January, first and second dekads of February in different parts of the country. The highest frequency of dry spells lasting for 10 or more days occurred in Matabeleland South followed by Manicaland and southern parts of Midlands. The longest dry spell of thirty-two (32) consecutive days occurred in Beitbridge during the first half of the season. These dry spells impacted negatively on crop development, particularly the early-planted crop that endured tong periods of moisture stress. There were, however, no evident dry spells of 10 or more days in the major maize producing provinces of Mashonaland West and Mashonaland Central since December 2002
Tropical Cyclones • The tropical cyclone season in the South-West Indian Ocean Basin stretches from November to April, the onset varying across the geographical positions. In Zimbabwe, the season normally starts in December and ends in March (4 months). However, these cyclones have influenced the country outside the period above. Also, only tropical cyclones in the Mozambique Channel have a direct bearing on Zimbabwe, with either floods or in-season prolonged dry spells. Either way, the results have negative impacts on agriculture, the mainstay of the country’s economy.
Tropical Cyclone Japhet (2003) • A low pressure area developed in the Mozambique Channel on the 21st of February and intensified into a tropical cyclone, Japhet, by 28 February. The cyclone then started moving SSW at 5 knots (9km/hr) with winds of 65 to 70 knots (120 to 130km/hr). The central surface pressure was about 965hPa.
Tropical Cyclone Japhet (2003) cont. • By the time it reached southeast Zimbabwe (near 30E and 20S) on the 4th, the maximum winds had been reduced to between 60 and 80km/hr. It however, caused a lot of rainfall over many parts of Zimbabwe. It continued to weaken and by 12:00 GMT on 5 March, the system had been downgraded to just being an ordinary low-pressure system. • IR image of TC Japhet centered over central Mozambique on the 3rd of March • (06:30 GMT)
Areas Affected • The depression nevertheless caused heavy falls in the south and eastern areas of Zimbabwe. An example is Masvingo, which recorded 204mm of rain in the 24 hours from 6 to 7 March. Rupike, also in the southeast, recorded an amount almost seven times its March mean in the 4 days from 3 to 7 March.
Prolonged Drought Periods • Zimbabwe, a largely agricultural country, has experienced almost successive drought/flooding periods, which in the past two decades have become more frequent and increased intensity. The food insecurity that occurred in the country in the past years was a cumulative consequence of recurrent disasters, which actually eroded the population's coping mechanisms.
Effects • Drought is the most economic damaging phenomena in the country with more than 2.5 Billion dollars loss since 1982. The following graph shows the top two economic losses since 1982.
