adminIN light of the debate on the weather forecast for the coming season owing to varying reports from the Meteorological Services Department and the Zimbabwe National Traditional Healers Association, Sunday News Senior Reporter Robin Muchetu (RM) interviewed the Met Department’s principal meteorologist Mrs Linia Mashawi Gopo (LG). Below is an excerpt of the interview:
RM: To start with, most people would like to know what normal, below normal and above normal rainfall is.
LG: Normal rainfall is when an area receives between 75 percent and 125 percent of its long-term average in a sub-season. Practically, when an area has a long term average of 400mm, it is said to have received normal rainfall when the area receives anything between 300 and 500mm of rainfall in that sub-season. When the outlook goes for normal rainfall with a bias towards below normal rainfall, it means that place is expected to receive anything between 300 and 500mm or below 300mm for that particular sub-season.
Last but not least, when the outlook goes for normal rainfall with a bias towards above normal rainfall it means there is a high likelihood of an area to receive anything between 300 and 500mm or above 500mm for that particular sub-season. Below normal rainfall is when an area receives less than 75 percent of its long term mean. Using the example above the area would have received below 300mm. On the other hand above normal rainfall is when an area receives more than 125 percent of its long term average. Again, using the long term average of 400mm, above normal rainfall means that an area would have received more than 500mm accumulation for that particular sub season.
Rainfall is highly variable; hence the need for each and every farmer to keep rainfall records for their specific areas. This helps in acquiring knowledge on area specific climate and also to keep track of the changes in climate. Long term daily rainfall data can be analysed to come up with area specific climate products such as mean onset of the rainfall, mean cessation of the rainfall, length of season, number of rainy days, length of season, seasonal total rainfall and number of dry spells. These climatic characteristics guide agricultural decision making if combined with the seasonal forecast as well the medium range (10 days) and short range forecasts (three days).
RM: How does the Met Department determine a normal rainfall?
LG: Seasonal Climate forecasting is a process of coming up with the seasonal forecast/outlook where a number of climate indicators are critically assessed and in addition a number of tools are used in coming up with the forecast. Scientists from Southern Africa meet every year for two weeks in a chosen host country to come up with a seasonal forecast for the Sadc region. Some of the Seasonal forecast indicators are as follows:
(a) Winds
Prevalence of cool south easterlies during the winter season is a good sign of normal rainfall over Zimbabwe while prevalence of north easterlies is a sign of reduced rainfall over Zimbabwe.
(b) Prior heating
A hot September and October is an indicator for a good summer rainfall season.
(c) Sea Surface Temperatures
Increased SSTs encourages evaporation, which leads a lot of moisture in the atmosphere and in turn supports cloud development leading to more rainfall. If SSTs are subdued there will be less evaporatute, less atmospheric moisture, reduced cloud development and in turn less rainfall.
(d) Westerly cloud bands
Prevalence of westerly cloud bands during the beginning of the season is a good indicator for normal rainfall.
(e) ENSO (El Nino La Nina events)
El Nino is mostly associated with less rainfall over Zimbabwe while La Nina is associated with more rainfall over Zimbabwe.
(f) teleconnections
What happens over East Africa during July-August-September has a bearing on the rainfall performance over Zimbabwe. Good rainfall over East Africa during this period is an indicator for normal rainfall over Zimbabwe.
RM: What is the highest and lowest amount of rainfall that the country has experienced before?
LG: The lowest rainfall in the country over the past 35 years was received during the 1991/92 rainfall season and the average for the country was 392mm. The highest average rainfall for the country over the same period was received during the 1999/00 rainfall season; the average rainfall for the country was 986mm. The 35 -year average rainfall for Zimbabwe is 708mm. The season 1999/00 was a La Nina year but what contributed the most to the rainfall was Cyclone Eline. For the 1991/92 rainfall season, it was an El Nino year.
RM: There are obvious changes in the seasons or weather patterns at the moment, please explain what is happening so that the public has an idea of what is being experienced.
LG: The summer rainfall season patterns have changed quite significantly in Zimbabwe. Generally this is witnessed by late onset of the rainfall season, increased frequency and length of dry spells in any one season, extreme weather events are on the increase (droughts, hail and floods), rainfall distribution is becoming more and more erratic and in most areas the summer rainfall season is ending earlier than normal.
The other visible changes are reduction in the number of rainy days and the seasonal total rainfall for the country is increasing slightly below. From an agro meteorological perspective; these changes have a direct impact on agricultural activities, both crops and livestock. Delayed onset of the rainfall season and early end of the season implies a short rainfall season; this has serious implications on agricultural decision making.
Rainfall distribution is very critical when it comes to crop productivity; if normal rainfall for a season is received over a few days then this poses a big challenge on the crops and livestock. The biggest challenge at hand is the fact that the rainfall distribution is becoming more and more erratic. The changes in the rainfall patterns are attributed mostly to Climate Change.
RM: Please clarify some of the misconceptions that you have come across pertaining to the changes in the seasons in Zimbabwe?
LG: There have been allegations of people who are said to be dissipating clouds in Mashonaland Central province. The farmers who see an aircraft during the rainy season and according to them whenever it passes their area clouds move from their area or they dissipate. So according to these farmers they say the rainfall pattern has changed due to deliberate efforts by human beings.
Most people do not understand the cloud seeding process hence some believe that cloud seeding alter rainfall patterns while others think that the MSD can undertake cloud seeding anytime and the rains will come. It is important to note that cloud seeding is a process that is done when the atmosphere is conducive (when there are seedable clouds).
Some people also believe that the seasons have changed due to failure to observe traditional norms like observing special days like “Chisi/Ukuzila”. Others are also attributing the changes in the rainfall pattern to failure to observe rain making ceremonies. Thus some people believe the ancestors are unhappy because people are no longer brewing the “Mukwerera” beer to appease the ancestral spirits to bring the rains.
RM: Are there any human activities that have a contribution in the change weather patterns? Please explain a few and their effects and how this can be curbed.
LG: Yes, there are human activities which indirectly contribute to the changes in the weather patterns; these include emission of greenhouse gases (eg Carbon dioxide), which create a blanket effect in the upper levels of the atmosphere. This blanket traps outgoing long wave radiation and hence causes extraordinary warming. Increase in temperatures significantly alters the rainfall patterns. Use of clean energy (in place of using coal people use solar) is highly recommended.
RM: What are Homogeneous rainfall regions?
LG: Zimbabwe can be put into three homogeneous rainfall regions. The regions were obtained using a principal component analysis and they differ for the OND and the JFM season. During the JFM season Regions 1 and 2 become larger while Region 3 shrinks becoming smaller than the size of the same region during the OND season. Region 3 is drier than the other regions while Region 1 is the wettest. The regions receive more rainfall during the JFM season than during the OND season. This is because during the JFM season the Inter-tropical Convergence Zone reaches its south-most position to increase rainfall activity over Zimbabwe.
RM: How do you intepret a forecast?
LG: The seasonal forecast for any region is given in terms of possibilities of each of the categories B, N and A occurring expressed as percentages. The category with the highest percentage will be the most favoured by the forecast. The probabilistic approach implies that the other categories may also be experienced and should not be ignored even though less likely.
Online Reporter