VERIFICATION OF THE ZAMBIA RAINFALL SEASONAL FORECASTS USING METEOROLOGICAL PRODUCTS FROM METEOSAT

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1 VERIFICATION OF THE ZAMBIA RAINFALL SEASONAL FORECASTS USING METEOROLOGICAL PRODUCTS FROM METEOSAT N. AIKAYO ZAMBIAN METEOROLOGICAL DEPARTMENT P.O. BOX, LUSAKA, ZAMBIA Abstract: Rainfall Seasonal forecasts verifications have proved to be rather a difficult exercise in Zambia, given the fact that there are very few reports of surface data from the rainfall reporting stations. This is mainly attributed to poor communication facilities. The current methods being used are decadal totals and monthly rainfall totals. These methods have proved to be inadequate due to inconsistence reports or data from stations. In this paper an attempt has been done to use the Meteosat products such as: forecast surface charts, 850hpa, 700hpa, 500hpa winds, satellite images, precipitation charts etc. These products usually indicate convergence boundaries such as the Inter-Tropical Convergence Zone (ITCZ) and the moist Congo air boundaries, which are the major rain producing mechanisms in Zambia. The forecast precipitation charts also plays an important role in trying to show areas with maximum rainfall, and are related to zones whose seasonal forecasts are either above, normal or below. On the other hand when there is a persistent high-pressure cell over the southwest part of the sub-continent (southern Africa), these systems are normally pushed towards the extreme north of the country and is usually during an El-Nino season. The images are used as observed data to verify the occurrence of rainfall or storms particularly to places where the actual amounts are not reported. In addition, during the presence of a tropical cyclone in the Indian Ocean depending on its location, the low pressure belt such us the ITCZ is forced to pass through it, thus maintaining a lot of rainfall over Zambia. This is because the ITCZ will be forced to remain stationary or oscillate about and this occurs normally during the mid-season of January, February and March. Empirical models for seasonal forecasts also make use of predictors such as sea surface temperatures and reasently introduced General Circulation Models (GCM) using the Model Output Statistics (MOS) climate predictability tool (CPT). The GCMs makes use of historical data such as precipitation, 850 and 700 hpa. Details and demonstrations explaining how this method is used will be presented with the text. 1. INTRODUCTION Realistic verification of the seasonal rainfall forecasts in Zambia can only be represented accurately if the initial data is whole available. Unfortunately, the network of rainfall reporting stations is insufficient to achieve an adequate density of observed data.however; Meteorological products from Meteorological Satellites (MSG1) give forecasts of pressure, winds and precipitation for five days or more. These can be used to verify with the actual amount of rainfall observed. For the past years, questions have been asked raised on the assessment of the rainfall and forecast performance, which plays a vital role if improvements are to be made. This paper intends to produce a viable method of verifying the probabilistic approach of the seasonal rainfall forecast produced by the Zambia Meteorological Department using; Decadal and monthly rainfall data versus surface and upper air charts from MSG1. Position of the cyclone and high pressure systems Position of the rain belts, the InterTropical convergence Zone (ITCZ) and the Congo Air Boundary (CAB). Forecast precipitation charts versus actual rainfall amounts. The four methods individually will assist to assess the forecast performance and risk value of the 2006 and 2007 rainfall forecast map which was issued in September 2006 in Lusaka, Zambia. 1

2 The major limitations of probabilistic forecast lay in the interpretation by the users especially those without any mathematical or statistical background. Furthermore, the uncertainty of the range of rainfall expected for each category or tercile has posed serious doubt in the user s mind whether to make decisions based on the forecast or resort to other means. Over the years user oriented techniques and the knowledge of the climatology of the country and its variation (extremes) with reference to the regional and global climate patterns has helped user s and scientists in the interpretation. 2. DATA MANAGEMENT Daily rainfall records of rain gauge stations across the country are provided to the national Meteorological centre (Lusaka) on regular basis. This provides a continuous inflow of data that undergoes quality control before any analysis is undertaken. 3. METHODOLOGY The decadal data collected during the first week of October 2006 indicated that most stations in the country recorded almost no rainfall. Actual rainfall started in November; this rainfall was plotted on the Zambian map and compared with the surface charts and upper air charts e.g. the 850 Hpa, 700Hpa and 500Hpa charts from Meteosat. This was carried out for each decade and then compiled up to the end of each month to the end of the season. December rainfall improved and comparison was with the Tropical cyclone BONDO.As more cyclones affected the region in January through to march, the rain belt commonly known as ITCZ formed and the relationship with the rainfall amounts was determined. The aim was focused on the synoptic systems which favor rainfall such as the convergence line, the ITCZ, CAB, Tropical cyclones, High pressure systems, cloudiness etc.thereafter the process of verification was determined using the actual amounts for that particular period. 4. RESULTS AND DISCUSSION Generally speaking October was dry; therefore the rainfall map was blank. November rainfall was mainly below normal as can be seen from fig.1. The rainfall was almost blank except for very very few places over the western parts of the country. The surface chart showed high pressure cells controlling dry easterly airflow over the country and thus no or less rainfall activities. The global forecast for ENSO events indicated EL-NINO conditions during the season in question. This was almost evidenced during OND season, but things improved throughout the JFM season and this was due to the occurrence of tropical cyclones which occurred most frequently. In December we had the following cyclone; BONDO, January: DORA, February: FAVIO, HUMBA, GAMEDE, DORA and ENOK.March, we had: GAMEDE and INDLALA. These cyclones influenced rainfall activities over Zambia by maintaining the convergence zones oscillating about the country. The results can be seen from the rainfall map versus synoptic systems and the cyclones on the Annex. February which had a lot of cyclone affecting the region induced a lot of rainfall as can be seen on the map of February versus Tropical cyclone and the ITCZ. In the case of March, the cyclones induced thick cloudy bands which caused a lot of rains and storms. 4.1: 2006/2007 Rainfall Summary October 2006 In the first dekad of October many stations had not received rainfall except for Senenga which had 118mm,Kaoma 86mm, Mbala 28mm,Sesheke and Kafironda had 13mm each. The rest of the country had received rainfall below 4mm By the end of October stations in the northern half had received rainfall above 30mm while stations in the southern half had rainfall below 20mm. Land preparation was underway in most parts the Country with some areas in North western and northern provinces having started planting. 2

3 November The country had received normal to below rainfall. Normal rainfall was recorded over Mwinilunga, Zambezi, Sesheke and Serenje. The rest of the country, especially the southern half, sunny to hot conditions prevailed. By the second and third dekad of November, the northern half of the country had received enough rainfall to support planting and germination of seeds. The rainfall picture changed to normal and above normal. Nevertheless, departure rainfall from normal in Lusaka, Central, Southern and part of Western provinces was still below normal. Land preparation was still underway in the southern half of the country while the northern half of the country, planting had reached the peak. (See Figure1) Fig.1 RAINFALL DEPARTURE MAP FROM 1ST JULY 2006 TO 30TH NOVEMBER Chiengi Mwinilunga Solwezi Chingola Nchelenge Mwense Mansa Milenge Kaputa Kawambwa Luwingu Samfya Mporokoso Chilubi Mpulungu Kasama Mpika Mbala Mungwi Chinsali Nakonde Isoka Chama Lundazi Lufwanyam a Ndola Chavuma Serenje Kabompo Masaiti Mpongwe Zam bezi Mufumbwe Kasempa Kapiri Mposhi Mkushi Lukulu Kabwe Nyimba Kaoma Mumbwa Chibombo Kalabo Mongu Chongwe Lusaka Itezhi-tezhi Luangwa Kafue Mazabuka Senanga Monze Siavonga Choma Shang'ombo Gwembe Sesheke Kalomo Kazungula Sinazongwe Livingstone Mambwe Petauke Chipata Katete Chadiza Below Normal Normal Above Normal December 2006 By Middle of December, most areas in southern half of the country had received rainfall. The planting activity commenced in the middle of December in Southern half of Zambia. The Northern half of the country started experiencing Heavy to Moderate rainfall with average rain days of 8. This development resulted into floods situations in places along the river banks. WRSI Distribution About 13 stations (mainly in the northern half of Zambia) have recorded the Water Satisfaction Index below 100 % due to excess water received to support the growth of Maize Crop, and about 17 stations still maintain 100% Water Satisfaction Index.(See Bar chart 1) 3

4 RAINFALL DEPARTURE MAP FROM 1ST JULY 2006 TO 31ST DECEMBER Mwinilunga Mansa Kaputa Mpulungu Nchelenge Mbala Mporokoso Nakonde Kawambwa Mungwi Isoka Mwense Luwingu Kasama Milenge Chinsali Chilubi Chama Samfya Solwezi Chingola Lundazi Lufwanyama Ndola Chavuma Kabompo Masaiti Serenje Mambwe Zambezi Mpongwe Mufumbwe Chipata Kasempa Kapiri Mposhi Mkushi Chadiza Lukulu Kabwe Petauke Nyimba Kaoma Mumbwa Chibombo Kalabo Mongu Chongwe Lusaka Itezhi-tezhi Luangwa Kafue Mazabuka Senanga Monze Siavonga Shang'om bo Sesheke Choma Gwembe Kazungula Below Normal Kalomo Normal Livingstone Above Normal Sinazongwe Chiengi Mpika Figure 2 Barchart 1 January 2007 Heavy to moderate rainfall spread over Zambia. Most places in the Southern half had recovered from rainfall deficit.normal to above Normal Rainfall Recorded The flood situation in western,northern and part of eastern provinces increased. As a result crops along the banks of the rivers were partly submerged in water. Planting of Maize crop in the southern half the country come to an end by the second dekad of January. However basal to top dressing fertiliser application was underway in the Southern and Northern Half of the country, respectively. WRSI Distribution The Water Requirement Satisfaction Index (WRSI) distribution indicates that 12 stations had maintained 100% index, 14 stations about 95%, 3 stations about 90% and 1 station 75% index as shown in Bar chart. The Water Requirement Satisfaction Index shows that most stations in the country have received more than normal water requirements to support the maize crop at its current stage. Figure 3 Barchart 2 RAINFALL DEPARTURE MAP FROM 1ST JULY 2006 TO 31ST JANUARY Kaputa Chiengi Mpulungu Nchelenge Mbala Nakonde Mporokoso Kawambwa Mungwi Isoka Mwense Luwingu Kasama Mansa Chilubi Chinsali Chama Mwinilunga Solwezi Samfya Milenge Chililabombwe Chingola Kitwe Mpika Lundazi Chavuma Lufwanyama Ndola Serenje Kabompo Masaiti Zambezi Mpongwe Mufumbwe Kasempa Kapiri Mposhi Mkushi Lukulu Kabwe Nyimba Kaoma Kalabo Mumbwa Chibombo Mongu Lusaka Chongwe Itezhi-tezhi Kafue Luangwa Namwala Mazabuka Senanga Monze Siavonga Shang'ombo Choma Gwembe Sesheke Kalomo Kazungula Sinazongwe Livingstone Mambwe Chipata Katete Chadiza Petauke Below Normal Normal Above Normal 4

5 February 2007 Tropical Flavio enhanced rainfall over Zambia. By the 1st and 2nd Deakd of February normal to above normal rainfall was recorded over Zambia. The flood situation along the river banks that had sources in the northern part of the country had reached its peak level. Towards the end of February a reduction in rainfall activities over Southern Half Zambia experienced, with places such as Lusaka, Mt Makulu and Livingstone, crops started experiencing Water Deficit. Crop had reached flowering and Finger filling stages in the southern and northern half of the Country, respectively. This stage was crtical to Maize crop,and required enough rainfall to support the full development of the crop. On contrally some areas in the southern half of the country started experiencing water stress. WRSI FEB 2007 The WRSI for stations in the northern half of the country has reduced for most stations by 5% due to excess rainfall experienced., The lowest WRSI of 85 and 87% were recorded over Ndola and Chipata,respectively. Lusaka CITY, Mt Makulu and Livingstone had experience a reduction in WRSI due to rainfall deficit. Figure 4 Bar chart 3 March 2007 Moderate to heavy rainfall continue being experienced over the northern half of the country while light to moderate rainfall was experienced over the southern half of the country. Normal to above normal rainfall had been received over Northern parts of Zambia with some stations such as Chipata, Ndola, Kasama,Misamfu and Mwinilunga, had recorded cummulative rainfall above 1500mm. The general picture in March was that normal to above normal rainfall was recorded over most parts of Zambia except Lusaka and Southern province districts which had below normal rainfall. At the end of the Month rainfall activities shifted to the northern parts of the country leaving the southern part with light to moderate showers. The Maize crop had attained finger filling to mature stages. Water Requirement Satisfaction Index (WRSI) The water requirement satisfaction index indicates that most stations in the northern half of the country have had the index drop from 100 % due to excess rainfall received. However, about 9 stations had maintained the index of 100%. Lusaka City Airport, Lusaka International Airport, Livingstone, Mt Makulu and Senega had received below 90% of water requirements. This indicates that there were pockets in these areas where crops suffered a water stress. Mt Makulu 5

6 station was the most hit station with the Water Requirement Satisfaction Index of 79% since the start of the season. April 2007 The first dekad of April signaled the end of rainfall activities over the southern half the country though scatterd showers and thunderstorms were still experienced. Rainfall activities confined to the northern half of the country in both the 1st and 2nd Dekad of April. The country closed the season with normal to above normal rainfall with pockets of Lusaka and southern provinces ended the wet season with rainfall below normal. The Maize crop had reached Maturity to full ripen stages over Zambia. Reports of early harvest were received over the southern half the country. Hazards Flood situation experienced in western, northern and along the valleys of eastern provinces had destroyed some crops, and displaced human beings and live- stocks. Heavy rains experienced over northern parts of the country had caused damage to public infrustructure such as roads, Schools and clinics. They were reports of cattle diseases in western and southern provinces which had since claimed life of animals. In some instances animal and man competed over land in places that are near by game reserves. As a result few deaths were recorded. 5. C ONCLUSIONS From the figures of the observed rainfall versus the products from the MSG1, it can be clearly being seen that the forecast products came out in agreement with the observed rainfall. The set back is that, the archiving system for our PUMA workstation is not available. I would like to further this research using more products from MSG1 for this coming season and the ones to come in future. The Country had received rainfall normal to above normal in the northern half the country, and normal to below normal in the southern half the country. The rainfall distribution had not been good especially over the southern half of the country. The just ended rainfall season (2006/2007) had not been better than the 2005/2006 season in terms of agriculture production. There are likely to be reduction in food production in the southern parts of Zambia while the northern part of the country is expected to have bumper harvest. The long dry spell and flood situation in some parts of the country, may threaten house food security in affected areas. 6. ACKNOWLEDGEMENT AND REFERENCES MSG1 (METEOSAT) Meteorological products from EUMETSAT Dr. S.J. Mason: Definition of Technical Terms in Forecasting Verification and Examples of Forecast verification scores. Professor C.Oludhe: Verification of September to December 2003 consensus climate outlook. Theodore M.Marguerite: Verification of the 2002/2003 SARCOF Season Rainfall outlook Maps. Zambia Meteorological Department: Crop weather Bulletins-Decadal Rainfall maps. 6