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- Author or Editor: M. G. El-Fandy x
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In the area extending from the Middle East to the southern Sudan the pressure distribution generally assumes typical seasonal patterns. Apart from winter (December to February) the main low-pressure system which has a direct control on the weather of the area under consideration is an oscillatory barometric minimum, which during the two transitional seasons (spring—March to May—and autumn—September to November) is centered over the central Sudan, and is referred to as the Sudan monsoon low. In October, which represents average conditions in autumn, for example, this low normally extends to about lat. 16°N, but continues towards north with a small inverted V-shaped arm projecting to the northern Red Sea. As winter starts the low acquires a southerly displacement and by January, which represents average conditions in winter, it becomes situated near the Abyssinian Lake Plateau. On the other hand in late spring and early summer the low moves from the central Sudan across Arabia to Persia, and by July it becomes a part of the Asiatic monsoon low which extends to the N.E. Sudan.
The movements of the Sudan low take place in the form of a series of oscillations the average track of which has two outstanding features, namely: (1) There is a remarkable tendency for the low to be located near tablelands. A similar feature is also observed with secondary depressions travelling over the E. Mediterranean in winter. (2) Outstanding northward oscillations of the low take place when middle-latitudes travelling depressions invade the E. Mediterranean from West.
It has also been observed that air currents, not necessarily obeying pressure-gradient, flow along the general run of the Abyssinian Plateau as a result of convergence into the Sudan low, and along the general run of the Persian Plateau (the Valley of Iraq) as a result of convergence into active depressions near Cyprus in winter. Such air currents have been referred to as “effective currents.”
The writer suggests that the image principle may enter into the explanation of these features. The theory which determines the type of motion produced when fluid is drawn off from the center of a revolving disc of incompressible fluid is made use of; but further mathematical treatment and dynamical considerations have been carried out, assuming the active oscillatory low as a slowly travelling cylinder whose axis represents a two-dimensional sink.
In the area extending from the Middle East to the southern Sudan the pressure distribution generally assumes typical seasonal patterns. Apart from winter (December to February) the main low-pressure system which has a direct control on the weather of the area under consideration is an oscillatory barometric minimum, which during the two transitional seasons (spring—March to May—and autumn—September to November) is centered over the central Sudan, and is referred to as the Sudan monsoon low. In October, which represents average conditions in autumn, for example, this low normally extends to about lat. 16°N, but continues towards north with a small inverted V-shaped arm projecting to the northern Red Sea. As winter starts the low acquires a southerly displacement and by January, which represents average conditions in winter, it becomes situated near the Abyssinian Lake Plateau. On the other hand in late spring and early summer the low moves from the central Sudan across Arabia to Persia, and by July it becomes a part of the Asiatic monsoon low which extends to the N.E. Sudan.
The movements of the Sudan low take place in the form of a series of oscillations the average track of which has two outstanding features, namely: (1) There is a remarkable tendency for the low to be located near tablelands. A similar feature is also observed with secondary depressions travelling over the E. Mediterranean in winter. (2) Outstanding northward oscillations of the low take place when middle-latitudes travelling depressions invade the E. Mediterranean from West.
It has also been observed that air currents, not necessarily obeying pressure-gradient, flow along the general run of the Abyssinian Plateau as a result of convergence into the Sudan low, and along the general run of the Persian Plateau (the Valley of Iraq) as a result of convergence into active depressions near Cyprus in winter. Such air currents have been referred to as “effective currents.”
The writer suggests that the image principle may enter into the explanation of these features. The theory which determines the type of motion produced when fluid is drawn off from the center of a revolving disc of incompressible fluid is made use of; but further mathematical treatment and dynamical considerations have been carried out, assuming the active oscillatory low as a slowly travelling cylinder whose axis represents a two-dimensional sink.
Forecasting thunderstorms in the Red Sea is an outstanding meteorological problem of low latitudes. This is notably the case in the central and southern regions of the Sea, where the frequency, duration and intensity of these storms attain their maxima. It has been shown that in general thundery conditions in this Sea arise from convergence processes such as the setting up of centers of convergence or the development of upper-air cyclones. Centers of convergence are set up by troughs in the upper westerlies or troughs in the upper NE-E current of the Sudan, and are usually followed by cyclonic developments. It has eventually been made clear that the upper winds (and not the usual pressure map) hold the clue to much of the weather in these fairly low latitudes.
Forecasting thunderstorms in the Red Sea is an outstanding meteorological problem of low latitudes. This is notably the case in the central and southern regions of the Sea, where the frequency, duration and intensity of these storms attain their maxima. It has been shown that in general thundery conditions in this Sea arise from convergence processes such as the setting up of centers of convergence or the development of upper-air cyclones. Centers of convergence are set up by troughs in the upper westerlies or troughs in the upper NE-E current of the Sudan, and are usually followed by cyclonic developments. It has eventually been made clear that the upper winds (and not the usual pressure map) hold the clue to much of the weather in these fairly low latitudes.
Long-term rainfall forecasting is used in making economic and agricultural decisions in many countries. It may also be a tool in minimizing the devastation resulting from recurrent droughts. To be able to forecast the total annual rainfall or the levels of seasonal floods, a class of models has first been chosen. The model parameters have then been estimated with an appropriate parameter estimation algorithm. Finally, diagnostic tests have been performed to verify the adequacy of the model. These are the general principles of system identification, which is the most crucial part of the forecasting procedure. In this paper several sets of data have been studied using different statistical procedures. The examined data include a historical 835-year record representing the levels of the seasonal Nile floods in Cairo, Egypt, during the period A.D. 622–1457. These readings were originally carried out by the Arabs to a great degree of accuracy in order to be used in estimating yearly taxes or Zacat (Islamic duties). The observations also comprise recent total annual rainfall data over Addis Ababa (Ethiopia) (1907–1984), the total annual discharges of Ethiopian rivers (including the river Sobat discharges at Hillet Doleib, Blue Nile discharge at Roseris, river Dinder, river Rahar, and river Atbara), equatorial lake plateau supply as contributed at Aswan during the period 1912–1982, and the total annual discharges at Aswan during the period 1871–1982. Periodograms have been used to uncover possible periodicities. Trends of rainfall and discharges of some rivers of east and central Africa have been also estimated.
Using the first half of the available record, two autoregressive integrated moving average (ARIMA) time series models have been identified, one for the levels of the seasonal Nile floods in Cairo, the second to model the annual rainfall over Ethiopia. The time series models have been applied in 1-year-ahead forecasting to the other half of the available record and give fairly promising results, thus indicating the adequacy of the fitted models.
Long-term rainfall forecasting is used in making economic and agricultural decisions in many countries. It may also be a tool in minimizing the devastation resulting from recurrent droughts. To be able to forecast the total annual rainfall or the levels of seasonal floods, a class of models has first been chosen. The model parameters have then been estimated with an appropriate parameter estimation algorithm. Finally, diagnostic tests have been performed to verify the adequacy of the model. These are the general principles of system identification, which is the most crucial part of the forecasting procedure. In this paper several sets of data have been studied using different statistical procedures. The examined data include a historical 835-year record representing the levels of the seasonal Nile floods in Cairo, Egypt, during the period A.D. 622–1457. These readings were originally carried out by the Arabs to a great degree of accuracy in order to be used in estimating yearly taxes or Zacat (Islamic duties). The observations also comprise recent total annual rainfall data over Addis Ababa (Ethiopia) (1907–1984), the total annual discharges of Ethiopian rivers (including the river Sobat discharges at Hillet Doleib, Blue Nile discharge at Roseris, river Dinder, river Rahar, and river Atbara), equatorial lake plateau supply as contributed at Aswan during the period 1912–1982, and the total annual discharges at Aswan during the period 1871–1982. Periodograms have been used to uncover possible periodicities. Trends of rainfall and discharges of some rivers of east and central Africa have been also estimated.
Using the first half of the available record, two autoregressive integrated moving average (ARIMA) time series models have been identified, one for the levels of the seasonal Nile floods in Cairo, the second to model the annual rainfall over Ethiopia. The time series models have been applied in 1-year-ahead forecasting to the other half of the available record and give fairly promising results, thus indicating the adequacy of the fitted models.
