Effects of Topography and Other Factors on the Movement of Lows in the Middle East and Sudan M. G. EL-FANDY

VOL. 31, No. 10, DECEMBER, 1950 375 Effects of Topography and Other Factors on the Movement of Lows in the Middle East and Sudan M. G. EL-FANDY Professor of Meteorology, Farouk I University, Alexandria, Egypt ABSTRACT 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. AVERAGE PRESSURE DISTRIBUTION THE average pressure distributions characteristic of the four seasons are represented by the maps of mean pressure for January, April, July and October given in FIGURE 1. They generally give a much better picture of the daily pressure situation than any similar map for an area in temperate latitudes. Except for winter, during which depressions or secondaries travelling eastwards are relatively frequent near the North African coast, departures from the mean usually take place in the form of movements or oscillations to N. and S. of the Sudan low, or local changes of pressure gradient due to the development of disturbances. FIGURE 1 shows that during the two transitional seasons (represented by April and October) a barometric low is situated over the central sudan. It will be referred to as the Sudan monsoon low. This low normally extends to about lat. 15-19 N, but continues with a small inverted V-shaped arm projecting northwards to the northern Red Sea. Excluding the coast of Egypt, Palestine, and Syria, January represents the season of the N.E. monsoon, which extends far south to the southern Sudan. In summer, as represented by July, the whole of Abyssinia and the southern Sudan are covered by a high pressure system, but over the N.E. Sudan there is an oscillatory barometric minimum. It is almost located to the N.N.W. of the Abyssinian Plateau and is a continuation of the Asiatic low. THE OSCILLATIONS OF THE SUDAN MONSOON LOW It is clear from FIGURE 1 that the Sudan monsoon low has its mean position changed from season to season. Day-to-day pilot-balloon observations reveal that these changes in the mean position of the low are also dynamical. In other

376 BULLETIN AMERICAN METEOROLOGICAL SOCIETY FIG. 1. Mean pressure-distribution characteristic of the four seasons. words they are not only due to thermal effects between land and sea, but also include the general wind system. During the months of January, April, July and October the low is respectively centered near the Lake Plateau (central Africa), Abyssinian Plateau, Iranian Plateau and Abyssinian Plateau again. Generally speaking, the movements of the Sudan monsoon low can be classified into two distinct types of oscillations. The first is the above described displacement of its center from near the Abyssinian Plateau and back again twice during the course of the year, the second includes a series of relatively small oscillations superposed on the annual track, as represented in FIGURE 2. These small oscillations are most noticeable in the two transitional seasons, notably in Spring before the formation of depressions of the "khamsin type" (El-Fandy, 1940). In Autumn they cause the spreading out of heat waves and thundery conditions over the N.E. generally (El-Fandy, 1948). Observations show that the so-called small oscillations accompany the passage of troughs of low pressure or secondaries associated with depressions travelling farther north over the E. Mediterranean or E. Europe. The average track of the annual march has two marked features: One is the outstanding tendency of the low to be centered near ranges of high mountains. The other is that this track has a component that follows the apparent position of the sun. THE DEVELOPMENT OF EFFECTIVE S.E. CURRENTS OVER IRAQ That active barometric lows are apparently attracted by tablelands is again observable with depressions travelling over the Mediterranean in winter. Depressions approaching from the W. Mediterranean almost always move towards the Balkans. It is only when these depressions are not active (in their last stages) or are moving

VOL. 31, No. 10, DECEMBER, 1950 377 FIG. 2. Oscillations of the Sudan monsoon low. very near to the North African coast, that they reach the E. Mediterranean, where they hold near Cyprus for a long time under the effect of the Plateau of Asia Minor. Under certain meteorological conditions these shallow lows near Cyprus become regenerated (El-Fandy, 1946). Such regenerated lows stay near Cyprus for several days, and are now known to be most active depressions that affect the weather of the whole of the area extending from Iraq to the southern extremity of the Sudan in winter. FIGURE 3 gives an example of a Cyprus low regenerated by the inflow of cold air from an anticyclone that extended over the Balkans. It gives the 0600 GMT maps of 10th to 12th January, 1934. Notice how the high pressure area over Iraq on the 10th and 11th was replaced by an independent low that developed on 12th as a result of the inflow of 4 'effective" S.E. currents (warm and damp in this case) along the general trend of the Iranian Plateau. The term 1 'effective current" is used by the author to indicate an air current induced dynamically to flow along the general trend of a plateau as a result of convergence into a neighboring low. Regenerated lows stationary near Cyprus for several days ultimately cause the inflow of "effective S.E. currents" over Iraq. These currents being warm and damp almost always give rise to the development of thundery conditions. Since the low is always centered near Cyprus and there is vigorous convergence into it during the period in which it is active (El-Fandy, 1946), it can be regarded as a two-dimensional sink of inward radial flow, as represented in FIGURE 4. The velocity q of the "effective current" at any point P{6) in the Valley of Iraq, along the general run of the Iranian Plateau (as represer ted in FIGURE 5 may be given by the equation (Milne-Thomson, 1938): m q = sin 20, (1) * a where m is the strength of the sink and a is its (vertical) distance from the general alignment of the plateau. This gives a possible application of the theory of images in its simplest form, but a different case will be discussed later when considering slowly travelling lows. In FIGURE 5 the general run of the plateau is regarded to a fair degree of approximation to be represented by the line DEFG, while the line KL gives the average position of the center of the low of Cyprus. Accordingly, the effective part of the plateau is at an average distance "a" from the sink which is equal to 900 kilometers. Moreover, sin 20=1 practically over central Iraq, so that q is greatest over central Iraq. The average velocity of the "induced current" FIG. 3. The development of an active low near Cyprus.

378 BULLETIN AMERICAN METEOROLOGICAL SOCIETY are nearly the average dimensions of the sector enclosing the warm air converging into the Cyprus low, the total air converging below 1.8 km into the low will be 1,200 + 500 Y X 15 X 1.8 (3) or m = 72,000 cubic kilometers per hour. The values of m given by equations (2) and (3) are comparable, which shows that the theory of images is applicable in this case. The velocity of the effective currents is, of course, the difference between the wind velocity actually measured (abnormally high under such conditions) and the gradient wind. EFFECTIVE CURRENTS ASSOCIATED WITH THE SUDAN MONSOON LOW FIG. 4. Cyprus low as a two-dimensional sink. at stations 773, 774, and 775 (in FIGURE 5) usually amounts to about 40 km/hr, and if the average depth of the S.E. current is taken to be 1.8 km (according to observation) then M = 900 X 1.8 X 40, (2) or m = 65,000 cubic kilometers per hour. On the other hand, if we assume an isallobaric component of 15 km/hr into the area of half an ellipse of radii 1,200 and 500 kilometers, which The Abyssinian Plateau represents a wall which lies to the E. of the Sudan monsoon low during the two transitional seasons. In these seasons the E. Sudan, and in particular the northern Red Sea, are outstanding areas which are again subject to the occasional inflow of "effective currents" when the Sudan monsoon low becomes active. The inflow of such currents is accompanied, on the maps, by the deepening of the inverted V-shaped northern arm of the Red Sea low and fall of pressure over the N. Sudan (El-Fandy, 1948). In the Sudan, however, pilot-balloon observations are useful in considering wind speed, and since some of the reporting centers have their altitudes slightly above 500 m, the average values of velocities measured at 1,000 and 1,500 m have been adopted. One of the active periods of the Sudan low was that of 21st to 23rd April, 1938. FIGURE 6 gives the 0600 GMT maps for this period and TABLE 1 Station 21st April 1938 22nd April 1938 23rd April 1938 V R V XR V R V XR V R V XR Haifa 27.5 1,340 37,000 Kareima 37.5 960 36,000 20 480 9,600 Khartoum 40.0 640 25,600 32.5 360 11,700 34 230 7,800 Kassala 42.5 730 31,000 35.0 590 20,600 29 590 17,000 El-Obeid 45.0 440 20,000 42.5 220 9,400 47 170 8,000 Malakal 5.0 70 350 37.5 340 13,400 19 590 11,200 Juba 20.0 880 17,600 13 1,060 13,800

VOL. 31, No. 10, DECEMBER, 1950 379 FIG. 5. Computation of mean SEly "effective currents" over Iraq (shaded area). FIGURE 7 shows the various centers of pilotballoon observations at that time. In TABLE 1 the average velocity V in km/hr the distance R from the center in kilometers and the value of V X R are given for each reporting station. These values are also represented diagrammatically in FIGURE 8, in which the average speeds are drawn against distance from the center of the low. Evidently the speeds reported from Kassala exceed the proper values corresponding FIG. 6. 0600 GMT charts, 21st-23rd April, 1938, over the Sudan.

380 BULLETIN AMERICAN METEOROLOGICAL SOCIETY THE IMAGE PRINCIPLE The writer suggests that the theory of images may enter not only into the above mentioned dynamic considerations but also into the explanation of the tendency of active lows to move towards tablelands. The theory which determines the type of motion produced when fluid is drawn off from the center of a revolving disc of incompressible fluid (Brunt, 1939) is made use of. If we think of the oscillating low as a cylinder from which air is removed upward at the axis (see FIG. 4), and regard the atmosphere as a solid rotating with the earth, then as the upward flow begins a cyclonic motion is attained; but a vortex is mathematically a sink of imaginary strength, and a sink is a negative source. Let a source be travelling with the velocity (Vx, Vv) at any instant, and let its position be at Z0, as represented in FIGURE 9. The complex potential of the motion (adopting the usual notations of Milne-Thompson) is: FIG. 7. Stations making pilot-balloon observations in the Sudan (1938). with these curves by about 12 km/hr on the 21st, 8 km/hr on the 22nd and 10 km/hr on the 23rd. These should represent the values of the effective currents over Kassala. W = m log (Z iy0) = - m log {(Z - iy0) 2 - x0 2 }, m log (Z0 + x0 iy0) dw _ - 2m(Z - iy0) dz ~ (Z - iy0) 2 - x0 r (W = <t> + i\fr\z = re ib). FIG. 8. "Effective currents" with the Sudan low, April 21-23, 1938; relation of wind speed to distance from center of low for each day shown separately.

VOL. 31, No. 10, DECEMBER, 1950 381 where II is the pressure at infinity and is not necessarily constant, p - n _ p ~ dt k 2 - FIG. 9. The image principle. or / H J Q = 2UmVx Urn 2 Tlpm 2 (P - = 2Hmp Vx - where is the velocity of the low away from the wall. For a sink the effect of motion is, therefore, to diminish the pressure on the wall by an amount F, where x0 x0 J o On the wall dw 2 my' dz / 2 + *o 2 ' dw _.di _ 2mO0Fs - y'vy) dt ~ dt dt ~ - (y' 2 + x0 2 ) ' \(f-dy' = 2m 2 Ilm 2 +0 Also 2TW f / oo 2x0 r _ 3 Jo (/ 2 y' 2 + *0 2 dy' = 2wnFx. Since by Bernouilli Theorem, + *o 2 ) 2, where y' = i{y yo), and :-y'vy q = velocity on the wall. f + i<z 2 " ~dt ^ + 0 = C(*) n = - + P Gf limp (S-4 (4) In the case of the monsoon low 2VX is small compared with m/xo, and the effect of convergence is to urge the low towards the plateau. ACKNOWLEDGMENT My deepest appreciation is due to Mr. A. Haque for valuable assistance regarding the mathematical treatment of the image principle. REFERENCES Brunt, D. (1939). Physical and Dynamical Meteorology. Cambridge University Press, pp. 299-302. El-Fandy, M. G. (1940). Q. J. Roy. Met. Soc.} Vol. LXVI, No. 286. El-Fandy, M. G. (1946). Q. J. Roy. Met. Soc.} Vol. LXXII, No. 314. El-Fandy, M. G. (1948). Q. J. Roy. Met. Soc., Vol. 74, No. 319. Milne-Thomson, L. M. (1938). Theoretical Hydrodynamics. MacMillan & Co., Ltd., London, pp. 208-209.