Chapter 10 WIND PAÏTtiKNS IN THK WESTERN DEbERT FARUUK EL-ÜAZ and R. W. WOLFE cJational Air and Space Museum Smitnsonian Institution Washington, D.C. 2Û30U AtíSTKAüT The Western Desert is part of the driest region on Earth, where the inaident solar radiation is capable of eva- porating 200 times the amount of precipitation^ For this reason wind is the main agent of erosion and deposition in a sompletely eolian environment. Data on wind velocity and direction are analysed in this paper to establish their relationships to sand transport and orientation of dunes. Surface wind data are swrmarized for 42 meteorological sta- tions between 15° and 35° N latitude and 15° and 41° E longitude. The summaries are presented in the form of graphs showing the patterns of sand-moving winds in wind roses, sand-drift potential in resultants, and streamlines. The basic patterns agree with the overall southward, direc- tion of prevailing wind, and thus of general sand transport directions. Variations from this general pattern are believed to be due to interaction between the wind and local topography. Prevailing wind directions in the Western Desert are also analyzed in terms of seasonal wind cir- culation patterns in North Africa. Because of the scarcity of data, it is recommended that automated meteorological stations be used to gather information cm local winds in the open desert, particularly in the southwestern part, which is important for analog correlations with Mars, INTKUUÜLTIOLSI As part of the eastern Sahara, the Western Desert is one of the driest places on Earth, The "aridity index" of much of this desert is 200, Which means tnat the incident solar energy is capable of eva- porating 2U0 times the amount of precipitation received (Henning and Fiohn, 1977). Because of this extreme aridity, wind is the main agent of sculpture in this desert. Wind» air moving more or less parallel to tne surface of the ground, dismantles scarps, deepens hollows and erodes exposed rocfcs. The products of erosion are either hurled in the atmosphere as dust, accumulated in the form of sand sheets and dunes, or are left beiiind as coarse lag deposits. Thus, an understanding of the prevailing wind directions and their variations is necessary to the study of eolian depositional and erosional features of this desert. In this paper we present a preii- miiLary analysis of the available wind data and their relations to known directions of sand dunes, -119- Much of the basic research on dune classifications and sand move- ment has been carried out in the Western Desert, Based on his obser- vations in this desert, Bagnold (1933; 1941; Bagnold and others, 1939) charted the major dune locations and discovered the basic prin- ciples of sand transport by wind. He attributed the main direction of dunes to a clockwise circulation of wind about a center near the Kufra Oasis in southeastern Libya. Bagnold's early observations have recently been supported by stu- dies of sand dune patterns as revealed by photographs taken from space (Fig, 10.1). Glfford and others (1979, p. 219) established that "the dune orientations change from north-northwesterly in the northern desert to north-northeasterly in the south. The dunes are intimately associated with scarps that bound numerous depressions in the Western Desert. This relationship is believed to result from the interactions between sand-carrying winds and scarps and other topographic variations." A preliminary investigation of sand-moving winds from 21 stations in Egypt by Wolfe and El-Baz (1979, p. 299) revealed two major wind regimes : "a narrow band of predominantly westerlies along the Mediterranean seacoast and generally north-northwesterlies throughout the Western Desert. Imposed upon these regional trends are local variations that may be attributed to topographic effects." It is important to note here that the northeasterly trend observed on the basis of dune orientations (Gifford and others, 1979) did not show up in the wind data analysis by Wolfe and El-Baz (1979). This is basically due to the lack of wind recording stations in the southwestern part of the desert, where the dunes veer towards a northeasterly orientation. In other localities, dune orientations were found to mimic prevailing wind direction. For this reason, the local topographic setting of meteorological stations in the Western Desert must be considered when using the wind data. The wind data used in this study were obtained from the U. S, National Climatic Center in the forra of U.S. Air Force N-type sum- maries (Summary Form 2 - surface winds). These summaries were pro- duced from observations by World Meteorological Organization (WMO) stations, and cover periods ranging from 7 to 13 years. GENERAL WIND CIRCULATION The wind circulation of northern Africa including the Western Desert of Egypt is influenced by many factors, especially the presence of seasonally serai-permanent high pressure areas and the location of the Inter-Tropical Convergence Zone (I.T.C.Z.), a discontinuity separating southeasterly and northeasterly winds (Griffiths and Soliman, 1972). The location of the I.T.C.Z. generally follows the path of the Sun with the advance of the seasons (Fig. 10.2) and may play the greatest role in determining the winds of southern Egypt and the Western Desert. -120- Figure lü.l Sand dune belts of the Western Desert of Egypt as mapped from Apollo-Soyuz photographs and Landsat images. (Modified from Gifford and others, iy79, p. 225). -121- JANUARY APRIL ,> ^ / ^^ r ^V^^-^/i X •••••• INTER TROPICAL CONVERGENCE ZONE iiiMiiiiiiiii OTHER CONVERGENCE ZONE JULY OCTOBER ¿LA-^VC^ v>t -^ V -r \i ? ÙJ\]1^ "^'•^ f\ **f ^ > MT"^""^ Figure 10.2 Wind circulation in North Africa as indicated by four months representing the seasons (After Griffiths and Soliman, 1972). 1. Winter (December-February) : During this season, the I.T.C.2. is far to Che south and wind circulation is dominated by the pre- sence of the large Saharan high situated in the wes tern Sahara and nioditied by the passage of depressions across tlie Mediterranean Sea in northern Egypt, The winds, then, are generally north-northwesterly. In northern Egypt, winds are south-southwesterly in advance of the Mediterranean depressions and Become northwesterly as the depressions pass. These two wind regimes are clearly seen in the wind map for January (Fig, 1Ü.2). Z. Spring (March-Hay): During these months, the l.T,C,Z, begins to move nortnward (Fig, L0.2) and depressions become less frequent and follow tracks that are more southerly than in winter. Often these depressions are accompanied by strong soutnerly winds and may cause severe sandstorms. 3. Summer (June-September) : The I,T.C,Z,, which reaches its northerly limit during July, is located in northern Sudan (Fig. 1U.2). During this season winds are generally northerly, even in nortnern ügypt. Generally, winds are stronger, blow more frequently, and are less variable than In other seasons. -Ill- 4. Autumn (October-November): Like Spring, Autumn is a transi- tion season witti the I.T.CZ, retreating southward. This retreat is more rapid than the Spring advance. With the exception of Aswan, this is the season of lowest wind speed (Griffiths and Soliman, 1972). INTERACTIONS OF WIND AND SANU Wind Data Surface wind summaries for 42 observing stations between 15° and I 35°^ and 15° and 41°E listed in Table 10.1 were selected for analysis l(Fig. lu.3). üf these, half (21 stations) are in Egypt, including the ¡. Sinai Peninsula. Only 9 stations cover parts of the V/estern Desert I including eastern Libya, but none are located in the southern and southwestern parts of the desert, A guide to these and other weather summaries for the entire world is given by Ownbey (1978). No details about the placement of anemometers or frequency and resolution of ooservation are included in the N~type summaries. However, World Meteorological Organization guidelines (WMO, ISfo. 8 TPß) I TaDie 10.1. Name and designation of 42 wind stations whose data were studied in this report. See locations in Figure 10.3. <STATION WMO No. STATION WMO No, 1. Abu named 62640 22. Gulf of Suez RS012 2. Abu Suer 62438 23, Helwan 62378 3. Agedabia 62055 24. Jeddah 40477 4. Alexandria 62318 25- Jerusalem 40184 5. Aswaa 62414 26. Kassala 62370 6. Atbara 62680 27. Kharga 62435 7. üadanah 40357 28. Khartoum 62721 ii. lianariya 62420 29. Kufra 62271 9. iäentna 62053 30. Luxor 62405 10, Cairo 62366 31. t'lanqabad 62393 11. Ûakhla 62432 32. Mersa Matruh 62306 12. Uerna 62059 33. Port Said 62333 13, bülat 40199 34. Port Sudan 62641 14, El-Ad em 62063 35. Quselr 62465 15. El-Arish 62336 36. Red Sea North RSOll lb. El-Tor 62459 37, Sal urn 62300 17. Faiyum 62318 38. Siwa 62417 18. Farafra 62423 39. Suez 62450 19. Gaza 62338 40, Tanta 62438 20. Gialo 62161 41. Turaif 40356 21. Giarabnb 62176 42. Wadi Haifa 62600 -123- iíigiire 10.3 Location map of 42 wind stations from which data were analyzed (bee Table 10.1 for name and number of indivi- dual stations). recommend placement of anemometers 10 m above the surface and recording winds to the nearest one knot and to the nearest 10° direction. The data are suiaraarized by month (in some cases an annual summary is available) and presented as frequency of occurrence in 15 (sometimes a) directional classes corresponding to points of the compass, and 5 to 9 velocity classes shown in Table 10.2. Calm and variable winds are tallied separately. Sand-Moving Winds The basic expressions for the laovement of sand by wind were developed by Üagnold (1941) based on field and laboratory observations.