J E T S T R E A M CLOUDS IN THE MIDDLE EAST (*) b y A. I. ELTANTAWY (**)

Summary - - A time cross-section was constructed for Cairo during the period i0-31 Dee. 57. The outstanding feature of this cross-section is the periodic existence of high wind maxima associated with high cloud formation. A method was suggested for the location of high wind centers from the thermal field only. Moreover, the climatological means of high clouds showed quick changes in the period of the Jet Stream migration. This has been taken as an evidence of the relation between the subtropical Jet Stream and high cloud formation in the region. Introduction ~ J e t S t r e a m clouds have been discussed before b y several authors m a i n l y for J e t Streams of higher l a t i t u d e s in Europe and N o r t h A m e r i c a where a close n e t work of observation stations allow for detailed investigation. A trial to s t u d y the clouds associating J e t Streams in the Middle E a s t area should have a different a p p r o a c h due to several considerations: 1) The scarcity of d a t a and the great distances between upper air stations which do n o t allow the a n a l y s t to locate the position o f the J e t S t r e a m axis with the required degree of accuracy. 2) The absence of frontal surfaces in m a n y cases of the Middle E a s t J e t Streams. Those J e t Streams without fronts were classed as (Sub-tropical J e t Streams) b y KOTESWAmtM (1), BANNON (2) and others (3). 3) The low content of water v a p o r over the sub-tropical ridge does not allow clouds t o form easily in the vicinity of this sub-tropical J e t Stream. 4) The r e l a t i v e l y higher levels o f the sub-tropical J e t S t r e a m t h a n the polar front J e t Stream. The present work, however, is a trial to find out if a n y correlation exists between these high level J e t Streams arid high cloud formation in the Middle E a s t area.

Survey of previous literature ~ I n 1951 SxWYER & ILETT (4) following a statistical procedure found a relation between the J e t S t r e a m and m e d i u m and high clouds. Amounts of high clouds of four octas were more frequent on the southern side of the J e t axis t h a n the n o r t h e r n side.

(*) Presented by ProL M. G. EL-FANDY. (**) Dept. of Astronomy, Faculty of Science, Cairo University, Giza, Cairo, Egypt (U.A.R.).

~ - 353 - SCHAEF~I~ 1952 (5) indicated t h a t certain t y p e s of m e d i u m and high clouds can be r e l a t e d to the J e t S t r e a m and can be t a k e n as evidence of its existence. He suggested certain other features of these J e t S t r e a m clouds as coherency of cloud p a t t e r n and high velocity of t h e m across the sky. I n 1955 SChAEfER & H~BERT (6) in a s t u d y of J e t S t r e a m clouds a t t r i b u t e d the c o m p l e x i t y of cloud formation to complex shear distribution and turbulence elements. The authors drew a vertical cross section on which t h e y analysed the v o r t i c i t y field. On this field t h e y superimposed isopleths of 11IC~ARDSO~'S numb e r (Ri). I t was indicated t h a t t y p i c a l J e t S t r e a m clouds form in a space where negative v o r t i c i t y is superimposed upon R i < 1 on the cross section. I n other words, these clouds form i a a space where two i n s t a b i l i t y criteria are fulfilled. I n 1957 Mc LEAN (7) studied statistically the cloud distribution in the vicinity of J e t Streams, he used an enormous b o d y of observations from reconnaissance flights, he came to the frequency of occurrence of each cloud t y p e in different positions r e l a t i v e to the J e t core. No trial - - to the present author's knowledge - - was made to discuss the cloud p a t t e r n associating the J e t S t r e a m in its southern position in the subtropics, or even to indicate t h a t such p a t t e r n existed. Cloud formation and vertical motion - - I t is logic to relate cloud formation especially at high levels to vertical motion. The p r o b l e m of the vertical motion p a t t e r n in the vicinity of the J e t S t r e a m has been the subject of m a n y discussions. The Chicago group in 1947 pictured a meridional circulation p a t t e r n with sinking to the South and rising to the N o r t h of the J e t axis. PALM~N & N~WTOrr i948 (S) indicated t h a t the p o t e n t i a l t e m p e r a t u r e field in t h e v i c i n i t y of the J e t S t r e a m suggested a reverse circulation with u p w a r d motion in the Southern side. This was also pictured b y NAMIAS & CLArr 1949 (9) in their t h e o r y of confluence. RIE~IL & TWEL~S 1953 (10) found t h a t the circulation p a t t e r n in the v i c i n i t y of the J e t S t r e a m studied b y t h e m was in accordance with the p a t t e r n suggested b y the Chicago group i.e. rising motion to the N o r t h and sinking to the South. F i n a l settlement was found b y MURRAY & DANIELS 1953 who indicated t h a t the circulation p a t t e r n is in accordance with the Chicago group p a t t e r n in the exit p o r t i o n and the reverse is true in the entrance or confluence portion of the J e t Stream. Data and m e t h o d - - High cloud observations at Cairo Station (3007 ' N 31~ ' E ) were collected for the period 10-31 Dec. 1957 at 0600 and 1200 U.T. These times were chosen due to the difficulty of high cloud observation at night. These observations were copied from the m o n t h l y weather r e p o r t of the State Met. Dept., Cairo (~). Upper air observations of Cairo radiosonde station were used for the same period and cross-checked on the radiosonde maps. Cairo radiosonde station takes two observations e v e r y 24 hours at 0000 and 1200 U.T. The u p p e r air observations were displayed in a time cross section for the whole period. Winds and t e m p e r a t u r e s were p l o t t e d in their positions on the cross section. The wind and t h e r m a l fields were analysed b y drawing isotachs a n d isotherms (Fig. 1).

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T h e t i m e c r o s s - s e c t i o n - - The use of time cross sections to study the changes in the upper wind and temperature fields has been emphasized b y several writers.~ A D a I i ~ o G i z z o L l 1954 (12) in a study of Jet Streams in a blocking situation used efficiently time cross sections and recommended their use in similar studies. i

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It is seen from the time cross section that Can'o in the period under consideration has b e e n o v e r h e a d e d b y s e v e r a l w i n d m a x i m a at c e r t a i n definite periods. These maxima

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- - 355 - in some observations the balloon failed to observe winds higher t h a n the 500 m b level. This was due to high winds in lower levels which pushed the balloon to the low angle range before higher levels were reached. The most p r o m i n e n t features of the time cross section can be s u m m a r i s e d in the following : 1) More or less periodic existence of a wind m a x i m u m over the station, the time period between each two m a x i m a being 3-4 days. 2) Vertically elongated isotach p a t t e r n s which in certain periods showed low level centers of m a x i m u m wind. These low level centers of high wind m i g h t be identical w i t h the low level J e t Streams discribed b y ARAKAWA (la). C o r r e l a t i o n b e t w e e n the w i n d t e m p e r a t u r e f i e l d s - - The isotherms were fluctuating in a w a v y fashion showing m a x i m a and minima, i t can be noticed t h a t , going in the direction of time the isotherms ascend steeply in the vicinity of the J e t centers and descend in the other side w i t h the m a x i m u m associating the center. I t is noticed t h a t : i) The a m p l i t u d e of these isotherm waves changes with height in some occasions. 2) The a m p l i t u d e is small in lower layers, b u t increases in the upper troposphere. 3) I n other occasions the a m p l i t u d e is quite large at the b o t t o m of the section. These amplitudes can be considered as a measure of the a c t i v i t y of passing fronts, so t h a t small or indistinct amplitudes in the lower layers denote the absence of fronts. The existence of these t e m p e r a t u r e fluctuations in the upper troposphere only is a t t r i b u t e d to the passage of a discontinuity surface at these levels. The idea of a J e t - f r o n t is suggested b y m a n y authors. ]~r & McLEA~ (i957) (14), in their work to construct a J e t S t r e a m model, a d o p t e d the idea of the existence of such a high level front and gave it the name (Jet-front). Is is noticed t h a t above the J e t centers the horizontal t e m p e r a t u r e g r a d i e n t is reversed. Joining the crests of the isotherm waves we get a line, the inclination of which gives the m e a n time lag of the wave crest w i t h height. This line n e a r l y hits t h e center of the m a x i m u m wind core. This m e t h o d affords a tool to locate the J e t center on such time cross-sections from the t e m p e r a t u r e field only. The rule is t h a t the J e t center falls on the line joining the isotherm wave crests j u s t below the level at which the horizontal t e m p e r a t u r e gradient is reversed. I t is noticed t h a t the t e m p e r a t u r e changes below the wind m a x i m a are pronounced in some occasions and insignificant in others the J e t centers being pronounced in the former occasions. T h e t r o p o p a u s e - - U n f o r t u n a t e l y the tropopause was n o t reached in m a n y days of the period considered, however, the 6bservations in the period 23-31 Dec. allow for its location in a continuous manner. The WMO definition of the tropopause has been a d o p t e d (The level above which the lapse r a t e falls to a value equal or less t h a n 2 ~ C per kin). Although the tropopause was no t located except for about one t h i r d of the period, it can be noticed from this p a r t t h a t it changes its height periodically in

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a wavy manner. These waves are in phase with the waves of the isotherms of the upper troposphere, the wave length is about 4 days. The lowest value of the tropopause height in the p e r i o d 23-31 Dec. 57 was about 11.5 kms with a mean of 12 kms.

The figure shows distinctly t h a t the fluctuations in the tropopause height are linked with the centers of high winds in the upper troposphere. The vertical distance between the tropopause and the centers of wind maxima is roughly a b o u t 1 k m which is about 40 nabs on the pressure scale. This vertical distance between the tropopause and the m a x i m u m wind is in agreement with previous findings of BAN~O~, and was theoretically enterpretted b y E. GOLD in 1953 (15).

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High cloud observations were collected for the synoptic times 0600 and 1200 GMT and tabulated in Table 1. Straight horizontal lines were drawn on the time coordinate showing the periods of high cloud formation. I t is to be noted here t h a t high cloud observations in all amounts were considered, a little a m o u n t of cloud m a y be of great significance in such s t u d y if we remember the low h u m i d i t y content i n the region as indicated above. The correlation between high clouds and centers of

- - 357 - wind m a x i m a proved to be most striking, each period of high cloud observations was also a period of a high wind center overhead. The coincidence in the period considered was almost complete without a single exception. H i g h clouds were observed in 6 periods each of which was characterised b y high winds in the u p p e r troposphere. Most of the high clouds observed were of the cirrus and cirrostratus t y p e s , cirrocumulus was observed in one occasion only on i l t h Dec. Climatological discontinuity in the normal Cloudiness - - I n the foregoing investigation it was found t h a t high clouds were closely linked w i t h the J e t stream. A l t h o u g h this was proved for a r a t h e r short period so t h a t it cannot be generalized as a rule, y e t it is believed t h a t this relations is valid t h r o u g h o u t the year. The i r r e g u l a r i t y of the u p p e r air observations in this region and their absence in m a n y days of the y e a r does not allow a similar procedure as the previous to be followed on an annual or even on a seasonal scale. Accordingly, another a p p r o a c h was tried depending upon climatological normals. The position of the subtropical J e t S t r e a m in different seasons was discussed b y m a n y writers in m a n y longitudes. The nearest longitude along which the m e a n position of the J e t S t r e a m is located is 45 ~ E. This position was first located b y F~OST in i953 (16) b y the construction of m e a n cross sections for each season. T h i s a u t h o r a t t r i b u t e d the sharp change in the m e a n m a x i m u m t e m p e r a t u r e and in .the height and t e m p e r a t u r e of the tropopause to the quick migration of the subtropical J e t S t r e a m n o r t h w a r d s in H a y and southwards in October. SUTCLIFFE & BA]NrqO~ (1954)(17) carried out a detailed s t u d y of the period of t r a n s i t i o n May and June, t h e y indicated a relation between the l a t i t u d i n a l change in the mean position o f the subtropical J e t S t r e a m and the onset of monsoon regime in India. I n t h a t work it is i n d i c a t e d t h a t the s u m m e r regime is set on all over the area in a r a t h e r short period (about 3 weeks). The mean position of the subtropical Jet Stream - - The m e a n position of t h e subtropical J e t S t r e a m was c o m p u t e d b y m a n y authors at different longitudes for different seasons b y means of constructing m e a n cross sections. Some writers e x t r a p o l a t e d these positions to areas where the d a t a do not allow for computation. Although there are some differences in the mean seasonal position and s t r e n g t h of the subtropical J e t S t r e a m in the E g y p t i a n zone, y e t most authors agree t h a t the subtropical J e t S t r e a m exists in this area about 25o N in winter and migrates t o t a l l y out of it in s u m m e r to be about 360 N [FRosT (1G) and JZ~KE~SO~ (ls)]. The cloud element - - The E g y p t i a n zone can he d e r i d e d into two distinct climatological zones, a n o r t h e r n zone to about 28 1/20 N which is affected b y the Mediterranean depressions, and a southern zone, south of this l a t i t u d e where desert climate predominates. I n the n o r t h e r n region usually, cloud forms are identical w i t h those of e x t r a t r o p i c a l depressions, b u t in m u c h smaller amounts. I n the southern region clouds are less frequent and are m a i n l y of the high types, accordingly t h e y can be related to the J e t Stream. I f this relations is true, the n o r m a l cloudiness in the southern region should fall s h a r p l y with the quick mov e m e n t of the J e t S t r e a m in H a y . The climatological means :of cloudiness for stations along the Nile v a l e y is published b y the State Meteorological D e p a r t m e n t , Cairo. These normals are pre-

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p a r e d b y t a k i n g the m e a n of 3 observations during the 24 hours, for periods v a r y i n g from 19 to 45 years (19). The normals for cloud amounts for each m o u t h are plott e d against t i m e in Fig. 2 for 5 stations along the Nile valley, s e p a r a t e d approxim a t e l y b y one degree latitude. The curves in this figure show the m e a n cloud amounts at different months for the 5 stations. ~Aswan 2 4 ~0 2 ' E 3 2 ~ 53" N 1901-45 ~Kharga

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F r o m these curves it is seen t h a t the changes in cloudiness t h r o u g h o u t the y e a r in the 5 stations are strikingly similar, each of these curves has a m a x i m u m in winter (December, or J a n u a r y , a n o t h e r m a x i m u m iu May, and a flat m i n i m u m in summer (June, July, August and September). The most striking common feature o f these 5 curves is the steep change between May and June, ad after September. The former one being more pronounced t h a n the latter. The association of this steep change in the n o r m a l cloud amounts (which are m a i n l y of the high types), and the quick migration of the subtropical J e t S t r e a m is considered a strong evidence of the relation between the subtropical J e t S t r e a m a n d high cloud formation in the area.

Acknowledgement The a u t h o r expresses his thankfulness to Prof. Dr. M.G. EL-Fi~DY for suggesting the problem, iris kind encouragement and rev i s i o n of this paper.

REFERENCES Q)~P. KO~rESWARAM: Indian Journal of Meteorology and Geophysics, 1953. - (2) j . K. BA~ON: M. M., Vol. 83, 1954. - - (3) Met. Discussion by the Met. O~ee Staff

359 - ( L o n d o n , M. M., J a n . , I953). - - (~) SAWYER & ILETT: M.M., vol. 80, 1951. - - (5) V. I. SC~AErEI~: Tellus, v o h 5, 1953. - - (6) V. I. SCaAEFEI~ & HUSEaT: Tellus, vol." 7, i955. - (7) Mc LEAN: B.A.M.S., vol. 38, ~qo. I0, 1957. - - (s) E. PAL~EN & C . W . NEWTON: if.M, vol. 6, 1948. - - (9)3. NAmAS & P. F. CLAPt: J. M., 6, 1948. - - (10) H. RIrHL & TWELES: Tellus, 5, 1953. - - (tl) M o n t h l y W e a t h e r R e p o r t , Met. Dept. Cairo, Dee. 1957. - - (12) ADmANO GAZZOI~A: Scientific Proceedings in Meteorology, R o m e 1954. - (13) H. AI~AKAWA: Tellus, vol. 3; 1951. __(14)ENDLICH & MC LEAN: J.M., vol. 14, 1957. - (15) E. GOlJ~: M.M., vol. 82, No. 973, J u l t 1953. - - (1~) Fl~oST: Professional Motes 1~o. 107, Met. Office, London, 1953. - - (17) R. C. SUTCLIFr~ & BArCNON: Scientific Proceedings i n Meteorology, R o m e 1954. - - (is) JINKINSON: Technical Yqote No. 19 of t h e W.M.O., 1955. - - (1~) Climatological Normals: Met. Dept. Cairo.

(Received 29th April 1960)