0 AIAA-81-0386 Microbursts as an Aviation Wind Shear Hazard T. T. Fujita, University of Ch ica go , 111.
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AIAA 19th AEROSPACE SCIENCES MEETl'NG January12-15, 1981/St. Louis, Missouri
.for permission to copy or republish, contact the American Institute of Aeron1ut1cs and Astronautlc.s • 1290 Avenue of the Americas, New Yark, N.Y. 10104 .. NOTES •• MICROBURSTS AS AN AVIATION WIND SHEAR HAZARD T. Theodore Fujita* The University of Chicago Chicago, Illinois
Introduction Aircraft operations, for both comfort and ABSTRACT safety , are closely related to the weather situ ations along the flight path. High pressure Since the Eastern 66 accident in 1975 at regions {anticyclones) are generally dominated JFK International Airport, downburst-related by fine weather, while turbulent weather occurs accidents or near-miss cases of jet aircraft in or near the frontal zone. Nonetheless, we have been occurring at the rate of once or cannot always generalize these simple relation twice a year. A microburst with its field, ships irrespective of the scales of atmospheric comparable to the length of runways, could disturbances. induce a wind shear that endan9ers landing or l ift-off aircraft. Latest near-miss landing The high pressure vs. fine weather relation {go around) of a 727 aircraft at Atlanta, Ga., ship is no longer valid in mesoscale high on 22 August 1979 implied that some micro pressure systems, cal led the "pressure dome" by bursts are so smal l that they may not trigger Byers and Braham (1949). They also defined the the warnina device of the anemometer network "pressure nose" to be a sma ll mesoscale high installed and operated at major U.S. airports. pressure spot at the foot of an intense down A second microburst example, obtained during draft. the NIMROD experiment in the Chicago area on 29 May 1978, is discussed in detail. The The scale-dependent airflows in frontal nature of microburst and its possible zones are summarized in Table 1. Large scal e detection by Doppler radar are discussed. weather systems, highs, lows, and fronts were Finally, plans for future studies of srna ll discovered by Norwegian meteorologists back in scale microbu rst phenomena are given. the 1920s. Since then mesoscale highs were mapped extensively by the U.S. Thunderstorm Project in 1946 and 1947. Table 1. Two types of atmospheric disturbances, high pressure and front, which operate in three distinct scales . This t abl e shows that the hori zontal scale of air motion alters the disturbance characteristics signif i cantly. In general , the smaller the scale the larger the wind shear.
H I G H P R E S S U R E S Y S T E M S
Large-scale High Mesoscale High Small Mesohigh (1000 km or larger) {10 to 100 km) (less than 10 km) •Anticyclone • Pressure dome • Pressure nose e Di verging wind • Diverging outflow • Downburst outflow • Large-scale subs idence • Sinking motion • Descending current • Clockwise rotation • Straight-line outflow • Strai9ht-line outburst • Fine weather • Bad weather •Rain or virga
F R 0 N T A L Z 0 N E S
Large-scale Front Mesoscale Front Microscale Front (100 km or l onger) (10 to 100 km) (less than 10 km) •Warm or co ld front • Gust front •Outburst front • Converging wind • Converging flow • Converg ing f l ow e Rising motion • Upward current • Upward current e Bad weather •Rain or no rain • Rain or dust
*Professor of Meteorology
Copyrlghl © American lnslllule of Aeronaulics and Aslronaulirs, Inc., 1981. All rights reserved. Gust fronts, one of the most active meso scale fronts, were studied by Charba (1974), Goff (1975) (1976), and Lee et al. (1978) . These studies revealed that gust fronts are characterized by the converging flow and the upward current with or without rain during the frontal passage (Refer to FRONTAL ZONES in Table l ) . The downburst wind, occurring near the foot of a· strong downdraft, was first identified by Fujita (1976) and studied further by Fujita and Byers (1977), Fujita and Caracena (1977), Fujita LARGE MESOSCALE WEATHER SYSTEM (1978), and Caracena and Maier (1979). These f---100 km--J studies indicated that the downburst is charac terized by a pressure nose, descending current, straight-line outburst, with rain or virga (Refer Terre Haute to HIGH PRESSURE SYSTEMS in Table 1). , Most researches on gust fronts were initiated and performed by the National Severe Storms Laboratory based intensively on the NSSL tower data, while downburst researches were pur sued at the University of Chicago. It should be SMALL MESOSCALE WEATHER SYSTEM noted that gust fronts and downbursts are entirely different, both in scales and in their i---1okm--I ~"'- effects upon the penetrating aircraft. ~~O s"'- The entire area of an airport will be AIRPoRr <::,\:) affected by a passing gust front, while a down burst may affect only a small section of a large airport because of its small dimensions and short life. The point frequency of downbursts is sig " OUTBURST FRONT nificantly lower than those of gust fronts. This is, probably, why a large number of gust fronts were recorded by the NSSL tower while Fig. 1. High pressure areas and associated downburst winds have not been recorded. Further frontal zones in three different scales. more, the characteristics of recorded winds Large scale highs accompany fine weather should vary according to the relative location while mesoscale highs, bad weather. between the tower and the downburst center. In view of small dimensions and short life, the downburst research at the University of Chicago has been based on (1) the aerial photo graphy and mapping of the downburst damage, and Wind shear which affects a moving object, (2) the low-level, short-range scan of Doppler such as an aircraft, can be expressed by moving radar of the NIMROD (Northern Illinois Meteoro the coordinate system with the aircraft, thus logical Research on Downburst) Network. referring to the Lagrangian coordinates. The purpose of this paper is to define the We choose x in the horizontal direction of nature of the diverging and frontal winds in the ground velocity, G; y, toward the left of relation to their horizontal scales and to assess the aircraft; and z, upward. Let L be the the wind shear events likely to be experienced length measured in the direction of the ground during traverses (Figure l l. velocity. The variation of wind velocity at the aircraft is given by
~ = G ..fil!. + .llf... 8 I 9L 9 I ( 2) 2. Definition of Wind Shear where the second term on the right side denotes the local variation of the vector wind caused The meterological definition of wind shear either by the formation of a new wind system or is the "local variation of the wind vector or by an advection of a wind system into the flight any of its components in a given direction", path. according to the Glossary of Meteorology (1959). Noting that L is a function of x and z, we By expressing the wind vector with W and write its x, y, and z components with u, v, and w, d x respectively, we write dL° = COi a and :~ = ein a (3) w= i u + j v + kw (l) where a denotes the elevation anole of vector G, where i, j, and k are unit vectors pointing which is ± 3°when an aircraft flies along the toward x, y, and z, respectively. glides lope.
2 Table 2. Extreme values (maxima and minima) and the shear of component winds, u, v, and w at the aircraft flying toward the direction of positive u. The underlined values and shear could cause difficulties in maintain ing the nominal flight path.
(+) u (HEAD OR TAIL~IIND) ( -) TAILWIND HEADWIND
(+) Tailwind Increase (Ii!l£) Headwind Decrease (~)
8u (0) Maximum Tailwind (Tmax) Maximum Headwind (Hmax) 8t Minimum Tailwind (Tmin) Minimum Headwind (Hmin) (-) Tailwind Decrease (Tdec) Headwind Increase (Hine)
(+) v (CROSSWIND) (-) CROSSWIND FROM RIGHT CROSSWIND FROM LEFT (+) Right Crosswind Increase ( Ri nc) Left Crosswind Increase (Linc) ~ (0) Maximum Right Crosswind (Rmax) Maximum Left Crosswind (Lmax) 8t Minimum Right Crosswind (Rmin) Minimum Left Crosswind (Lmin) (-) Right Crossw ind Decrease (Rdec) Left Crosswind Decrease (Ldec)
(+) w (VERTICAL WIND) (-) UPDRAFT DOWNDRAFT ( +) Updraft Increase (Uinc) Downdraft Decrease (Odee) 8w (0) Maximum Updraft (Umax) Maximum Downdraft (~) 8t Minimum Updraft (Umin) Minimum Downdraft (Dmin) (-) Updraft Decrease (Udec) Downdraft Increase (D ine)
During the penetration of a typical gust Pu tting Eqs. (1) and (3) into Eq. (2), we front in Figure 2, tailwind changes into he adwind have regardless of the direction of the traverse. The rate of the tailwind increase and the headwind decrease is generally small, occurring during the ll'.. + j + (4) 81 A B k C approach and receding phases of the traverse. where
8u ·(au au au) A = 1 G-cosa+ G-slna + - (5) = -at ax e z a 1
8v ( av B = F· = J Ga;cosaav +Git 1lna +atav ) (6)
C h k(G 8wcosa +Gaw sina + ~) = 81 = ax az at ( 7) Tmox
Terms A, B, and C denote the variation of tail wind, crosswind, and vertical wind, respectively. u Component Wind % Quantities u, v, w, and A, B, C, can be I Hrna• either positive, zero, or negative, resulting in the 18 types.of wind shear presented in Table HEA6WIND 2. Of these, the underlined events could ca use 1 the loss of altitude when they are excessive either individually or when combined. Umax w Component Wind 3. Gust Front Gust front i s the frontal zone of advancing od•c DOWNDRAFT cold air characterized by a sudden increase in ! the windspeed followed by gusty winds. The source of the cold air with strong gusts is pre Fig. 2. Wind-shear events expected to dominantly thunderstorms which frequently form a occur during a traverse through a gust sq ua 11 l i ne. front.
3 At or near the frontal boundary, tailwind changes into headwind very rapidly. Vertical winds encountered are mostly upward unless an aircraft fl ·; ~s through a roll cloud (or roll) which ma· · r., 4. Downburst Tmax u Component Wind The tenn "downburst" was first used by Fujita (1976) in his analysis of the Eastern 66 accident in 1975 at JFK International Airport. It was defined as "A localized, intense downdraft Hmox with vertical current exceeding a downward speed HEADW I ND of 12 fps or 720 fpm at 300 ft above the sur l face . " This downward speed is comparable to the rate of descent of a jet aircraft during its w Component Wind final approach. ~~~~- ~ Fig. 4. Wind-shear events expected to occur during a traverse through a down burst. Subsequent studies showed that downbursts descend with precipitation which sometimes evap orates as "virga" before reaching the ground. Three photographs in Figure 3 show three stages of the downburst,which are descending, contact, and outburst stages. It should be noted that the downward current turns into an outburst current as it hits the surface. An aircraft which traverses a downburst near the surface could lose its altitude due to the low-level wind shear. A schematic drawing {Figure 4) presents a low-level traverse from left to right. If an aircraft flies through the center of a downburst, it will not be affected by the crosswind. Of a number of events experienced by the aircraft, increase of downdraft (Dine), decrease of head wind (Hdec), maximum downdraft (Dmax), and increase of tailwind (Tine), contribute to the loss of altitude. Dangerously, however, all of these events occur one after another or simul taneously resulting in a significant sinking which may or may not be arrested for a success ful fly out. Downburst is accompanied by changes in vector winds in a small area, which could affect aircraft operations seriously. The basic rule of the events is "the smaller the scale, the faster the time variations . " Fig. J. Three stages of downburst phenomenon. Therefore, the wind systems comparable to The descending stage (top) , contact stage glidepaths or runways are the inducer of dan (middle), and outburst stage (below). gerous wind shear. 4 4. Microburst On June 24, 1975, two Eastern flights were affected by two separate microbursts. Eastern 902 The horizontal dimensions of mesoscale dis flew through on the right side of a microburst, turbances corrmonly referred to in meteorology are experiencing right drift and serious wind shear 10 to 100 km . Squall lines, gust fronts, and (Figure 6) . large thunderstorms are examples of mesoscale weather systems. Studies since the JFK accident in 1965 re w km vealed that the typical scale of downbursts is /OOkm _J 100 EASTERN 902 (L-10111 BASED ON EXHI BIT 13-C I I Feet ' :... 700 '"" ,,,,,'l ' 600 I~ 1 500 I 1 FLE W INTO HEAVY RAIN ,fJ ,.. 40 0 I I PUSHED DOWN WHILE DRIFTING RIGHT I 300 IAS 120 kl, POWE R APPLI ED I , I I 200 100 OUTBUR ST ~.--~-,.----,---,-~---,-~-,-~----,~--,,-~c=ErNT~E=R--,-~-,..~-;r-<.._..--,-·~···~...... __,o 10,000' 8,000 6,000 4,000 21)00 ROckowoy O Runway Blvd. Fig. 6. Flight path of Eastern 902 on June 24, 1975, at JFK airport. Wind-shear events are shown in the right side figure. EASTERN 66 (727J BASED ON EXHIBIT 13-D I I Feet WINOSHlfLO WIPER ON I' 700 I I I I 600 I WIPER AT HIGH SPEED ,I I ... 500 C/) I~ a:: l~Q:- ;:) APPROACH1 LIGHTS VISIBLE I 4 00 en ,~w 0 ,:f a:: I 300 (..) ,'~(q.,,,. I I 2DD '·· ..• I 0~ , ~·UNwA.'f. ..v 1s1e LE I . ··... 100 I 1.4 sec, 1 N·1n 4.~ IMPACT 10,000' 8,000 6,000 2,000 Rdckowoy O Runwoy O Blvd. Fig. 7. Flight pa.th of Eastern 66 on June 24, 1975, at JFK airport. Wind-shear events are shown in the right side of the figure. 5 Eastern 66 flew through the center of the 5. Warm-sector Mi croburst second microburst. Headwind decrease (Hdec) coupl ed with the maximum downdraft (Dmax) was so When a microburst descends toward the ground, severe that it crashed 2,400 ft short of the run the condition of the low-level atmosphere affects way threshold (Figure 7). the airflow characteristics of the descending air. On August 22, 1979, the low-level wind shear Stationary Microburst alert system at Atlanta Airport did not trigger an alert. On its final approach to 27L, Eastern A schematic view in FigurelO depicts the 693 encountered a strong downflow which could be three stages of a microburst which keeps descend caused by a microburst in its contact stage ing over one particular spot on the earth. At (Figures 8 and 9~ first the downflow air deflects horizontally at high speed during the outburst stage. ATLANTA AIRPORT ) eLLWSAS ANEMOMETER ®NWS Atlanta Office As the cold air accumulates beneath the microburst, the cushion of cold air prevents the r-- ,/'------"'"""''·~orth successive downflow from hitting the ground to _)/ ~ ·,,, !< spread violently. During this cushion stage, f~-- ,,1 -,.\.... TE l>MINAL BLDG ''"\ , the extreme winds weaken and mi croburst becomes I l ' less dangerous. 1 Northwest 'ON A~?'o'lle ter \. _ _ / • 08 ° ',.:: 26 4 / I I .,., New TerminoJ I -: I Under Construction STAT IONARY MICROBURST I ., 311: East Sorr;est 09L • IC eJd ~ 27R. I 09Rc======Celitil==' ='='t===H1~2~7~L~-:o...::-~-=-;::-~· -~,~GL~•P£~AA~T~HI Contact Stage I o n.l'fti ',..1_,-----,, e south ' ,,,.,., VJ ~y ...... ,... ' - ...... ,,..,,,. 1-75 outburst stf!J)/ge / I ,I ' Fig, 8 . Distribution of the LI,WSAS anemometers at Atlanta Airport as of '', ' August 22, 1979. DESCENDING STAGE Fig , 10, Three stages of a s tationary fA6~ microburst. 1910 OM CONTACT STAGE ~ 19 I 2 .11<•J.-Ob 128 2000' -~ Traveling Microburst Role of dH~•nfi lJOtp./__ .,. .. ~{i~3l 1000' OL 4!52 _...... \,/ When a mic roburst travels, the descending current does not descend on to the cushion of the Ob 1742 cold air. Instead, the successive downflow - des cend on to the ground ahead of the cold air. Meanwhile, the cold air, being left behind, ac.ts li ke a slide, which deflects the downflow into a slanted flow (Fi gure 11). 3000' A traveling microburst on May 29, 1978, was ""i.<>.!.'- 1915 OLl742 __ _ 2000' depicted by a Doppler radar of NIMROD (Northern Rot• ot descenl, 27fp1_,,,-- IOOO' Illinois Meteorological Research On Downburst) . -- /- , DISSIPATING STAGE Obl28 -- ...... _ .. .. Against our initial expectation, the height of ion.mi the maximum wind was extremely low , less than 50 m above the ground. Fig. 9. Aircraft locations in relation to a microburst which descended between Figure 12 denotes the Doppler radar measured the outer mar ker and runway 271 u-component windspeed with its peak, 32 m/sec or Between 1910 and 1915 GMT, August 22, 62 kts. This peak wind wa s located 1300 m or 1979. 4300 ft ahead of the ce nter of the downflow. 6 Wind ve l ocities in the vertical plane MICROBURST (Figure 13 ) reveal that the downfl ow speed of \\~,,~~--)TRAVELING 17 m/ sec at 900 m above the ground increased into 32 m/ sec horizontal fl ow at 50 m (or less) above ''<\.\ ~ Contact Stage the ground. Such an acceleration was resulted by the squeezed outflow layer above the ground ahead of the cold air. 6. Cold-sector Microburs t Outburst Stage j Cold-sector mi crobursts descend beh ind cold fronts or gust fronts where the surface is covered with cold air. Figure 14 shows an example of the cold sector microburst which occurred near the O'Hare International Airport after the passage of a gust front. The gust front was more than 100 km long accompani ed by the warm-sector winds f rom south westerly directions . Temperature to the north of the gust front was 5°to 7°C colder. Fig, 11. Three s tages of a traveling microburst. The microburst descended throuqh the cushion of col d air caus ing an abrupt increase U-COMPONENT WINDSPEED and GROUND-RELATIVE FLOW 213604 - 213 703 CDT MAY 29, 1978 ~ m // 184 7- 1848 CDT JUNE 7, 1978 OOWHFLOW >.000 ~\.!:~ .. ,,. •oo I CE!lfEll w 10111/l u 10 "'"" ' ~ ~ · ~ • I ,)\,I ~._'\. ' oez ~ ' ) ' \ .- I _"\) .!.. flQ \ .• i ~- (1 , 194 1MICROfJUltST .., • ~~Z:- ~. · ~/ ~ ' Hore :!::''!', .' ~ 2,000 ~ ~ Co Id '].,. Secto r tjfa 1/.'\. ~~ .... ~ \ ' : 1 ,109 ...... ~~· 21~· ~ ~ "'\' ~~ - ~- ,, '-...~ '' v n. '-tt 1 \ • ' "' • • ~ " •oo .... ~o -~~·8:' ~c, ~' ~ 2,.,· 2'. '!;: _:_ ,,~ri~-<- 300 "--...... t_f) GI' "' ./- S ~<4.B ~• ------/26.I I 200 -...... 4 • _.,..,..,.. ~2 .• J W o r m Sec115 tor1 --, - o 100 ...... _.! 27.St ~ ....,28.2 / 1 /"c --· ~~.../I/750· • 20 Yorkville J . / J ?...... 1...... 'p___ 20._1 __lf.._ _ __,,,."' Fig, 12. Vertical cross section of u component windspeed of the May 29, 1978 micr oburst obtained by the Yorkville Fig, 14. A cold- srctor micr oburst det ected Doppler radar i n the NIMROD Network. 15 km t o the west of t he O'Hare Inter national Airport by the NIMR OD Doppler radar at O'Hare , GROUND-RELATIVE VELOCITIES 213604 - 21370 3 CDT MAY 29, 1978 JUNE: 7, 1978 m PAM-7 OOWHFLOW ~000" O'Hare Microburst 900 \ -,!5 15 17 15 15 • CE~TER 17 w IOrnlt u 10 intstc -.f.5 15 16 I ' 16 t5mlste OUTBURST \ j'7 16 FllOHT Gust Front \ 15 100 ... 15 15 I \ \ \ 16 j,, 16 V5 .. I • \ 15 15 16 17\ 2 \ \ i I vs i 16 ~5 " 16 500 15 15 !.5 19 18 1 15 ' \ \ i 16 16 ~HOSE i 15 15 15 18 zo 17 \ \ 16 I \ i 15 ,, 11 20 20 300 18 17 : I, 000 I \ '\ I 11 17 • 1 21 22 21 16 200 20 2 1 zz I \ '\ ! 18 28 29 28 25 zo / 100 ! 17 23 I I 21 30 32 30 25 1~/ 0 i A 8 C 0 E F G J K ...... Hm .~ 24 ...... 22 Fig. lJ. Vertical cross section of the Ai r Temperoture -- ~g t otal wind.speed of the micr oburst in Fig . 12. The maximum winds peed of 32 m/s is seen 1.3 km ahead of the down Fig, 15 . Wind variation at PAM stat ion flow center at 50 m AGL. Windspeed No . 7 r ecorded during the passage of both decreases t oward the outburst front . gust f r ont and microburst i n Fig. 14 7 .....-----,...-.,.....---.----,,...... ,.--.,..------. , ..M 1835- 1853 COT JUNE 7, 1978 800 400 Fig. 17. Two hypothetical traverses through the center of the microburst in Fig. 14 and 15. The u-component wind speeds were obtained by the NIMROD radar at O'Hare Fig. 16, Schematical cross sections of a cold sector microburst as it sinks into the cold air behind a gust front. The foregoing example suggests strongly that a ground-based network, no matter how exten in the windspeed measured by the anemometer of s~ve and dense their station distribution may be, PAM station No . 7. The station was affected by w1ll probably fail to detect the extreme winds two disturbances; first, by the ~ust front of the cold-sector microburst. The hiah winds on (mesoscale) and second, by the microburst (miso the ground (Fig. 17) occur where the downflow scale). The gust-front wind showed an abrupt hits the ground, away from the location of the increase followed by a gradual decrease while extreme winds aloft. the microburst wind lasted only two minutes with its 38 kt peak speed. The cold- sector microburst must sink into the cold air before reaching the surface. The descending current will first make contact with 7. Detection of Microbursts the top of the cold air moving toward the gust by Doppler Radar front. Then it interacts with the cold air, pushing forward and downward (Figure 16). Examples of wind systems presented in this paper indicate clearly that the microburst i s During the mature stage, a pocket of the the inducer of the wind shear whi ch causes the extreme wind will be located above the cushion loss of altitude at low levels. of the cold air. The vertical wind shear, vertical gradient of horizontal wind, beneath A network of surface anemometers are very the pocket of the extreme wind i s so large that effective in depicting advancing gust fronts surface anemometers may not register the extreme for warning purposes. It is unlikely, however, wind aloft. that microbursts can be detected by anemometers in time for alerting pi l ots to take proper Doppler radar measurements of the micro actions. burst in Figures 14 and 15 revealed that the extreme wind of 22 m/sec (43 kt) was located The short- lived , small scale microburst 250 m (820 ft) above the ground while a surface ca n be detected by one Doppler radar once we anemometer was measuring only 4 to 5 m/sec (8 to know the downburst signatures at various alti 10 kt) winds. tudes. The heights of .the maximum wind are estimated to be 10 to 50 m (30 to 100 ft) AGL Schematic penetration of two aircraft, the for warm-sector microbursts and 100 to 300 m one descending and the other ascending along the (300 to 1000 ft) AGL for cold-sector micro 3 glidepaths are presented in Figure 17. Both bursts. aircraft will experience a headwin d increase (Hine) followed by a dangerous headwind decrease It is essential to co nduct a basic-research (Hdec). Finally, the aircraft will fly through experiment in an around a major airport to obtain the region of the maximum downdraft (Dma x) 1 m/s Doppler signatures of all types of microbursts. to 5 m/s in the downflow speeds. Since the rate of descent and that of ascent are approximately Figure 18 describes the di stribution of 3 m/s, a combination of the wind-shear events Doppler radars and surface ~nemometers for con could result in a serious difficulty in main ducting such an experiment. Five location taining the altitude. identi fied with letters M1 through M5 are where microburst-related incidents occurred in and It should be emphasized that the surface around the five major airports at JFK, Denver, anemometer recorded the peak windspeed of 13 m/s Tucson, Philadelphia, and Atlanta during the four minutes after the passage of the extreme past five years. wind aloft. 8 References Byers, H.R . and R.R. Braham Jr. (1949) , The Thunderstorm. U.S . Government Printing Office, Washington D.C., 287 pp . Caracena, F. and M. Maier (1979) Analysis of a micr oburst in the FACE meteorological mesonetwork, Preprints, Eleventh Conf, on Severe Local Storms , Kansas City, Amer. Meteor. Soc. , 279-286 . Charba, J.P. (1974) Application of gravity current model to analysis of squall-line gust front, 6 OOPDltr Aodor YA0 1111 1nsondt • P AM Mon. Wea. Rev., 102, 140-156. O WtlOlt · s•r < W•de· ono lt MOY •t Comt,o 197f> - 79 Mttrobu'' '' Rtlotrwt to Run1111-ors M .1.r rK1, M 1100 11, M Jl , M.1PHLI, M t tA1 :..1 Fujita, T.T. (1976) Spearhead echo and downburst near the approach end of a John F . Kennedy Airport Fig, 18. A schematic view of a network for runway, New York City. SMRP Res . Paper No. 137, obtaining the signatures of microburst . A University of Chicago, 51 pp. Doppler radar (CP.-4) is placed in the air port ground to perform )60-deg scans for Fujita, T.T. (1978) Manual of downburst identifi detecting winds along the glide slopes . The cation. SMRP Res. Paper No. 156 , University of second Doppler (CP-3) and the third one Chicago, 104 pp. NTIS PB-286o48. (CP-2) will provide us with the data for performing dµal and triple Doppler analyses. Fujita, T.T. (1979) Objectives , operation , and Doppler-measured winds are compared with results of Project NIMROD. Preprints , Eleventh the surface winds measured by 27 PAM stat Conf. on Severe Local Storms, Kansas City, Amer, inons placed in and around the airport. Meteor . Soc. , 259-266. Fujita, T.T. and H.R . Byers (1977) Spearhead echo and downburst in the crash of an airliner. Mon. Wea. Rev., 105 , 129-146. 8. Conclusions Fujita , T.T. and F. Caracena (1977) An analysis of three weather-related aircraft accidents . Bull. Both gust fronts and microbursts are the Amer . Meteor, Soc ., 58 , 1164- 1181. inducers of the l ow-level wind shear which en dangers aircraft operations during the landing Goff, R.C. (1975) Thunderstorm outflow kinematics and takeoff phases. and dynamics . NOAA Technical Memorandum , ERL NSSL 75, 63 pp. Nevertheless, these two types of disturbances are quite different from each other in terms of Goff, R. C. (1976) Vertical structure of thunderstorm their airfl ow structure. Gust front is a meso outflows, Mon . Wea. Rev. , 104, 1429- 1440. scale frontal zone while a downburst accompanies a mesosca le high-pressure area. Glossary of Meteorology (1959) Amer. Meteor. Soc ., 638 pp. A gust front can be detected efficiently by a network of ground-based anemometers, while Lee, J.T. , J. Stokes , Y. Sasaki, and T. Baxter (1978) microbursts can hardly be detected by anemometers Thunderstorm gust fronts - observations and in time for an effective warning. However, a modeling, FAA-RD-78- 145 , 116 pp. single Doppler radar with proper display systems will be capable of monitoring both 9ust fronts a~d microbursts within 25 to 30 km range of an airport. In order to advance the basic knowledqe of the mic roburst phenomenon, aiming toward the desi9n of a fool-proof detection system, it is necessary to operate a fact-finding network of Doppler radar and ground-based anemometers . Acknowledgements This research has been supported by grants from NSF/ATM 79-21260, NOAA/NESS NA80AA-D:00001, and NASA NGR 14-001-008. 9 -- NOTES --