Characteristics of Microbursts in the Continental United States

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Wolison - Characteristics of Microbursts in the Continental United States

scale gust fronts - abrupt shifts in wind direction with corresponding increases in wind speed. Rather than encountering downbursts, they believed that the aircraft had encountered the turbulent leading edges of oufflows from large-scale storm systems and the strong, but unidirectional, horizontal winds just behind them. Part of this argument was based on de- tailed analyses of windfields in springtime tornadic storms and of 'squall lines in Oklahoma, in which no small-scale downdrafts were found [8,91. In his early papers [ 10,111, Fujita explained Fig. 1 - Downdrafts from a thunderstorm can be haz- the differences between downbursts and gust ardous to an airplane during a takeoff. (Redrawn from fronts, especiallywith regard to the wind shear Ref 2). hazard they posed for aviation. Nonetheless, skepticism of the microburst as a distinct phe- nomenon persisted. This skepticismpoints out on the aircraft), then it is a downburst [6]. the crucial importance of differentiating be- A few years after the downburst was defined, tween storm types that occur in different parts the term "microburst**was created to distin- of the country at different times of the year. It guish small downbursts (0.8 to 4.0 km in hori- also highlights the need for understanding the zontal scale) from larger ones [7].(No reason changes in surface wind shear patterns that based on aerodynamic principles or fluid dy- occur as these storms evolve. namics sets 4 km as the upper limit of a rnicro- The Federal Aviation Administration (FAA) burst, but the 0.8-km minimum does have a developed the anemometer-based Low Level fundamental basis. A downdraft smaller than Wind Shear Alert System (LLWAS)[ 121 in 1976, 0.8 km in horizontal scale is experienced by at the recommendation of NSSL scientists. The aircraft as turbulence.) An example of a thun- first LLWASs were installed at six airports in derstorm microburst is shown in Fig. 2. 1977; by 1982 over 50 more systems were in The introduction to the meteorological com- place, and by 1986 an additional 50 systems munity of the downburst concept met with had been installed. Before LLWAS, airportstyp- some controversy and resistance. Many scien- ically had only one wind-sensing device, located tists wondered if there was adifferencebetween either at the air traffic control tower or approx- the downburst and the thunderstorm down- imately centerfield, which was incapable of de- draft. Confusion still remains over what exactly tecting winds in the critical approach and de- the term describes; the observational studies parture comdors. NSSL personnel observed presented here show that several distinct that aircraft were sometimes brought in for phenomena can qualify. landing when there were tailwinds on the run- Despite resistance to a term that was devel- ways, rather than headwinds. This situation oped specifically to connote a hazard to avia- occurred when, for example, gust fronts were tion, the meteorological community and espe- moving across an airfield. cially researchers at the National Severe With an LLWAS, five additional anemometers Storms Laboratory (NSSL)were concerned with are located around an airport periphery. Their preventing accidents such as the one at J.F data plus the data from the centrally located Kennedy Airport. However, some scientists be- anemometer are transmitted to a computer, lieved that the wind shear-related aircraft acci- which evaluates wind differences between the dents attributed by Fujita to downbursts were outlying and centerfield sensors. Air traffic actually caused by aircraft penetration of larger- controllers then warn pilots about detected

The Lincoln Laboratory Journal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States wind shifts of high strength. The system was National Center for Atmospheric Research designed to detect the wind shear associated (NCAR) [ 131; and, most recently, project MIST with gust fronts, not the newly formed, highly (Microburstsand Severe Thunderstorms)near divergent oufflows (microbursts)hm thunder- Huntsville, Alabama in 1986 [14]. storms directly over the airport. Apparently it After both NIMROD and JAWS, the downbwst was felt that if a thunderstorm was present in was redefined to encompass newly observed the middle of the airport it would not be neces- phenomena. After NIMROD the downburst was sary to tell the air traffic controllers, since that redefined as "an outburst of damaging winds is where the control tower is located. on or near the ground [ 151, where "damaging However, Fujita remained convinced that winds" referred to winds of at least 18 mls. short-lived, highly divergent surface outflows Microbursts were simply small-scale wind from unusually strong, small-scaledowndrafts events of this magnitude. posed a serious threat to aviation. He directed During JAWS, many more microbursts were three research projects, in different parts of the found, so the emphasis was shifted according- country, using Doppler radars, instrumented ly. The microburst was redefined as having a aircraft, and a network of automatic weather "differential Doppler velocity across the diver- stations. They were project NIMROD (Northern gence center greater than or equal to 10 mls Illinois Meteorological Research on Down- and the initial distance between maximum bursts) near Chicago in 1978 [7];project JAWS approaching and receding centers less than or (JointAirport Weather Studies)near Denver in equal to 4 km" [ 161. This definition, now widely 1982 in cooperation with researchers from the accepted, encompasses weaker but still highly

Fig. 2 - Photographs of a wet microburst on I July 1978 near Wichita, Kansas, taken at 10-to 60-s intervals, looking south. A curling motion showing a vortex with a horizontal axis is visible near the left edge of the outburst flow. (Copyrighted photos by Mike Smith; reproduced from Ref 25.)

The Lincoln Labomtoy &umal, Volume 1, Number 1 (1988) Wolfbon - Characteristics of Microbursts in the Continental United States

0.5-1.0 km Humid Unsaturated Ground - Liquid Water Accumulates Precipitation Descends In Growing Cumulus Cell From Cloud, Evaporation Updraft Below Cloud Cools Air Fig. 3 - Two stages in the lifetime of an air mass thunderstorm cell that produces a microburst. The light blue color represents the visible cloud boundary; the darker blue color represents the radar return from the region of liquid water within the cloud. In the earlystage of development (left), the cell is filled throughout with rising air. Notice that the first precipitation radar returns form aloft. About 20 min later (right), updrafts and downdrafts coexist within the cell and rain falls. The microburst outflow is associated with the rain at the surface. divergent meteorological phenomena Although meteorologists for a number of years. These the rapidity with which microbursts develop include those found in gust fronts, warm and and their short duration were recognized as cold air mass fronts, [etc].... Most [of these] are significant parts of their hazard to aviation, predictable, sometimes hours in advance." none of these definitions incorporated time They also noted, "Scientists have recently constraints. begun to recognize the importance of storm Shortly after takeoff at New Orleans Interna- downdrafts that are unusually small in hori- tional Airport, Pan American World Airways zontal cross section and that are of short dura- Flight 759 crashed in July 1982, and all 149 tion. Such downdrafts have been called micro- persons on board and eight persons on the bursts." ground died [17,18].This crash, caused by a The final report of the committee made sev- microburst, gave anew impetus to the meteoro- eral recommendations for the detection and logical investigation of microbursts. ANational prediction of low-altitude wind shear. A key Academy of Sciences Committee for the Study recommendation was that the FAA "take im- of Low-Altitude Wind Shear and Its Hazard to mediate action to develop a pulsed Doppler Aviation was formed under the sponsorship of radar system that can be used to observe weath- the FAA. er conditions at and around airport terminals. The final report of that committee [ 191 stated* This terminal radar system should be able to "Some wind shears have been understood by operate with a high degree of automation and to

The Lincoln Labomtory Jownal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursl in the Continental United States provide information on low-altitudewind shear, a1 Weather Studies (FLOWS) Project in 1984. turbulence, and rainfall intensity." The FAA-Lincoln Laboratory transportable test- The committee also noted the inadequacy of bed radar (FG2)took data in Memphis, Tennes- the FAA's LLWAS system for detecting micro- see, from April through November, 1985. The bursts but, since the LLWAS was the only sys- radar moved to Huntsville, Alabama in 1986, tem available for wind shear detection, the where it was operated fromApri1through Octo- committee made the recommendation that ber of that year. "every effort should be made to assess and Memphis and Huntsville were chosen as the improve its performance." first two sites because of the high frequency of thunderstorms there, especially during the High-Quality Weather Radar Data summer, and because no Doppler radar data In 1982, MIT Lincoln Laboratory began to had previously been collected in that part of the develop a pulse Doppler weather radar testbed country. Microbursts were indeed found and data sets were collected suitable for use in an that could be used to detect hazardous weather-- - - in en route and terminal airspace [20,21].Many automatic microburst detection system. Most challenging technical issues have been ad- microbursts in Memphis and Huntsville were dressed in the course of developing aradar that caused by collapsing phase downdrafts of iso- can operate as a Terminal Doppler Weather Radar (TDWR). Since, to detect microburst outflows, the radar has to scan at low elevation angles, ground clutter returns must be filtered [22].Some clut- C ter will still be present after filtering (residue e from very strong returns or moving clutter),so an automated data editing procedure based on site-specific clutter maps is needed [23].Dop- pler velocity aliasing and range aliasing of dis- tant echoes can occur with the pulse Doppler system, algorithms for selection of the radar pulse repetition frequency [24]and for clean- Scale of Miles ing up the recorded data are required. And finally,the system must include algorithms for automaticallydetecting gust fronts and microburst wind shear hazards based on the Doppler radar measurements. This last task was especially Micult because microburst parent storm structurevaries, both across the country and with the time of year in a given part of the country. Moreover, almost no Doppler radar data (of sufficientquality for use in algorithm development)had been collected. The data sets that had been collected in research experiments like NIMROD and JAWS Scale of Miles spanned only one season of the year in one area, and the data collection strategies were inconsistent. Fig. 4 - a) Radar echoes on a day of random air mass To collect high-qualityDoppler weather radar thunderstorms. b) Radar echoes on a day of squall line thunderstorms. The radial lines and arcs indicate the data on thunderstorms, Lincoln Laboratory azimuths and ranges from the radar site. (Redrawn from started the FAA-Lincoln Laboratory Operation- Ref 37.)

The Lincoln Laboratory Journal, Vdume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

lated, air mass thunderstorms, and were ac- the wind shear thresholds for alarms [28]. companied by very heavy rain. As the Table Since the National Academy of Sciences Com- shows, these microburst storms appear to be mittee made its recommendations, microburst very similar to the ones that have caused nu- wind shear has caused one more aircraft acci- merous aircraft accidents [25,26]. dent - the crash of Delta 191 at Dallas/Ft. The FAA commissioned scientists at NCAR Worth in August 1985 [29,30].The FAA soon in 1984 to investigate how much a change of thereafter received funding from Congress to LLWAS wind-processing algorithms or network move forward with the operational TDWR pro- geometry could improve the system's ability to gram [31,321. detect microburst wind shear. NCAR found that The FG2 radar has been moved to Denver substantial improvements were possible - by where, during the 1987 microburst season, increasing the number of anemometers in the many excellent data sets with l-min surface array, by distinguishing between microburst update rates and coverage of upper level storm and gust front events (whichpose very different structure were gathered. These data are being types of aviation hazards), and by fine-tuning used to test and refine the TDWR microburst

l able - ~lrcraftAcciaents Attr~~utaole I Specifically to Microburst Wind SheaP Wind Diameter Rain Surface Location Date Speed Weather I (knots) fkm) Kano, Nigeria 24 Jun 1956 >20 3 Heavy Small-scale outflow cell 32/11 /2 Pago Pago, 30 Jan 1974 22-35 3 Heavy Heavy rain showers near airport 96/5/0 ~amoa nearby New Yoik 24 Jun 1975 22-35 5-10 Heavy Hot smoggy day, sea breeze. 112/12/0 light, moderate, & heavy rain numerous small cells, "spearhead" echo 8 X 32 km 1 Denver 7 Aug 1975 >25 2 Light Numerous scattered showers 0/15/119 small and weak, cell broke into 2, thunder heard, "spearhead" echo 8X16km Doha, Qatar 14 May 1976 28 6 Yes Thunderstorms of unknown 45/15/4 strength Philadelphia 23 Jun 1976 39 4 Yes Headwind increase in front of 0/86/20 shower; scattered showers & thunderstorms in cold sector near warm front, growing "spearhead" echo Tuscon 3 Jun 1977 28 2 None Numerous cumulonimbi around O/O/All airport; gust front passed earlier with 49 knots surface wind speed New Orleans 9 Jul 1982 >30 2 Heavy Scattered showers, 7 gust fronts 152/9/0 nearby, recent growth of convective cloud tops Dallas 2 Aug 1985 45-70 4 Very Scattered small cells initiated on 130/31/0 heavy gust front out of larger cell to northwest, very hot day, cloud top of microburst cell 23 Kft (ques- tionable: National Transportation Safety Board reported 40 to 50 Kft [271) I 'All accidents with fatalities occurred in or near thunderstorms with heavy rain. Modified from Ref. 25. L-3= fatalities. I = iniuries. U = uniniured.

The Lincoln Labomtory Journal, Volume 1. Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States recognition algorithm, developed at MIT Lin- acceleration equation is: coln Laboratory [33,34],and the TDWR gust front recognition algorithm, developed at NSSL [35],for the types of storms found in the Denver area. (Verticalacceleration equals thermal buoyancy Lincoln Laboratory, NSSL, and NCAR will be minus precipitation loading minus nonhydro- cooperating this summer in a real-time dem- static pressure gradient.) W is the vertical ve- onstration of the TDWR system at Denver's locity of an air parcel; t is time; g is the gravita- Stapleton International Airport with FG2 as tional acceleration;T is the temperature of the the primary sensor. The demonstration will environment; AT is the temperature difference involve providing low-altitude wind shear in- between the air parcel and the environment; 2 formation to air traffic controllers on detected and i are the mass mixing ratios (kilograms of microbursts and gust fronts that threaten to water per kilogram of air) of liquid water and impact airport operations. A new enhanced ice, respectively, in the air parcel; z is the verti- LLWAS system, which has greatly improved cal coordinate; P' is the pressure perturbation performance over the original system, will also from a hydrostatic basic state; and po is the be operating during the TDWR demonstration. basic state density. The first term on the right side of Eq. 1shows that, if a thunderstorm air parcel is colder than the ambient air, the thermal buoyancy is nega- STRENGTH OF THE THUNDERSTORM tive and the acceleration is downward. The OUTFLOW second term shows that any amount of precipitation acts to accelerate the thunderstorm air Before reviewing meteorological studies on parcel downward. The third term shows that, microbursts produced by thunderstorms, it is when the perturbation pressure increases with instructive to examine the factors that affect height, the force on the thunderstorm air par- the speed of thunderstorm oufflows. The mate- cels is directed downward. This term becomes rial in this section provides a simple mathe- large only in very unusual situations - such matical framework that is helpful in under- as occur near a tornado. standing the emprical evidence presented in These forces contribute to the vertical accel- the remainder of this article. eration that, over time, builds the speed of the Two factors primarily determine the speed downdraft. The horizontal oufflow of the micro- of thunderstorm oufflows: the speed of the burst forms when the downdraft impacts the downdraft that impacts the surface and spreads surface. horizontally (with roughly the same speed as Because water phase changes occur, the first the downdraft),and the temperature of the out- two terms on the right side of Eq. 1 are not flow air. Even if the downdraft air reaches the independent. Ice may melt, with an associated surface with essentially zero velocity it will cooling due to the latent heat of fusion, as the spread horizontally, as a gravity current, if it is air parcel moves vertically through the freezing colder than the environment. Other factors that level. Liquid water may evaporate, with an even also influence the strength of the thunder- greater associated cooling due to the latent storm oufflow include the oufflow depth, which heat of vaporization, when it comes into con- is influenced in part by storm geometry (eg, tact with unsaturated air. Evaporation occurs, linear vs circular),and the horizontal momen- for example, when dry midlevel air is entrained tum of the air that originally feeds the down- into a thunderstorm or when rain falls into the draft. unsaturated air below cloud base. The connec- The first factor, the speed of the downdraft, tion between thermal buoyancy and precipita- depends on the forces that accelerate the down- tion loading provides a way to evaluate their draft air vertically. The approximate vertical relative effects on the downdraft [36].

The Lincoln Laboratory Journal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

Descending Microburst

Fig. 5 - Four stages of microburst at Andrews Air Force Base. Stage 1 (descending): midair microburst descends. Stage 2 (contact): microburst hits the ground. Stage 3 (mature): stretching of the ring vortex intensifies the surface wind speeds. Stage 4 (breakup): runaway vortex rolls induce burst swaths. (Redrawn from Ref 41.)

If a liquid water mixing ratio 1 is evaporated, very efficient way to create strong downdrafts. the temperature deficit is A similar forcing occurs when ice melts, but this effect is proportionately smaller because of the smaller associated latent heat. The second factor influencingthe strength of where L, is the latent heat of vaporization of the thunderstorm oufflow is the difference in water and Cp is the specific heat of air at con- temperature between the oufflow air and the stant pressure. The negative buoyancy, AT/T, ambient surface air. This can be expressed by is roughly equal to 10 1. Therefore,when evapo- the equation for the propagation speed of shal- ration occurs, the downward acceleration due low density currents: to the weight of the water is replaced by a downward acceleration due to the colder air that is ten times larger! This result shows that evaporating rain is a where V is the speed of the leading edge of the

The Lincoln Labomtory cFownaL, Volume 1, Number 1 (1 988) Wolfson - Characteristics of Microburst. in the Continental United States density current, AT is now the temperature dif- Air Mass Thunderstorms ference between the environment and the density current, h is the outflow depth, and k is the One of the first parent cell types to be asso- internal Froude number (ratio of the inertial ciated with microbursts was the isolated cumu- force to the force of gravity). The Froude num- lonimbus cloud. Although called simply "air ber is greater than 1initially, depending mainly mass thunderstorms" at the time (1949),Byers on the downdraft speed, but with time tends and Braham [37]measured very strong, small- toward a value somewhat less than 1 (-- 0.77). scale divergent surface oufflows that would Aside from any horizontal momentum de- today be classified as microbursts (eg, "When rived from the vertical velocity, Eq. 2 shows the cold downdraft of a cell reaches the surface that the deeper and colder the oufflow, the layers of the atmosphere, it spreads out in a higher the speed at which it will spread. The cooling of thunderstorm air is basically due to water phase changes. Thus if evaporation be- gins in a rainshaft as it fallsbelow cloud base, it PPI 2 will cool the already downward moving air. But the resulting negative buoyancy force may not act long enough to increase the vertical velocity substantially (cloudbase is often only 1km agl). Nonetheless, as shown by Eq. 2, the outflow strength can still be augmented by the cooling.

OBSERVATIONAL STUDIES OF MICROBURSTS FLOWS FL-2 A rapidly growing number of meteorological ,09/218'86 19:02:51 studies on downbursts and microbursts have PPI 2 EL 0.0 been performed since Fujita developed this term in 1976. In studies both before and after 1976, authors occasionally described damaging wind phenomena without specifically dis- cussing the hazard to aviation. This section categorizes those studies, as well as studies specifically of downbursts and microbursts, into four meteorologically distinct types: air mass thunderstorms; bow echo downbursts; shallow, high-based cumulonimbus clouds; and rnicroburst lines. This categorization is a prerequisite to achiev- Fig. 6 - FL-2 PPI scan (at 0.O0 elevation) of a microburst ing two goals: the discovery of exactly what storm in Huntsville, Alabama on 2 1 September 1986 at conditions or dynamical interactions lead to 19:02:51 GMT (Greenwich mean time). Top: Radar re- the development of unusually strong down- flectivity field, which measures the amount ofprecipita- drafts and/or the development of unusually tion. Bottom: Doppler velocity field. The Doppler velocity is negative toward the radar and positive away from the small-scale, high speed oufflows; and the de- radar in the radial direction. The labelled azimuth line, at velopment of automated algorithms (eg,for use 24 1O, passes directly through the center of the micro- in the TDWR) that use this base of knowledge burst. A "classical"microburst divergent outflow signature (dipole pattern of 12 m/s flow away from the radar for accurate early detections,and perhaps even- and 8 to 10 m/s flow toward the radar) is visible between tually predictions, of microburst wind shear. the 5-km and 15-km range rings.

The Lincoln Laboratory Journal, Volume 1. Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

FLOWS FL-2 098'2 146 18:54:27 RHI 1 A2 241.8 DZ dBZ 58 1 45 40 1 35 38 1 25 1 28 15 10 m 5 - 8 rn ae -5 FLOWS FL-2 0%.*21#*86 19.00:84 EHI 1 7 241.8 d BZ : 5s 1; 50 1 4s 1 40 1 35 1 38 1 29 28 m 15 = 10 m S rn 8 = -5 FLOWS FL-2 098'218'86 19: 03: 49 RHI 1 AZ 241.0 DZ d BZ 55 58 45 40 3s 30 25 28 '!1s

Fig. 7 - Time sequence of FL-2 RHI scans (at 24 7 O azimuth) through a microburst storm in Huntsville, Alabama on 2 1 September 1986. Above: Radar reflectivity fields. The background grid is Right: Doppler velocity fields, contoured at 3 m/s inter- labelled in kilometers. The core of high reflectivity (light vals, in the lowest 1 km. The background grid spaces are blue) drops in altitude from the first to the second panel; 0.1 km in the vertical by 0.5 km in the horizontal. Nega- in the third panel, it has dropped still farther (the micro- tive velocities represent flow towards the radar andpos- burst outflow is strongest here). The upper level reflec- itive velocities, away. The outflow is stronger awa y from tivity has also decreased markedly in the third panel. the radar than toward it because the microburst fell into a preexisting outflow that was moving away from the radar. In the secondpanel, vortices are set up in advance of, and at the leading edge of, the outflow. In the third panel, the outflow has become thinner (200 m deep), broader, and has increased in speed; the highest speed winds were at the lowest sampled altitude. The tran- sient vortices have dissipated, leaving the microburst outflow and one vortex at the leading (outbound) edge of the outflow pool.

The Lincoln Laboratory Journal, Volume 1, Number 1 (1 988) Wolfaon - Characteristics of Microbursts in the Continental United States

dge of eeexinin'g Outflow 1.o

nE Y, E 0.5 I-i!

0 6 7 11 - 13 16 17 19 21 Range (km)

The Lincoln Lctbomtory &uma& Volume 1, Nurnber 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

Sudbury, Massachusetts, collected Doppler radar data of awindstorm in which a "brief phe- nomenon" associated with heavy rain caused straight-linewind damage "confined to a region less than 1.5 square miles in area" [37].Radar operators failed to recognize the damage potential since no characteristic severe storm radar signature was present, such as the famous "hook" echo (created as raindrops are drawn into a tornadic thunderstorm circulation). A subsequent examination of the data showed a disorganized multicell air mass storm with one large, tall cell and a weak echo region at the surface in the area of highest winds. An analysis of a dual microburst event that (s) Time occurred in the Florida Area Cumulus Experi- ment (FACE)weather station network revealed Fig. 8 - Aircraft-measured vertical velocity of the air (red) and precipitation water content (blue) are plotted that the cell that produced the microbursts for one pass through a microburst storm on 10 August was, again, one of the tallest within a disorga- 1985 in Memphis, Tennessee. Aircraft altitude was nized multicell storm complex [38]; it was 0.66 km. (Redrawn from Ref 45.) forced more vigorously at the surface in the convergence zone of two colliding outflow fashion similar to that of a fluid jet striking a boundaries. The "spearhead" shape of the ra- flat plate"). dar echo was attributed to rapid growth of new Figure 2 shows an air mass thunderstorm cells on the advancing edge of a storm. (The that isjust reaching the mature stage - rain is microburst that caused the crash of Eastern 66 beginning to fall from the base. Figure 3 sche- at J.F. Kennedy Airport in 1975 also came from matically illustrates this stage, as well as an a spearhead-shaped echo.) The microbursts, earlier stage in the development of such a cell. lasting less than 5 min, were associated with Air mass thunderstorms usually form in the heavy rain and embedded in a storm-scale afternoon, in calm, hot, humid air masses; there downdraft that continued for over 30 min. Care- is little or no vertical shear of the horizontal ful analysis of the synoptic-scale (ahorizontal wind. These thunderstorms can occur in most size of roughly 500 to 2,500 km)situation re- parts of the country during the "heat waves" of vealed conditions favorable for thunderstorm the summer months. development, as well as very dry air at midlevels Air mass storms, which form as randomly in the atmosphere. scattered thundershowers, are distinct from Although these storms had 30 mls surface "squall lines" or "frontal" thunderstorms (dis- oufflow speeds, a downdraft speed estimation cussed later),which appear in organized linear technique [40]predicted gusts of less than 19 patterns (Fig. 4). Considering the number of mls. Additional sources of negative buoyancy fatalities that have occurred in accidents relat- were proposed [39]: the melting of large quan- ed to microburst wind shear, air mass storms tities of ice; efficient entrainment of dry mid- are the most hazardous form of low-altitude level air into the downdraft without mixing with wind shear. Therefore, the primary research updraft air; and precipitation loading, although question is how to distinguish, in advance, air the observed precipitation rates were too low to mass thunderstorms that will produce violent account for the discrepancy. oufflows from those that will produce oufflows None of these factors completely explained of ordinary strength. the large difference between probable down- The Air Force Geophysics Laboratory in draft speeds and observed oufflow speeds.

The Lincoln Labomtory Journal.,Volume I, Number 1 (1988) Wolt.on - Characteristics of Microbursts in the Continental United States

Through analysis of a microburst that caused Figure 6 shows a plan-positionindicator (PPI) damage at Andrews Air Force Base, through Doppler-radar scan taken during the maximum visual and multiple Doppler observations of oufflow of the Huntsville event. In a PPI radar JAWS microbursts, and through laboratory scan, the antenna elevation angle is fixed and simulations with cold descending air currents, the azimuth angle is varied. Figure 7 shows a Fujita showed that a well-defined rotor existed time sequence of range-height indicator (RHI) at the leading edge of microburst oufflows, scans through this event. In an RHI scan, the which could explain the measured wind speeds antenna azimuth angle is fixed and the eleva- [411* tion angle is varied. The sequence of photographs in Fig. 2 shows The time of the last RHI in Fig. 7 corresponds the development of a horizontal vortex at the as closely as possible to that of the PPI shown oufflow edge. Fujita hypothesized that, through in Fig. 6. The small vortices that developed vortex-tube stretching and the resultant "spin- when the microburst formed dissipated rapidly, up" at the leading edge of an expanding oufflow, leaving the largest, fastest wave traveling out- a weak or moderate downdraft could produce ward at the head of the oufflow current. The strong surface winds, which would appear in presence of a well-developed leading oufflow small patches along the oufflow boundary as wave was the rule, rather than the exception, the vortex tube separated (Fig. 5).He suggested for microbursts observed in Memphis and that embedded vortices in an oufflow pose an Huntsville. additional wind shear threat to aviation, and A key radar-detectable precursor of a micro- that the microburst-related crash of Delta 191 burst oufflow is a descending reflectivity core at DallasIFt. Worth may have been caused by in a collapsing thunderstorm cell [42,43].This the downward motion on the backside of one of effect can be seen in the sequence of RHIs these vortices [29]. shown in Fig. 7. The descending reflectivity The conditions that encourage the develop- core, together with the very high rainfall rates ment of high-speed horizontal vortex rolls and and radar reflectivity levels observed in these how often these conditions occur are unknown. storms, suggests that precipitation loading In an air mass thunderstorm microburst ob- plays a central role in forcing the intense served during the FLOWS Project with the FL2 downward vertical acceleration. radar (Huntsville, Alabama, 21 September Analyses of surface weather station data 1986), horizontal vortices were excited in a [44] collected during the FLOWS project in preexisting oufflow pool when fresh oufflow Memphis show a significant correlation be- from a newly forming microburst impacted the tween surface rainfall rate, which was extreme- surface. ly heavy at times, and the strength of the peak

Tall Echo Bow Echo Stage Comma Echo Stage Rotating \ L

Evolution of Bow Echo

Fig. 9 - Evolution of bow echo proposed by Fujita. In this model a downburst thunderstorm produces a bow echo as the downflow cascades to the ground. The horizontal flow of a weakening downburst induces a mesoscale circulation, which distorts the initial line echo into a comma-shaped echo with a rotating head. (Redrawn from Ref 53.)

The Lincoln Labomtory JoumaL Volume 1, Number 1 (1988) Wolibon - Characteristics of Microbursts in the Continental United States 7

Fig. 10 -A large thunderstorm, which is producing an outflow. The base of the cloud shows some structure that indicates storm rotation, such as occurs before a tornado forms. (Copyrighted photo by A. & J. Verkaik.)

microburst outflow winds. In nearly every case, 6 g/m3 was slightly less than the contribution however, the outflow current was significantly from the observed temperature deficit of 2.3OC colder than the surface air that it was displac- (42%due to water loading and 58%to tempera- ing. The cold temperature of the outflow cur- ture deficit). rent indicates that evaporation, and to some Even if dry air is entrained into the precipita- degree melting, must contribute to the negative tion core at high levels, little evaporative cool- buoyancy and the resulting outflow speeds. ing can occur because the air is too cold. In fact, The analyses show that the peak microburst the temperature deviation in the downdraft may outflow speeds are also comlatedwiththe tem- actually be positive above the freezing level perature deficit of the outflow. [46],because the cooling from the sublimation The role of precipitation loading in forcing a of hail is too small to compensate for the ef- microburst was investigated during a Memphis fects of compressional heating. As the core de- storm. In this experiment, an airplane measured scends, the effects of evaporative cooling be- cloud liquid water content and vertical velocity. come much more important. In every pass through the storm "the strong At upper levels in the region of liquid (frozen) downdrafts were found in close association water accumulation, precipitation loading is with the areas of heavy precipitation loading" the dominant forcing mechanism that initiates [45],but the correlation between vertical veloc- the the collapse of the cell. However, cooling ity and liquid water content was by no means due to water phase changes during the descent perfect (Fig. 8).At the flight altitude within the of the core also plays a significant role in pro- storm (660 m agl), the negative buoyancy con- viding the forcing that produces the extraordi- tribution from a mean liquid water content of nary outflow speeds of the few cells that pro-

The Lincoln Labomtory &wnal. Volume 1. Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States duce microbursts. Examples have been pub- the leading edge and within the microburst out- lished [47]of visually impressive, high-reflec- flow occur commonly and are associated with tivity (> 60 dBz) thunderstorm rainshafts that very strong surface winds. produced only weak outflows. Among the aircraft accidents attributed to Significant evaporation can take place with- microburst wind shear, the greatest number of out altering the general appearance of a radar fatalities have occurred during those in which echo, which makes it Mcult to use radar data heavy rain was present. In some cases, the rain to determine whether negative thermal buoy- was so heavy that it may have caused the aero- ancy is forcing adowndraft. The smallest drops dynamic performance of the aircraft to deterio- evaporate first and most efficiently, but they rate, which would have compounded the prob- contribute relatively little to the reflectivity, lem caused by the wind shear [48-501. which is proportional to the sixth power of the An investigation of the microburst that raindrop diameter. Also, because reilectivity caused the crash of Eastern 66 at J.F. Kennedy measurements are displayed on a log scale, the Airport showed that the wind shear spectrum reduction in liquid water content (ie,the reduc- contained high energy at the aircraft9sreso- tion in downward forcing from precipitation nant frequency [51].By producing sudden os- loading) associated with a reduction in radar reflectivity of 55 to 50 dBz is almost six times as great as the reduction in liquid water content associated with the change from 40 to 35 dBz. In summary, air mass thunderstorms with very strong collapsing phase downdrafts and subsequent oufflows are microbursts. Condi- tions conducive to the development of air mass thunderstorms occur in most parts of the country during the summer months. But not every air mass thunderstorm cell produces an oufflow that is strong enough to be a microburst. In essentially every case, these storms are characterized by very heavy rainfall concentrated in an area of small horizontal extent and by large decreases in temperature at the surface. It is possible that the presence of dry air at midlevels in the atmosphere is required to per- mit enough evaporation to occur to sufficiently cool the downdraft and oufflow air. The convection that creates the microbursts is often initiated by convergence at the edge of older outflows, so microburst surface flow patterns are often embedded in larger storm outflows. Thus the microburst-inducing convection often appears in the form of multicell storms. Storms with overshooting tops have greater Fig. 1 1 - Damage pattern left in a pine forest after a energy levels than other storms. Furthermore, tornado moved through the area. Tornado damage is their cores contain more ice, which adds to the shown by the swath of missing trees in the upper portion of the picture; a microburst knocked down the trees in negative buoyancy as the downdrafts pass the lower part of the picture. (Photo courtesy of T.T. through the freezing melting level. Vortices at Fujita, The University of Chicago.)

The Lincoln Labomtory Journal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

Fig. 12 - Virga descending from the base of benign-looking cumulus cloud. This virga shaft indicates a small-scale downdraft that could produce a microburst if it impacts the surface. (Copyrighted photo by A. & J. Verkaik.)

cillations in airspeed and height about the or in the "rotating head" [54].Figure 10 shows a glideslope, this resonance may have seriously photograph of a downburst-producing cell. deteriorated the aircraft's performance, addi- The bow-shaped echo is generally part of a tionally compounding the problem caused by synoptic-scale squall line [55,56],a mesoscale the wind shear. (horizontalsize of roughly 25 to 500 km)linear Air mass thunderstorms produce the most echo configuration or cluster [57-591, or a dangerous type of microbursts. These storms combination of supercell and weaker storms combine a deadly set of factors: frequent oc- [60,611. currence; highly divergent outflows with em- Satellite analyses have shown general cloud bedded vortices; small, insignificant-looking top warming before a downburst forms, indicat- cells that produce microbursts; and very heavy ing collapse of the cell [ 151. A hole may appear rain. atthe edge of the echo at midlevels around 5 km [7].In general, this reflectivity "notch" appears Bow Echoes and Downbursts on the upwind (at midlevels) side of the storm Another type of echo with which downbursts system. are associated was identified by Fujita [52]in A downburst-producingbow echo storm that 1978 as the "bow" echo. This type of storm developed in southeastern Kansas was studied takes the shape of a "spearhead echo" during [59] with airborne Doppler radar data, taken its strong downburst stage and sometimes de- near the weak echo region of the bow -just velops a "weak echo channel" at low levels in after damaging surface winds had occurred. the area of strongest winds (Fig. 9). Tornadoes Negative buoyancy created by melting and evap- sometimes develop on the cyclonic-shear(coun- oration in the lowest 2 to 3 km of the storm terclockwise flow) side of the area of high winds caused strong downward acceleration in the

The Lincoln Labomtory Jownal Volume 1, Number 1 (1 988) Wolfaon - Characteristics of Microbursts in the Continental United States large stratiform rain region behind the bow. A from, the tornado damage path (wherethe trees study of a similar storm [61]showed a strong are actually missing). This pattern was caused inflow from the rear of the storm directly into by a small-scale downdraft, which is thought to the vertex of the "bow" at 5 km,apparently in be essentially the same as the "occlusion" response to this type of large-scale downdraft. downdraft found in a high-resolution numerical The downdraft generated a strong low-level out- model of the tornadic region in a supercell flow, which reached damaging speeds when storm [62]. smaller-scale embedded downdrafts of only Organized downburst storms occur through- moderate intensity were superimposed. out the continental United States at times of Damage surveys [ 15,52,58] revealed that the year when synoptic-scale instabilities and small microbursts and tornadoes, twisting frontal storms dominate the weather patterns. downbursts, and other rotational and divergent In the central part of the country, these storms wind patterns coincidentally occurred. This led typically occur in the spring and fall; farther to the hypothesis that the vertical pressure north, they appear in the early and late summer. gradient set up by strong rotation at low levels In summary, bow echo storms develop in can dynamically force a small-scale downdraft environments characterized by moderate ver- or microburst 1561. tical shear of the horizontal wind, instability or Figure 11 shows the damage caused to a conditional instability, and abundant moisture. forest of pine trees when a tornado moved In the cases analyzed, a layer of dry air was through the area. Notice the small burst pat- found at midlevels. A bow echo storm is gener- tern of flattened trees close to, but distinct ally part of a larger mesoscale or synoptic-scale

Fig. 13 - A ring of dust generated by the outburst winds of a microburst 27 km east of Stapleton International Airport on 14 July 1982. (Reproduced from Ref 25.)

The Lincoln Laborntory rfoumal, Volume 1, Number 1 (1988) Above Dry Ground Subcloud Level Air

Rain Falls Mixing With Dry Negatively Buoyant From Cloud Subcloud Air Air Accelerates with High Base Causes Evaporation Downward and Cooling Little or No Rain Reaches Ground

Fig. 14 - Various stages in the development of a microburst from a shallow, high-based cumulus cloud. Little or no rain is reaching the ground. This type of parent cloud and the microbursts it produces are common in the Denver area during the summer. storm complex or frontal line storm, has high Thus even though these storms are hazardous radar reflectivity levels (at least 50 dBz), pro- to aviation, the hazardous regions are predict- duces downbursts that are quite large (typi- able and avoidable with currently available cally 20 km or more across),and often contains meteorological information. embedded microbursts and tornadoes. The large-scale downdraft is driven by the Shallow, High-based Cumulonimbus cooling due to evaporation and melting as dry Clouds environmental air enters a storm. This process A great deal of attention has been given to leads to the formation of the weak echo regions microbursts that originate from benign-look- behind the bow. The downward flux of horizon- ing, high-based (3to 4 km agl), shallow (2 to 3 tal momentum from midlevels is also impor- km deep) cumulus congestus or stratocumu- tant in accountingfor high surface wind speeds lus clouds. One of these clouds is shown in Fig. in some cases. Smaller embedded microbursts 12. can be produced in a variety of ways. These clouds often have glaciated tops and In general, these storms are long-lived and lack the rapidly rising convective towers, thun- have fairly predictable paths. Moreover, their der, and lightning of typical lower-based cumu- appearance is sufficientlythreatening that air- lonimbus clouds [63], although some small craft rarely, if ever, try to fly through them. convective turrets occasionally appear [64].

The Lincoln Labomtory Jownal Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

Virga (wisps of precipitation that evaporate be- An example of FG2 radar data collected dur- fore reaching the ground) is commonly visible ing one of these dry microbursts is shown in below cloud base but little or no rain reaches Fig. 15. The maximum reflectivity in the cell is the ground [65].Therefore, these microbursts only 20 dBz, yet the differential velocity in the are called "dry" or "virga" microbursts. oufflow (20 mls over 3 km)is quite strong. The Figure 13shows a ring of dust "kicked up" by evolution of the surface-flow field typical of a microburst oufflow with no visible rainshaft nearly all microbursts observed during JAWS feeding it. A schematic illustration of this type is schematically illustrated in Fig. 16. of dry microburst-producing cloud is given in Fig. 14. A 1952 paper briefly mentioned the "dry thunderstormover the plateau area of the Unit- FLOWS FL-2 ed States" [66].This type of storm was charac- 0-12.47 21 :37:59 terized in a 1954 paper with, in retrospect, PPI 1 EL d.S amazing accuracy 1671. The dry microburst was P z dB2 also documented [68],where it was noted that 65 60 the storm's damaging oufflow qualified as a I 55 I 50 downburst. The 1982 paper correctly predicted I - 45 I 48 (asthe JAWS investigators quickly confirmed) I 35 I 30 I 25 that this type of storm is much more common 20 I 15 than was generally recognized at the time. I 10 The characteristics of the environment in L 5 which this type of microburst forms have been successfully described. Studies [63,69]show that a deep, dry subcloud layer (dew point depression greater than 30°C) with a nearly dry FLOW FL-2 adiabatic (neutrally stable) lapse rate is com- 06/ 128.*87 21 z37.59 mon. A moist layer around the 500-mbar level PPI 1 EL 8.5 (5.6 km msl [mean sea level]) nearly always Y occurs. Winds typically have a strong westerly component and increase with height. A simple rule was discovered that predicted the days on which dry microbursts would occur [69].On 26 of the 30 days that had dry microbursts, the dew point depression (the difference between the temperature and the dew point temperature)was greater than 8OC at 700 mbar (3.0 km msl) and less than 8OC at 500 mbar. Radar and flow characteristics of this type of storm have been documented [ 16,25,70-731and Fig. 15 - Reflectivity and Doppler velocity PPI data of a summarized [74].These microbursts all formed "dry" microburst collected with the FL-2 radar in Den- between 1300 and 1900 MDT (mountain day- ver, Colorado on 12 June 1987 at 2 1:37:59 GMT. The light time) with 75% occumng between 1400 high reflectivity regions associated with regions of 0 m/s Doppler velocity are ground-clutter targets that and 1700 MDT. Reflectivity values were always remained in the data after filtering. The microburst, less that 30 dBz at 500 m agl, in stark contrast enclosed by the red rectangle, has a maximum reflectiv- to the high reflectivity values (50 to 60 dBz) ity of 20 dBz. The Doppler velocity field shows flow of roughly 10 m/s toward the radar, and 10 m/s away found at the surface in air mass thunderstorm from the radar, giving a differential velocity of 20 m/s microbursts. over a distance of about 3 km in the microburst.

The Lincoln Laboratory Jownal,Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

-Scale (km) Fig. 16 - Vertical cross section of the evolution of a microburst wind field, based on Denver area data. T is the time of initial divergence at the surface. The shading refers to the vector wind speeds. (Taken from Ref 16.)

Observations based on all microbursts in speed is linearly proportional to the initial ra- JAWS (approximatelyhalf were associated with dius of the rainwater region. virga or light rain) show that there is no correla- This same model was used to study the role tion between radar reflectivity or surface rain- of ice-phase microphysics in determining the fall rate and the subsequent strength of the downdraft and oufflow strength of dry micro- oufflow [75]. Rainfall rates never exceeded bursts [77]. Experiments were performed in 3 in/h, and on only 6 days was the rainfall rate which the precipitation dropped at the top of associated with microbursts above 1in/h. The the model consisted of either rain, graupel strong surface oufflow winds typically lasted (granularsnow pellets or soft, spongy hail), or from 2 to 5 min, with speeds between 10 m/s snow at each of three cloud base precipitation and 20 m/s. The surface temperature was just rates. as likely to rise as to fall [25],and by as much as Greater amounts of precipitation were found 3OC. to be linked to stronger downdrafts and surface It has been hypothesized that the combina- oufflows. These variations were much larger tion of the deep, dry, neutrally stable subcloud than those attributable to the different forms of layer, which permits cold air near cloud base to precipitation with the same water content. continue to accelerate all the way to the sur- However, for a given precipitation rate, rain face, and the weak updrafts, which produce generally produced the strongest downdraft small precipitation particles that evaporate and and graupel produced the coldest, strongest melt efficiently, allows the very strong down- surface oufflow. drafts to form [68]. Simple one-dimensional To compensate for the descending air in a [36]and two-dimensional [76]numerical mod- microburst, convergence must develop at or els have confirmed this hypothesis. above the downdraft initiation level. The down- The two-dimensional (&symmetric) model ward motion and convergence increase the ver- results revealed that the vertical velocity de- tical vorticity in the same region. A schematic creases appreciably as the width of the rain- model of this overall microburst flow pattern is shaft increases (which is to be expected since shown in Fig. 17. the hydrostatic pressure balance in the atmos- Significant convergence, including sinking phere inhibits broad-scale vertical motion),but of the visible cloud into the downdraft region that the resulting surface oufflow speeds in- has been observed, as has increased rotation crease only slightly. This result is applicable to coincident with the downdraft and reflectivity any isolated downdraft; the cylindrical geome- core. These upper-level velocity features, de- try and mass continuity alone determine that tectable with Doppler radars, can give an early the ratio of the oufflow speed to the downflow indication of microburst formation and can

The Lincoln Laboratory Journal. Volume 1, Number 1 (1 9881 Wolfson - Characteristics of Microbursts in the Continental United States

increase confidence in a surface microburst shown in Fig. 18. detection. Microburst lines originate from high-based In summary, all observations and simula- shallow cloud lines. These cloud lines are often tions indicate that downward acceleration from initiated by the surface convergence lines that negative buoyancy, generated as precipitation develop daily over the Rocky Mountains [82].It falls from cloud base and evaporates (ormelts), has also been suggested that the cloud lines leads to the observed downdraft speeds in the may form in response to eddy flow patterns microbursts originating from shallow, high- forced by the mountains, similar to Von Kar- based cumulonimbus clouds. The conditions man vortex streets, that are set up parallel to suitable for the formation of this type of micro- the prevailing winds [25,83].(Von Karman vor- burst have mainly been observed in the high tex streets are long staggered rows of vortices, plains east of the Rocky Mountains during the where each vortex of one row has equal but summer months. However, they can certainly opposite circulation to each vortex of the other occur elsewhere. row, created in the wake of a long cylinder as The downdrafts are probably initiated by pre- fluid flows past.) The lines generally have em- cipitation loading within the elevated clouds. bedded centers of divergence at the surface, Model results show that the narrowest down- coincident with local maxima in the radar re- drafts (exceptingdowndrafts less than 1km in flectivity field. A single microburst may have a horizontal scale) produce the most divergent lifetime of about 15 min, but a microburst line and thus the most hazardous outflows. Not typically lasts for about an hour. only are the vertical velocities strongest, but Microburst lines have a severe impact on air- the outflow winds are nearly as strong as those port operations primarily because they are from larger storms, even though the horizontal long-lived and propagate slowly (mean speed scale is smaller. 1.3 mls). However, this also implies that they The actual hazard to aviation of this type of can be more easily predicted. Through the use microburst has been assessed through obser- of a three-dimensional numerical flow simula- vations of air trdc response at Stapleton In- tion [84],it has been shown that merging micro- ternational Airport in Denver [78,79].Aircraft burst outflows, such as would be present in a do fly through microbursts at Stapleton, and microburst line, may pose an even greater pilot reports of encountered wind shear are used to warn subsequentflights. Because these microbursts occur only in the afternoon (day- light hours),and because they are often marked by virga below cloud base, pilots can sometimes avoid flying through them.

Microburst Lines During the JAWS project, it was found that two or more microbursts could occur simul- taneously, forming a line [80].This led to the definition of the "microburst line" (811 as con- sisting of two or more microbursts, being at least twice as long as wide (between velocity maxima on either side of the line),and having a velocity differential in the cross-line direction meeting microburst criteria. A microburst line Surface Microburst may be nearly homogeneous along its length or

may be made up of distinct, discrete micro- Fig. 17 - Overall microburst flow pattern in Denver. bursts. The basic microburst line structure is (Redrawn from Ref 70.)

The Lincoln Laboratory Journal, Volume 1, Nurnber 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

identification of the microburst line as a new storm type arose from observations of weather phenomena near the Rocky Mountains, sug- gesting orographic influences in the organization of this storm type. The primary concern for aviation appears to be the severe impact that a slow-moving large-scale storm with embedded divergent oufflows has on airport operations.

SUMMARY r' .CS)- Several distinct phenomena can cause strong a I Surface surface outflows that qualify as microbursts. At the largest scales, organized downburst Horizontal Distance (km) storms occur in association with mesoscale or (a) Vertical Structure as Viewed synoptic-scale linear radar echo configura- From End of Line tions, in environments characterized by moderate vertical wind shear and strong thunderstorm potential. The strength of the observed Outflow oufflow is determined by the strength of the [h*?Boundary vertical velocity and the downward flux of horizontal momentum, and may be influenced by Intensifying the nearly two-dimensional, linear storm geom- Microburst etry. Because the storms are large-scale, long- Scale Winds lived, infrequent, and severe, aircraft are generally able to avoid them. 4 1 Boundary of When there is little vertical shear of the hori- \ r\-&+~axirnurn Horizontal zontal wind, but similar conditional instability, Wind Velocities isolated air mass thunderstorms form. In hot and humid conditions,the strength of the out- A 4 flow from these storms is determined by evap- Viewpoint in Part (a) orative cooling both in cloud and below cloud base, as well as by precipitation loading, espe- (b) Horizontal Structure cially at upper levels. As the oufflow pool ex- of Outflow pands rapidly, strong straight-line winds form in association with the leading edge vortex roll. Fig. 18 - Basic microburst line structure. (Redrawn from Ref 79.) For a number of reasons, these microburst- producing air mass storms pose the greatest hazard to aviation: relatively high frequency; danger to aviation than solitary oufflows for rapid development; small-scale, very strong two reasons: the effective divergent outflow oufflows; and lack of translational motion. depth increases and thus so does the total Moreover, storms that are identical in appear- amount of hazardous airspace; and the in- ance, at least visually and on conventional air- creased horizontal pressure gradients can lead craft radar, are successfullyflown through on a to even stronger, more divergent oufflows. regular basis. In summary, the strength of a microburst Between the isolated thunderstorm and the line oufflow and the corresponding hazard to large, organized storm are the other forms of aviation vary tremendously. Although micro- loosely organized multicell storms. These bursts have been observed to form in groups or storms, with closely spaced echoes that merge "families" in other parts of the country [52],the to form a "spearhead" appearance on low-res-

The Lincoln Laboratory Journal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States olution radar scopes, may be similar to the mi- high plains east of the Rocky Mountains during croburst lines found near Denver; however, the summer. The surface reflectivity values of they form without any orographic organization. these microbursts are low, but the oufflows are Strongforcing of updrafts can occur as the out- often just as strong as those arising from high- flow from a nearby decaying cell triggers the reflectivity air mass thunderstorms. enhanced growth of new cells. Cells that form The development at Lincoln Laboratory of later in the "chain" appear to grow faster and FG2 - a sophisticated, highly capable Doppler taller, perhaps because more humid air is en- weather radar - and the collection of data with trained into their updrafts. The downdrafts and that radar for the FLOWS project in Memphis, oufflows are comespondingly stronger,increas- Huntsville, and Denver have dramatically in- ing the forcing for the next cell, and so on. To creased our understanding of the characteris- the extent that these multicell storms are larger tics of microbursts in the continental United and longer-lived than isolated storms, they are States. Using this increased understanding of easier for air traffic to avoid. But because of microbursts, the varied phenomena that have their explosive growth, they are unpredictable, been called microbursts can now be divided so air space that was a safe distance away from into distinct categories. such a storm complex one minute could be This review has presented a first attempt at inundated with microburst wind shear the next categorizing storms along lines that are me- minute. teorologically meaningful and that consider The microbursts that arise from shallow, their relative hazard to aviation. This categori- high-based cumulonimbus clouds can only zation is an essential first step towards discov- occur in an environment with a deep, dry adia- ering exactly which atmospheric conditions batic mixed layer. Sufficient moisture must be and dynamic interactions lead to the-develop- available aloft to sustain a downdraft all the ment of microbursts - so meteorologists can way to the surface, even in the face of strong predict their occurrence. The categorization of evaporation. These microbursts occur as iso- microbursts will also aid the development of lated cells, or in clusters of two or more as automated algorithms for the TDWRs that util- microburst lines. Suitable conditions for their ize this knowledge to make accurate early de- development have mainly been observed in the tections and predictions of microbursts.

The Lincoln Laboratory Journal, Volume 1, Number 1 (1988) Wolfmon - Characteristics of Microbursts in the Continental United States

REFERENCES 22. D.R. Mann, J.E. Evans, and M.W. Merritt, "Clutter Sup- 1. M.M. Wolfson, "Characteristics of Microbursts Ob- pression for Low-Altitude Wind Shear Detection by served in the Continental US," Reprints, 15th Con. on Doppler Weather Radars," Preprints, 23rd Conf. on Severe Local Storms, American Meteorological So- Radar Meteorology,American Meteorological Society, ciety, 372 (1988). R9 (1986). 2. P.E. Viemeister, The Lightning Book Original edition: 23. D.R. Mann, "TDWR Clutter Residue Map Generation Doubleday, Garden City, NY (1961);Second edition: and Usage," Project Report ATC-148, MIT/Lincoln MIT Press, Cambridge, MA (1972). Laboratory; FAA Report No. DOT/FAA/PM-87-26 3. 0. Stewart, Danger in the Air, Philosophical Library, (1988). New York (1958). 24. S.C. Crocker, "TDWR PRF Selection Criteria," Project 4. T.T. Fujita and H.R. Byers, "Spearhead Echo and Down- ReportATC-147,MIT/Lincoln Laboratory; FAA Report burst in the Crash of an Airliner," Mon. Weather Rev. NO. DOT/FAAPM-87-25 (1988). 105, 129 (1977). 25. T.T. Fujita, The Downburst - Microbwst and Macro- 5. E.J. Fawbush and R.C. Miller, "ABasis for Forecasting burst, Satellite and Mesometeorology Project, Dept. of Peak Wind Gusts in Non-frontal Thunderstorms," Bull. Geophysical Sciences, University of Chicago (1985). Am Meteorol. Soc. 38, 14 (1954). 26. R.E. Rinehart. J.T. DiStefano, and M.M. Wolfson, "Pre- 6. T.T. Fujita and F. Caracena, "An Analysis of Three liminary Memphis FAAILincoln Laboratory Opera- Weather-related Aircraft Accidents," Bull. Am Meteo- tional Weather Studies Results," Project Report ATC- roLSoc. 68, 1164 (1977). 141, MIT/Lincoln Laboratory; FAA Report No. DOT/ 7. T.T. Fujita, "Objectives, Operation, and Results of Pro- FMPM-86-40 ( 1987). ject NIMROD," Reprints, 11th Con. on Severe Local 27. National Transportation Safety Board, "Aircraft Acci- Storms,American Meteorological Society, 259 (1979). dent Report: Delta Air Lines, Inc., Lockheed L1011- 8. Eh. Brandes, "Flow in Severe Thunderstorms Ob- 385-1, N726DA, DallasIFt. Worth Int. Airport, Texas, served by Dual-Doppler Radar," Mon. Weather Rev. August 2, 1985," Report No. NTSB/AAR-86/05 (1986). 106, 113 (1977). 28. F.W. Wilson, Jr. and J. A. Flueck, "A Study of the 9. P.S. Ray, "Triple-DopplerObservations of a Convective Methodology of Low-altitude Wind Shear Detection Storm," J. Appl. Meteoml. 15, 879 (1978). with Special Emphasis on the Low Level Wind Shear 10. T.T. Fujita, "Downbursts and Microbursts - An Avia- Alert System Concept," JAWS NCAR Report No. 02-85, tion Hazard," Preprints, 11th Conf. on Radar Meteo- FAA Report No. DOT/FAAPM-86/4 (1986). rolog y, American Meteorological Society, 94 ( 1980). 29. T.T. Fujita, DFW Microburst, Satellite and Mesomete- 11. T.T. Fujita, "Microbursts as an Aviation Wind Shear orology Project, Dept. of Geophysical Sciences, Uni- Hazard," Proc. of the 19th Aerospace Sciences Mtg., versity of Chicago (1986). AIAA-8 1-0386,American Institute for Aeronautics and 30. F. Caracena, R. Ortiz, and J. Augustine, "The Crash of Astronautics (1981). Delta Flight 191 at Dallas-Ft. Worth International Air- 12. R.C. Goff, "Low-Level Wind Shear Alert System port on 2 August 1985: Multiscale Analysis of Weather (LLWSAS),"FAA-RD-80-45, Federal Aviation Adminis- Conditions," NOAA Technical Report ERL 430-ESG 2 tration, Washington, DC (1980). (1986). 13. J. McCarthy, J.W. Wilson, and T.T. Fujita, "The Joint 31. J.E. Evans and D.H. Turnbull, "The FAA-MIT Lincoln Airport Weather Studies Project," BulL Am Meteorol. Laboratory Doppler Weather Radar Program," Pre- Soc. 63, 15 (1982). prints, 2nd Int. Conf. on theAviation Weather System, 14. J. Dodge, J. Arnold, G. Wilson, J. Evans, and T. Fujita, American Meteorological Society 76 (1985). "The Cooperative Huntsville Meteorological Experi- 32. G. Zorpette, "The Menacing Microburst," IEEE Spec- ment (COHMEX)," Bull. Am Meteorol. Soc. 67, 417 trum 23, 50 (1986). (1986). 33. M.W. Merritt, "Automated Detection of Microburst 15. T.T. Fujita and RM. Wakimoto, "Five Scales of Airflow Windshear for Terminal Doppler Weather Radar,"Pmc. Associated with a Series of Downbursts on 16 July SPIE, Vol. 846, Digital Image Processing and Visual 1980," Mon. Weather Rev. 109, 1438 (1981). Communications Technologies in Meteorology, The 16. J.W. Wilson, R.D. Roberts, C. Kessinger, and J. International Society for Optical Engineering, 61 McCarthy, "Microburst Wind Structure and Evaluation (1987). of Doppler Radar for Ahport Wind Shear Detection," J. 34. S.D. Campbell, "Microburst Precursor Recognition Climate AppZ. Meteorol. 23, 898 (1984). Using an Expert System Approach," Preprints, 4th Int. 17. T.T. Fujita, "Microburst Wind Shear at New Orleans Conf. on InteractiveInformation and Processing Sys- International Wrt,Kenner, Louisiana on July 9, temsfor Meteorology, Oceanography, and Hydrology, 1982," SMRP Research Paper 199, University of Chi- American Meteorology Society, 300 (1988). cago (1983). 35. P. Sanford, A. Witt, and S. Smith, "Gust FrontIWind 18. F. Caracena, RAMaddox, J.F.W. hudom, J.F. Weaver, Shift Detection Algorithm," Project Report ATC-146, and R.N. Green, "Multi-scale Analysis of Meteorologi- MIT/Lincoln Laboratory; FAA Report No. DOT/FM cal Conditions AffectingPan Am World Airways Flight PM-87-24, in press (1988). 759," NOAA Technical Memorandum ERLESG-2, En- 36. R.C. Srivastava, "A simple model of evaporatively driv- vironmental Sciences Group, Boulder, CO (1983). en downdraft: Application to a microburst downdraft," 19. National Research Council, Low-altitude Wind Shear J. Atmos. Sci 42, 1004 (1985). and Its Hazard to Aviation, National Academy Press 37. H.R. 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The Lincoln Laboratory Journal, Volume 1, Number 1 (1988) Wolfson - Characteristics of Microbursts in the Continental United States

40. D.S. Foster, "Thunderstorm Gusts Compared with 59. K.R. Knupp and D. P. Jorgensen, "Case Study Analysis Computed Downdraft Speeds,"Mon. Weather Rev. 86, of a Large-scale and Long-lived Downburst Producing 91 (1958). Storm,"Preprints, 14th Conf. on Severe Local Storms, 41. T.T. Fujita, "Andrews AFB Microburst: Six Minutes American Meteorological Society, 301 (1985). after the Touchdown of Air Force One," SMRP Re- 60. F. Caracena, "A Comparison of Two Downbursts of search Paper, 205, University of Chicago (1984). Different Meso Scales," Preprints, Con. on Weather 42. R.D. Roberts and J.W. Wilson, "Precipitation and Kin- Forecasting and Analysis and Aviation Meteorology, ematic Structure of Microburst-producing Storms," American Meteorological Society, 293 (1978). Preprints, 22nd Con. on Radar Meteorology, Ameri- 61. J.M. Schmidt and W.R. Cotton, "Bow Echo Structure can Meteorological Society, 71 (1984). and Evolution within a High Plains Meso-P Squall 43. R.D. Roberts and J. W. Wilson, "Nowcasting Micro- Line," Preprints, 14th Conf. on Severe Local Storms, burst Events Using Single Doppler Radar Data," Pre- American Meteorological Society, 77 (1985). prints, 23rd Conf. on Radar Meteorology. American 62. J.B. Klemp and R. Rotunno, "A Study of the Tomadic Meteorological Society, R14 (1986). Region Within a Supercell Thunderstorm,"J. Atmos. 44. M.M. Wolfson, J.T. DiStefano, and T.T. Fujita, "Low- Sci 40, 359 (1983). altitude Wind Shear Characteristics in the Memphis, 63. R.M. Wakimoto, "Forecasting Dry Microburst Activity TN Area Based on Mesonet and LLWAS Data," Pre- over the High Plains," Mon. Weather Rev. 1 13, 1131 prints, 14th Con. on Severe Local Storms, American ( 1985). Meteorological Society, 322 (1985). 64. M.R. Hjelmfelt, R.D. Roberts, and H.D. Orville, "Obser- 45. DA. Burrows and L.F. 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American Meteorological Society, 5 1 (1983). 67. W.R. Krumm, "On the Cause of Downdrafts from Dry 48. R.E. Dunham, G.M. Bezos, C.L. Gentry, and E. Melson, Thunderstorms over the Plateau Area of the United "Two-dimensional Wind Tunnel Tests of a Transport- States," Bull. Am Meteorol. Soc. 36, 122 (1954). type Airfoil in a Water Spray,"AIAA-85-0258, American 68. J.M. Brown, K.R. Knupp, and F. Caracena, "Destructive Institute for Aeronautics and Astronautics (1985). Winds from Shallow, High-based Cumulonimbi," Pre- 49. R.J. Hansman, Jr. and A. Craig, "Low Reynolds Num- prints, 12th Con. on Severe Local Storms, American ber Tests of NACA 64-210,NACA 0012 and Wort- Meteorological Society, 272 (1982). mann FX67-K170 Airfoils in Rain," J. Aircraft 24* 69. F. Caracena, J. McCarthy, and J. A. Flueck, "Forecast- 559 (1987). ing the Likelihood of Microbursts along the Front 50. J.K. Luers and P. 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Fujita, "Tornadoes and Downbursts in the Context 72. C.K. Mueller and P.H. Hildebrand, "Evaluation of Mete- of Generalized Planetary Scales," J. Atmos. Sci. 38, orological Airborne Doppler Radar. Part 11: Triple- 1511 (1981). Doppler Analyses of Air Motions," J. Atmos. Oceanic 54. B.E. Smith and J.W. Partacz, "Bow-echo Induced Tor- Technol. 2, 381 (1985). nado at Minneapolis on 26 April 1984,"Preprints, 14th 73. K.L. Elmore, "Evolution of a Microburst and Bow- Conf. on Severe Local Storms, American Meteorolog- shaped Echo during JAWS," Preprints, 23rd Conf. on ical Society, 81 (1985). Radar Meteorology, American Meteorological So- 55. J.T. DiStefano, "Analysis of a Thunderstorm Down- ciety, JlOl (1986). burst," S. M. Thesis, Dept. of Meteorology and Physical 74. C.J. Kessinger, R.D. Roberts, and K.L. Elmore, "A Sum- Oceanography, MIT (1983). mary of Microburst Characteristics from Low-reflec- 56. M.M. Wolfson, "Doppler Radar Observations of an tivity Storms," Preprints, 23rd Conf. on Radar Meteo- Oklahoma Downburst," Preprints, 21st Con. on Ra- rology, American Meteorological Society, J105 (1986). dar Meteorology, American Meteorological Society, 75. J. McCarthy, R.D. Roberts, and W.E. Schreiber,"JAWS 590;and S. M. Thesis, Department of Meteorology and Data Collection, Analysis Highlights, and Microburst Oceanography, MIT (1983). Statistics," Preprints, 21st Conf. on Radar Meteorol- 57. J.R. Cooley, "Microburst at Hickory Comers, Michi- ogy, American Meteorological Society, 596 (1983). gan," Preprints, 23rd Conf. on Radar Meteorology, 76. S.K. Krueger and R.M. Wakimoto, "Numerical Simula- American Meteorological Society, 5113 ( 1986). tion of Dry Microbursts," Preprints, 14th Conf. on 58. G.S. Forbes and R.M. Wakimoto, "A Concentrated Out- Severe Local Storms, American Meteorological So- break of Tornadoes,Downbursts and Microbursts, and ciety, 163 (1985). 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The Lincoln Laboratory Journal. Volume 1, Number 1 (1988) Wolfwn - Characteristics of Microbursts in the Continental United States

77. S.K. Krueger, R.M. Wakimoto, and S.J. Lord, "Role of Ice-phase Microphysics in Dry Microburst Simula- tions," Preprints, 23rd Conf. on Radar Meteorology, The author thanks Dr. T.T. Fujita of the University of American Meteorological Society, 573 (1986). Chicago and Dr. E. Kessler, former director of NSSL, 78. L. Stevenson. bbThunderstormImpact on Denver Air for numerous enlightening discussions about micro- Traffic Control Operations and the Role of NEXRAD," MIT, DOT-TSC-RSPA-84-1, US Dept. of Transportation, bursts, and Dr. K.A. Emanuel of Dr. J.E. Evans Washington, DC (1984). of Lincoln Laboratory, Dr. D.S. Zrnic of NSSL, and 79. L. Stevenson, "The Stapleton Microburst Advisory Mr. D.H. Turnbull of the FAA for their constructive Se~viceProject - An Operational Viewpoint," DOT- comments. This work was sponsored by the FAA. FAA-PM-S5/21, US Dept. of Transportation, Washing- ton, DC (1985). 80. C.J. Kessinger, M.R. Hjelmfelt, and J.W. Wilson, "Low- level Microburst Wind Structure Using Doppler Radar and PAM Data," Preprints,21 st Conf. on Radar Meteor- ology, American Meteorological Society, 609 (1983). 81. M.R. Hjelmfelt and R. D. Roberts, "Microburst Lines," Preprints, 14th Conf. on Severe Local Storms,Ameri- can Meteorological Society, 297 (1985). 82. J.W. Wilson and W.E. Schreiber, "Initiation of Convec- tive Storms at Radar-observed Boundary Layer Con- vergence Lines," Mon. Weather Rev. 1 14.25 16 (1986). 83. E.C. Peterson, "JAWS Microbursts Occurring from MARILYN M. WOLFSON is a Orographic Vortex Streets," Preprints, 14th Conf. on member of the Air Traffic Severe Local Storms, American Meteorological So- Surveillance Group, where ' ciety, 256 (1985). she studies aviation-haz- 84. J.R. Anderson, K.K. Droegemeir, and R.B. Wilhelmson, ardous weather events. She "Simulation of the Thunderstorm Subcloud Environ- is currently completing a ment," Preprints, 14th Con$ on Severe Local Storms, P~Din meGorolo& at MIT. American Meteorological Society 147 (1985). Marilyn received a BS in atmospheric science from the University of Michigan and an SM in meteorology from MIT. Before she began graduate school, Marilyn worked as a Summer Fellow at the NASA Goddard Institute for Space Studies.

The Lincoln Laboratory Journal, Volume 1, Number 1 (1988)