STORMSCALE WEATHER (I!)

by Lee Edson

In Mosaic Volume 13 Number 6 of 1982, we wrote: "Mesoscale, or stormscale, weather systems are the ones

that do the most damage. Predicting them has been beyond reach." A decade later, after extraordinary and ingenious effort

and the best that technology can offer, prediction of these

omelike clouds huddle, block-; ing out the sun. The sky

, darkens and glowers. High • winds whip through the trees.

Somewhere in the skies, invisible to ca­sual observers, a powerful storm system forms, propelling moist and warm air into collision with its dry, cold counter­part Convection currents are created that bring thunderclaps and lightning, then rain and hailstones that can grow as big as baseballs. Now gust fronts race across the terrain* The air swirls, circles, and dervish-like transforms itself into a black, forbidding, rotating funnel. It touches the earth, and a deadly tornado is born.

Each year some region of the conti­nent is bedeviled by a sudden, violent thunderstorm, flash flood, microburst, tornado, or other unruly weather change that leaves casualties and multimillion dollar property damage. In 1989, in a burst of unusual weather, the eastern half of the United States suffered over 800 tornadoes in a four-month period, the second worst year in American his­tory. One killer tornado that November leveled a 25-mile stretch of homes and businesses in Huntsville, Alabama, injur­ing 400 and leaving 18 dead. Then the storm system whipped north to New-burg, New York, where the high winds

knocked down a school cafeteria wall, crashing nine children to death.

Over the years weather tragedies like these have stirred meteorologists into looking for new knowledge and insights into atmospheric flow that might help them more accurately to predict the on­set of stormscale, or mesoscale, weather conditions and of the abrupt, relatively small, short-duration violent storms they produce. So far, despite the develop­ment of satellite-enhanced meteorology and the promise of the computer to turn the art of forecasting into science by re­solving all the mysteries of atmospheric dynamics, there has been little success; the stormscale phenomenon has defied and continues to defy detailed predic­tion. Indeed stormscale prediction today consists essentially of the TV weather­man's pointing to a storm on his radar display, drawing an arrow in the direc­tion of movement, and noting where the disturbance is likely to occur next. Unfortunately, half the storms on the screen will have died an hour later, and a new set will have developed in unex­pected locations outside the weather­man's arrow.

There are several reasons for this fail­ure of prediction. First is the difficulty in pinning down initial atmospheric con­ditions, which in the environment of a

single thunderstorm can take as many as ten million variables (10 variables at 100 foot intervals in each of three dimen­sions) to characterize at a given mo­ment. The second difficulty concerns nu­merous regional variables, both surface and atmospheric, that influence storm development, and make prediction of local events even more uncertain. In ad­dition, many observing systems in cur­rent use are not precise enough to pin­point conditions that specify the origin of an event. Conventional radars, for in­stance, like those deployed in the Na­tional Weather Service, are unable to spot the start of a rotating thunderstorm or mesocyclone, which may generate a tornado. Finally, in today's state of the art, there are relatively few observations available on a small enough scale to measure the tiny wind motions needed to resolve the storm scale phenomenon and accurately predict when and where a thunderstorm will strike.

New dewices

Despite these negatives factors many atmospheric scientists and meteorolo­gists refuse to believe that the picture is

44 MOSAIC Volume 23 Number 2 Summer 1992

"Storms on a Deadly Scale" remains among the sciences knottiest problems.

omelike clouds huddle, block­ing out the sun. The sky darkens and glowers. High

winds whip through the trees.

#

as gloomy as portrayed. In rebuttal to Mark Twain's classic aphorism that we all talk about the weather but don't do anything about it, the modern meteor­ologist points to the recent development and application of new meteorological devices, many of them just coming on line, that are more versatile, sophisti­cated, and sensitive than the traditional barometers, thermometers, balloon-carrying rawinsondes, or other devices and instruments now in use.

Among the newer devices that are coming of age the most noteworthy is weather Doppler radar, which uses the well-known Dopple effect to measure wind speed in the radar beam direction with good accuracy and with high space and time resolution. Ground-based "wind profilers—a special kind of re­mote-sensor radar—also collect contin­uous data on the changes of wind speed and direction at closely spaced altitudes over depths of the atmosphere up to 15 kilometers or greater. Other sensitive instruments such as thermodynamic profilers provide information on the ver­tical distribution of temperature and moisture. The data from these sources are fed for analysis into faster and more powerful computers. Finally, aircraft equipped with Doppler radar and other devices are now available to bring ob­servers in close communication with the center of storm-scale phenomena.

With such advanced instrumentations and with the help of improved mathe­matical models, the operational meteo­rologist can call up and diagnose the three dimensional dynamics of an approaching storm on a workstation screen and can predict its direction and evolution with greater accuracy and con­fidence than ever before. Although this is an advance of considerable signifi­cance, Kevin Droegemeier, deputy di­rector of the Center for the Analysis and Prediction of Storms at the University of Oklahoma in Norman, the newest center of its kind, believes that the meteorolo­gist may soon do even better. "Our mis­sion is to develop techniques for the practical prediction of short-lived and small-scale weather events, especially those involving intense convection," Droegemeier says.

Edson is a freelance science writer in Stamford, Connecticut, who has written on many subjects for Mosaic. His most recent contribution was "B Physics and B Factories" in Volume 22 Number 2.

Convection, the rise of warm air cur­rents through a colder environment, is of course the natural generator of thun­derstorms. With new radar networks under development and with new math­ematical models soon to be deployed, Droegemeier expects to be able to pre­dict in close detail the location, intensity, and type of precipitation of an existing storm, from one to six hours hence, and to accurately forecast a new thunder­storm two hours in advance, in ample time to warn people to take necessary precautions. He also contends that he will be able to depict the evolution of existing storms three to six hours ahead.

If this new predictive capacity should come to pass, it will mark striking prog­ress in the relatively young science of mesoscale meteorology, and in under­standing its relationship to meteorology and the atmospheric sciences in gen­eral. As a discipline, meteorology dates back to the early 1900s when Vilhelm Bjerknes and the Norwegian school first described weather as a global battle be­tween cold and warm fronts. Meteorol­ogist LP. Richardson of Great Britain provided the mathematical basis of weather forecasting. In the forties, John von Neumann, the eminent mathemati­cian, led meteorology into the world of computers and opened up a new vista for the growing science.

The introduction of radar in weather observation in the 1950s and the advent of globe-circling satellites in the 1960s contributed significantly to data acquisi­tion. The meteorologist could add new data points to the conventional network of observation that led to the construc­tion of weather maps and synoptic charts and that provided the basis for TV displays of global cloud and wind movements. Tracking the large-scale processes, however, could not reveal the site and timing of the next thunder­storm. That was left to the complexities of mesoscale analysis and prediction.

The stormscales

Mesoscale, or midscale, meteorology joined the parent discipline in the fifties and identified its turf as weather phe­nomena that are typically too small in extent (smaller than the State of Okla­homa, for instance) to be captured by the upper air and satellite soundings used routinely by the National Weather Service. It also covered phenomena that were too large to be adequately meas­ured by line of sight observations of

events at a single fixed location. Today, meteorologists find it conven­

ient to break down the mesoscale into three subscales: alpha (200 to 2,000 kil­ometers), beta (20 to 200) and gamma (2 to 20). Each domain is a specialty with its own niche of research and model building. The alpha scale, for instance, embraces frontal and land/sea breeze circulations that set the stage for thun­derstorm development; the beta scale covers supercell thunderstorms and in­cludes banded precipitation structures such as squall lines, and the gamma scale deals with shorter duration and smaller severe storms such as torna­does, microbursts, and cloudbursts.

Meteorologists have recently recog­nized, however, that in predicting thun­derstorms and related phenomena the scales are interlinked and work to­gether. Even motions on very small scales, such as a single cloud that lasts for only an hour or so, can play a role in many large-scale weather events that transverse continents and oceans and may persist for days or weeks. Although the large-scale processes do not by themselves totally determine the initia­tion of a thunderstorm in Kansas, say, or Oklahoma, they do tend, according to the scientists, to establish the in­stabilities that are released sequentially to the smaller scale storms, while the latter feed back their own modifications to affect the largescale weather systems.

Meteorologist Edward Zipser of Texas A & M has studied multiple scales in weather and their linkages in the de velopment of severe storms in the South­west On a typical day in April or early May, he notes, the synoptic scale (1000 km) weather predictions from global forecasters, usually two to three days in advance, reveal the first signs of ap­proaching stormy weather in trough, which can bring contrasting dry and cool, and warm and wet air masses into closer contact and thus increase the buoyant instabilities. As the storm devel-ops, disturbances occur at the 100-kilometer scale on the dryline, a sharp, volatile boundary structure that arises when western desert air spilling over the Rockies meets moist Gulf of Mexico air moving north. Such a meeting may lead to the formation of a thunderstorm. Once thunderstorms form, radar can identify the regions where the air is starting to rotate on the scale of a few kilometers, a phenomena that is often the precursor to an intense 100-meter

46 MOSAIC Volume 23 Number 2 Summer 1992

gests that throughout the interplay the mesoscale's function is primarily that of setting the stage for a severe thunder­storm and often acting to initiate it. He theorizes that the synoptic and the me-soscale processes provide the buoyancy necessary to lift warm air into cooler re­gions and thus to create the deep moist convective state that is the key to weather phenomena ranging from lash foods at one end of the mesoscale to microbursts at the other end. Focusing on these convective events, and analyz­ing them in terms of the other scales,

scale tornado* As the storm reaches and passes max­

imum intensity, the scale interactions tend to reverse. The storms expand their regions of influence, often forming long precipitating anvils. Individual storms tend to amalgamate, forming squall lines, or the circular arrays known as mesoscale convective systems. These storm activity centers then generate im­portant changes in the larger synoptic and climate scales as they decay.

Charles Doswell of the National Se­vere Storms Laboratory in Norman, sug-

MOSAIC Volume 23 Number 2 Summer 1992 47

underlies much of the work of modem mesoscale meteorologists in their at­tempts to improve the accuracy of pre­dicting local weather.

Consider lash loods, for instance. In recent years sudden devastating tor­rents of rain that overwhelm gully or slope and drown everything in sight have caused more deaths than torna­does have. In 1976, as an example , the Big Thompson River near Loveland, Col­orado, overflowed during a heavy storm, creating a flood that roared down the Big Thompson Canyon, killing 154 peo­ple. A year later a series of thunder­storms in Johnstown, Pennsylvania dropped 10 inches of rain in a small re­gion of the Conemaugh River Basin, cre~ ating a flash flood that took 77 lives*

The conwectiwe system

How does one know where the heavy rainfall will concentrate and why, mete­orologically, it should persist longer than normal? Can science quantify the rainfall in advance? To answer these questions scientists have looked to the recently discovered mesoscale convec-tive system, or MCS, a type of storm system which provides much of the

48 MOSAIC Volume 23 Number 2 Summer 1992

midwest's beneficial rainfall during the growing season. In its violent state it is well known for producing locally severe storms and accompanying flash floods.

The MCS takes many forms, but the largest and longest lived—indeed the Cadillac of the class—is the Mesoscale Convective Complex (MCC). It was first described by Robert Maddox of the Na­tional Oceanic and Atmospheric Admin­istration in 1979 following a study of en­hanced infrared photographs of clouds taken by geostationary satellites. Maddox discovered that the cloud tops towering above this type of storm could grow to form a very cold slab (32 de­grees C below zero) spread over an area of a hundred thousand square kilome­ters and persisting for periods of more than twelve hours.

From this finding Maddox tracked the convective currents that initially rise from the sun-warmed mountain slopes to regions of thermodynamic instability that occur when warm air is topped by upper colder air. Maddox also studied the way in which the air circulation was maintained through the various phases of the life cycle of these storm systems as they advance into the adjacent plains. He found that the slow-moving multicel­lular nature of the mesoscale convective complex, and its relatively long duration, can process moisture into precipitation quite efficiently and can generate heavy rainfall over small areas. In the case of the Johnstown Flood, which was created by an MCC, thunderstorms formed and reformed along the periphery of the MCC, taking similar trajectories that led to the intense flooding.

Charles Chappell of the National Cen­ter for Atmospheric Research describes the basic process as the formation of a "quasi-stationary convective event" in which meteorological processes on sev­eral scales work together synergistically to keep the storm stationary for several hours. In many cases the parent meso­scale storm system will stand still while the individual convective cells that com­pose it may move in concert along sim­ilar paths and produce pulsating heavy rains. In other cases the entire storm may move away, only to have a new storm system reappear in the same spot, thus concentrating rainfall in a relatively small area.

Da-lin Zhang of McGill University in Montreal and Michael Fritsch of Penn State University, at University Park, have been working on stimulating vari-

m.

ous flash flood events using computer models. In simulating the Johnstown flash flood they have been able to repro­duce many of the main features of the storm, such as the mesoscale surface pressure patterns and the rainbands that

MOSAIC Volume 23 Number 2 Summer 1992 49

produced the heavy rainfall in the early part of the storm. Fritsch acknowledges, however, that their computer model does not yet have enough resolution to predict the specific time and intensity of the flood* 'The new models we are de-veloping will define the state of the at­mosphere with sufficient resolution to be able to predict these small-scale events/' Fritsch says.

An integrated assault

Meanwhile, information on MCCS and their rainfall characteristics is accumu­lating. In 1985, in a landmark experi­ment sponsored by NSF and NOAA, known as PRE-STOEM (Preliminary Regional Ex­periment for Stormscale Operational and Research Meteorology), Bradley Smull and his colleagues at NOAA'S Me™ soscale Research Division in Boulder, Colorado, zeroed in on various aspects of the internal structure of the MCC that struck Kansas and Oklahoma that year. Using data from a special sounding bal­loon network, wind profilers, and both surface and airborne Doppler radar (the first time such a wide variety of instru­ments had been combined to study these complex storms), Smull found that

o z

ift^

Classic storms. A sine - -Florida; a squall line (bottom) at sunset at UKiahoma City, Uklahoma.

50 MOSAIC Volume 23 Number 2 Summer 1992

the mature MCC could comprise a veri­table microcosm of contrasting environ­ments and weather. These could range from supercell thunderstorms to au­tumn-like chilling rains, all apparently cooperating with one another to create a longer-lasting circulation than any sin­gle storm in the complex.

SmulFs finding suggests that MCCS may interact closely with synoptic-scale frontal boundaries along which they travel, alternating patterns of air low, temperature, and moisture to create the characteristic bands of precipitation of these storms. The flash flood potential of the MCC arises from a series of events occurring both within and around the storm. Early in the storm, for instance, moisture is confined to the lower level of the atmosphere. As the MCC develops its owe circulation, it modifies the entire environment of the storm. The low-level moist air moves upward, reaching the upper levels of the troposphere and em­bedding the component thunderstorms in an environment more conducive to the production of heavy rainfall

These new findings about MCCS and flash-flooding have already been utilized in operational meteorology. BillTogstad of the National Weather Service in Min­neapolis was able to detect the coming of an MCC flash flood south of Minneap­olis in August 1991 in sufficient time to issue a warning. The flash flood arrived on schedule and created heavy damage, but no lives were lost. Fatalities might have occurred without the warning.

Squall lines

In recent years scientists have at­tempted to look more closely at another less threatening flash-flood-producing storm system, the squall line or lines of thunderstorms. These occur through­out the tropics and midlatitudes and are often preceded by a sharp roll-like line of clouds followed by sudden wind gusts and a brief heavy downpour, which may produce as much as 30 millimeters of rain in 30 minutes. The heavy initial downpour is often followed by light to moderate rainfall from the anvil cloud that characteristically trails the squall front Flash floods may follow.

Not all squall lines produce flash flooding, however. Some squall line storms in middle latitudes, known as derechos {derecho is Spanish for straight ahead), are dangerous for other rea­sons. They can generate severe, wide-ranging surface winds of fifty to a hun­

dred kilometers an hour that are long lived and can do considerable damage. Bob Maddox and his colleagues at the National Severe Storms laboratory have studied the wind patterns in derechos while Morris Weisman at NCAR in Boul­der and William Colton at Colorado State University in Fort Collins have used computer simulations to investi­gate the related bow-shaped segments of convective lines known as bow ech­oes. They discovered that the most dam­aging winds usually occur when the sys­tems encounter unusual amounts of low-level thermal instability. Instability, along with wind shear that increases in intensity with height, gives rise to the

• '< • Joseph Klemp (below) of . - ' , :ation (circular-ribbon arrows)

-. . > '.' flow; barbed lines, the cold air

bow echoes. The early detection of low-level jets of wind and upper troposphere polar jets may help forecasters in iden­tifying atmospheric conditions that sup­port the development of these strange wind phenomena.

In another investigation, Richard Johnson of Colorado State University, found a special pressure feature, called a wake low, in the trailing edge of a squall line storm. It produced light rain­fall with strong surface winds. Johnson reports that strong low-level wind shear can occur in this region and pose poten­tial threats for aircraft Indeed, this light rain regime is now regarded as being as dangerous to aircraft as heavy rain zones because of the high winds associ­ated with it Other studies have also shown the existence of wake lows in the GuM of Mexico, where they have pre­sented problems for offshore oil-drilling.

Tornadoes

Under certain conditions severe squall line thunderstorms can generate individual tornadoes, the most dramatic and most intensively studied mesoscale phenomenon of atmospheric power and violence. Tornadoes are usually associ­ated with relatively more isolated but long-lived thunderstorms that are com­posed of a single giant cell or supercell This type of tomadic thunderstorm be­gins with a broad and intense updraft entering its southeast flank that rises vertically and then turns cyclonically,

MOSAIC Volume 23 Number 2 Summer 1992 51

that is, counterclockwise, in the outflow region. Such a storm frequently harbors a mesocyclone, the precursor of the spi­ralling tornado. Hailstones in such thun­derstorms can grow as large as grape­fruit

Just what fluid dynamic conditions are required for the formation of a tornado within a mesocyclone are still a matter of speculation. Careful 3 dimension numerical modeling of tornadoes, how­ever, based on recent radar data and supplemented by observations and mov­ies from storm intercept teams and tor­nado chasers, have provided the base for a general theory of the tornado mechanism. According to tornado mod­eling specialists Joe Klemp and Rich Rotunno of NCAR, the wind shear in the tornado-producing supercell storm sets up forces that result in the formation of narrow tubes of spiralling air, or vorti­ces. These vortices are embedded in the shear and run horizontally through the storm between the various layers of air. Like a string of paddle wheels in a stream, the vortices turn faster as the wind whips up speed, causing some of them to tilt upward into the updraft at the center of the storm and to form two vertical vortices, one rotating and the other counterrotating.

As more air is drawn inward and the storm becomes more concentrated, the whole system starts rotating faster, like ice skaters drawing their arms closer to their bodies to achieve greater spin. The air motion is so rapid that a mesocyclone is formed that can spin as fast as 60 miles an hour. Decending through the cloud base as it grows, the mesocyclone brings with it a rainfree rotating wall cloud. This is what makes the meso­cyclone visible. If the wind shear is weak and high-energy surface air runs out, the storm will simply dissipate and the wall cloud attached to it will die without ever forming a tornado. If the shear is strong and the storm continues to feed on high-energy surface air, however, a small col­umn of air ahead of a strong downdraft may start rotating along the edge of the wall cloud and eventually spin off, form­ing a classic funnel-shaped tornado.

Under some circumstances the entire wall cloud may be swiftly transformed into a truly gigantic tornado, which can take an entire town by surprise. On April 10, 1979, a towering thunderstorm ex­tending over 15 kilometers high pro­duced one of the largest tornadoes ever sighted. It touched down in Wichita

Falls, Texas, where 60 people were killed before the storm subsided.

The growing understanding of the or­igins and evolution of tornadoes has led to a promising new method of tornado forecasting. It is based on the measure ment of the potential of a storm to gen­erate a helical circulation in the rising air inside the tornado-producing clouds. Helical circulations arise from wind shear when the strength and velocity of winds at low levels reach certain delned values compared to those at upper lev­els. Recent research indicates that the resultant rotation of the storm's updraft stabilizes turbulent flows in the meso­cyclone and enhances its longevity.

Using as a parameter the value of he-licity—a mathematical term that com­bines the component of vorticity or spin and its velocity in the direction of the moving storm—Douglas Lilly, director of CAPS at the University of Oklahoma, and Robert Davis-Jones at the National Severe Storms Laboratory have devel­oped a set of mathematical equations that describe the growth and dissipation of the tornado helix. With the appropri-ate input, these equations, it was thought, could be used to serve as a pre­dictor of mesocyclones and tornadoes. This possibility was tested during the spring of 1990 in a series of experiments conducted at the National Weather Service Forecast Office in Norman. The method proved to be successful, more so than previous forecasts using alterna­tive techniques. Further experiments are now taking place.

Helicity has also been shown to be the

key factor in the development of torna­does in hurricanes that have made land­fall This type of tornado occurs fre quently and can be quite destructive. Since it was found to grow in an envi­ronment whose buoyant instability is much weaker than that seen in the tor­nado environments of the Great Plains and the Midwest, it was considered to be highly anomalous. Recent studies by E. W. McCaul, Jr., a NASA research sci­entist in Huntsville, Alabama, however, have shown a strong correlation b e tween helicity and tornado intensity in landfalling hurricanes. This has helped to confirm the importance of helicity in tornado activity and forecasting.

Microbursts

Tornadoes are not the only small-scale offshoots of violent thunderstorm behavior. Another phenomenon at the low end of the mesoscale—under 20 kilometers—is the microburst, with its thunderstorm-spawned diverging winds that emerge close to the ground and are a hazard to aviation. This phenomenon first came into the limelight in June 1975, when an Eastern Airlines 707 from San Juan crashed at John F. Kennedy Airport just short of the runway, killing all 113 people aboard. The disaster shocked the meteorological community. Was it pilot error or an unexplained weather phenomenon? To answer this question, the airline operations officials called in T. Theodore (Ted) Fujita of the University of Chicago, a pioneer in di­saster research who had studied every­thing from blast damage in Hiroshima to air crashes in South America to tor­nadoes in the midwest

With the help of satellite photographs and eyewitness testimony Fujita spent a year reconstructing and analyzing the weather components that caused the JFK tragedy. Fujita found that the storm started simply enough in the New Jersey area with a weak cold front that spawned several thunderstorms as it moved slowly north toward Long Island. Within an hour however the storm complex had developed unusual features which Fujita describes as including gentle winds at the southern end of the airport and "un­known wind extremes" at the northern end where the plane was to land. The JFK air controllers, not obtaining the full airport wind picture on their wind sen­sors, which were facing the seaward end of the airport, called for a routine landing on runway 22L

52 MOSAIC Volume 23 Number 2 Summer 1992

This was an appropriate decision. Indeed, during the twenty-two minutes prior to the accident 12 airplanes made successful approaches along runway 22L A number of the pilots detected a wind problem on landing, but only a few admitted later that they thought it was serious enough to report it to the tower. They all landed without incident. Not so with flight 66, Eastern's thirteenth flight of the day.

As the ill-fated plane approached the runway, the pilot had no inkling that the winds above his landing area were join­ing together to create a huge downdraft, a rash of air to the ground that spreads like water over a hosed driveway. His instruments reported only a head wind at 500 feet above the runway and a light thunderstorm moving in. As with the earlier flights, this seemed not espe­cially worrisome.

But minutes later as the 707 trimmed its speed for normal touchdown, the plane suddenly experienced a doubling of its sinking speed. The evidence indi­cates that the pilot saw a sudden in­crease in lift and air speed on his ap-proach and, never realizing it was due to head winds caused by the outflow from the downdraft, responded by reducing his engine power. Once the plane passed the center of the down-draft, however, the head wind became an equally strong tail wind. The extra air speed and lift disappeared and the plane sank. The pilot was unable to compen­sate for the sudden loss of speed and altitude by boosting the engines or tak­ing other measures and the plane stalled and crashed, killing all aboard.

Fujita found that the wind shear— rapid changes in horizontal wind speed and direction—experienced by the air­craft as it crossed the center of the down-draft was "spectacular." And when it was combined with the effects of air rushing downward, the effect produced was far beyond the usual intensity of down-drafts. Concluding that it was a hitherto unreported hazardous weather event of great severity, he invented a new word for it: downburst. This phenomenon now embraces microbursts (four kilo­meters across and smaller) and macro-bursts (larger than four kilometers.) Downbursts are characterized on a radar screen by distinctively shaped echoes known as spearhead echoes.

Fujita looked into the origins of two other major crashes in the late 1970s and again identified the culprit as the

downburst. These findings provoked controversy and some skepticism among meteorologists at first. Eventu­ally they had a strong impact on U.S. aviation officials. The FAA, for instance, soon installed throughout the country more than 100 systems of low-level sen­sors at airports that were designed to detect evolving downburst cells. These sensors are currently being enhanced to detect microburst precursors that were able to evade the earlier detection sys­tems. Some experts believe that air­borne Doppler radars can provide better detection of wind shear. Such devices— which employ forward scanning to de­termine changes in wind velocity—are being developed for the FAA and NASA by several industrial laboratories.

In the mid-eighties, through the NSF-sponsored NCAR, the FAA established a Doppler weather radar program at Stapleton International Airport in Denver, which experiences many microbursts a year. The program was integrated with other instrumentation systems to provide automatic detection and warning of low-altitude wind shear. A check of the program in 1988 showed that the integrated system could accu­rately provide critical weather informa­tion that could be relayed to the pilots by the controllers. As a result, the FAA has let contracts for 47 systems of this type at major U.S. airports.

Electrical activity

Meteorologists' focus on the mesa-scale convective system has also shed new light on a familiar characteristic of a severe storm, its electrical structure and lightening activity. For one thing, scientists have found that lightening is not a simple electrical dipole arc be­tween cloud and earth or between cloud and cloud, but is much more compli­cated. A lightening discharge, for in­stance, may be affected by the internal wind in the core of the storm, by the collisions of raindrops in the region, and by the activity of the convective cells in the storm. These forces can affect the frequency and polarity of the strikes to ground and contribute to their danger.

Interestingly, lightening potential may be dangerous even if no thunder­storm is present An unusual instance of this occurred in March 1987 when an Atlas/Centaur space vehicle carrying a naval communications satellite was launched from Cape Canaveral A min­ute after launch it was unexpectedly

struck by a series of lightening bolts that seemed to come from nowhere. The electric current of the bolts caused a false signal that created havoc in the spacecraft's digital computer, which led to the vehicle's breakup.

In the subsequent investigation, NASA scientists revealed that even though there were no lightening discharges prior to the time of launch, the clouds were charged to a surprisingly high de­gree. What seems to have happened is that the vehicle itself triggered a light­ening discharge, which developed into several follow up cloud-to-ground flashes. The lightening traversed the en­tire length of the vehicle and exited through the rocket plume.

This unusual accident caused NASA to reevaluate its method of assessment of weather conditions prior to launch. The event also confirmed something the avi­ation industry had known—namely, that a vehicle can trigger lightening, not merely intercept it during a storm. Moreover it emphasized that these haz­ards can occur under weather condi­tions thought to be safe. This in turn has led the industry to provide special pro­tective measures for aircraft and to pay increased attention to nonthunderstorm but hazardous flying weather.

Steven Rutledge at Colorado State University and Donald R. MacGorman at the National Severe Storms Labora­tory, have studied further details of the electrification process in thunder­storms, beginning with the 1985 Okla­homa-Kansas PRESTORM project Using a unique network of lightening recorders, they found that some storms are so highly electrified that they were able to count 26,000 lightening strikes to ground in eight hours in northern Texas and Oklahoma. One of the mysteries has been that lightening flashes between earth and clouds can reverse polarities, going from the normal negative cloud-to-earth flash to the unusual positive cloud-to-earth flash. The reasons for this continue to be studied.

In 1988 Rutledge and Earle Williams of MIT also began a two-year investiga­tion of tropical thunderstorms that have provided some additional insights into the relation between the mesoscale con­vective system and the electric field. Known as DUNDEE (Down Under Dopp­ler and Electricity Experiment) the proj­ect took place at the northern tip of Aus­tralia in the vicinity of Darwin. This area experiences some of the world's heavi-

MOSAIC Volume 23 Number 2 Summer 1992 53

est rainfall attended by high incidences of lightening*

Rutledge and Williams discovered, among other things, that lightening flashes over land in the Darwin vicinity occur ten times as often as over the sur­rounding Indian Ocean but that no sin­gle parameter like cloud height can be used to predict the flash rate, at least in the tropical monsoon area. They also found predominantly positive cloud-to-ground flash polarity in the stratiform regions of the tropical cloud clusters. They suggest that the reversal of polar­ity in this case may be explained by the presence of a strong mesoscale updraft and associated generation of charge differences by microphysical processes arising from a mix of water and ice. Fi­nally, Rutledge and Williams note that the frequency of lightening flashes is di­rectly related to the convective vigor of the storm system measured by its con­vective available potential energy (CAPE) .

Insight to application

Can these new discoveries and in­sights concerning mesoscale weather phenomena be used to improve the pre­diction of thunderstorms? To what ex­tent will the new observational systems being planned augment the process? The answer to the second question is that they can to quite an extent, it seems. For one thing, meteorologists no longer will have to depend on scattered data collected from grid points that are tens or even hundreds of kilometers apart. A national program (jointly sponsored by the Department of Commerce, the De­partment of Defense, and the Depart­ment of Transportation) and known as Nexrad (next generation radar) is under-way to provide the first comprehensive high-resolution, on-line Doppler radar network in the United States. Planned for completion in the mid-1990s it will cover the nation, utilizing an assembly of 175 Doppler radars that will provide some of the data at spacings (down to a few kilometers in critical areas near cit­ies and airports) required to analyze and predict thunderstorms and other meso­scale phenomena.

Each Doppler radar station will be de­signed to incorporate three compo­nents: the familiar scanning radar dish in its radome, which collects and trans­mits the basic data—echo reflectivity (that is, power reflected by the storm), wind speed, and spectrum width; the radar product generator, which pro­

cesses the data, and the principal user processor, the work station where they are converted into meteorological prod­ucts and deployed for evaluation. Reflec­tivity data, for instance, are digested by the computer to bring forth hourly rain­fall accumulation as well as such infor­mation as past storm tracks and future projections of storm motion, all digitized and visualized. The velocity data tells the meteorologist whether a mesocyclone and a tornado are a probable product. And the spectrum width provides infor­mation on the key elements of wind shear and turbulence in the area.

The first Nexrad station was placed in operation in 1990 near Oklahoma City. Hardly had the dish been set into scan­ning mode when radar reported winds moving toward the station on one side of the storm and away from it on the other—a sign of rotation in the storm. A tornado warning went out 19 minutes before the tornado became visible. When it touched down near the town of Laverae, it remained on the ground long enough to leave a swath of damage one mile wide and three miles long, but there were no casualties. "Our forecast was pretty good," says Ron Alberty, Director of Nexrad's Operational Sup­port Facility, who notes that the national average lead time with the old conven­tional radar is only a few minutes, often zero. Then he adds: "But I think we can do even better." Interestingly, another tornado did strike the same day, the pre­cursors of which were noted 24 minutes before touchdown. Elsewhere, out of range of the Nexrad station, four torna­does ravaged Kansas and Oklahoma kill­ing 17 people.

When Nexrad is fully operative (and that may take longer than expected b e cause of federal budget cuts), the radars will be supplemented by wind profilers located strategically in remote rural areas of the country, to avoid false r e flections from passing planes or other interference. Designed to detect the small changes in the index of refraction of the eddies moving with the wind, the wind profiler utilizes a pattern of radar pulses that are directed into the atmos­phere on three fixed beams and in con­trolled sequence. The eddies scatter the energy, and the return signals are picked up by the wind profiler's receiv­ers and translated into wind direction and speed.

Wind profilers were long used as re­search tools before they went into prac­

tical operation in 1982. Since then they have been refined enough to measure the effect of wind eddies as gentle as those caused by a butterfly's wing. The new wind profiles—generated every ten minutes compared to the present twelve hours between successive releases of balloon-borne sondes—-will provide the detailed wind picture that has long been missing from weather data-collection. NOAA has established a demonstration network of wind profilers that will be used this year for further research and for testing operational concepts.

Models

Whether these additions are enough to significantly improve thunderstorm forecasts will depend ultimately on ad­vances in numerical simulation models. These types of models are computer-generated, mathematical representa­tions of the dynamics of weather as an atmospheric event proceeds into the fu­ture from an initial state as determined by observations. The future states are calculated by integrating over time the values in a grid of points in space.

The current thunderstorm models are simulation models that are generally in­adequate for making the leap to forecast­ing because they are only designed to resolve a specific research problem and are often modified to accommodate the particular objective of the instigator. Moreover, some existing models utilize numerical solution techniques that do not work on the small scales needed for thunderstorm prediction. Even more im­portantly, they do not have the capability to continuously assimilate the necessary updated information from Doppler ra­dars and wind profilers.

Model makers believe they can over­come some of these limitations in cur­rent simulation models by building into the models more accurate formulations of the physics of the storm and its envi­ronment The science of cloud physics, the science that addresses complex particle interactions within clouds, can tell the forecasters something of the po­tential structure and movement of the cloud over given regions and enable them to correlate this information with the intensity and direction of a storm over a given area.

Some progress with mesoscale pre­dictive models has already been made. One such model, tested for its predictive value in microbursts, is a 3 dimension cloud model developed in 1986 by Jerry

54 MOSAIC Volume 23 Number 2 Summer 1992

Straka of the University of Oklahoma and John Anderson of the University of Wisconsin. In a program known as the Cooperative Httntsville Meteorological Experiment the scientists compared the model simulations with the actual atmo­spheric data from several thunder­storms in the Huntsville, Alabama re-gion during June and July of 1985. They concluded that if sufficiently dense sur­face and upper air data were put into the model, the evolution of the simulated storm appeared to be in good agreement with the real storm, thus opening up the possibility for the irst time of using a cloud model to detect the precursors of the microburst thunderstorm To test this prospect the FAA sent soundings on six storms to Straka, challenging him to identify the storm with the microburst. Straka was able to make the proper iden­tification in each case.

Other models have also shown good potential for mesoscale prediction* The most widely used mesoscale model is the Penn State-NCAR model, which was developed 18 years ago by Tom Warner of Penn State and Rick Anthes of NCAR and has been continuously refined and

tested. Currently regarded as ideal for beta-scale studies, it is expected to be further modified for higher resolution prediction problems.

The CAPS model, now under design at the University of Oklahoma, has advan­tages of its own. It has a fully interactive menu-driven, window interface for mon­itoring the progress of the prediction as events unfold and allows insertion of new data as it becomes necessary.

To determine the thermodynamics of a storm system and for analyzing its three-dimensional structure, the CAPS model, like any other numerical model, requires data of a type that is not directly observable by the Doppler radar. These data include the wind velocity in three directions and the pressure distribution in the storm. A similar problem exists with the satellite system which mea­sures the radiation leaving the tops of the clouds, from which temperature and water vapor can be inferred but fails to quantify moisture directly. The relevant data have to be deduced or retrieved by the computer and then assimilated, a process requiring considerable mathe­matical operation.

MOSAIC Volume 23 Number 2 Summer 1992 55

In the case of the CAPS model, the the­oretical underpinnings for the assimila­tion of data were established by French and Soviet mathematicians and were translated into the connective storm en­vironment by Douglas Lilly and Tzvi Gal-Chen at the University of Oklahoma and John Lewis at the National Severe Storms Laboratory. These scientists turned to a process in science of match­ing estimate and reality that goes back conceptually to the ancient Greeks. Today it is known in mathematics as control theory. It has been applied in fields as far ranging as cybernetics, bal­listics, and biology and to such other problems as predicting the trajectory of Haley's comet

Control theory

The use of control theory in determin­ing and optimizing the initial state of the atmosphere—a process known in mete­orology as initialization—has intrigued a number of meteorological research­ers. At the University of Oklahoma, Dr. Dawn Wolfsberg-Flicker, working with single Doppler radar observations, sug­gested using the so-called mathematical adjoint method to assess the consis­tency of prediction and observations. In this method two sets of equations—one for the observed variables of the storm over time and the other, its adjoint, for the predictive variables—are integrated over time and space and then matched, with changes made to bring the pre­dicted values in line with the observed. The result is to produce a four-dimen­sional dynamic portrait of the changing atmosphere. Unfortunately, this adjoint method initially proved to be rather in­efficient and expensive in terms of com­puter time. The reason: the large num­ber (2,000) of repeated calculations required to retrieve and match the un­observed variables to real atmospheric data without error.

Juanzhen (Jenny) Sun of Lanzhou University in the People's Republic of China, now working with CAPS and NCAR in Boulder, improved on Wolfsberg-Flicker's formulation. Her strategy re­moves 75 percent of the error in the first 10 iterations and the balance of the error in 50 subsequent iterations.

Other computer model initialization strategies are under development. At Penn State, for example, Tom Warner, Nelson Seaman, and David Stouffer have concentrated on a mathematical technique known as the Newtonian Re­

laxation Technique or colloquially among meteorologists as "nudging." This technique—which enables the me teorologist to modify predictions as new data appear—is advantageous for developing a four-dimensional data as­similation procedure suitable for short-term predictions. Indeed, Penn State's Michael Fritsch believes that this ap­proach is "the one most likely to allow7

rapid progress toward a viable, short-term mesoscale forecasting system for aviation." Additional models, incorporat­ing variations of this technique, are being developed at NOAA, NCAR, and at several universities, notably Colorado State, Drexel University, University of Wisconsin, Florida State, and the Uni­versity of Illinois.

All these advances on various fronts have created rising expectations among many mesoscale meteorologists that they are on the threshold of removing many of the barriers to accurate stormscale prediction. More conserva­tive weather scientists, however, are less sanguine, especially in calculating how long it will take. They are in good his­torical company. Arnold Sommerfeld, the great German theoretical physicist, for instance, once said wistfully that b e fore he died he hoped to understand two phenomena: quantum dynamics and tur­bulence. Sommerfeld died in 1924. Just before his death he was very close to understanding quantum dynamics, but admitted sadly that turbulence was still an impenetrable mystery.

Today, almost 70 years later, quantum dynamics has experienced such extraor­dinary progress that physicists routinely use the theory. Turbulence, on the other hand, is still an enigma wrapped in the elusive disorderly nonlinear behavior of the atmosphere that continues to defy mathematical analysis and remains a major obstacle to weather predictions. Nonetheless, the rapid progress in mes­oscale meteorology during the last few years, combined with the advances ex­pected in the next decade, offer atmo­spheric scientists renewed confidence in the future. They are convinced that for the first time they are possessed of the tools likely to lead to the age of accurate thunderstorm prediction. •

The National Science Foundation has contributed to the support of the research described in this article principally through several programs in its Division of Atmospheric Sciences.

56 MOSAIC Volume 23 Number 2 Summer 1992