RAIN MAN: A Different Kind of Hurricane Hunter
All my life I have yearned to experience the blinding fury of a hurricane and the ironic clarity of its
eye. For years, first as a boy locked indoors by worried parents during several hurricane passages up
the Eastern Seaboard, and later as a meteorologist, I was never quite able to realize this dream. But
in science, chance discoveries often lead us down unexpected paths. Thus, it was only by joining the
ranks of the isotope hunters that I finally became a hurricane hunter as well.
Not long ago, isotope hunters were a small and relatively obscure group of scientists who, with little
or no fanfare, trekked to the far ends of the Earth, and even into space, in search of their subtle prey-atoms with different numbers of neutrons than normal. Back home in the lab, they used painstaking
care to analyze their isotopes in samples of wood, water, ice, rock, meteorites, and precious gems.
From astronauts collecting rocks in space to climatologists drilling for ice cores in Antarctica, isotope
hunters have been piecing together the history of our planet and providing us with clues to its future.
So, it should not be surprising that watching isotopes as they course through rainfall in hurricanes
may enable us to discover a trick or two about the internal workings of these magnificent and
devastating storms.
Nature's Clocks and Thermometers
Isotopes serve as the premier clocks and thermometers of ancient events because the seemingly tiny
difference of a neutron or two enables scientists to trace an object's past history. For example,
radioactive isotopes of uranium and carbon serve as clocks because they are unstable and decay into
other atoms or isotopes at fixed rates. The older a rock layer or an artifact such as a wood carving,
the lower the concentration of the original radioactive material.
By contrast, stable isotopes do not change with time. Instead, they fractionate or separate to some
degree during processes such as evaporation and condensation. And since the degree of separation or
fractionation depends on temperature, stable isotopes serve as nature's thermometer.
For example, Oxygen 18 (O-18) has 10 neutrons--two more than the most common form of oxygen,
O-16. Those two extra neutrons actually make the atom heavier, and heavy isotopes do not
evaporate as easily as their lighter counterparts. Thus, when water evaporates from the ocean, every
gram of vapor contains about one percent fewer O-18 atoms than sea water.
But once the air cools enough for clouds to form, the heavy isotopes condense more readily. The
more the air cools the more vapor is squeezed from the air, and the lower the concentration of the
heavy isotopes in the remaining vapor and precipitation. Since the polar ice sheets mark the end of
the condensation chain, not only are they the coldest places on Earth, but they contain the smallest
concentrations of O-18 of all waters on Earth. And snow that fell during the Ice Ages contained even
less.
Thus, wherever ancient waters are preserved, the abundance of O-18 represents the premier archive
of past temperature. But even though stable isotopes are best known as proxies of past climates,
their message accumulates one storm at a time. And rooting out their meteorological message is what
drew me to hunting isotopes.
The Call
My life as an isotope hunter began in 1980 when Jim Lawrence, a stable isotope geochemist, sought
me out for an explanation of what could cause isotopes to vary in storms. Jim, who had always
secretly loved weather, had discovered that major East Coast snowstorms passing south of New York
City had much less O-18 than storms that tracked farther north.
I quickly showed that when tropical air slides over a warm frontal surface, heavy isotopes
concentrate in the precipitation and separate as if they had been placed in a giant refinery. We
realized that isotopes provide an odd but unique perspective for studying snowstorms.
In 1984, Jim relocated to the University of Houston. Needless to say, there aren't many snowstorms
down in southern Texas, but there sure are hurricanes! Jim, who collects more data than the IRS,
began going out into the paths of Texas hurricanes, planting rain collectors (PVC pipes sealed at the
bottom and open at the top) in the ground or on sturdy fence posts.
Starting with Hurricane Gilbert in 1988, Jim found that hurricanes and tropical storms have much less
O-18 than other tropical or summer rains, and even less than some major East Coast snowstorms. We
then began the slow process of trying to explain why.
Hurricanes as Isotope Refineries
Hurricanes act as giant isotope refineries. Air laden with isotope-enriched vapor spirals in toward the
center at low levels. It rises in spiral bands or in the eyewall, where it produces torrential precipitation
as it ascends to great heights. Near the tropopause, the air turns out and exits the hurricane, leaving
almost all the water behind.
As air rises through the clouds, the heavy O-18's are squeezed out first and are most concentrated in
the rain that forms at low levels. Rains from storms with low cloud tops are rich in O-18 because their
clouds simply quit squeezing at modest heights. But because hurricanes keep squeezing water from
their towering clouds, they produce ice with little O-18 at high altitudes. In thunderstorms, much of
this ice escapes through the anvil and evaporates, but under the hurricane's extensive cloud canopy
most of the ice falls and reaches the ground where it dilutes the O-18 concentration of the rain.
In addition, falling raindrops acquire O-18 from the water vapor in the surrounding air. The first rain to
reach the ground in any storm is laden with O-18, but in the process it has removed O-18 from the
vapor that remains in the air.
In storms without organized circulation, much of the depleted vapor simply mixes into the surrounding
atmosphere. But in the organized circulation of hurricanes, O-18's are progressively stripped from the
vapor during its inward journey toward the eyewall. The depleted vapor eventually rises to produce
precipitation with less O-18 than before.
We were excited when our rain samples confirmed our expectations that O-18 concentrations tend to
decrease toward the centers of hurricanes.
In the Queue
Because hurricanes are three-dimensional storms, there were many features that could only be
explained by collecting rain and vapor samples above ground level. So in the fall of 1993, we
contacted Bob Burpee, then director of NOAA's Hurricane Research Division (HRD), who invited us to
fly with them. My almost-forgotten dream of flying into a hurricane was revived.
It took a year to design and install a window panel for the NOAA aircraft with tubes to the outside that
allowed us to collect both rain and water vapor. And because 1994 was such a poor year for Atlantic
hurricanes, it was only at the last moment that Jim got the call to join HRD in Puerto Vallarta, Mexico.
They would fly into Hurricane Olivia in the Eastern Pacific. The flights were conducted two days in a
row and proved to be a scientific success.
On the first day, as Olivia intensified, the hurricane thrust all the high-level ice far out from the center.
As a result, rain in and near the eyewall had substantially higher O-18 concentrations than on the
second day, when Olivia was filling and had much weaker updrafts. This allowed ice from high levels to
fall almost directly below where it formed. Consequently, the rain contained almost as little O-18 as
the snow in Greenland or Antarctica. Jim was ecstatic.
But I was heartbroken. Not only had I been left behind to wait for another year, but Hurricane Olivia
turned out to have one of the most spectacular eyewalls the meteorologists had ever seen.
By contrast, the 1995 hurricane season turned out to be too active, too early. The hurricanes came so
fast and furious it seemed the planes were always in the air, so it was well into August before our
instruments were reinstalled.
This meant we were grounded for August. So when Hurricane Felix threatened North Carolina, Jim and
I drove there, setting down rain samplers all over the place. Then we waited, and waited, and waited.
For two days, Felix dawdled near the coast without striking the mainland. Finally it moved out to sea,
a surfer's delight but a great letdown for us. Yet the tinkling rains we collected from Felix's extreme
outer fringes contained as much O-18 as normal summer rains. This confirmed our hypothesis that
hurricane rains remove heavy isotopes from the water vapor as the air spirals inward.
Flying at Last
On Friday, September 29, 1995, with the season rapidly drawing to a close and my hopes of flying
dwindling, Opal began to stir. On Saturday, I was put on standby. If only one plane flew, Jim would
once again get the nod. But because this hurricane would probably take aim at the United States, both
planes were readied.
~~~~~~~~
By Monday, October 2, the likely landfall for Opal was Louisiana, and I was put in a state of medium
alert. On Tuesday morning, this became high alert, and the call came at 2:30 p.m. when the updated
forecast made it a go. Two hours later I was on a commercial flight from Newark to Tampa. There, I
joined Jim, who had driven from Houston to Tampa installing rain samplers all along the way.
Our leisurely dinner that evening in Tampa with the HRD meteorologists gave no warning of how
hectic things become when hurricanes are involved. We were scheduled to fly at 8:00 p.m.
Wednesday evening. But overnight on Tuesday, Opal deepened explosively when it passed over a
warm water eddy in the Gulf of Mexico and accelerated toward its eventual landfall at Navarre Beach,
Florida.
Flight time was upped to 12:00 noon and we raced to McDill Air Force Base. Then, as we were setting
up in the hangar, flight time was upped once again to 11:00 a.m. This happened so fast that HRD's
second team couldn't get to Tampa and I had no time for dry runs. It also meant that only one plane
would take off, and the one seat would once again rightly go to Jim.
The next few minutes were totally chaotic. I can't describe how downcast I felt as I raced with Jim to
the plane, helping him carry all our equipment, expecting to have to get right back off immediately
and wait in the hanger for the next 12 hours while everyone else went into the hurricane. Suddenly,
Jim told me I was going and he was staying on ground because I needed the training. But that didn't
make me feel much better because there was a good chance I would ruin the data. Then,
miraculously, Hugh Willoughby, HRD's new director, found room on board for both of us. At last I was
going into a hurricane.
Nature's Coliseum
Jim immediately began collecting vapor samples as the plane climbed to 16,000 feet, and I was soon
in the swing of things. Once the data collection was working, my nervousness vanished. At no time
was I nervous about the potential danger of flying into a hurricane. I was simply too excited. I was
prepared to get sick and throw up as the necessary price of the process.
But the flight turned out to be only mildly bumpy, with modest turbulence. And because Jim and I
worked together, we each had some precious free moments in the cockpit for a bird's eye view of the
eye, the Grand Canyon of cloud systems.
When we entered the eye it was as if we were standing on the stage of a gigantic coliseum, five
hundred times higher and larger than Yankee Stadium. Below us, Opal's center was marked by a
smooth cloud shaped like a giant chocolate kiss. Just inside the base of the eyewall, concentric rows of
stratocumulus clouds formed an inner circle. The rows telescoped outward and upward to much larger
circular rows in the eyewall that stretched high above us into the distance.
Opal's eye was almost shut by a deck of high clouds, but a few glints of sunshine penetrated to
highlight the scene. Opal was a weakening, but still ferocious monster. Its inner circle of clouds raced
along at a stunning speed. The ocean churned incessantly. Turquoise patches that gleamed like jewels
in the sunlight formed where the wind had decapitated the waves. And even though we were too high
to identify individual waves, I could see how the winds shredded the ocean surface, lacing it with milelong trails of spray and spume that blurred the boundary between air and sea. Later, the memory of
that sight would help me explain why the rain in Opal's crescent-shaped eyewall contained so much O18.
We tracked Opal all day, weaving through its eye 10 times as it moved northward across the gulf, until
it plowed into the coast of Florida. Then, we turned toward home to a magnificent sunset, while on the
ground below, Opal produced $3 billion in damage and claimed 59 lives. The next morning Jim headed
back to Houston, taking two days to gather the rain samples from all the collectors he had installed in
the ground, while I flew back to New York to teach, arriving in class feeling like Indiana Jones.
Validation
During that time my daughter, Elise, had joined the isotope hunt, and carefully collected rain samples
from Opal's embers. Her samples showed that the spatial pattern of isotopes matched Opal's growing
asymmetry as it transformed into an extratropical cyclone.
Jim's rain and vapor samples taught us even more about isotopes and hurricanes. A day before Opal
began its move out of the gulf, a modest squall line had passed through eastern Texas and Louisiana.
Jim quizzed me on the results. "How much O-18 do you think the Texas squall line had?"
"Plenty," said I. "It had unimpressive cloud tops, and it occurred far from Opal and long before it hit
the coast."
"Wrong again!" Jim said.
The rain and vapor associated with this squall line contained so little O-18, we suspected that Opal had
something to do with it. This was very strange because it meant that the wind had to be blowing out
from the vicinity of Opal. Sure enough, trajectories of air that wound up in eastern Texas early on
October 3 pointed right back to the central Gulf of Mexico where on the 1st and 2nd, several
mesoscale convective systems had popped up as Opal took form. We then hypothesized that the
mesoscale convective systems that prime the atmosphere for hurricanes start removing the heavy
isotopes.
Jim confirmed our hypothesis by traveling down to Puerto Escondido, Mexico, in July, 1998. His
samples showed that developing tropical depressions and storms flush much of the O-18 out of the
water vapor long before the system reaches hurricane status.
Finally, we were incredulous to find that the rain 20,000 feet above sea level in Opal's eyewall had
almost as much O-18 as rain at sea level. Then I recalled the spray and spume and realized that as it
evaporates it fills the air with O-18-enriched vapor. The high O-18 concentration implied that a
significant fraction of the water for the rain in Opal's eyewall came from the spray and spume.
Since Opal, I have been privileged to fly into two more hurricanes: Hortense in 1996 and Mitch in
1998. Hortense passed out to sea after strafing Puerto Rico. While it quietly disappeared as a news
item, the O-18 content of the vapor samples in the eye suggest that the air had leaked in from the
eyewall.
By contrast, Mitch was both a human catastrophe and an isotopic disaster. The instrumented plane
had just been grounded, and in the rush, the window panel was not transferred to my plane. As a
result, I could not collect a single drop of the torrential rains that doused us as we lurched through the
monstrous killer storm over the Caribbean Sea near Honduras.
Grounded: Down but Not Out
Jim and I only get the call to fly into one or two storms a year. Other scientists, and increasingly the
national press, stake claims for the precious few seats. But even though being grounded keeps us
down, it doesn't keep us out. When a hurricane threatens the mainland, we drive to intercept it.
Reaching the coast well before its arrival, we install our samplers at the highest points and then
retreat inland until the storm has passed.
While neither of us have felt at risk personally, we have seen some impressive damage and had our
share of minor incidents. In retrospect, some are almost funny. During the landfall of Hurricane
Georges in 1998, Jim's windshield wiper failed and he had to collect rain samples in a driving rain with
his head out the window like Jim Carrey in the movie Ace Ventura, Pet Detective. When Hurricane
Bonnie came ashore in 1998, I watched an eerily silent lightning display near Norfolk, Virginia, only to
discover it was the transformers blowing.
During Hurricane Floyd last year, I put down collectors from New York to South Carolina, threading my
way along almost empty roads, avoiding the horrendous evacuation traffic heading out of Charleston.
After Floyd passed, I retraced my route under clear skies, but the swollen rivers transformed my trip
back home into a maze of detours tainted by the faint smell of dead livestock. I had counted on
flooding but never imagined that such large sections of land and major highways would be inundated.
I arrived home to a final surprise. In Floyd's vanguard, a coastal front had formed, dumping 11 inches
of rain right over our home in Upper Saddle River, New Jersey. The small stream in my backyard rose
from its normal five inches to five feet and recarved the stream bed. Further downstream, water levels
rose 15 feet after a dam burst. Homes along the river were trashed, but amazingly, no one drowned.
A Rare Vintage
The results from Floyd look intriguing. From Florida to Massachusetts, O-18 content increased as Floyd
approached and decreased to very low values once the storm center and the coastal front passed by.
Oddly, Wilmington, North Carolina, directly in Floyd's path, had the highest O-18 content, implying
that much of the condensation took place at relatively low heights. This matches the scenario painted
by Weatherwise Highlights columnist Lee Grenci, who pointed out in the January/February 2000 issue
that much of Wilmington's rain fell well ahead of Floyd from a combination of surface friction and a
tropical version of a warm front.
Because isotopes can only reveal their story after they have been analyzed in the lab, they cannot be
used in real time to help diagnose a storm's evolution. At the moment, when we still have much to
learn, the meteorological fascination of isotopes lies in the fact that they can illustrate so many
aspects of the lives of hurricanes.
The O-18 in emeralds can tell us whether they were mined in South America or Asia. The O-18 in wine
can tell us both the region and the year in which the grape was grown. Indeed, if you could taste the
O-18 from a hurricane, you would realize that each one has its own rare vintage, and you would be
able to read not only where the storm has been, but also what processes led it there. How intriguing it
is to consider how much more isotopes can tell us, and where else they may lead us.
Fronts act
as
isotope
refineries.
As tropical
air
rides
over
a
warm
front,
heavy O18s
(shown in
orange)
condense
preferentially. As a result, the further from the front,
the less O-18 in the rain and snow.
The author takes the pilot's seat during a brief lull in
Hurricane Opal.
The eye of Hurricane
gargantuan stadium.
Sunset from inside Opal.
Georges
resembles
a
In 1998, the author flew into deadly Hurricane Mitch, which killed thousands of people in Central America.
Above and opposite page: Trajectories of snow and ice crystals in 1994's Eastern Pacific Hurrican Olivia, based on dual Doppler
radar analyses of winds by John Gamache of the Hurrican Research Divission. On September 24, strong updrafts and outflow
thrust snow far out from the eyewall so rain intercepted by the plane (#1) came from low levels. The resulting rain had more O18.
As circulation weakened on the 25th, accumulated snow near the cloud tops fell to flight level, so much of the rain intercepted by
the plane (#20) originated at high levels. The resulting rain had less O-18.
~~~~~~~~
By Stanley David Gedzelman
Stanley David Gedzelman is a professor of meteorology at the City College of New York and a
Weatherwise executive editor.
Source: Weatherwise, Nov/Dec2000, Vol. 53 Issue 6, p26, 6p