Hunting Hurricanes: NASA Seeks Answers in African Dust
By Halverson, Jeffrey B
THE OFFICIAL TALLY FOR THE 2006 ATLANTIC HURRICANE SEASON STANDS AT nine named storms, slightly below the average of 10 per year. Of its own accord, the statistic hardly seems unusual; nine is just a slight dip below long-term expectations. But when you compare this number with the activity level of the previous year-the blockbuster season that spawned 28 named storms and many new records-the juxtaposition is exceptional. The average annual number of Atlantic hurricanes undergoes a slow rhythmic oscillation over decades, alternating between highs and lows. But the threefold reduction in activity in 2006, including the landfall of just one marginal hurricane in the United States, occurred smack in the middle of what many believe is an ongoing cycle of enhanced Atlantic tropical activity. And this is doubly interesting because the dip occurred during what might otherwise appear to be a steady upward trend in the frequency of hurricanes, nudged along by accelerated climate warming.
In an effort to gain insight into the behavior of Atlantic hurricanes and understand the forces that cause such differences, in 2006 NASA launched a mission to study the birth of hurricanes. Many of the powerful late-fall storms that take aim at the U.S. Gulf Coast and eastern seaboard are born over Africa. The goal of the mission-dubbed NAMMA for NASA African Monsoon Multidisciplinary Activities-was to fly high-altitude research aircraft into the maw of early cloud disturbances to tease out the precise mechanisms by which a storm’s spin becomes organized. In addition, mission scientists hoped to gain a better understanding of how Saharan dust might curb the disturbances’ formation into hurricanes.
Out of Africa
The question of how to unravel the processes by which innocuous tropical disturbances transform into deadly hurricanes is one that tropical meteorologists have been trying to answer for a century. The metamorphosis of these disturbances into hurricanes is called tropical cyclogenesis. Understanding this process involves divining not just the myriad steps that occur within the storm clouds themselves, but also the manner in which the larger storm environment influences the growth of rotating storms.
The vast, central stretches of equatorial Africa trigger some 60 to 70 tropical wave disturbances each year. These linear zones of low pressure with embedded storm energy exit Africa and move steadily west, but on average only about 10 percent become named tropical systems, a category that includes both tropical storms and hurricanes. While some of those systems mushroom rapidly into full- fledged tropical cyclones, other seedlings exhibit tentative signs of organized spin-a sputtering of the tropical heat engine-and then fizzle.
Hurricanes thrive off water vapor and are characterized by torrential rainfall, so it might seem odd that an Atlantic hurricane’s development depends partly on its relationship with desiccated, dusty air masses that sweep off Africa. However, for years hurricane researchers suspected a relationship between the Saharan Air Layer (SAL) and wave disturbances struggling to become hurricanes. Many studies suggested that the SAL acts to suppress storm development. In the past several years, scientists at the NOAA Hurricane Research Division have pioneered much of this early research, flying their Gulfstream IV aircraft into veils of yellow- tinged air over the western and central Atlantic. Satellites also monitored the trajectory of the giant dust clouds billowing off Africa.
Launching NAMMA
In 2006, NASA launched an experiment to use a legion of new scientific tools to penetrate the dust layer and examine how the SAL upsets the gestation of hurricanes. The plan for NAMMA was to take NASA’s high altitude DC 8 aircraft to the western edge of Senegal, where vortices embedded within atmospheric disturbances called tropical easterly waves first mingle with tendrils of Saharan air. Using the new technology, mission scientists could then sample the storm-dust interface in ways that had never been possible before.
In one of the major SAL dust outbreaks during NAMMA, an instrumented BAe-146 aircraft operated by scientists at the United Kingdom Meteorology Office departed Dakar, Senegal, to rendezvous with the DC-8 over the Atlantic. The BAe-146 carried sensors specially designed to sample the composition and size of the microscopic dust particles embedded within the SAL. Figure 1 on the preceding page illustrates some of the close choreography as these two aircraft flew side-by-side collecting data on the dust.
The NASA DC-8 is a flying laboratory that cruises at normal jetliner altitudes (31,000-39,000 feet) and speeds (450-500 knots). But in place of rows of seats there are racks of meteorological sensors and computers (Figure 2, preceding page). Meteorological probes hang beneath the wings and along the sides of the fuselage. Circular glass ports are built into the fuselage’s top and bottom so that laser beams and radiation sensors have a clear view of the atmosphere. There is a tube that issues from the lower empennage where dropsondes, or instrumented packages on a parachute, are ejected at high pressure from the aircraft. The flight scientist charts the course of the eight-hour mission, optimizing and fine- tuning the track to intercept meteorological targets of interest in line with specific science objectives. For a given mission, the primary objective might be to fly a rosette-shaped survey pattern of intersecting flight lines across the vortex of a developing hurricane. Or it might be a series of long flight legs transecting the SAL layer at different altitudes. It also might be a slow downward spiral through a heavily precipitating cloud cluster, in order to sample different areas of freezing and condensing water. Or it might be a perfectly timed satellite "underpass," in which the NASA aircraft flies beneath the orbital track of a satellite. During NAMMA, one of these underpass missions coincided with the orbit of the NASA satellite Calypso, which measures the thickness and horizontal spread of dust layers from space.
During the flight, there is constant chatter between the flight scientist, the plane’s navigator, the various instrument scientists, and the pilots. The flight scientist can conduct a real-time conversation with one or more mission scientists on the ground, using a satellite linked utility on a laptop called X-Chat. The ground scientists generally have better access to the latest satellite, radar, and lightning detection datasets and can help vector the aircraft to suitable areas of interest. During NAMMA, the mission scientists and flight scientists used a customized Google- Earth application called the Real Time Mission Monitor (RTMM), developed by NASA, to keep track of the aircraft in relation to the various cloud systems and dust clouds (Figure 3, preceding page).
The DC-8′s instrument payload was designed to measure just about every conceivable facet of the atmosphere. There are highly sensitive hygrometers designed to measure minute quantities of water vapor, down to several parts per billion. During NAMMA, a sophisticated Doppler weather radar, which was a prototype for an instrument to fly on a future satellite, scanned the skies below for heavy rain. Various probes captured two-dimensional video images of microscopic cloud and precipitation particles, ranging from tiny ice crystals to larger-rimed graupel particles and raindrops. A detector was used to identify the chemical composition of Saharan dusts and the sizes of the various particles. A vertically pointing laser beam mapped the intervening layers of water vapor and dusts within the SAL with exquisite detail.
Hurricane Hunting Off Senegal
During the last two weeks of August 2006, a team of NAMMA researchers that included Ed Zipser of the University of Utah, Gerry Heymsfield of NASA’s Goddard Space Flight Center, Robbie Hood of NASA’s Marshall Space Flight Center, and Jeffrey Halverson of the University of Maryland, Baltimore County, embarked on a one-month deployment out of the Cape Verde Islands (Figure 4). The scientists, who were directed by Ramesh Kakar at NASA’s headquarters, flew missions into seven different wave disturbances emerging off Senegal. As the waves emerged, the NASA DC-8 intercepted them and followed their development in the direction of the United States. Farther downstream, four of the waves developed into named storms. These included the Category 1 Ernesto, Tropical Storm Debby, and Category 3 Hurricanes Gordon and Helene. It was fortunate that NAMMA scientists were able to study a variety of different types of systems, because it is important to understand why some of the African waves failed to mature into rotating disturbances. It was also critical to fly back-to-back missions on two consecutive days into as many of the waves as possible; this allowed mission scientists to examine hurricanes at different stages of their genesis. The more detailed snapshots of this process that scientists can obtain, the more complete the picture of each system’s unique birth story.
On August 23, the DC-8 embarked on an eight-hour survey flight of a wave that had emerged off the coast of Senegal. It came off the coast complete with a rotating column of air, the incipient vortex that is critical for developing an eye. NOAA’s Tropical Prediction Center considered the disturbance organized enough to warrant the title of Tropical Storm Debby, with a maximum sustained wind of 45 knots. Scientists were able to take a snapshot from the RTMM of this system showing the temperatures of the cloud tops. The colder the clouds, the deeper they are, and in this manner the location of the most intense thunderstorm cells can be identified. This is important for not only scientific purposes, but also for aircraft safety. Debby was fairly compact, with modestly tall thunderstorm tops west of Cape Verde. The DC-8 flew west to intercept the storm, passing through the estimated center on three occasions and sampling the atmosphere in all quadrants surrounding the center. Using a combination of flight-level winds and vertical wind profiles obtained from dropsondes, the scientists found an embedded vortex struggling to survive. Its upper level was displaced well to the southeast of the lower level circulation, and rotation in the low levels was highly asymmetric, with strong winds on the northeast side and weak winds to the southwest. Thunderstorm activity in and around the center diminished throughout the mission. Large amounts of Saharan dust were found encroaching on the storm’s core from all sides. Fragmented and lopsided, Debby was struggling to shed her veil of dry, dusty air. She would struggle west for several more days. NASA then tossed the baton downstream to NOAA, which used its Gulfstream research aircraft to study the SAL interaction with Debby. In this manner, scientists maintained good scientific continuity with this storm, which then fizzled completely on August 27th. Why Debby did not thrive, and the role the SAL might have played in her demise, are two of questions that NAMMA and NOAA the critical questions that NAMMA and NOAA scientists hope to answer. On September 12, the DC-8 flew another sortie into a strong, rotating African disturbance (Figure 5). While there was a dust layer present on the north side of the disturbance, the vortex appeared to be strong and intensifying. In fact, rotation had been tracked across Africa and the various weather radar networks measured vigorous thunderstorm activity. Figure 5 suggests that by the time the system emerged off the coast, the spin was much better organized than in the case of Debby. The Tropical Prediction Center labeled this system Tropical Depression 8, a precursor to tropical storm status. The DC-8 flew into an especially intense cluster of thunderstorms in the northwest quadrant that gave scientists a bumpy ride and resulted in a lightning strike on the DC-8. The system did not yet possess the all-important warm core that defines a hurricane. But within 24 hours, Tropical Storm Helene was born. On the 16th she became a hurricane, before rapidly intensifying to Category 3 status on the 19th. This became the eighth and nearly final named storm of the 2006 Atlantic hurricane season.
From Dust to Dust
Why exactly did the system in NAMMA’s first sortie fizzle into nothingness while the second sortie’s storm morphed into Hurricane Debby? The scientists from NAMMA believe that the SAL plays a major role in whether or not a system develops into a hurricane. There are numerous hypotheses on how African dust might foil hurricanes.
The dust particles themselves likely impact the microscopic growth of cloud droplets and precipitation particles and alter the proportions of super-cooled water and ice that develop within clouds. This in turn impacts how heat energy is released in ascending turrets of air, since phase changes of water (such as vapor condensing into liquid or liquid freezing into ice) liberates varying amounts of latent heat.
But the answer probably does not lie solely with the microscopic dust particles themselves. Because it originates over the world’s most expansive subtropical expanse of desert, the air layer in which these tiny aerosols are suspended is hot and dry. It also contains substantial wind shear-that is, wind that increases its speed with height. The air mass presents a hostile environment for waves struggling to build a vertical core of rotating wind. Strong shear can fragment or otherwise interrupt the fragile vortex, or decouple it from the clusters of thunderstorms that supply energy. The thunderstorms fall victim to the dry air. The air mass is so moisture-starved that relative humidities on the order of 10 percent are common, compared with the moist 85 percent humidity that characterizes the layer just above the ocean. As dry air is entrained deep into clouds, precipitation evaporates and cools the air in and around the clouds. Since one of the key early steps in creating a hurricane is to generate a plug of anomalously warm air inside the vortex center (the so-called "warm core" within the eye), an atmosphere that cools instead of warms strikes a major blow against genesis.
Assessing the Data: Demystifying Tropical Cyclogenesis
The two Atlantic tempests that NAMMA scientists flew into during their mission vividly portray the contrasting downstream fates of wave disturbances that incubate over western Africa. In May 2007 the NAMMA scientists will meet in Baltimore to study terabytes of data collected during the mission. They will discuss hypotheses regarding vortex formation, the development of warm cores, the impact of the SAL, and how the various cloud microphysical processes likely influenced storm development. Some exciting and completely new theories are likely to emerge from the meeting, and with this improved physical understanding, future computer models that simulate hurricane development and intensification will produce more skillful forecasts.
But the general consensus that is emerging is this: Atlantic cyclogenesis is a delicate balancing act, a seesaw of factors involving ocean temperature, wind shear, dry and warm Saharan air layers, organized clusters of thunderstorms, and half a dozen other influences. As nascent African whirls cross the Atlantic, they thread in and out of regions both favorable and hostile to continued growth. NASA and NOAA scientists have now collected goldmines of data in a variety of locations throughout the tropical oceans of the western hemisphere (see sidebar). This includes data collected on missions on both sides of the Atlantic, where the tall mountains of Central America can trigger storm formation on the far western side, and Saharan air might suppress storm growth on the eastern side. The true challenge will be to identify a core set of tropical atmospheric processes universally required for cyclogenesis, versus processes that appear to have a strong regional dependence. New technologies have finally cracked open the mysterious puzzle that holds clues to tropical cyclogenesis, sparking brighter prospects for better predicting the most powerful storms on Earth.
NASA’s Hurricane History
For more than a decade, NASA has sponsored a series of research missions to investigate intensification of Atlantic hurricanes. The experiments began with missions flown out of Florida in 1998 and 2001. During July 2005, NASA operated its ER-2 stratospheric aircraft out of Costa Rica. The plane flew over two powerful early- season storms in the Gulf of Mexico, Hurricanes Dennis and Emily. It also collected data on the genesis of Tropical Storm Gert in the Caribbean and Tropical Storm Eugene in the eastern Pacific. NASA has always partnered with NOAA’s Hurricane Research Division during these campaigns. There is great synergy in bringing these two agencies together: NOAA operates its turboprop aircraft in the lower and mid-level regions of the storms, while NASA jets access the uppermost levels. Through carefully orchestrated vertical aircraft separation and overlapping survey patterns, as many as four NASA- NOAA research aircraft can simultaneously investigate entire storms from 70,000 feet down to the ocean surface.
The NAMMA experiment conducted by NASA and NOAA was one component of a much broader, international effort to understand summertime weather systems percolating during the western Africa monsoon. This is the AMMA part of the "NAMMA" acronym. Vast networks of weather radars, upper air balloons, surface weather stations, and satellite sensors were operated across Africa during the summer and fall of 2005. The goal was to investigate the origins and structure of African easterly waves. The waves produce much of the region’s rainfall during their transit across the country and therefore have major socioeconomic impacts on the region’s water balance.
Contributing editor JEFFREY B. HALVERSON is an associate professor of geography at the University of Maryland, Baltimore County. He was also a mission scientist on the NAMMA expedition.
Copyright Heldref Publications May/Jun 2007