Alternative Science & History News

By Halverson, Jeffrey B 

From RedOrbit.com

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.  For a link to the story please click below.

http://redorbit.com/news/science/967300/hunting_hurricanes_nasa_seeks_answers_in_african_dust/index.html