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Hurricane Dynamics

Hurricanes are nearly circular, warm-core vortices, characteristically 1000 km in radius. Strong winds and precipitation are concentrated near, but not at, their centers. Coincident with strongest winds, convective updrafts, fed by inward spiraling surface winds, release latent heat drawn from the ocean to power the storm. The clear eye, 15-30 km in radius, contains the axis of vortex rotation and is surrounded by this ring of strong winds. The eye is characterized by the warmest temperatures in the vortex, low humidity, low pressure, in extreme cases > 10% below that in the undisturbed tropical atmosphere, and calm winds at the very center. The hurricane's eye presents one of the truly magnificent vistas offered to human sight, and the hurricane's workings delight the mind as an intricate and subtle problem in hydrodynamics.

A typical radar image shows the essential structure of a hurricane: a clear eye enclosed by a ring of clouds, which is in turn surrounded by inward spiraling bands of convection. These features are common to all tropical cyclones. Hurricanes take their circular shape from the orbits of air moving in gradient balance around the low atmospheric pressure at the vortex center.

A cross section of a hurricane in the radius-height plane shows that the primary swirling flow is maintained by a radial and vertical secondary circulation. Under the influence of friction, the counter-clockwise spiraling air near the surface converges toward the lower pressure and rises around the eye in cumulus clouds. Although the inflowing air loses angular momentum to the sea, it gains heat stored as water vapor. The vapor condenses in the updrafts, releasing the stored heat and causing intense precipitation. This latent heat release is the power source for the storm. At 2-6 km altitude the updrafts entrain additional air, which is replaced by horizontal convergence that supplies both mass and angular momentum needed to spin up the wind. The updrafts may rise as high as 16 kilometers where the air flows outward to the environment. The source of the storm's energy is heat drawn from the warm sea surface at 28-29°C and returned to the surrounding upper atmosphere at -70°C.

In the center of the hurricane is the cloud-free eye. The clouds that enclose the eye form the eyewall. These clouds draw air from the eye at low levels, causing descent, drying, and warming inside the eye. As the air warms, it becomes less dense so surface pressure must fall. The clouds also draw air inward from outside the eye, thus concentrating the counter-clockwise rotation and increasing the swirling winds. In a hurricane, the strongest winds are near the surface and just outside the eyewall. An unanticipated use of the new GPS sondes was direct observation of low-level wind jets at the inner edge of the eyewall.

Hurricane Opal's northward careen across the Gulf of Mexico, showing schematically the storm's rapid deepening as a result of interaction with a "digging" mid-latitude trough and it's passage over the deep pool of warm water in an eddy spun off from the Gulf Stream.

As the experience in Hurricane Opal of 1995 illustrates, intensity predictions compare poorly to the skill exhibited by track forecasts. Forecasters at the National Hurricane Center use a half dozen models to guide track forecasts. For intensity forecasting, only one model has been available (until recently) and that was based on climatology and persistence. These rudimentary tools reflect the challenge of forecasting intensity. It is a difficult problem and it is the focus of continuing research. AOML scientists recently developed a more complete statistical hurricane intensity prediction model that includes the effects of variations in sea-surface temperature and upper tropospheric turbulent transports. Although this model is available in real time and has had some success, there is still much to be learned before accurate intensity forecasts become routine.

A typical hurricane intensifies slowly, remaining in Category 1 or reaching 2 or even 3 before it runs ashore or drifts north out of the tropics. The strongest hurricanes, such as Andrew in 1992, intensify rapidly and go from Category 1 or 2 to Category 4 or 5 in just a day or two. The process that initiates rapid intensification may begin with atmospheric waves that form on the hurricane vortex at altitudes of greater than 12 kilometers (where the new jet will fly). These waves result from interaction with nearby low-pressure systems. The waves, through a complicated chain of cause and effect, intensify the cumulus clouds that are involved in the energy conversion mechanisms of a hurricane.

Shear of the environmental wind appears to be the mechanism by which atmospheric teleconnections modulate Atlantic hurricane activity and is generally thought to have a significant role in day-to-day intensity changes of individual storms. Airborne Doppler and reflectivity radar data collected during AOML research flights into Hurricane Olivia show that an environmental shear > 10 m s^-1 imposes a wavenumber-one structure on the eyewall convection. Individual cells form ~45 to the right of the down-shear direction, reach maturity with reflectivities > 45 dBZ on the left side of the shear vector, and have largely rained out by the time they detach from the eyewall as they advect back to the right side of the shear. Similar observationally-based synthesis point the way to a more accurate understanding of rapid intensification and ultimately to better forecasting of the intensity of the most dangerous hurricanes.

AOML research has produced accurate, timely maps of surface winds in hurricanes. These maps, which are based upon aircraft observations, are invaluable for preparation of forecasts before landfall and for damage assessment after landfall. The maps are now prepared routinely whenever a hurricane threatens land. A particularly important application of these products is the rapid identification of the most severely devastated areas; this enables disaster response teams to reach them quickly. The same techniques are now being applied for reconstruction of historical storms.

The Stepped Frequency Microwave Radiometer (SMRF) is a promising tool for direct measurement of surface winds. This instrument has flown aboard research aircraft for more than 20 years and has finally matured enough to transition to operations. It works by sensing the increase in apparent microwave brightness temperature of the water surface caused by more foam as the wind increases. Because rain in the air column between the airplane and the surface also produces a frequency-dependant contribution to the upwelling microwave radiation, the radiometer needs to look at several different frequencies to correct for the precipitation effect and to deduce rainfall rate as a byproduct.