Hurricanes are large storms that form, and intensify, in warm ocean waters. Hurricanes are nearly circular storms 200-600 km across and about 15 km high. Hurricanes are not frontal systems, as is common in mid latitudes. Instead, they are characterized by a center of extremely low surface air pressure with a steep pressure gradient from the center outward producing a series of closed concentric isobars. Sea-level air pressure varies from near the atmospheric mean value of 1013 mb in the environment (at a radius of 1000 km from the storm center) to below 900 mb in extreme cases. The lowest minimum pressure ever recorded in the center of a hurricane was 870 mb.
In response to the pressure difference, winds flow from the perimeter toward the center of the storm, but friction (and other forces) causes the wind to follow a spiral path.
Fig. 2. Streamlines for wind in a typical mature hurricane at the 950 mb level (from
Hurricane Inez in 1966).
(Adapted from Anthes 1982)
Wind cannot indefinitely spiral inward toward the eye of a hurricane. At a distance of about 400 km from the storm center, the inflowing air becomes convergent, the mean vertical motion is upward, and cumulus convection begins.
Spiral Cloud Bands. Spiral bands of clouds radiate outward from the center of the hurricane. The bands form near the eye and propagate radially outward. Some investigators have shown that the bands do not rotate about the storm center, but instead remain in the quadrant in which they formed. Precipitation occurs in these well defined bands, rather than uniformly throughout the hurricane.
Fig. 3. Satellite Image of Hurricane Floyd, September 14, 1999, 12:59 UT, showing spiral cloud bands. Data from NOAA GOES-8 satellite.
Image produced by Hal Pierce, Laboratory for Atmospheres, NASA Goddard Space Flight Center.
The dominant clouds in the spiral bands are towering cumulus, and cumulonimbus. However, cirrostratus typically form a crown over the hurricane, and may spread outward as far as 800 to 1000 kilometers. Precipitation can be quite heavy in the bands (a typical rate is 30 mm/h). Liquid water contents reach values as high as 5 g/m3. In regions of heavy precipitation the temperature may decrease slightly, but on the whole there is little difference in temperature across the bands.
Hurricane spiral bands consist of individual convective cells associated with small-scale cumulus convection. The thunderstorms within the spiral bands are organized into regions of rising and sinking air.
Fig. 4. Vertical cross section showing air movement associated with cumulus convection in spiral cloud bands.
Graphic by Robert Simmon, NASA GSFC.
In contrast to the motion of the band, which moves considerably slower than the mean wind speed, the individual cells tend to move with the mean wind in the layer in which they are embedded. These convective cells, which have typical lifetimes of 20-40 min, are a result of a general lifting within the band of convectively unstable air. New thunderstorms form on the upwind (inner) side of a band, travel through the band and dissipate on the downwind (outer) side.
Hurricane Eye Wall. At a certain distance, which varies from 10 to 100 km in most storms, the inflowing air suddenly turns upward in a ring of intense convection surrounding the center. This ring or wall of convection is called the eye wall and it is here that the strongest wind speeds and heaviest precipitation occur. Rainfall rates can exceed 20 mm/hr in the eye wall.
Fig. 5. Rainfall rate from TRMM PR for a Pacific Typhoon in 2000.
Image courtesy of the National Space Development Agency of Japan (NASDA).
Hurricane Eye. Inside the radius of the eye wall, the winds and precipitation decrease rapidly. At the center light winds and generally descending air comprise the hurricane eye, which is one of the most characteristic features of the tropical cyclone. Eyes range in size from 8 km (5 mi) to over 200 km (120 mi) across, but most are approximately 30-60 km (20-40 mi) in diameter. The eye is so calm because the strong surface winds that converge towards the center never reach it. The coriolis force deflects the wind slightly away from the center, causing the wind to rotate around the center of the hurricane (the eye wall), leaving the exact center (the eye) calm.
Why is the eye clear? An eye becomes visible when some of the rising air in the eye wall is forced towards the center of the storm instead of outward -- where most of it goes. This air is coming inward towards the center from all directions. This convergence causes the air to actually sink in the eye. This sinking creates a warmer environment and the clouds evaporate leaving a clear area in the center (see warm core formation below). Although subsidence in the eye is sometimes sufficiently strong to produce clear skies, there are often shallow cumulus clouds near the surface. The eye is easily recognized in radar photographs and satellite pictures unless a cirrus canopy covers the top of the storm.
Winds at different heights in a hurricane. The strength of winds blowing in toward a hurricane eye varies with elevation above the sea surface. The strongest inflow occurs at an elevation of about 500 m. As the air is pulled closer to the storm center by the strong radial pressure gradient, its rotational velocity increases rapidly. This increase is a consequence of the partial conservation of angular momentum.
Fig. 6. Vertical structure of wind in an average Pacific typhoon. Positive isotachs
indicate wind moving in a cyclonic direction, negative isotachs indicate wind moving
in an anticyclonic direction.
(Adapted from Anthes, 1982)
Warm core formation. During the intensification of a tropical storm into a hurricane, the low pressure cell at the center of the storm is transformed from what is called a cold-core low into a warm core low. In a cold core low, a trough of low pressure is observed from the surface to the upper troposphere, and ascent of air occurs thoughout the air column (see Fig 7A.). In a cold core low, we have relatively cold air in the lower layers and relatively warm air aloft. Intensity of the low pressure system is maximum at the height where the system changes from being relatively cold to being relatively warm. In a warm-core low, a weak ridge of high pressure occurs at about 500 mb, and descent of air occurs above the ridge. A trough of low pressure, and ascent of air, occur at the surface.
Fig. 7. Vertical structure of air pressure and air movement in a (A) cold-core low (B) warm-core low.
(Adapted from From & Staver, 1979)
During the initial development stages of a hurricane, as the radial and tangential wind speeds increase, so does the magnitude of the low-level moisture convergence and the rainfall. Water vapor condenses within the storm, releasing latent heat of vaporization, and the heated air rises. The latent heat released by these convective cloud clusters warms the upper troposphere (500 ® 200 mb). Expansional cooling of the rising air triggers more condensation, release of even more latent heat, and a further increase in buoyancy. Deep convection produces a dense cirrus overcast at the tropopause.
During this intensification process, clusters of intense thunderstorms, called convective bursts, occur. A single thunderstorm within a convective burst is known as a hot tower. Hot towers are cumulonimbus towers with cloud tops higher than 37,000 ft (~15 km).
The relationship between the occurrence of convective bursts, warm core formation and sudden intensification of the hurricane is significant. In a warm-core low, a weak ridge of high pressure occurs about 2.5 to 3.0 km above the surface (~500 mb level); subsidence occurs above the ridge, and ascent below (see Fig 7b and Fig.10). The warm core develops through the action of 100-200 cumulonimbus towers releasing latent heat of condensation; about 15 per cent of the area of cloud bands is giving rain at any one time. Observations show that although these 'hot towers' form less than 1 per cent of the storm area within a radius of about 400 km (230 miles), their effect is sufficient to change the environment.
Fig. 9. Temperature and isobars in a warm-core low pressure system.
(Adapted from Moran & Morgan, 1997)
Hot towers result in strong subsidence in the eye of the hurricane. The descending motion starts at the top of the warm core and penetrates gradually downwards.
Fig. 10. Descending air in the eye of a hurricane. The vertical scale of the hurricane
is greatly exaggerated.
In the eye, adiabatic warming of descending air accentuates the high temperatures, although since high temperatures are also observed in the eyewall cloud masses, subsiding air can only be one contributory factor. The region of warmer temperatures at the core (the warm core) results in lower surface pressure and a consequent intensification of winds. The severity of winds, rain, and pressure deficiency at the center of the system increases fast after it has become a warm core vortex. If forecasters can identify a warm core in satellite imagery, they have a good indication that the storm is going to intensify.
Fig. 11. Vertical transect of precipitation rate through the eye of
Hurricane Bonnie, as measured by TRMM PR. Hurricane Bonnie's
cumulonimbus storm clouds towered 18 km (59,000 ft) from the eye
of the storm. After these hot towers were observed, Bonnieęs central
pressure dropped from 977 to 957 mb in 24 hours.
The height in this image is exaggerated for clarity, and colors correspond to
precipitation from blue (light) to red (heavy). (Image by Greg Shirah, GSFC
Scientific Visualization Studio; Data courtesy TRMM Project.)
The warm core is vital to hurricane growth because it intensifies the upper anticyclone (see below), leading to a "feedback' effect. When the surface pressure decreases, a larger pressure gradient is formed, and more air converges towards the center of the storm. This creates more surface convergence and causes more warm moist surface air to rise above the surface. This air, as it cools, condenses into clouds. While it does this, it releases even more latent heat and intensifies the upper level high pressure. This enhancement of a storm system by cumulus convection is termed Conditional Instability of the Second Kind, or CISK.
Upper level flow and the anticyclonic jet. As air leaving the center of the hurricane reaches an elevation of about 12 km above sea level (200 mb level), it produces high pressure near the tropopause, and divergent flow away from the focus of the hurricane. This upper level outflow circulates in an anticyclonic direction, typically at a radius of 300 km, and is termed the anticyclonic jet.
The anticyclonic jet supports the ascent of air at the center of the hurricane, and maintains dropping air pressure, which, in turn, induces more rapid convergence of air at the surface. The consequent uplift surrounding the developing eye leads to additional condensation and release of latent heat, an example of positive feedback. If the divergence at the top of the air column is greater than the convergence at the surface, surface pressure will decrease. Hence, surface winds will increase, as more air is rushed in to fill the gap. As long as the supply of energy can overcome forces such as friction, the intensity of the vortex will keep increasing. If the upper atmosphere can be sufficiently warmed and filled with moisture, cumulonimbus clouds can continue to grow up to the tropopause level, thus both covering a larger area of the storm with clouds, and allowing air to be transported aloft more easily. It is important that the mechanism exhausting the storm be sufficiently strong enough to carry the air away from the storm, so that it will not be circulated back into the storm at lower levels. A large-scale outflow at 200 mb, is therefore, critical to the intensification of tropical cyclones. A hurricane may further intensify if the jet stream, or a large pocket of cold air passes over at 40,000 ft. Both will enhance the storm's exhaust and accelerate its circulation.
Fig. 13. Upper level features that intensify the circulation inside a hurricane.
(Adapted from Whipple, 1987)
Ocean Thermal Energy. Tropical cyclones can have a profound influence on the temperature of the ocean over which they travel. The altered ocean temperature, in turn, can feed back and alter the character of the cyclone. The inner region (0-240 km) of the average hurricane consumes on the order of 150 x 106J m2/ (4000 cal cm2/d) of ocean sensible and latent heat energy. In the image below we see the wake of Hurricane Bonnie after it crossed the Atlantic, leaving a cooler trail of water in its wake. Hurricane Danielle, following behind, crossed Bonnie's path and was affected by the loss of ocean thermal energy available to fuel it.
In the scene below, clouds (acquired by GOES) have been made translucent to allow an unobstructed view of the surface. Notice Hurricane Bonnie approaching the Carolina Coast (upper left) and Hurricane Danielle following roughly in its path (lower right). The ocean surface has been falsely colored to show a map of water temperature, measured by the TRMM Microwave Imager (TMI). Dark blues are around 75 deg F, light blues are about 80 deg F, greens are about 85 deg F, and yellows are roughly 90 deg F. As Hurricane Bonnie crossed the Atlantic, it left a cooler trail of water in its wake. As Hurricane Danielle crossed Bonnie's path, the wind speed of the second storm droped markedly, as available energy to fuel the storm's engine dropped off.
Fig. 15. Cooler SST in the wake of Hurricane Bonnie, as detected by the TRMM
Microwave Imager.
Image courtesy TRMM Project, Remote Sensing Systems, and Scientific Visualization Studio,
NASA Goddard Space Flight Center.
