16.1: Tropical Cyclone Structure

Tropical cyclones are made of thunderstorms. Near the center (core) of the tropical cyclone is a ring or circle of thunderstorms called the eyewall. This is the most violent part of the storm with the heaviest rain and the greatest radar reflectivity (Fig. 16.2). The tops of these thunderstorms can be in the lower stratosphere: 15 to 18 km high. Thunderstorm bases are very low: in the boundary layer. Thus, Tropical cyclones span the tropical-troposphere depth.

Figure 16.2 Radar reflectivity PPI image of Hurricane Katrina, taken from a research aircraft at altitude 2325 m on 28 Aug 2005 at 1747 UTC while it was over the Gulf of Mexico. Dark orange regions in the eyewall correspond to 35 to 40 dBZ radar reflectivity. (Image courtesy of US DOC/NOAA/AOML/ Hurricane Research Division.)

The anvils from each of the thunderstorms in the eyewall merge into one large roughly-circular cloud shield that is visible by satellite (Fig. 16.1). These anvils spread outward 75 to 150 km away from the eye wall. Hence, tropical cyclone diameters are roughly 10 to 20 times their depth (Fig. 16.3), although the high-altitude outflow from the top of some asymmetric tropical cyclones can reach 1000s of km (Fig. 16.4).

In the middle of the eyewall is a calmer region called the eye with warm temperatures, subsiding (sinking) air, and fewer or no clouds. Eye diameter at sea level is 20 to 50 km. The eye is conical, with the larger diameter at the storm top (Fig. 16.5).

Spiraling out from the eye wall can be zero or more bands of thunderstorms called spiral bands.

Figure 16.3 Hurricane Isabel as viewed from the International Space Station on 13 Sep 2003. The hurricane eye is outlined with the thin black oval, while the outflow from the eyewall thunderstorms fills most of this image. For scale comparison, the white circle outlines a nearby thunderstorm of depth and diameter of roughly 15 km. [Image courtesy of NASA - Johnson Spaceflight Center.]

Figure 16.4 NOAA GOES satellite-derived 3-D rendering of hurricane Floyd at 2015 UTC on 15 Sep 1999, showing asymmetric outflow of this Category 4 storm extending 1000s of km north, past the Great Lakes into Canada. The eye is in the NW Atlantic just to the east of the Georgia-Florida border, USA. [Image by Hal Pierce is courtesy of the Laboratory for Atmospheres, NASA Goddard Space Flight Center.]

Figure 16.5 Vertical slice through a tropical cyclone. Green, yellow, orange, and red colors suggest the moderate to heavy rainfall as seen by radar. Light blue represents stratiform (St) clouds. The lighter rain that falls from the lower stratiform cloud deck of real tropical cyclones is not shown here. Tropopause altitude ≈ 15 km. Tropical-cyclone width ≈ 1500 km. Fair-weather trade-wind cumulus (Cu) clouds are sketched with grey shading. Arrows show radial and vertical wind directions.

Figure 16.6 Zoomed view of double eyewall in Hurricane Rita, as viewed by airborne radar at 1 km altitude on 22 Sep 2005 at 1801 UTC. Darker greys show two concentric rings of heavier rain. Darkest grey corresponds to roughly 40 dBZ. [Modified from original image by Michael Bell and Wen-Chau Lee.]

Sometimes these spiral rain bands will merge to form a temporary second eyewall of thunderstorms around the original eye wall (Fig. 16.6). The lighter rain between the two eyewalls is called the moat.

During an eyewall replacement cycle, in very strong tropical cyclones, the inner eyewall dissipates and is replaced by the outer eyewall. When this happens, tropical cyclone intensity sometimes diminishes temporarily, and then strengthens again when the new outer eyewall diameter shrinks to that of the original eyewall.