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.
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.
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.