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Tropical Cyclogenesis

Storm Center

Eye of the Storm

A tropical cyclone (TC) rotates counter-clockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere, due to the Coriolis force of the Earth rotation (explained in the preceding page of this article).

The vertical axis of this horizontal rotation is considered the center of the TC. The area around this central axis is called the eye of the storm.

The eye of the storm, immediately around the central axis of the TC rotation, is calm and has the lowest air pressure.

The eye of the storm is separated from the rest of the TC by an eye wall of upward convection that is the strongest convection of the storm, packing the highest winds of the TC, right next to the calm eye.

The top of the TC eye may be open to the sky, showing blue sky above during the daytime, or stars at night, if you are in the eye looking up. If the eye of the storm is clear like that, looking down at the eye (from spacecraft), the ocean or land beneath the TC may be seen in the eye.

Figure 1:  The eye of Hurricane Ian approaching the west coast of Florida, Sept. 2022. Storm rotation is counter-clockwise (in the Northern Hemisphere). Convection clouds can be seen along the eye wall, upwind (south) from the coast. The eye has sparse clouds, allowing view of the ocean below. [NASA]

Figure 2:  Night time view, looking up from aircraft within the eye of Hurricane Lee, 7 Sept. 2023, while the eye was well-formed and not cloud covered, with lightning in the eye wall. Star light is visible in the night sky in later frames of the video. Small lights in some other frames are cockpit window reflections. Video courtesy of DOD: The appearance of U.S. Department of Defense (DoD) visual information does not imply or constitute DoD endorsement. [DOD]

Eyewall Replacement

A smaller eye makes the TC stronger by conservation of angular momentum (see the Rotation page of this article), but a TC eye cannot get too small since that would, for example, not allow adequate perimeter for stronger eye wall convection.

Not a problem for an intensifying TC, because as intensification shrinks an already small TC eye, a second eye wall begins to form outside of the smaller eye. That causes a temporary weakening of the TC, causing the eye to be absorbed by the new outer eye wall which becomes the new eye of the storm. TC intensification resumes after eye replacement.

Eyewall Convection Towers

Tropical cyclones that are more powerful, due to ideal conditions (see the Introduction of this article), are more symmetrical, with convection towers (cumulonimbus overtopping) ringing the outside of the eye wall.

Following is a visible light animation of the eye of Hurricane Michael:

Figure 3:  The eye of Hurricane Michael approaching the NW Florida coast, after sunrise on the 10th of October 2018, showing convection towers (cumulonimbus overtopping) around the eye. [NOAA]

In this animation, it is easier to see the cumulonimbus overtopping in the early morning animation frames, due to extra shading from the lower sunlight angle. When the Sun is higher in the sky, after the initial animation frames, there are less shadows to help see the convection towers.

Shadow Enhancement

One way to help overcome shrinking shadows from higher Sun position, for preliminary analysis, is to use image processing to emphasize shadows.

Consider the following satellite image of Tropical Cyclone Jasmine:

Figure 4:  Tropical Cyclone Jasmine, on the 9th of February 2012. [NASA]

The clouds around the eye are not easily discernable in that image.

To enhance shadows in that image, we opened the image in Gimp (free image processing program), and selected Colors > Desaturate > Color to gray (with “Enhance Shadows” checked), producing the following image that emphasizes shade gradients to make the clouds more discernable:

Figure 5:  The preceding figure, processed in Gimp to enhance shadows.

This type of image processing is biased depending on sunlight angle, but could be useful for quick preview of storm characteristics.

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Tropical Cyclogenesis

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Cumulonimbus Clouds
Cirrus Clouds
Storm Rotation
Eye of Storm (this page)
Jova & Lee 2023

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