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Tropical Cyclogenesis

Cumulonimbus Clouds

Figure 1:  Cumulonimbus cloud, raining near Galveston Texas. [Matthew T Rader]

Cumulonimbus clouds, also called thunderclouds or convection clouds, are the towering clouds that produce thunderstorms and severe weather. These are the tallest clouds, drawing moist air vertically upward (convection).

Figure 2:  Types of clouds, with Cumulonimbus (abbreviated Cb) at left. [Wiki]

Cumulonimbus clouds require a vertical temperature gradient for humid air to convect upward: the atmosphere must be sufficiently cooler at increasing altitudes to provide bouyancy for convection.

Anvil Top

At high enough altitudes above the Earth, the atmosphere becomes too warm for clouds to form. At that altitude, further vertical convection is stopped (lacking a temperature gradient for bouyancy), but convection from below continues, causing the top of the convection cloud to bunch up and spread out horizontally as an “anvil”.

Figure 3:  Large cumulonimbus cloud, with anvil top. [Simon Eugster]

Figure 4:  Aircraft view of a large cumulonimbus cloud with anvil. [Shawn]

Overshooting Top

If convection is strong enough, upward momentum can cause the updraft to surpass the top of the anvil, forming an overshooting top. The duration of the overshoot dome may indicate how severe the thunderstorm is.

Figure 5:  Spacecraft view of a cumulonimbus cloud with overshooting top, oblique viewing direction. [NASA]

Figure 6:  Cumulonimbus cloud with sharp overshoot, viewed from above. Low sun angle casts shadow that helps identify the updraft overshoot. [NASA]

Figure 7:  Cumulonimbus clouds in various stages of development (click on image to view larger in a new browser window). [NASA]

Geostationary Infrared

The preceding photographs and satellite images are in the daytime. Daylight is the visible light range, which is the band of wavelengths a human eye can see (as colors).

Those type of images show varied details depending on the position of observation and depending on the position of the Sun (discussed later).

Another problem, with this type of imagery, is that it does not acquire images at night, when there is no sunlight to reflect to the observation sensor.

This type of imagery is referred to as passive because it does not generate its own light, unlike for example radar systems, which are active (generate their own “illumination” that is reflected back).

Another type of passive imagery acquisition type is infrared (IR) systems. These sensors, on a geostationary orbit platform, are usually lower resolution than the visible light sensors, but allow passive acquisition of images without sunlight.

A passive IR system that provides less detail because it is lower resolution may still be useful, because it is better than nothing at night, plus it has been found to allow users to deduce more information by using animation.

For example, while it may be difficult to identify some cumulonimbus clouds in geostationary broad band IR satellite imagery using still images, animating a temporal sequence of the images allows for more identification of cumulonimbus clouds because the temporal behaviour of Cb clouds is obvious in animation sequences.

Animation opens in new window
Figure 8:  Animation sequence of Hurricane Franklin, 28 August 2023.  Night (predawn) is lower resolution infrared (grayscale). Daytime (after sunrise) is visible light (sunlight reflectance, higher resolution). [CIRA]

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Tropical Cyclogenesis

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

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