Rapid deepening

Hurricane Charley nearing landfall after its rapid deepening phase

Rapid deepening, also known as rapid intensification, is a meteorological condition that occurs when the minimum sea-level atmospheric pressure of a tropical cyclone decreases drastically in a short period of time. The National Weather Service describes rapid deepening as a decrease of 42 millibars in less than 24 hours. However, this phrase is liberally applied to most storms undergoing rapid intensification.

Necessary conditions

In order for rapid deepening to occur, several conditions must be in place. Water temperatures must be extremely warm (near or above 30°C, 86°F), and water of this temperature must be sufficiently deep such that waves do not churn deeper cooler waters up to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well — for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.

Explosive intensification

Cyclone Giri nearing landfall after going through explosive intensification

Explosive intensification is a more extreme case of rapid deepening that involves the central pressure of a tropical cyclone deepening at a rate of at least 2.5 millibars (0.074 inHg) per hour for a minimum of 12 hours. Explosive intensification is rather rare, as conditions must be exceedingly favorable for cyclone intensification. Explosive intensification occurs regularly in the West Pacific basin, with the greatest frequency off the north coast of Australia; however, it has occurred numerous times in the Atlantic basin, as with 2008's Hurricane Ike with the hurricane undergoing a 24 millibar pressure drop in 3 hours. Hurricane Igor in 2010 is another prime example. It is rare in the North Indian Ocean, but Cyclone Giri is a good example of a storm going through explosive intensification in this basin. In these cases, however, the pressure was estimated via satellite presentation, not measured directly.

Instances

In 2004, Hurricane Charley was approaching the west coast of Florida as a category two storm on the Saffir-Simpson scale of hurricane strength. When just off the coast, however, its sustained winds rapidly increased from 110 to 150 mph (along with a pressure drop from 965 to 941 mbar) in only three hours. Charley caused unprecedented destruction in the Punta Gorda area, and inflicted major damage across the state of Florida.

Hurricane Wilma at record intensity

In 2005, the minimum central pressure of Hurricane Wilma dropped from 981 to 882 mbar in 24 hours, including a drop of 53 mbar in less than 6 hours. This is by far the fastest intensification of any Atlantic hurricane, and possibly the fastest intensification for any system in recorded history. Typhoon Forrest in 1983 may have strengthened faster, deepening from 976 to 876 mbar in just under 24 hours, but pressure estimates for this storm were much less accurate. 2005 was also notable in that two other storms, Katrina and Rita, also underwent episodes of extremely rapid intensification.

In 2006, the minimum central pressure of Typhoon Chebi in the Western Pacific dropped 75 mbar in 24 hours, including a 60 mbar pressure drop in 6 hours, as it intensified from a tropical storm to a category four equivalent typhoon in one advisory.

In 2010, Cyclone Ului intensified from a tropical storm to a Category 5-equivalent in just 24 hours.

See also

• Tropical cyclone
• Bomb (meteorology)
• Annular hurricane