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.^[1] 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.^[2]

Explosive intensification

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.^[1] Explosive intensification is comparatively 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 a few instances have been recorded, such as the 1999 Odisha Cyclone, the strongest storm on record in the basin, undergoing a 28 millibar pressure drop in 6 hours and Cyclone Giri undergoing 24 millibar drop in 6 hours.^[3] In these cases, however, the pressure was estimated via satellite presentation, not measured directly.^{[citation needed]}

Instances

In 1983, Super Typhoon Forrest underwent one of the most explosive deepening events ever recorded with the pressure dropping from 975 to 885 millibars as it went from a tropical storm to a category 5 super typhoon in 24 hours.^[4][5]

In 1992, Hurricane Andrew was just a minimum hurricane a day before making landfall in the Bahamas when it began to rapidly deepen into a category 5 hurricane right up to its landfall at peak strength. It weakened significantly over the island chain, but it soon hit open water again and rapidly intensified back into a category 5 hurricane just hours before making a second landfall at category 5 strength in Florida.^[6]

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.^[7]

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.^[8] 2005 was also notable in that two other storms, Katrina and Rita, also underwent episodes of extremely rapid intensification.^{[citation needed]}

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.^[9]

In 2007, Hurricane Felix rapidly intensified from a tropical depression to a Category 5 hurricane in 51 hours with a wind speed rise of 85knots/98mph in just 24 hours.^[10]

In 2010, Cyclone Ului intensified from a category one tropical cyclone to a category 5 severe tropical cyclone in 36 hours.^{[citation needed]}

In 2011, Typhoon Muifa intensified from a tropical storm to a category 4 typhoon in just 12 hours.^[11]

In 2012, Typhoon Vicente explosively deepened from a category 1 typhoon to a category 4 typhoon in just 18 hours less than 200 kilometers offshore of Hong Kong, before making landfall as a category 4 typhoon.^[12]

In that same year, Typhoon Sanba explosively deepened from a category 2 into a category 5 typhoon with the pressure dropping 80 millibars in one day.^[13]10 days later, Typhoon Jelawat underwent a very similar explosive intensification event to Typhoon Sanba.^[14] As the 2012 Pacific Typhoon Season entered the final month of the year, Typhoon Bopha went from a tropical storm to a category 4 typhoon in 24 hours while it was less than five degrees from the equator.^[15]

In 2013, Typhoon Utor intensified from a category 1 into a category 4 super typhoon with a 55 millibar pressure drop in just one day. Later that year, Typhoon Usagi very explosively deepened from a severe tropical storm to a category 5 super typhoon with the pressure dropping 70 millibars in less than a day. Also, Typhoon Francisco underwent a severe tropical storm to a super typhoon during late October 18 to 20. After Francisco, Typhoon Lekima rapidly deepened by 75 millibars in about 21 hours. It was a category 1 typhoon then became a category 5 super typhoon several hours later.^{[citation needed]} Later in November, Tropical storm Haiyan explosively intensified into a Category 5 super typhoon in 24 hours, dropping 95 millibars becoming the strongest storm of the year.

In the same year, Cyclone Phailin intensified from a Category 1 to a Category 5 tropical cyclone in about 23 hours on October 11.^{[citation needed]}

In the same year, Hurricane Raymond became a Category 3 hurricane in less than a day with the pressure dropping 54 millibars in the same time period.^{[citation needed]}

• A couple of storms also underwent rapid intensification.

See also

• Tropical cyclone
• Bomb (meteorology)
• Annular tropical cyclone

References