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Rapid intensification ( RI) is any process wherein a tropical cyclone strengthens very dramatically in a short period of time. Tropical cyclone forecasting agencies utilize differing thresholds for designating rapid intensification events, though the most widely used definition stipulates an increase in the maximum sustained winds of a tropical cyclone of at least in a 24-hour period. However, periods of rapid intensification often last longer than a day. About 20–30% of all tropical cyclones undergo rapid intensification, including a majority of tropical cyclones with peak wind speeds exceeding .
Rapid intensification constitutes a major source of error for tropical cyclone forecasting, and its predictability is commonly cited as a key area for improvement. The specific physical mechanisms that underlie rapid intensification and the environmental conditions necessary to support rapid intensification are unclear due to the complex interactions between the environment surrounding tropical cyclones and internal processes within the storms. Rapid intensification events are typically associated with warm sea surface temperatures and the availability of moist and potentially unstable air. The effect of wind shear on tropical cyclones is highly variable and can both enable or prevent rapid intensification. Rapid intensification events are also linked to the appearance of and bursts of strong convection within the core region of tropical cyclones, but it is not known whether such convective bursts are a cause or a byproduct of rapid intensification.
The frequency of rapid intensification has increased over the last four decades globally, both over open waters and near coastlines. The increased likelihood of rapid intensification has been linked with an increased tendency for tropical cyclone environments to enable intensification as a result of climate change. These changes may arise from warming ocean waters and the influence on climate change on the thermodynamic characteristics of the troposphere.
Tropical cyclones frequently become more axisymmetric prior to rapid intensification, with a strong relationship between a storm's degree of axisymmetry during initial development and its intensification rate. However, the asymmetric emergence of strong convection and near within inner core of tropical cyclones can also portend rapid intensification. The development of localized deep convection (termed "convective bursts") increases the structural organization of tropical cyclones in the upper troposphere and offsets the entrainment of drier and more stable air from the lower stratosphere, but whether bursts of deep convection induce rapid intensification or vice versa is unclear. Hot towers have been implicated in rapid intensification, though they have diagnostically seen varied impacts across basins. The frequency and intensity of lightning in the inner core region may be related to rapid intensification. A survey of tropical cyclones sampled by the Tropical Rainfall Measuring Mission suggested that rapidly intensifying storms were distinguished from other storms by the large extent and high magnitude of rainfall in their inner core regions. However, the physical mechanisms that drive rapid intensification do not appear to be fundamentally different from those that drive slower rates of intensification.
The characteristics of environments in which storms rapidly intensify do not vastly differ from those that engender slower intensification rates. High sea surface temperatures and oceanic heat content are potentially crucial in enabling rapid intensification. Waters with strong horizontal SST gradients or strong salinity stratification may favor stronger air–sea fluxes of enthalpy and moisture, providing more conducive conditions for rapid intensification. The presence of a favorable environment alone does not always lead to rapid intensification. Vertical wind shear adds additional uncertainty in predicting the behavior of storm intensity and the timing of rapid intensification. The presence of wind shear concentrates convective available potential energy (CAPE) and helicity and strengthens inflow within the downshear region of the tropical cyclone. Such conditions are conducive to vigorous rotating convection, which can induce rapid intensification if located close enough to the tropical cyclone's core of high vorticity. However, wind shear also concurrently produces conditions unfavorable to convection within a tropical cyclone's upshear region by entraining dry air into the storm and inducing subsidence. These upshear conditions can be brought into the initially favorable downshear regions, becoming deleterious to the tropical cyclone's intensity and forestalling rapid intensification. Simulations also suggest that rapid intensification episodes are sensitive to the timing of wind shear. Tropical cyclones that undergo rapid intensification in the presence of moderate () wind shear may exhibit similarly asymmetric convective structures. In such cases, outflow from the sheared tropical cyclone may interact with the surrounding environment in ways that locally reduce wind shear and permit further intensification. The interaction of tropical cyclones with upper-tropospheric troughs can also be conducive to rapid intensification, particularly when involving troughs with Shortwave trough and larger distances between the trough and the tropical cyclone.
Within environments favorable for rapid intensification, stochastic internal processes within storms play a larger role in modulating the rate of intensification. In some cases, the onset of rapid intensification is preceded by the large release of convective instability from moist air (characterized by high equivalent potential temperature), enabling an increase in convection around the center of the tropical cyclone. Rapid intensification events may also be related to the character and distribution of convection about the tropical cyclone. One study indicated that a substantial increase in stratiform precipitation throughout the storm signified the beginning of rapid intensification. In 2023, a National Center for Atmospheric Research study of rapid intensification using computer simulations identified two pathways for tropical cyclones to rapidly intensifying. In the "marathon" mode of rapid intensification, conducive environmental conditions including low wind shear and high SSTs promote symmetric intensification of tropical cyclone at a relatively moderate pace over a prolonged period. The "sprint" mode of rapid intensification is faster and more brief, but typically occurs in conditions long assumed to be unfavorable for intensification, such as in the presence of strong wind shear. This faster mode involves convective bursts removed from the tropical cyclone center that can rearrange the storm circulation or produce a new center of circulation. The modeled tropical cyclones undergoing the sprint mode of rapid intensification tended to peak at lower intensities (sustained winds below ) than those undergoing the marathon mode of rapid intensification.
Because forecast errors at 24-hour leadtimes are greater for rapidly intensifying tropical cyclones than other cases, operational forecasts do not typically depict rapid intensification. Probabilistic and deterministic forecasting tools have been developed to increase forecast confidence and aid forecasters in anticipating rapid intensification episodes. These aids have been integrated into the operational forecasting procedures of Regional Specialized Meteorological Centers (RSMCs) and are factored into tropical cyclone intensity forecasts worldwide. For example, the Rapid Intensification Index (RII)a quantification of the likelihood of rapid intensification for varying degrees of wind increases based on forecasts of environmental parametersis utilized by RSMC Tokyo–Typhoon Center, the Australian Bureau of Meteorology (BOM), and the NHC. An intensity prediction product is being developed at RSMC La Réunion for the South-West Indian Ocean based on tools developed in other tropical cyclone basins. The Rapid Intensity Prediction Aid (RIPA) increases the consensus intensity forecast provided by the JTWC's principal tropical cyclone intensity forecasting aid if at least a 40% chance of rapid intensification is assessed and has been used since 2018. The JTWC reported that a large increasing trend in the probability of rapid intensification assessed using RIPA was associated with higher likelihoods of rapid intensification. The JTWC is also experimenting with additional rapid intensification forecasting aids relying on a variety of statistical methods. Intensity forecasting tools incorporating predictors for rapid intensification are also being developed and used in operations at other forecasting agencies such as the Korea Meteorological Administration and the Indian Meteorological Department.
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