Observational analysis of tropical cyclone formation (original) (raw)
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Journal of the Atmospheric Sciences, 2007
This is the first of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. The primary TC-LAPS vortex enhancement mechanism is presented in Part I, the entire genesis process is illustrated in Part II using a single TC-LAPS simulation, and in Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS.
Tropical-Cyclone Formation: Theory and Idealized Modelling
2010
This report summarizes work completed since IWTC-VI that contributes to an improved understanding of tropical-cyclone formation and a potentially useful perspective for forecasters and researchers. The topic of tropical cyclone formation has been the subject of considerable active research in the past four years. Much of the recent research reviewed here points to a unified view of the genesis and intensification process. It provides also a basis for some new tools to aid in forecasting tropical-cyclone formation. 2.1.1. Introduction The most up-to-date review of the topic is that given by Tory and Frank (2010), which forms a chapter of the recent WMO-sponsored update on "Tropical Cyclones: From Science to Mitigation" (2010, Kepert and Chan, Eds.). Much of the work described below has been published since their review went to press. Before embarking on this review, it is appropriate to discuss some relevant terminology. The glossary on the Hurricane Research Division's website 1 uses "tropical cyclone as the generic term for a nonfrontal synoptic-scale low-pressure system over tropical or subtropical waters with organized convection (i.e. thunderstorm activity) and a definite cyclonic surface wind circulation (Holland 1993)." Notably, this definition does not invoke any wind threshold. The same glossary defines a tropical depression as a tropical cyclone with maximum sustained surface winds of less than 17 m s -1 (34 kt, 39 mph) and, in the Atlantic and Eastern Pacific Basins, a "tropical storm" as a tropical cyclone with surface winds between 17 m s -1 and 33 m s -1 .
Western North Pacific Tropical Cyclone Formation and Structure Change in TCS-08
2012
The long-term goal of this project is to develop a better understanding of mesoscale and synoptic-scale processes associated with the entire life cycle of tropical cyclones in the western North Pacific. The inability to correctly identify tropical cyclone formation over the period of 24 h-48 h poses a threat to shore and afloat assets across the western North Pacific. Furthermore, once a tropical cyclone has formed the predictability of structure changes during intensification of tropical cyclones is very low, which is due to complex physical processes that vary over a wide range of space and time scales. Periods of reduced predictability occur throughout the tropical cyclone life cycle, which includes the decaying stage. Because decaying tropical cyclones often transition to a fast-moving and rapidlydeveloping extratropical cyclone that may contain gale-, storm-, or hurricane-force winds, there is a need to improve understanding and prediction of the extratropical transition phase of a decaying tropical cyclone. The structural evolution of the transition from a tropical to an extratropical circulation involves rapid changes to the wind, cloud, and precipitation patterns that potentially impact maritime and shore-based facilities. OBJECTIVES A primary objective is to increase understanding of the formation of a tropical cyclone from what may have been a disorganized area of deep convection or a weak pre-existing cyclonic disturbance. Over the monsoon environment of the tropical western North Pacific, pre-tropical cyclone disturbances range from low-level waves in the easterlies to large monsoon depressions. An objective of this project is to define factors that impact the large-scale atmospheric and oceanic controls on tropical cyclone formation. A long-term goal is to understand the relative role(s) of mesoscale processes in organizing a pretropical cyclone disturbance such that it may begin to intensify as a tropical cyclone. A specific
Geophysical Research Letters
We analyze the spatiotemporal variations of tropical cyclone (TC) genesis and associated environmental conditions over the western North Pacific with a series of data science techniques, including Gaussian kernel estimator, wavelet, cross-wavelet coherence, and regression analyses. There are significant semiannual and annual variations of TC genesis over the northern South China Sea and oceanic areas east of the Philippines. Variations on the El Niño-Southern Oscillation timescale are prominent between 5-10°N, 155-160°E. With reconstructed TC series on those frequencies, we further quantify the influences of environmental variables on the primary TC signals over western North Pacific. Over northern South China Sea and oceanic areas east of the Philippines, 40% of the reconstructed TC variance can be explained by vertical shear of zonal wind, relative humidity, and absolute vorticity. The reconstructed TC series near (160°E, 7°N) has strong but varying in-phase relationships with El Niño-Southern Oscillation, which provides deeper insight into their nonstationary and nonlinear relations than their modest correlation. Plain Language Summary Many previous studies about tropical cyclone formation mainly focused on basin-wide or global scale characteristics. This study provides a comprehensive analysis of the spatial and temporal distributions of tropical cyclones formation in the western North Pacific. We use observational and reanalysis data to investigate the spatial patterns of tropical cyclone formation in the western North Pacific. Over the South China Sea and oceanic areas east of the Philippines, tropical cyclone formation has strong half-yearly and yearly variations. Over the southeastern part of western North Pacific, tropical cyclone formation has strong 4-to 8-year variations, which are closely associated with El Niño-Southern Oscillation. We also explore the linear and nonlinear relationships between tropical cyclone formation and favorable formation variables spatially and temporally. The primary variability of tropical cyclone formation can be considerably explained by the change of wind with height, moisture, vorticity, and the El Niño-Southern Oscillation. In the southeastern part of western North Pacific, tropical cyclone formation reveals strong nonlinear and nonstationary relationships with the change of wind with height and vorticity.
Atmosphere-Ocean Coupling in Tropical Cyclones
2009
The advent of numerical weather prediction tropical cyclone (TC) models has demonstrably improved the forecasts of TCs over the last decades. But to establish useful warning systems for TCs, it is necessary to accurately predict both storm track and intensity. Whereas TC tracks are determined almost exclusively by their large-scale atmospheric environment, storm intensity is influenced to a greater degree by smaller-scale features in both the atmosphere and ocean. The factors that control the intensity of TCs are still poorly understood, leading to limited reliability in forecasts of TC intensity evolution. Variability in TC intensity originates from two sources: internal variability and environmental interactions. There are three critical aspects of the environmental interactions: 1) the dynamical and microphysical processes near and at the sea surface that influence the turbulent exchange of heat and momentum between the ocean and atmosphere, 2) vertical and horizontal transport o...
The importance of low-deformation vorticity in tropical cyclone formation
Atmospheric Chemistry and Physics, 2013
Studies of tropical cyclone (TC) formation from tropical waves have shown that TC formation requires a wave-relative quasi-closed circulation: the "marsupial pouch" concept. This results in a layerwise nearly contained region of atmosphere in which the modification of moisture, temperature and vorticity profiles by convective and boundary layer processes occurs undisturbed. The pouch concept is further developed in this paper. TCs develop near the centre of the pouch where the flow is in near solid body rotation. A reference-frame independent parameter is introduced that effectively measures the level of solid-body rotation in the lower troposphere. The parameter is the product of a normalized Okubo-Weiss parameter and absolute vorticity (OWZ). Using 20 yr of ERA-interim reanalysis data and the IBTrACS global TC database, it is shown 95 % of TCs including, but not limited to, those forming in tropical waves are associated with enhanced levels of OWZ on both the 850 and 500 hPa pressure levels at the time of TC declaration, while 90 % show enhanced OWZ for at least 24 h prior to declaration. This result prompts the question of whether the pouch concept extends beyond wave-type formation to all TC formations worldwide. Combining the OWZ with a low vertical shear requirement and lower troposphere relative humidity thresholds, an imminent genesis parameter is defined. The parameter includes only relatively large-scale fluid properties that are resolved by coarse grid model data (>150 km), which means it can be used as a TC detector for climate model applications. It is also useful as a cyclogenesis diagnostic in higher resolution models such as real-time global forecast models.
2012
The overarching objectives of this research project are to obtain an improved understanding of the formation, intensification, predictability and structure change of tropical cyclones in the Western Pacific region. These new insights will ultimately improve forecast guidance for U.S. Naval operations in this region. APPROACH During this past year the PI and his research group have developed and further substantiated a new model for the phenomenon of secondary eyewall formation (SEF), a process that occurs frequently in mature tropical cyclones and is a continued forecast priority for storms threatening Naval and DOD operations in the Western North Pacific sector. The new model is based on a newly articulated paradigm of tropical cyclone intensification (discussed further below) that the PI has developed in collaboration with his distinguished international colleague, Professor Roger Smith from the University of Munich. Because of space constraints, only pertinent background information is provided to explain the basis for the new approach followed by a summary of some of the key new findings on the SEF problem. Other research supported by this grant is discussed in preceding annual reports and via the PI's website listed above. Recent developments in tropical cyclone intensification theory A new paradigm of tropical cyclone intensification and hurricane boundary layer dynamics has been developed by the PI and his distinguished scientific colleague, Professor Roger Smith. The intensification paradigm begins with the recognition of the inherent three-dimensional nature of the intensification process and emphasizes the aggregate effects of the rotating deep convective structures that drive the spin-up process (
ON TROPICAL CYCLONES Topic 2 . 1 : Tropical-Cyclone Formation : Theory and Idealized modelling
2010
This report summarizes work completed since IWTC-VI that contributes to an improved understanding of tropical-cyclone formation and a p otentially useful perspective for forecasters and researchers. The topic of tropical cyclone formatio n has been the subject of considerable active research in the past four years. Much of the recent research reviewed here points to a unified view of the genesis and intensification process. It provide s also a basis for some new tools to aid in forecas ting tropical-cyclone formation.
Current Topics in Tropical Cyclone Research [Working Title]
Current Topics in Tropical Cyclone Research, 2020
During the last decade, there has been concern that the frequency or intensity of tropical cyclones (TCs) has increased. Also, climate models have shown varying results regarding the future occurrence and intensities of TC. Previous research from this group showed there is significant interannual and interdecadal variability in TC occurrence and intensity for some tropical ocean basins and sub-basins. This work examines global TC occurrence and intensity from 2010 to 2019 and compares this period to the same quantities from 1980 to 2009. The data used here are obtained from publicly available TC archives. Globally, the number of TC occurring over the latest decade is similar to the previous decade. However, while the 40-year trend shows an increase in TC, only intense hurricanes have shown an increase. The Atlantic Ocean and North Indian Ocean Basins show increases in TC activity, especially intense storms. The Southern Hemisphere and West Pacific Region show decreases in TC activit...