Gerard Kilroy

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

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Roger K. Smith

Royal Veterinary College

Michael T. Montgomery

Naval Postgraduate School

Norman Wildmann

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Antonia Englberger

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Linus Wrba

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Jeffrey D. Thayer

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Minhee Chang

Seoul National University

Andreas Dörnbrack

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

V. P. M. Rajasree

Norwegian University of Science and Technology

NČ

Nina Črnivec

Columbia University

Cooperative Institutions

China Meteorological Administration

Ludwig-Maximilians-Universität München

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

National Oceanic and Atmospheric Administration

Texas A&M University

Bureau of Meteorology

NSF National Center for Atmospheric Research

Chinese Academy of Meteorological Sciences

University of Illinois Urbana-Champaign

Columbia University

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The effects of initial vortex size on tropical cyclogenesis and intensification

Quarterly Journal of the Royal Meteorological Society (2017)

Gerard Kilroy Roger K. Smith

Three high‐resolution numerical simulations are carried out to investigate the effects of initial vortex size on tropical cyclogenesis and intensification, starting with a relatively weak initial vortex with a maximum tangential wind speed of 5 m s −1 , but located at different radii. In all simulations, there is a progressive organization of convectively induced, cyclonic relative vorticity into a monopole structure. As the initial vortex size is increased, the organization occurs later. In addition, the size of the vorticity monopole, the sizes of the inner and outer core tangential wind circulations and the lifetime intensity of the vortex all become larger. An explanation for this behaviour is offered in terms of a boundary‐layer control of the overturning circulation. The findings are consistent with those of previous studies that bypass the genesis stage and start with appreciably stronger initial vortices.

Tropical cyclogenesis

Cyclogenesis

Circulation (fluid dynamics)

10.1002/qj.3134

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Citations (23)

Numerical studies of tropical convection

Gerard Kilroy

Idealized numerical model experiments are presented to investigate the convective generation of vertical vorticity in a tropical depression. The calculations are motivated by observations made during the recent PREDICT field experiment to study tropical cyclogenesis, and by a desire to understand the aggregation of vorticity debris produced by deep convection in models of tropical cyclogenesis to form a monopole vortex. One aim is to isolate and quantify the effects of low to mid level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. Another aim is to isolate the effects of a unidirectional boundary layer wind profile on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. A third aim is to isolate the effects of a vortex boundary-layer wind profile on tropical deep convection, focussing especially on the morphology of vertical vorticity that develops. The growing convective updraughts, that are initiated by a near surface thermal perturbation, amplify locally the ambient rotation at low levels by more than an order of magnitude and this vorticity persists long after the updraught has decayed, supporting the results of an earlier study. The results of calculations with dry air aloft do not support a common perception that the dry air produces stronger downdraughts. In calculations where the vertical wind shear changes sign at some level near the top of the boundary layer, as occurs in warm-cored disturbances such as tropical depressions or tropical cyclones, it was found that the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This feature is quite unlike the structure in a typical middle-latitude `supercell' storm. These results provide an essential first step to understanding the interaction between deep convective elements in a tropical depression or tropical cyclone. An increase in the magnitude of boundary-layer shear was found to have the dual effect of weakening the development of the initial thermal, which is detrimental to vertical vorticity production by stretching and tilting, while at the same time increasing the magnitude of horizontal vorticity that can be tilted. The results provide a basis for appraising a recent conjecture concerning the role of storm splitting in explaining the contraction of the eyewall in tropical cyclones.

Tropical cyclogenesis

Positive vorticity advection

Cyclogenesis

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Tropical cyclogenesis: Controlling factors and physical mechanisms

Tropical Cyclone Research and Review (2023)

V. P. M. Rajasree Xi Cao Hamish A. Ramsay Kelly M. Núñez Ocasio Gerard Kilroy

In this review, advances in the understanding of the controlling factors and physical mechanisms of tropical cyclogenesis (TCG) are summarized from recent (2018-2022) research on TCG, as presented in the Tenth International Workshop on Tropical Cyclones (IWTC-10). Observational, theoretical, and numerical modeling studies published in recent years have advanced our knowledge on the influence of large-scale environmental factors on TCG. Furthermore, studies have shown clearly that appropriate convective coupling with tropical equatorial waves enhances the development chances of TCG. More recently, illuminating research has been carried out on analyzing the mechanisms by which oscillations and teleconnections (El Niño Southern Oscillation (ENSO) in particular) modulate TCG globally, in association with changes in the sea surface temperature (SST). In addition to this, recent research has diligently addressed different aspects of TCG. Multiple studies have reported the applicability of unified theories and physical mechanisms of TCG in different ocean basins. Recently, research has been carried out on TCG under different flow pattern regimes, dry air intrusion, importance of marsupial pouch, genesis of Medicanes, wind shear, convection and vertical structure. Furthermore, studies have discussed the possibility of near equatorial TCG provided that there is enough supply of background vertical vorticity and relatively low vertical wind shear. Progress has been made to understand the role of climate change on global and regional TCG. However, there are still significant gaps which need to be addressed in order to better understand TCG prediction.

Tropical cyclogenesis

Teleconnection

Cyclogenesis

10.1016/j.tcrr.2023.09.004

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Citations (18)

Why Do Model Tropical Cyclones Grow Progressively in Size and Decay in Intensity after Reaching Maturity?

Journal of the Atmospheric Sciences (2015)

Gerard Kilroy Roger K. Smith Michael T. Montgomery

Abstract The long-term behavior of tropical cyclones in the prototype problem for cyclone intensification on an f plane is examined using a nonhydrostatic, three-dimensional numerical model. After reaching a mature intensity, the model storms progressively decay while both the inner-core size, characterized by the radius of the eyewall, and the size of the outer circulation—measured, for example, by the radius of the gale-force winds—progressively increase. This behavior is explained in terms of a boundary layer control mechanism in which the expansion of the swirling wind in the lower troposphere leads through boundary layer dynamics to an increase in the radii of forced eyewall ascent as well as to a reduction in the maximum tangential wind speed in the layer. These changes are accompanied by changes in the radial and vertical distribution of diabatic heating. As long as the aggregate effects of inner-core convection, characterized by the distribution of diabatic heating, are able to draw absolute angular momentum surfaces inward, the outer circulation will continue to expand. The quantitative effects of latitude on the foregoing processes are investigated also. The study provides new insight into the factors controlling the evolution of the size and intensity of a tropical cyclone. It provides also a plausible, and arguably simpler, explanation for the expansion of the inner core of Hurricane Isabel (2003) and Typhoon Megi (2010) than that given previously.

Eye

Diabatic

Typhoon

Rainband

10.1175/jas-d-15-0157.1

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Tropical cyclone life cycle in a three‐dimensional numerical simulation

Quarterly Journal of the Royal Meteorological Society (2021)

Roger K. Smith Gerard Kilroy Michael T. Montgomery

Abstract An idealized, three‐dimensional, numerical simulation of tropical cyclone evolution in a quiescent environment on an f ‐plane is used to explore aspects of the cyclone's life cycle in the context of the rotating‐convection paradigm. In the 20‐day simulation, the vortex undergoes a life cycle including a gestation period culminating in genesis, a rapid intensification phase, a mature phase, a transient decay and re‐intensification phase, a second mature phase and a rapid decay phase. During much of the life cycle, the flow evolution is highly asymmetric, although important aspects of it can be understood within an azimuthally averaged framework, central to which are a boundary‐layer control mechanism and a new ventilation diagnostic. The boundary‐layer control mechanism provides an explanation for the gradual expansion of the inner core of the vortex. The ventilation diagnostic characterizes the ability of deep convection within a given radius to evacuate the mass of air ascending out of the boundary layer within that radius. The transient decay and re‐intensification phase is not associated with an eyewall replacement cycle, but rather with a hitherto undescribed process in which the eyewall becomes fragmented as a rainband complex forms beyond it. This process is interpreted as an interplay between the boundary layer and ventilation. The final rapid decay of the vortex results from the ever increasing difficulty of deep convection to ventilate the air exiting the boundary layer. Any unventilated air flows radially outwards in the lower troposphere and leads to spin‐down because of the approximate conservation of mean absolute angular momentum. If found in real cyclones, such transience or final decay might be erroneously attributed to ambient vertical wind shear. The results support the hypothesis that, even in a quiescent environment, isolated tropical cyclone vortices are intrinsically transient and never reach a globally steady state.

Eye

Rainband

10.1002/qj.4133

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Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

Journal of the Atmospheric Sciences (2014)

Roger K. Smith Gerard Kilroy Michael T. Montgomery

Abstract The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation. The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a time-dependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 10° and 30°N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations.

Diabatic

Inflow

10.1175/jas-d-14-0044.1

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Citations (52)

A numerical study of deep convection in tropical cyclones

Quarterly Journal of the Royal Meteorological Society (2016)

Gerard Kilroy Roger K. Smith

Idealized numerical model simulations are used to investigate the generation and evolution of vertical vorticity by deep convection in a warm‐cored vortex of near‐tropical‐storm strength. Deep convective updraughts are initiated by thermal perturbations located at different radii from the vortex axis. It is found that, as the location of the thermal perturbation is moved away from the axis of rotation, the updraught that results becomes stronger, the cyclonic vorticity anomaly generated by the updraught becomes weaker, the structure of the vorticity anomaly changes markedly and the depth of the anomaly increases. For an updraught along or near the vortex axis, the vorticity anomaly has the structure of a monopole and little or no anticyclonic vorticity is generated in the core. Vorticity dipoles are generated in updraughts near or beyond the radius of maximum tangential wind speed and this structure reverses in sign with height. In all cases, the vorticity anomalies persist long after the initial updraught has decayed. Implications of the results for understanding the vorticity consolidation during tropical cyclogenesis are discussed. The effects of eddy momentum fluxes associated with a single updraught on the tangential‐mean velocity tendency are investigated and a conceptual framework for the interpretation of these eddy fluxes is given. The simulations are used to appraise long‐standing ideas suggesting that latent heat release in deep convection occurring in the high inertial stability region of a vortex core is ‘more efficient’ than deep convection outside the core in producing temperature rise in the updraught.

Tropical cyclogenesis

Anticyclone

10.1002/qj.2895

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Citations (21)

Recent advances in research on tropical cyclogenesis

Tropical Cyclone Research and Review (2020)

Brian H. Tang Juan Fang Alicia M. Bentley Gerard Kilroy Masuo Nakano

This review article summarizes recent (2014–2019) advances in our understanding of tropical cyclogenesis, stemming from activities at the ninth International Workshop on Tropical Cyclones. Tropical cyclogenesis involves the interaction of dynamic and thermodynamic processes at multiple spatio-temporal scales. Studies have furthered our understanding of how tropical cyclogenesis may be affected by external processes, such as intraseasonal oscillations, monsoon circulations, the intertropical convergence zone, and midlatitude troughs and cutoff lows. Additionally, studies have furthered our understanding of how tropical cyclogenesis may be affected by internal processes, such as the organization of deep convection; the evolution of the "pouch" structure; the role of friction; the development of the moist, warm core; the importance of surface fluxes; and the role of the mid-level vortex. A relatively recent class of idealized, numerical simulations of tropical cyclogenesis in radiative-convective equilibrium have highlighted the potential importance of radiative feedbacks on tropical cyclogenesis. We also offer some recommendations to the community on future directions for tropical cyclogenesis research.

Tropical cyclogenesis

Cyclogenesis

10.1016/j.tcrr.2020.04.004

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Citations (46)

Dependence of tropical cyclone intensification rate on sea‐surface temperature

Quarterly Journal of the Royal Meteorological Society (2016)

Nina Črnivec Roger K. Smith Gerard Kilroy

The dependence of tropical cyclone intensification rate on the sea‐surface temperature (SST) is examined in the prototype problem for tropical cyclone intensification on an f ‐plane using a three‐dimensional, non‐hydrostatic numerical model. The effects of changing the SST are compared with those of changing the latitude examined in a recent article. It is found that the dependence of intensification rate on latitude is largest when the SST is marginal for tropical cyclone intensification (26 °C) and reduces in significance as the SST is increased. Further, at a given latitude, intensification begins earlier and the rate of intensification increases with increasing SST, on account of a significant increase of surface moisture fluxes from the warmer ocean. These higher fluxes result in higher values of near‐surface moisture and equivalent potential temperature, leading to a larger radial gradient of diabatic heating rate in the low to middle troposphere above the boundary layer. This larger radial gradient leads to a stronger overturning circulation, which in turn leads to a stronger radial import of absolute angular momentum surfaces and therefore more rapid spin‐up. These arguments invoke the classical axisymmetric spin‐up mechanism. Non‐axisymmetric issues are touched upon briefly.

Diabatic

Hydrostatic equilibrium

Lapse rate

10.1002/qj.2752

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Citations (47)

The role of heating and cooling associated with ice processes on tropical cyclogenesis and intensification

Quarterly Journal of the Royal Meteorological Society (2017)

Gerard Kilroy Roger K. Smith Michael T. Montgomery

A recent idealized numerical study of tropical cyclogenesis and subsequent intensification using warm‐rain‐only microphysics is extended to examine the modifications brought about by a representation of ice processes. It is found that the time taken to reach cyclogenesis is more than twice that in the equivalent warm‐rain‐only simulation. The subtle reasons for the difference in the length of the gestation period are discussed. A mid‐level vortex forms during the early gestation period when ice processes are present, but not when warm‐rain‐only processes are present. Axisymmetric balance calculations show that the spin‐up of this mid‐level vortex is related to the different spatial distribution of diabatic heating rate in the presence of ice, which leads to a system‐scale radial influx of absolute vorticity in the middle troposphere. The tropical‐cyclone vortex that forms in the simulation with ice is similar to that in the warm‐rain‐only simulation, with the strengthening frictional boundary layer exerting a progressively important role in focusing inner‐core deep convection. This vortex develops in situ on a much smaller scale than the mid‐level vortex and there is no evidence that it is a result of the mid‐level vortex being somehow carried downwards, as has been suggested previously by some researchers. Some implications of the results in relation to previous theories of tropical cyclogenesis are discussed.

Tropical cyclogenesis

Cyclogenesis

Diabatic

10.1002/qj.3187

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Citations (18)