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- Author or Editor: Greg J. Holland x
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Abstract
The dynamics of tropical cyclone motion are investigated by solving for instantaneous motion tendencies using the divergent barotropic vorticity equation on a beta plane. Two methods of solution are presented a direct analytic solution for a constant basic current, and a simple numerical solution for more general conditions. These solutions indicate that cyclone motion can be accurately prescribed by a nonlinear combination of two processes. 1) an interaction between the cyclone and its basic current (the well known steering concept), and 2) an interaction with the Earth's vorticity field which causes a westward deviation from the pure steering flow. The nonlinear manner in which these two processes combine with the effect of asymmetries in the steering current raise some interesting questions on the way in which cyclones of different characteristics interact with their environment, and has implications for tropical cyclone forecasting and the manner in which forecasting techniques are derived.
Abstract
The dynamics of tropical cyclone motion are investigated by solving for instantaneous motion tendencies using the divergent barotropic vorticity equation on a beta plane. Two methods of solution are presented a direct analytic solution for a constant basic current, and a simple numerical solution for more general conditions. These solutions indicate that cyclone motion can be accurately prescribed by a nonlinear combination of two processes. 1) an interaction between the cyclone and its basic current (the well known steering concept), and 2) an interaction with the Earth's vorticity field which causes a westward deviation from the pure steering flow. The nonlinear manner in which these two processes combine with the effect of asymmetries in the steering current raise some interesting questions on the way in which cyclones of different characteristics interact with their environment, and has implications for tropical cyclone forecasting and the manner in which forecasting techniques are derived.
Abstract
The analytic predictions of tropical cyclone motion by Holland are shown to be in very good agreement with observations in the Australian southwest Pacific region. These results indicate that a combined linear asymmetric advection and divergence of earth and cyclone vorticity provides the main mechanism for tropical cyclone motion. It is also shown that an accurate prediction requires a consideration of horizontal and vertical asymmetries in the wind field. Hence, care needs to be taken in defining a steering current.
Abstract
The analytic predictions of tropical cyclone motion by Holland are shown to be in very good agreement with observations in the Australian southwest Pacific region. These results indicate that a combined linear asymmetric advection and divergence of earth and cyclone vorticity provides the main mechanism for tropical cyclone motion. It is also shown that an accurate prediction requires a consideration of horizontal and vertical asymmetries in the wind field. Hence, care needs to be taken in defining a steering current.
Abstract
A thermodynamic approach to estimating maximum potential intensity (MPI) of tropical cyclones is described and compared with observations and previous studies. The approach requires an atmospheric temperature sounding, SST, and surface pressure; includes the oceanic feedback of increasing moist entropy associated with falling surface pressure over a steady SST; and explicitly incorporates a cloudy eyewall and a clear eye. Energetically consistent, analytic solutions exist for all known atmospheric conditions. The method is straightforward to apply and is applicable to operational analyses and numerical model forecasts, including climate model simulations.
The derived MPI is highly sensitive to the surface relative humidity under the eyewall, to the height of the warm core, and to transient changes of ocean surface temperature. The role of the ocean is to initially contribute to the establishment of the ambient environment suitable for cyclone development, then to provide the additional energy required for development of an intense cyclone. The major limiting factor on cyclone intensity is the height and amplitude of the warm core that can develop; this is closely linked to the height to which eyewall clouds can reach, which is related to the level of moist entropy that can be achieved from ocean interactions under the eyewall. Moist ascent provides almost all the warming above 200 hPa throughout the cyclone core, including the eye, where warm temperatures are derived by inward advection and detrainment mixing from the eyewall. The clear eye contributes roughly half the total warming below 300 hPa and produces a less intense cyclone than could be achieved by purely saturated moist processes.
There are necessarily several simplifications incorporated to arrive at a tractable solution, the consequences of which are discussed in detail. Nevertheless, application of the method indicates very close agreement with observations. For SST < 26°C there is generally insufficient energy for development. From 26° to 28°C SST the ambient atmosphere warms sharply in the lower troposphere and cools near the tropopause, but with little change in midlevels. The result is a rapid increase of MPI of about 30 hPa °C−1. At higher SST, the atmospheric destabilization ceases and the rate of increase of MPI is reduced.
Abstract
A thermodynamic approach to estimating maximum potential intensity (MPI) of tropical cyclones is described and compared with observations and previous studies. The approach requires an atmospheric temperature sounding, SST, and surface pressure; includes the oceanic feedback of increasing moist entropy associated with falling surface pressure over a steady SST; and explicitly incorporates a cloudy eyewall and a clear eye. Energetically consistent, analytic solutions exist for all known atmospheric conditions. The method is straightforward to apply and is applicable to operational analyses and numerical model forecasts, including climate model simulations.
The derived MPI is highly sensitive to the surface relative humidity under the eyewall, to the height of the warm core, and to transient changes of ocean surface temperature. The role of the ocean is to initially contribute to the establishment of the ambient environment suitable for cyclone development, then to provide the additional energy required for development of an intense cyclone. The major limiting factor on cyclone intensity is the height and amplitude of the warm core that can develop; this is closely linked to the height to which eyewall clouds can reach, which is related to the level of moist entropy that can be achieved from ocean interactions under the eyewall. Moist ascent provides almost all the warming above 200 hPa throughout the cyclone core, including the eye, where warm temperatures are derived by inward advection and detrainment mixing from the eyewall. The clear eye contributes roughly half the total warming below 300 hPa and produces a less intense cyclone than could be achieved by purely saturated moist processes.
There are necessarily several simplifications incorporated to arrive at a tractable solution, the consequences of which are discussed in detail. Nevertheless, application of the method indicates very close agreement with observations. For SST < 26°C there is generally insufficient energy for development. From 26° to 28°C SST the ambient atmosphere warms sharply in the lower troposphere and cools near the tropopause, but with little change in midlevels. The result is a rapid increase of MPI of about 30 hPa °C−1. At higher SST, the atmospheric destabilization ceases and the rate of increase of MPI is reduced.
Abstract
Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure.
Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Niño teleconnections.
Abstract
Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure.
Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Niño teleconnections.
Abstract
An analytic model of the radial profiles of sea level pressure and winds in a hurricane is presented. The equations contain two parameters which may be empirically estimated from observations in a hurricane or determined climatologically to define a standard hurricane; example are given. The model is shown to be generally superior to two other widely used models and is considered to be a valuable aid in operational forecasting, case studies and engineering work.
Abstract
An analytic model of the radial profiles of sea level pressure and winds in a hurricane is presented. The equations contain two parameters which may be empirically estimated from observations in a hurricane or determined climatologically to define a standard hurricane; example are given. The model is shown to be generally superior to two other widely used models and is considered to be a valuable aid in operational forecasting, case studies and engineering work.
Abstract
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Abstract
The mechanisms associated with tropical cyclone recurvature are investigated using a five-level primitive equation model and an idealized environment with characteristics observed in cyclone recurvature conditions. All cyclones moved generally with the flow in the lower and middle troposphere, but the precise motion occurs by a combination of divergence and of advection in both the horizontal and the vertical. The horizontal advection arises from a combination of the initial environmental flow and local changes from rearrangement of the potential vorticity field by cyclone-environment interaction (the so-called,β effect). The balance between these mechanisms changes as the vortex recurves. Since the gradients of potential vorticity increase sharply poleward of the subtropical ridge, this is the preferred region for development of an anticyclonic gyre. This gyre is advected eastward and becomes the dominant anticyclonic system. Recurvature is aided by horizontal deformation of the cyclone in the vicinity of this gyre, and by the manner in which the vertical tilt of the vortex and local divergence fields vary as it moves through a changing vertical wind shear of the environment. Recurvature is sensitive to the degree of diabatic heating and to small meridional changes in the initial vortex location.
It is shown that recurvature can occur through an initially unbroken subtropical ridge, but that the presence of a midlatitude trough substantially enhances the potential for recurvature. However, while changes in the upper troposphere are indicative of recurvature potential, recurvature is accomplished largely by lower-tropospheric changes. An important component of this change is the development of a major anticyclone poleward and eastward of the cyclone. A recent observational study by Ford et al. concurs with this finding.
Abstract
The mechanisms associated with tropical cyclone recurvature are investigated using a five-level primitive equation model and an idealized environment with characteristics observed in cyclone recurvature conditions. All cyclones moved generally with the flow in the lower and middle troposphere, but the precise motion occurs by a combination of divergence and of advection in both the horizontal and the vertical. The horizontal advection arises from a combination of the initial environmental flow and local changes from rearrangement of the potential vorticity field by cyclone-environment interaction (the so-called,β effect). The balance between these mechanisms changes as the vortex recurves. Since the gradients of potential vorticity increase sharply poleward of the subtropical ridge, this is the preferred region for development of an anticyclonic gyre. This gyre is advected eastward and becomes the dominant anticyclonic system. Recurvature is aided by horizontal deformation of the cyclone in the vicinity of this gyre, and by the manner in which the vertical tilt of the vortex and local divergence fields vary as it moves through a changing vertical wind shear of the environment. Recurvature is sensitive to the degree of diabatic heating and to small meridional changes in the initial vortex location.
It is shown that recurvature can occur through an initially unbroken subtropical ridge, but that the presence of a midlatitude trough substantially enhances the potential for recurvature. However, while changes in the upper troposphere are indicative of recurvature potential, recurvature is accomplished largely by lower-tropospheric changes. An important component of this change is the development of a major anticyclone poleward and eastward of the cyclone. A recent observational study by Ford et al. concurs with this finding.
Abstract
The observed tendency for tropical cyclones to meander about a longer-term track with periods of several days and amplitudes around 100 km is investigated. An analysis of 26 cyclones in the western North Pacific Ocean does not support the theories by Syono and Futi that tropical cyclone track oscillations occur from excitation of inertial oscillations. The observations and related numerical modeling studies also do not support the vortex patch and rotating cylinder theories by Yeh and Kuo. It is suggested that many meanders occur from interactions with mesoscale vortices and convective systems within the cyclone circulation. This hypothesis is supported by a case study of the effects of mesoscale convective complexes that developed in Typhoon Sarah (1989).
Abstract
The observed tendency for tropical cyclones to meander about a longer-term track with periods of several days and amplitudes around 100 km is investigated. An analysis of 26 cyclones in the western North Pacific Ocean does not support the theories by Syono and Futi that tropical cyclone track oscillations occur from excitation of inertial oscillations. The observations and related numerical modeling studies also do not support the vortex patch and rotating cylinder theories by Yeh and Kuo. It is suggested that many meanders occur from interactions with mesoscale vortices and convective systems within the cyclone circulation. This hypothesis is supported by a case study of the effects of mesoscale convective complexes that developed in Typhoon Sarah (1989).
Abstract
The interactions between a barotropic vortex and an idealized subtropical ridge environment on a beta plane are examined and compared to the well-documented case of a single vortex with no environmental flow. First, the problems and advantages of several potential partitioning methods are discussed and then a three-part partition is chosen. Substantial variations are found from the single vortex case. In particular, the familiar gyres associated with the propagation of a single vortex are markedly distorted and relocated by the environment.
A vorticity budget is presented to help isolate the physical mechanisms. This analysis indicates that the major processes are associated with interactions with the gradients of absolute vorticity in the environment. Other nonlinear mechanisms can also be of significance in specific cases.
Abstract
The interactions between a barotropic vortex and an idealized subtropical ridge environment on a beta plane are examined and compared to the well-documented case of a single vortex with no environmental flow. First, the problems and advantages of several potential partitioning methods are discussed and then a three-part partition is chosen. Substantial variations are found from the single vortex case. In particular, the familiar gyres associated with the propagation of a single vortex are markedly distorted and relocated by the environment.
A vorticity budget is presented to help isolate the physical mechanisms. This analysis indicates that the major processes are associated with interactions with the gradients of absolute vorticity in the environment. Other nonlinear mechanisms can also be of significance in specific cases.
Abstract
The implied heating and potential vorticity generation in tropical cyclone rainbands is derived from observed vertical motion profiles. High levels of potential vorticity generation are found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of potential vorticity for the system as a whole and may have implications for cyclone intensity.
Abstract
The implied heating and potential vorticity generation in tropical cyclone rainbands is derived from observed vertical motion profiles. High levels of potential vorticity generation are found in the stratiform rain regions, sufficient to generate substantial wind maxima along the bands within a couple of hours. Such generation may represent a significant source of potential vorticity for the system as a whole and may have implications for cyclone intensity.
