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Greg J. Holland

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.

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Greg J. Holland

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.

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Greg J. Holland

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.

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Greg J. Holland

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.

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Greg J. Holland

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.

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Greg J. Holland and Mark Lander

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).

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Yuqing Wang and Greg J. Holland

Abstract

The beta drift of diabatic vortices is investigated with a three-dimensional primitive equation model with simple physical parameterizations. The vertical coupling mechanism discussed in Part I is extended to include the effects of diabatic heating and moist processes. The results show that the motion and evolution of the diabatic vortices can substantially differ from those of adiabatic vortices.

The anticyclone at the upper troposphere tends to propagate equatorward and westward due to the Rossby wave dispersion. But the continuous regeneration of an anticyclonic PV anomaly by diabatic heating keeps the upper-level anticyclone in a band stretching from the vortex core to several hundred kilometers equatorward and westward. Downward penetration of the circulation associated with these anticyclonic PV anomalies reduces the westward motion of the diabatic vortices by the vertical coupling mechanism discussed in Part I. This also rotates the lower-level beta-gyres anticyclonically, resulting in a more poleward asymmetric flow over the lower-level vortex core. As a result, diabatic vortices with a deeper and stronger outflow-layer anticyclone move in a more poleward direction than do the equivalent adiabatic baroclinic or barotropic vortices.

The asymmetric divergent flow associated with convective asymmetries within the vortex core region deflects the vortex center toward the region with maximum convection. Evolution of both the asymmetric convection and the vertical coupling may result in meandering vortex tracks.

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Greg J. Holland and Yuqing Wang

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.

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Yuqing Wang and Greg J. Holland

Abstract

The dynamics of the movement of an initially axisymmetric baroclinic vortex embedded in an environment at rest on a beta plane is investigated with a three-dimensional primitive equation model. The study focuses on the motion and evolution of an adiabatic vortex and especially the manner in which vertical coupling of a tilted vortex influences its motion. The authors find that the vortex movement is determined by both the asymmetric flow over the vortex core associated with beta gyres and the flow associated with vertical projection of the tilted potential vorticity anomaly. The effects of vortex tilt can be large and complex. The secondary divergent circulation is found to be associated with the development of potential temperature anomalies required to maintain a balanced state. The processes involved strongly depend on the vertical structure, size, and intensity of the vortex together with external parameters such as the earth rotation and static stability of the environment. As a result, simple relationships between vortex motion and the vertical mean relative angular momentum are not always applicable.

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Yuqing Wang and Greg J. Holland

Abstract

The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating.

The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or with the layer mean between 350 and 900 hPa, but the stronger tropical cyclone vortices move with a relatively deeper layer mean flow. In addition to advection by the environmental flow, most vortices propagate to the left of the vertical shear due to downward penetration of the circulation associated with the upper-level anticyclonic potential vorticity (PV) anomalies that are displaced downshear.

Diabatic and moist processes can substantially modify the adiabatic vortex motion by both the vertical transport of potential vorticity associated with diabatic heating and the development of convective asymmetries within the vortex core region. Diabatic heating can either substantially enhance the leftward motion tendency or result in a rightward motion relative to the vertical shear, depending on the vertical structure and intensity of the vortex and its environment. This occurs by transport of anticyclonic PV to the upper troposphere and cyclonic PV to the right of the vortex center relative to the vertical shear in the lower troposphere. A rightward motion tendency to the boundary layer flow is found to arise from enhanced heat fluxes from the ocean on the higher wind side of the vortex center. Cumulus convection is substantially enhanced on the downshear side of the vortex center due to the interaction between the vortex circulation and the vertical shear in the environmental flow. The asymmetric divergent flow associated with these convective asymmetries affects the vortex motion by deflecting the vortex to the region with maximum convection.

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