Abstract

The ability of dry tropical-cyclone-like vortices to resist vertical shear is discussed. An idealized model calculation is presented in which a dry vortex remains nearly upright during 4 days under the influence of environmental vertical shear. It is shown that the outer portion of the vortex tilts more strongly than the inner core and that the pattern of vertical velocity is related to the vertical tilt of the outer portion of the vortex. This result is discussed with relation to observations of the location of convection in tropical cyclones. An alternative definition of the vortex center is proposed for cases in which the vertical tilt of the vortex is of importance. The average vertical shear across the center of the vortex is shown to depend on both the vortex tilt and the presence of large-scale potential vorticity asymmetries in the outer regions of the vortex. The average vertical shear is a function of time and of the area of the circle over which the averaging is carried out. Thus, the initial environmental shear may not be a reliable measure of the vertical shear felt by the vortex at later times.

Abstract

Many studies have highlighted the importance of recurving tropical cyclones (TCs) in triggering Rossby waves. This study investigates the impact of western North Pacific (WNP), south Indian Ocean, and North Atlantic recurving TCs on the amplitude and frequency of synoptic-scale Rossby wave packets (RWPs) over a 30-yr period. The results indicate a significant increase of RWP frequency downstream of WNP and south Indian Ocean TCs. A statistically significant RWP amplitude anomaly downstream of these TCs suggests that RWPs, which are associated with TCs, are stronger than those that generally occur in midlatitudes. North Atlantic TCs do not seem to be associated with a statistically significant increase in RWP frequency and amplitude downstream.

Processes that contribute to Rossby wave amplification are identified by creating composites for WNP TCs with and without downstream development. Potential vorticity, eddy kinetic energy, and quasigeostrophic forcing diagnostics highlight dynamical mechanisms that contribute to the synergistic interaction between the TC and the midlatitude flow. The existence of an upstream Rossby wave favors a downstream development. Diabatically enhanced upper-level divergent flow that can be attributed to the nonlinear interaction between the TC and the midlatitude flow impedes the eastward propagation of the upstream trough, amplifies the downstream ridge, and intensifies the jet. The amplified midlatitude flow provides upper-level forcing, which helps to maintain the predominantly diabatically driven divergent flow.

Forecast uncertainties that are related to these complex TC–midlatitude flow interactions may spread into downstream regions. A climatological analysis of ensemble reforecast data emphasizes the importance of TC–midlatitude flow interactions and Rossby wave amplification on downstream predictability.

Abstract

The extratropical transitions of Hurricanes Felix and Iris in 1995 are examined and compared. Both systems affected northwest Europe but only Iris developed significantly as an extratropical system. In both cases the hurricane interacts with a preexisting extratropical system over the western Atlantic. The remnants of the exhurricanes can be identified and tracked across the Atlantic as separate low-level potential vorticity (PV) anomalies. The nature of the baroclinic wave involved in the extratropical transition is described from a PV perspective and shown to differ significantly between the two cases.

The role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated. Iris moved into a region of strong shear. The high PV tower of Iris developed a marked downshear tilt. Felix moved into a vertically sheared environment also but the shear was weaker than for Iris and the PV tower of Felix did not tilt much.

Iris maintained its warm-core structure as it tracked across relatively warm water. It moved into the center of a large-scale baroclinic cyclone. The superposition of the two systems gave rise to strong low-level winds. The resulting strong surface latent heat fluxes helped to keep the boundary layer equivalent potential temperature (θ_e ) close to the saturated equivalent potential temperature of the underlying sea surface temperature. This high equivalent potential temperature air was redistributed in the vertical in association with deep convection, which helped maintain the warm core in a similar way to that in tropical cyclones.

Felix did not maintain its warm-core structure as it tracked across the Atlantic. This has been shown to be linked to its more poleward track across colder water. It is argued that negative surface fluxes of latent and sensible heat decrease the boundary layer θ _e , resulting in low-cloud formation and a decoupling of the cyclone boundary layer from the the deep troposphere.

In order to forecast these events there is a need for skill in predicting both the nature of the large-scale baroclinic wave development and the structural evolution of the exhurricane remnants.

Abstract

A three-dimensional model is developed, based upon the recently derived asymmetric balance (AB) formulation of Shapiro and Montgomery, to study the evolution of rapidly rotating vortices, including hurricanes. A particular advantage of the AB theory, unlike other balanced models, is its ability to incorporate divergence of the same order as the vorticity. The main assumption of the AB theory is that the squared local Rossby number ≪1, where the squared local Rossby number is defined by the ratio of the orbital frequency squared to the inertial stability. The AB theory leads to a set of prognostic equations that are manipulated so that the first- and second-order local time tendencies can be evaluated diagnostically at a given time. Using the diagnostic version of the AB equations the potential vorticity (PV) distribution from a primitive equation (PE) model is inverted to obtain the corresponding balanced height and wind fields. As far as the authors are aware, this is the first time that the AB equations have been solved in three dimensions.

A calculation is described in which the PE model is initialized with an axisymmetric barotropic vortex in a vertical shear flow. Vertical shear leads to a wavenumber 1 asymmetry in the PV distribution. Associated with this asymmetry is a component of flow across the vortex center, which has an influence on the vortex motion. In this calculation the PE model provides not only the PV distribution but also the data to test the accuracy of the newly derived AB theory. The wavenumber 1 distributions of the radial, tangential, and vertical velocity fields diagnosed using the AB theory are compared with the results of the PE model. The agreement in amplitude and orientation is found to be good. The relative error between the amplitude maxima of the velocities in the PE calculations and the diagnostically derived AB fields is comparable with the maximum size of the squared local Rossby number. Although the main assumption of the AB theory is not strictly satisfied in these calculations, meaningful comparisons can be made between the PE results and the AB solutions. Presenting the results of the velocity fields in the moving coordinate system and use of the piecewise inversion makes it possible to isolate the influence of the upper-level PV anomaly on the lower-level part of the vortex and the influence of the lower-level PV anomaly on the upper-level part of the vortex.

In a further calculation a vortex is initialized in a horizontal shear flow and diabatic heating and friction are included. The prescribed heating is related to the boundary layer convergence. The heating produces strong vertical gradients in the tangential wind so that the PV of the symmetric vortex becomes negative after 24 h. As in the nonlinear balance equations, the AB formulation requires the PV to be positive in order to be able to find a solution.

A comparison between the velocity fields of the PE model and the diagnostically derived AB solutions after 12 h shows a good agreement in amplitude and orientation at lower levels but significant differences in amplitude at upper levels. At upper levels a vortex has not developed after 12 h and the standard Rossby number is the appropriate measure of the validity and accuracy as in the quasigeostrophic approximation. As in the case with no heating the agreement between the velocity components of the AB and PE model depends on the magnitude of the squared local Rossby number or standard Rossby number.

Abstract

In this study the structure and evolution of singular vectors (SVs) for stable and unstable hurricane-like vortices in background flows with horizontal shear are investigated on f and β planes using a nondivergent barotropic model. With increasing shear strength, the singular values for stable vortices increase and the sensitive regions extend farther away from the vortex. The formation of β gyres leads to significant changes in the SV structure but has only weak influence on the singular values. For sufficiently strong anticyclonic shear, the initial SVs are aligned with streamlines connected to stagnation points. The evolved SVs are dominated by dipole structures, indicating a displacement of the vortex. The displacement is caused by the circulation associated with the initial SV perturbation outside of the vortex core, which grows by untilting and unshielding. This process is strongly enhanced by anticyclonic background shear. For both cyclonic and anticyclonic shear, the displacement by the perturbation circulation causes an additional displacement that is proportional to the shear strength. The shear-enhanced barotropic growth mechanism in stable vortices results in singular values that are comparable to those for unstable vortices without background shear. Perturbation growth involving the normal mode in barotropically unstable vortices suffers from background shear. The shear-induced modifications of the outer vortex regions cause a strong decrease of the singular value with increasing shear. For sufficiently strong shear, the SVs for unstable vortices grow by the same mechanism as for stable vortices.

Abstract

The interaction of a tropical cyclone and a zonally aligned tropopause front is investigated in an idealized framework. A nondivergent barotropic model is used in which the front is represented by a vorticity step, giving a jetlike velocity profile. The excitation of frontal waves by a cyclone located south of the front and the impact of the wave flow on the cyclone motion is studied for different representations of the cyclone and the jet. The evolution from the initial wave excitation until after the cyclone has crossed the front is discussed. The interaction becomes stronger with increasing jet speed. For cyclone representations containing negative relative vorticity, anticyclones develop and can influence the excitation of frontal waves significantly. Resonant frontal waves propagating with a phase speed matching the zonal translation speed of the cyclone are decisive for the interaction. The frontal wave spectrum excited by a cyclone on the front is dominated by waves that are in resonance in the initial phase. These waves have the largest impact on the cyclone motion.

Abstract

The influence of frontal waves on the interaction of a tropical cyclone and a tropopause front is investigated in an idealized framework. In a nondivergent barotropic model the front is represented by a vorticity step with a superimposed sinusoidal perturbation. This gives rise to a jet that meanders to the north and south and can be viewed as a sequence of upper-level troughs and ridges. The model evolution depends sensitively on the position of the cyclone relative to the troughs and ridges. Here a bifurcation point is identified that is located on the trough axis at a distance where the zonal speed of the background flow equals the phase speed of the wave. Arbitrarily small displacements from this position determine whether a cyclone is advected toward the front or repelled. Only a limited range of wavelengths can lead to track bifurcations. The largest effects are obtained for resonant frontal waves propagating with a phase speed matching the initial zonal translation speed of the cyclone. Weak and large-scale vortices can be disrupted when approaching the bifurcation point, where they are exposed to continuously strong shear deformation.

Abstract

Measures of the variability among ensemble members from the National Centers for Environmental Prediction ensemble prediction system are examined with respect to forecasts of the extratropical transition (ET) of Typhoon Nabi over the western North Pacific during September 2005. In this study, variability among ensemble members is used as a proxy for predictability. The time–longitude distribution of standard deviations among 500-hPa height fields from the ensemble members is found to increase across the North Pacific following the completion of the extratropical transition. Furthermore, the increase in ensemble standard deviation is organized such that an increase is associated with the extratropical transition and another increase extends downstream from the ET event. The organization and amplitude of the standard deviations increase from 144 h until approximately 72–48 h prior to the completion of the extratropical transition, and then decrease as the forecast interval decreases.

An empirical orthogonal function analysis of potential temperature on the dynamic tropopause is applied to ensemble members to identify the spatial and temporal organization of centers of variability related to the extratropical transition. The principal components are then used in a fuzzy cluster analysis to examine the grouping of forecast sequences in the collection of ensemble members. The number of forecast groups decreases as the forecast interval to the completion of the ET decreases. However, there is a systematic progression of centers of variability downstream of the ET event. Once the variability associated with the ET begins to decrease, the variability downstream of the ET event also begins to decrease.

Abstract

The extratropical transition (ET) of tropical cyclones often has a negative impact on the predictability of the atmospheric situation both around the ET event and farther downstream. The predictability of five ET cases of different intensities in the North Atlantic and the western North Pacific is investigated using the ECMWF ensemble prediction system. The variability in the ensemble members is regarded as a measure of the predictability. Plumes of forecast uncertainty spread downstream of each ET event. Initialization times closer to the ET events yield higher predictability of the downstream flow independent of forecast lead time.

Principal component analysis and fuzzy clustering is used to assess the variability in the ensemble members and to identify groupings of the members that contribute in a similar way to the variability patterns. Applying the method to the potential temperature on the dynamic tropopause reveals a characteristic variability pattern in all five cases that is closely related to the synoptic patterns of the ET events. Clusters that contribute in a similar manner to the variability patterns exhibit similar ET developments in the different cases. A probability can be assigned to a given group based on the number of members in the group. The number of clusters decreases with shorter forecast intervals and the difference between the clusters becomes less marked. This indicates an increase of predictability.

The usefulness of ensemble prediction is highlighted in the weak ET cases in that a low probability is assigned to the erroneous deterministic forecasts.

Abstract

The extratropical transition (ET) of Hurricane Hanna (2008) and Typhoon Choi-Wan (2009) caused a variety of forecast scenarios in the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). The dominant development scenarios are extracted for two ensemble forecasts initialized prior to the ET of those tropical storms, using an EOF and fuzzy clustering analysis. The role of the transitioning tropical cyclone and its impact on the midlatitude flow in the distinct forecast scenarios is examined by conducting an analysis of the eddy kinetic energy budget in the framework of downstream baroclinic development. This budget highlights sources and sinks of eddy kinetic energy emanating from the transitioning tropical cyclone or adjacent upstream midlatitude flow features. By comparing the budget for several forecast scenarios for the ET of each of the two tropical cyclones, the role of the transitioning storms on the development in downstream regions is investigated. Distinct features during the interaction between the tropical cyclone and the midlatitude flow turned out to be important. In the case of Hurricane Hanna, the duration of baroclinic conversion from eddy available potential into eddy kinetic energy was important for the amplification of the midlatitude wave pattern and the subsequent reintensification of Hanna as an extratropical cyclone. In the case of Typhoon Choi-Wan, the phasing between the storm and the midlatitude flow was one of the most critical factors for the future development.