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.

Abstract

A nondeveloping tropical disturbance, identified as TCS025, was observed during three intensive observing periods during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment. The low-level circulation of the disturbance was relatively weak, asymmetric, and displaced a considerable distance from the midlevel circulation. An ensemble of high-resolution numerical simulations initialized from global model analyses was used to further examine TCS025. These simulations tended to unrealistically overdevelop the TCS025 disturbance. This study extends that work by examining the impact of assimilating in situ observations of TCS025 and dual-Doppler radial velocities from the airborne Electra Doppler Radar (ELDORA) using the Data Assimilation Research Testbed (DART) ensemble data assimilation system.

The assimilation of observations results in a more accurate vortex structure that is consistent with the observational analysis. In addition, forecasts initialized from the state of the ensemble after data assimilation exhibit less development than both the control simulation and an ensemble of forecasts without prior data assimilation.

A composite analysis of developing and nondeveloping forecasts from the ensemble reveals that convection was more active in developing simulations, especially near the low-level circulation center. This led to larger diabatic heating rates, spinup of the low-level circulation from vorticity stretching, and greater alignment of the low- and midlevel vorticity centers. In contrast, nondeveloping simulations exhibited less convection, and the circulation was more heavily impacted by vertical wind shear.

Abstract

An analysis of in situ observations from the nondeveloping tropical disturbance named TCS025 revealed that a combination of unfavorable system-scale and environmental factors limited further development. In this study, a multiphysics ensemble of high-resolution simulations of TCS025 are analyzed and compared. A simulation that overdeveloped the TCS025 disturbance is compared with one that correctly simulated nondevelopment and reveals that convection was stronger and diabatic heating rates were larger in the developing simulation. This led to continued spinup of the low-level circulation primarily through vorticity stretching. In contrast, convection was much weaker in the nondeveloping simulation, and after an initial period of deep convection, average vorticity tendencies from stretching became weakly negative, which allowed for the frictional spindown of the low-level circulation.

Convective-scale differences identified early in the simulations appear to have resulted from the explicit representation of graupel in the developing simulation. The net impacts resulting from these differences in convection are manifest in the average diabatic heating profiles that are important for determining the developmental outcome. Additional simulations are conducted whereby the diabatic heating rates are artificially adjusted. Relatively small changes in the diabatic heating rate led to significantly different outcomes with respect to storm development, and the degree of overdevelopment is largely dictated by the diabatic heating rate. These findings suggest the correct representation of convective processes and associated diabatic heating are necessary to adequately forecast tropical cyclogenesis, especially for systems near a threshold of development like TCS025.

Abstract

Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system.

The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature ( ) air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

Abstract

Two characteristic midlatitude circulation patterns (labeled northwest and northeast) are found to be associated with extratropical transition (ET) of tropical cyclones over the western North Pacific Ocean. Although in both cases the tropical cyclone moves poleward ahead of a midlatitude trough, the primary midlatitude circulation is either that trough or is a large quasi-stationary cyclone to the northeast of the poleward-moving tropical cyclone. Transition into a northwest pattern typically results in the development within 36 h of an intense extratropical cyclone that moves north–northeast. A tropical cyclone that moves into a northeast pattern enters into strong zonal flow between the primary midlatitude circulation and the subtropical ridge to the southeast. These systems move rapidly eastward and do not intensify significantly during the 36 h following transition.

In Part I of this study, the ET of Typhoon (TY) David (1997) and the ET of TY Opal (1997) were investigated in terms of the formation of extratropical cyclone features. In this study, the same cases of ET are examined to define interactions between the decaying tropical cyclone and these two general synoptic environments in terms of the distributions of heat and momentum fluxes and generation of kinetic energy between the cyclone and the environment. During transition into either circulation pattern, the tropical cyclone is initially impacted by upper-tropospheric eddy angular momentum fluxes associated with the juxtaposition of the midlatitude circulation. During transition into a northwest pattern, the tropical cyclone couples with the midlatitude baroclinic zone such that low-level eddy heat fluxes contribute to the extratropical cyclone development. During transition into a northeast pattern, the strong zonal flow seems to prevent a direct interaction between the decaying tropical cyclone and the primary midlatitude circulation. Eddy heat fluxes do not increase and minimal baroclinic development occurs.

The two types of extratropical transition also have significant differences in the generation of kinetic energy during the reintensification as an extratropical cyclone. Extratropical transition into a northwest pattern results in barotropic and baroclinic production of kinetic energy through direct solenoidal circulations that result from the coupling of the tropical cyclone and midlatitude trough. The movement of a tropical cyclone toward the large, quasi-stationary extratropical cyclone in the northeast pattern results in barotropic destruction of kinetic energy that inhibits significant reintensification.

Abstract

This study of extratropical transition of western North Pacific tropical cyclones (TCs) addresses the reintensification stage during which the TC remnants develop as an extratropical cyclone. The hypothesis examined here is that reintensification depends on the interaction between the midlatitude circulation contributions from mid- and upper-level dynamic processes, low-level thermal processes from the decaying TC, and upper-level outflow characteristics from the decaying TC. Reintensification occurs when the combination of the dynamic and thermodynamic processes define a region that is favorable for extratropical cyclone development.

The midlatitude circulation contribution to reintensification is characterized by comparing a control forecast made with an atmosphere-only version of the Coupled Ocean–Atmosphere Mesoscale Prediction System with a simulation in which the TC has been removed (NOTC). The midlatitude contribution is favorable if a significant extratropical cyclone forms in the NOTC simulation. A neutral midlatitude contribution is defined to occur when weak or no extratropical cyclogenesis occurs in the NOTC simulation. Finally, an unfavorable midlatitude contribution occurs when anticyclogenesis is predicted in the NOTC simulation. The TC contribution to the reintensification is characterized in a similar manner by assessing the different reintensification characteristics between a control simulation and the NOTC simulation.

Within favorable, neutral, and unfavorable midlatitude categories, the initial vortex is displaced to achieve increased (decreased) interactions between the TC remnants and midlatitude development region and, thus, more (less) reintensification. The displaced-vortex simulations indicate an interaction between the TC and the midlatitude circulation may shift the location of the development region and vary the relative contributions from various physical processes to the development. Reintensification is favored when the upper-level TC outflow enhances the equatorward entrance region of a downstream jet streak, and when the TC remnant circulation interacts with the lower-tropospheric baroclinic zone. Thus the interaction is not a static process, but a dynamic process in which both the TC and midlatitude circulation have a contribution.

Abstract

Data obtained during two aircraft observing periods (AOP) from the TCM-93 mini field experiment are used to describe the transformation between 5° and 10°N of a large depression in the western North Pacific monsoon trough into a tropical cyclone over a 36-h period. The transformation is defined to occur in three stages. Although a large mesoscale convective system (MCS) was present along the eastern periphery of the monsoon depression during the preorganization stage characterized by observations from the first AOP, the overall convective organization of the broad circulation is weak. The structure of the MCS provided a midlevel subsynoptic contribution to the vorticity of the monsoon depression and contributed to a shift in the center of the monsoon depression circulation between 800 and 600 mb toward the MCS location. However, the presence of unsaturated downdrafts associated with the MCS perturbed the low-level thermodynamic conditions and contributed to the rapid decay of the MCS. Slow intensification of the monsoon depression circulation during the preorganization stage is primarily due to favorable interactions with large-scale mean and eddy circulations at both upper and lower levels. The overall convective signature was observed in hourly satellite imagery to become more organized during a 24-h period between the two AOPs. This organization stage was characterized by the formation of a new MCS near the midlevel circulation of the decaying MCS from the preorganization stage. Satellite imagery indicates that the broad monsoon depression began to organize around the new MCS and the outer convection started to be oriented in large principle bands. During the transformation to a tropical storm during the second AOP, the outer principal bands appear to separate the inner circulation of the monsoon depression from the large-scale monsoon trough environment. Convection rapidly develops along the periphery of the inner circulation that now contains a vigorous central updraft and high values of equivalent potential temperature that extend to the middle troposphere. Although several episodes of MCS generation and decay occurred throughout the development of the monsoon depression, it is hypothesized that the subsynoptic processes in the MCS during the first AOP and the MCSs that formed immediately following the second AOP contributed to the concentration of the monsoon depression center and transformation to a tropical cyclone.

Abstract

A multiple-set canonical correlation analysis (MCCA), which can be used to study atmospheric motions by analyzing the relationships among more than two sets of data fields, is proposed. By using the product or squared product of correlation matrices as the optimization criterion, this method generalizes the two-set canonical correlation analysis (CCA) and reduces the complications associated with the supermatrix approaches previously proposed in statistical textbooks. The final optimization equations can be greatly simplified to involve weighting functions of one field at a time. Furthermore, excluding or emphasizing correlations between special field pairs based on physical considerations can be easily implemented.

The method is identical to a supermatrix approach based on maximizing the product of canonical correlation coefficients when the individual canonical correlation matrices are perfectly diagonal. This would be true for idealized data that contain only orthogonal motion systems so that all datasets are perfectly correlated. In such a case, all supermatrix methods will also converge to the same solution. In real cases, cross-component correlations will occur, and their largest values, called largest residual correlations (LRCs), are a crude measure of the validity of the approximation. When LRCs are small compared to the corresponding canonical correlation coefficients, the results are reliable. Otherwise, solutions of different methods diverge and are all doubtful.

A statistical textbook example illustrates that solutions obtained are comparable to those from the supermatrix methods, and the relative LRCs are about 20%. A meteorological application example shows that, compared to the two-set CCA, the proposed MCCA gives a more powerful concentration of variance in the leading modes and higher canonical correlation coefficients. The resultant relative LRCs are small throughout all leading modes, apparently because meteorological data contain highly correlated variations.

The proposed technique nay also be applied to the singular-value decomposition analysis to allow a multiple-set singular-value decomposition analysis to be used on mart than two sets of data fields.

Abstract

During a 10-day period in the Tropical Cyclone Motion (TCM-93) field experiment over the tropical western North Pacific, tropical cyclone formation occurred in association with persistent deep convection that was observed over low-level, north-oriented confluent flow between a large monsoon gyre to the west of a strong subtropical ridge. The convection was also modulated by a strong diurnal cycle with a convective maximum just before dawn and a convective minimum during the late afternoon. Observations from two aircraft observing periods (A0Ps) during two consecutive daytime periods identified three distinct mesoscale convective vortices (MCVS) in the persistent deep convection. During the initial AOP (AOP-1A), a well-defined mesoscale circulation at 500 mb was located directly above the strong low-level, south-southwesterly confluent flow. However, reduction in convection and associated midlevel forcing during the convective minimum period contributed to the decay of the MCV before it could penetrate downward through the strong low-level flow to tap ocean surface energy sources.

During the second AOP (AOP-1B), which was approximately 24 h after AOP-1A, two MCVs were identified by aircraft observations. A northern MCV, which dissipated shortly after the AOP, had a structure similar to the observed MCV in AOP-1A and was also located directly above the strong low-level north-oriented flow. A second midtropospheric MCV over the southern portion of the aircraft operating area extended down to 850 mb and was located in the cyclonic shear of the low-level flow. Although convection over the large area was decreasing during the diurnal minimum, several convective cells formed and grew in association with local low-level confluence between the low-level MCV circulation and the large-scale flow. In contrast to AOP-1A, this convection persisted and acquired a rotation as part of a northward-moving circulation that can he traced to a small low-level mesoscale circulation in satellite visible imagery approximately 10 h after the AOP as the same circulation observed over the southern region of AOP-1B. Satellite visible imagery documents the explosive convective development associated with the low-level circulation that led to the formation of Tropical Storm Ofelia. It is concluded that the southern MCV in AOP-1B was able to persist because of its extension to low levels, which was linked to its location on the cyclonic shear side of the strong low-level flow.

Abstract

This study first examines the tropical cyclone (TC) intensity response to its cold wake with time-invariant, stationary cold wakes and an uncoupled version of COAMPS-TC, and second with simulated cold wakes from the fully coupled version. The objective of the uncoupled simulations with the time-invariant cold wake is to fix the thermodynamic response and to isolate the dynamic response of the TC to the cold wake. While the stationary TC over a cold wake has an immediate intensity decrease, the intensity decrease with a long trailing wake from the moving TC was delayed. This time delay is attributed to a “wake jet” that leads to an enhanced inward transport of moist air that tends to offset the effect of decreasing enthalpy flux from the ocean. In the fully coupled version, the TC translating at 2 m s⁻¹ generated a long trailing cold wake, and again the intensity decrease was delayed. Lagrangian trajectories released behind the TC center at four times illustrate the inward deflection and ascent and descent as the air parcels cross the trailing cold wake. The momentum budget analysis indicates large radial and tangential wind tendencies primarily due to imbalances among the pressure gradient force, the Coriolis, and the horizontal advection as the parcels pass over the cold wake. Nevertheless, a steadily increasing radial inflow (wake jet) is simulated in the region of a positive moisture anomaly that tends to offset the thermodynamic effect of decreasing enthalpy flux.