Abstract

The movement of a tropical cyclone into the midlatitudes involves interactions among many complex physical processes over a variety of space and time scales. Furthermore, the extratropical transition (ET) of a tropical cyclone may also result in a high-amplitude Rossby wave response that can extend to near-hemispheric scales. After an ET event occurs over the western portion of a Northern Hemisphere ocean basin, the high-amplitude downstream response often forces anomalous midlatitude circulations for periods of days to a week. These circulations may then be related to high-impact weather events far downstream of the forcing by the ET event. In this study, downstream development following ET events over the western North Pacific Ocean is examined. Local eddy kinetic energy analyses are conducted on four cases of North Pacific tropical cyclones of varying characteristics during ET into varying midlatitude flow characteristics during 15 July–30 September 2005. The goal is to examine the impact of each case on downstream development across the North Pacific during a period in which these events might increase the midlatitude cyclogenesis across the North Pacific during a season in which cyclogenesis is typically weak. Four typhoon (TY) cases from the summer of 2005 are chosen to represent the wide spectrum of variability in ET. This includes a case (TY Nabi 14W) that directly resulted in an intense midlatitude cyclone, a case in which a weak midlatitude cyclone resulted (TY Banyan 07W), a case in which the decaying tropical cyclone was absorbed into the midlatitude flow (TY Guchol 12W), and a case (TY Saola 17W) in which the tropical cyclone decayed under the influence of strong vertical wind shear. The variability in downstream response to each ET case is related to specific physical characteristics associated with the evolution of the ET process and the phasing between the poleward-moving tropical cyclone and the midlatitude circulation into which it is moving. A case of downstream development that occurred during September 2005 without an ET event is compared with the four ET cases.

Abstract

The development of extratropical cyclone structural characteristics that resulted from the extratropical transition of Typhoon (TY) David (1997) and TY Opal (1997) over the western North Pacific is examined. David moved poleward ahead of a midlatitude trough that was moving eastward as the dominant midlatitude circulation feature over the western North Pacific. During the transition, David coupled with the midlatitude trough, which led to the evolution of an intense cyclone that became the primary circulation over the North Pacific. Although Opal also moved poleward ahead of a midlatitude trough, the principal midlatitude feature over the western North Pacific was a preexisting stationary cyclone over the Kamchatka peninsula. During transition, Opal weakened and became a secondary cyclone to the preexisting primary North Pacific cyclone.

The structural characteristics of the evolving extratropical cyclone with respect to each case are examined in the context of the interaction between a vortex and a baroclinic zone using vector-frontogenesis diagnostics for the Lagrangian rate of change of the magnitude and direction of the horizontal gradient of potential temperature. In this framework, total frontogenesis is divided into components that define the magnitude and rotation of the potential temperature gradient. The initial evolution of extratropical cyclone features for both cases was dominated by warm frontogenesis due to the large amount of warm advection on the east side of the decaying tropical cyclone and the deformation field defined by the poleward movement of the tropical cyclone. However, large differences between the components of rotational frontogenesis for David and Opal are observed that are related to the subsequent reintensification of David and weakening of Opal. The differences are attributed to the different midlatitude circulation characteristics into which each tropical cyclone moved. The pattern of rotational frontogenesis associated with TY David reinforced the dynamical support for the coupling of David with the midlatitude trough, which resulted in the development of an intense extratropical cyclone. During the transition of Opal, maximum rotational frontogenesis occurred over the region where Opal interacted with the preexisting midlatitude cyclone. This weakened the coupling between Opal and the midlatitude trough and prevented the development of a separate extratropical cyclone.

One of the unresolved aspects of forecasting extratropical transition is to define when transition has occurred. Although the final extratropical cyclone characteristics may vary greatly from case to case, increased warm frontogenesis seems to be consistent during the initial change from tropical to extratropical characteristics. Therefore, evolution of a frontogenesis parameter is calculated for each case from before transition, through transition, and after transition. In both cases, the rate of increase in frontogenesis peaks at a time that may be defined as the transition time.

Abstract

The basic structure of the variability of the large-scale circulations over the tropical western Pacific is investigated with respect to its influence on tropical cyclone characteristics. A vector empirical orthogonal function analysis and fuzzy cluster algorithm are applied to a 9-yr dataset to define six recurrent 700-mb circulation patterns that represent large-scale variabilities associated with the monsoon trough and subtropical ridge. Five of the cluster patterns, which contain 48% of the sample, define combinations of active (inactive) monsoon trough and strong (weak) subtropical ridge circulations. The sixth cluster, which contains 26% of the data sample, depicts small deviations from the long-term climatology. After the cluster centers are defined, the fuzzy cluster coefficients are used to identify a seventh cluster, which contains the remaining 26% of the circulation patterns that could not be classified within any of the original six clusters. The 700-mb circulation patterns are physically consistent with outgoing longwave radiation anomalies and the 200-mb streamfunction and velocity potential anomalies. Active and inactive monsoon trough patterns are related to large-scale velocity potential anomalies over the tropical western Pacific and Indian Ocean basins. Anomalous cyclonic circulations are found to be regions of anomalous convergence at 700 mb, divergence at 200 mb, and enhanced large-scale convection. Anticyclonic anomalies are regions of anomalous 700-mb divergence, 200-mb convergence, and reduced large-scale convection. Variability of the subtropical ridge is associated with large-scale, 200-mb streamfunction anomalies that are related to variations in the midlatitude longwave pattern.

Tropical cyclone activity is found to be related significantly to the variability of the monsoon trough described within the cluster framework. Active (inactive) periods are found to occur when the large-scale circulation anomalies are contained within clusters that represent an active (inactive) monsoon trough. However, grouping of clusters based exclusively on the variability of the monsoon trough does not adequately account for the variability in tropical cyclone track types. Comparisons between observed tropical cyclone track characteristics and the cluster definition at the time the tropical cyclone reaches tropical storm strength identify a statistically significant relationship between track type (straight-moving versus recurving) and the individual five cluster patterns that describe the variability of the monsoon trough and subtropical ridge. No relationships are found between tropical cyclone characteristics and the cluster that represents small deviations from the climatological mean or the cluster that is defined to contain circulation patterns not classified in any of the original six clusters. It is concluded that the cluster patterns define the basic structure of large-scale circulation variability over the tropical western Pacific and that these structures are related to tropical cyclone characteristics.

Abstract

The variability of the large-scale circulation over the tropical western North Pacific is described within a framework defined by recurrent 700-mb circulation patterns that were defined by a fuzzy cluster analysis. Individual cluster patterns (defined in Part 1), which represent instantaneous depictions of the circulation variability, define favorable and unfavorable regions for tropical cyclone genesis and preferred track types. The fuzzy cluster coefficients, which describe the time variability of 700-mb large-scale circulation anomalies, are used to identify the basic persistence properties of the recurrent, anomalous circulation patterns. It is found that recurrent circulation patterns that are defined by small anomalies (i.e., close to the center of the cluster analysis phase space) are less persistent than recurrent patterns that represent distinct circulation anomaly patterns. Furthermore, the persistence of a particular sequence of anomaly maps that pass through a cluster is dependent upon the size of the cluster coefficients, which define how well the cluster pattern represents individual anomaly maps.

Analysis of transitions between clusters reveals that a rather limited set of transition paths exist. The most significant transition paths occur across a boundary within the cluster analysis phase space that separates circulation patterns that represent an active monsoon trough from patterns that represent an inactive monsoon trough. Physical descriptions of the significant transition paths are based upon 700-mb and 200-mb streamfunction and velocity potential anomalies, and anomalies of outgoing longwave radiation. The primary transition paths are found to be dependent upon interrelationships between several spatial and temporal scales of atmospheric variability. Furthermore, specific relationships were found to be critical for determining which transition path is followed. Secondary transition paths, which occur less frequently, are more dependent upon regional characteristics such as circulations within the tropical upper-tropospheric trough.

Physical associations between cluster patterns and tropical cyclone characteristics that were defined in Part I remain intact during transitions between the individual clusters. This is a significant result since the variability of the large-scale circulation within the cluster framework, which is defined by the cluster membership coefficients, can be used to infer sequences of persistent or transitioning circulation patterns. The potential application of the cluster framework for estimation of the stability of large-scale atmospheric circulation patterns and expected durations and transition paths is discussed in relation to the predictability of tropical cyclone characteristics.

Abstract

A mesoscale convective system (MCS) embedded in the circulation of Typhoon (TY) Robyn was investigated by two aircraft missions during the tropical cyclone motion (TCM-93) mini field experiment. The MCS initially formed north of the typhoon center, but was rapidly advected to the west by the strong midlevel easterlies located between Robyn and the subtropical ridge to the north. Coincident with the occurrence of the MCS, the motion of the typhoon changed from west-northwestward to a slow drift to the north. The structure of the MCS is investigated to examine whether the changes in motion of TY Robyn could be related to a midtropospheric vortex circulation in the MCS.

During the mature stage, the MCS has a convective region and an extensive stratiform region. A vigorous updraft in the convective region is tilted southward by the primary circulation around TY Robyn. Below the tilted updraft, descent in a mesoscale downdraft contributes to drying in the low levels, with a shallow surface layer of divergent flow from a weak cold pool. These features are below the MCS stratiform region, which is also forced to be south of the convective region by the circulation along the western side of TY Robyn. A potential vorticity maximum near 500 mb extends downward to 800 mb at the very southern edge of the MCS stratiform region, which was approximately 5° longitude west of the center of TY Robyn. During the decay stage of the MCS, the strengthening circulation of TY Robyn results in strong midlevel wind shear that prevents the maintenance of an upright potential vorticity center in the MCS stratiform region.

The track change of Robyn during the period is assessed relative to a potential Fujiwhara-type interaction with the MCS versus a change in the large-scale steering. Although the TY Robyn circulation clearly had an effect on the MCS, the MCS circulation is judged to be too weak, too shallow, and separated too far from TY Robyn to be responsible for the observed track changes. It is concluded that the changes in speed and direction of the typhoon arc caused by a large-scale circulation pattern that results in a combination of weak net environmental flow that is oriented to the north because of Robyn's location at the eastern edge of the western North Pacific monsoon trough.

Abstract

Factors that contribute to intraseasonal variability in western North Pacific tropical cyclone track types are investigated. It is hypothesized that the 700-mb large-scale circulation can affect tropical cyclone track characteristics by enhancing or excluding genesis in certain regions, and concurrently prohibiting or favoring recurving versus straight tracks. A track-type climatology indicates that genesis location alone may explain some of the variability in track type. Although some genesis regions have no preference for straight-moving or recurving tracks, a formation north of 20°N or east of 150°E and north of 10°N favors a recurvature track. These recurving storms are classified as recurving-north, and recurving storms that form in regions with nearly equal probability of straight or recurving tracks are classified as recurving south.

A compositing technique is used to define anomalous 700-mb large-scale circulations that exist during the formation of tropical cyclones that subsequently follow either a straight track or one of the two types of recurring tracks. Anomalous circulations associated with extended periods that do not contain any tropical cyclones are also identified. Physically and statistically different anomalous large-scale circulation patterns exist at the time of genesis for storms following each track type and for inactive periods. The large-scale anomalies describe variations in the positions and intensities of the monsoon trough and subtropical ridge. During genesis of straight-moving and recurving-south storms, anomalous large-scale horizontal cyclonic shear exists throughout the South China Sea and Philippine Sm During straight-moving storms, cyclonic shear increases because of anomalous easterlies along the southern boundary of an enhanced subtropical ridge. During recurving-south storms, anomalous equatorial westerlies and cross-equatorial flow from the Southern Hemisphere act to increase the cyclonic shear.

The track-type climatology is used to predict the subsequent track type based only on genesis location. In a second scheme, the distributions of anomalous 700-mb zonal wind components in 5°latitude bands averaged between 100° and 140°E are used to predict the most likely track type. The large-scale 700-mb anomalies at genesis time determine the subsequent track type in a majority of cases. The skill of this simple scheme exceeds that from the climatological probability of track type.

Abstract

The decision to prepare for an oncoming hurricane is typically framed as a static cost:loss problem, based on a strike-probability forecast. The value of waiting for updated forecasts is therefore neglected. In this paper, the problem is reframed as a sequence of interrelated decisions that more accurately represents the situation faced by a decision maker monitoring an evolving tropical cyclone. A key feature of the decision model is that the decision maker explicitly anticipates and plans for future forecasts whose accuracy improves as lead time declines. A discrete Markov model of hurricane travel is derived from historical tropical cyclone tracks and combined with the dynamic decision model to estimate the additional value that can be extracted from existing forecasts by anticipating updated forecasts, rather than incurring an irreversible preparation cost based on the instantaneous strike probability. The value of anticipating forecasts depends on the specific alternatives and cost profile of each decision maker, but conceptual examples for targets at Norfolk, Virginia, and Galveston, Texas, yield expected savings ranging up to 8% relative to repeated static decisions. In real-time decision making, forecasts of improving information quality could be used in combination with strike-probability forecasts to evaluate the trade-off between lead time and forecast accuracy, estimate the value of waiting for improving forecasts, and thereby reduce the frequency of false alarms.

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.