Patrick A. Harr

The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions

Abstract A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition ...

Extratropical Transition of Western North Pacific Tropical Cyclones: An Overview and Conceptual Model of the Transformation Stage

Abstract Extratropical transition (ET) in the western North Pacific is defined here in terms of two stages: transformation, in which the tropical cyclone evolves into a baroclinic storm; and reintensification, where the transformed storm then deepens as an extratropical cyclone. In this study, 30 ET cases occurring during 1 June–31 October 1994–98 are reviewed using Navy Operational Global Atmospheric Prediction System analyses; hourly geostationary visible, infrared, and water vapor imagery; and microwave imagery. A brief climatology based on these cases is presented for the transformation stage and the subsequent cyclone characteristics of the reintensification stage. A three-dimensional conceptual model of the transformation stage of ET in the western North Pacific Ocean is proposed that describes how virtually all 30 cases evolved into an incipient, baroclinic low. The three-step evolution of the transformation of Typhoon (TY) David (September 1997) is described as a prototypical example. Four importa...

The “Year” of Tropical Convection (May 2008–April 2010): Climate Variability and Weather Highlights

The representation of tropical convection remains a serious challenge to the skillfulness of our weather and climate prediction systems. To address this challenge, the World Climate Research Programme (WCRP) and The Observing System Research and Predictability Experiment (THORPEX) of the World Weather Research Programme (WWRP) are conducting a joint research activity consisting of a focus period approach along with an integrated research framework tailored to exploit the vast amounts of existing observations, expanding computational resources, and the development of new, high-resolution modeling frameworks. The objective of the Year of Tropical Convection (YOTC) is to use these constructs to advance the characterization, modeling, parameterization, and prediction of multiscale tropical convection, including relevant two-way interactions between tropical and extratropical systems. This article highlights the diverse array of scientifically interesting and socially important weather and climate events assoc...

Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part I: Evolution of Structural Characteristics during the Transition Process

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 an...

Large-Scale Circulation Variability over the Tropical Western North Pacific. Part I: Spatial Patterns and Tropical Cyclone Characteristics

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 physi...

Tropical Cyclone Track Characteristics as a Function of Large-Scale Circulation Anomalies

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 fol...

Predictability Associated with the Downstream Impacts of the Extratropical Transition of Tropical Cyclones: Case Studies

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 rela...

Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part II: The Impact of Midlatitude Circulation Characteristics

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 form...

Northwestward-Propagating Wave Patterns over the Tropical Western North Pacific during Summer

Abstract The possible relationship between northwestward-propagating wave disturbances and tropical cyclones over the tropical western North Pacific during summer is studied using data assimilated by the navys global model during May–September 1989–91. A multiple-set canonical correlation (MCC) analysis is applied to the 850-hPa meridional (v) component over a core domain covering the western Pacific. The analysis seeks the maximal geometrically averaged correlation between 12 consecutive twice-daily fields. Two MCC components, with a 90° phase difference and comparable variances that combine to nearly one-third of the total variance, describe the northwestward-propagating pattern with a period near 8–9 days. Upstream of this steady northwestward-propagating pattern there is a weaker, westward propagation along 5°N that may be traced back to 170°E. The surface pressure cell advancing east of the Philippines is consistent with low-level winds for a circulation in gradient wind balance. It has a zonal wave...

Predictability Associated with the Downstream Impacts of the Extratropical Transition of Tropical Cyclones: Methodology and a Case Study of Typhoon Nabi (2005)

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 o...