Robert E. Hart

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 ...

A Cyclone Phase Space Derived from Thermal Wind and Thermal Asymmetry

Abstract An objectively defined three-dimensional cyclone phase space is proposed and explored. Cyclone phase is described using the parameters of storm-motion-relative thickness asymmetry (symmetric/nonfrontal versus asymmetric/frontal) and vertical derivative of horizontal height gradient (cold- versus warm-core structure via the thermal wind relationship). A cyclones life cycle can be analyzed within this phase space, providing substantial insight into the cyclone structural evolution. An objective classification of cyclone phase is possible, unifying the basic structural description of tropical, extratropical, and hybrid cyclones into a continuum. Stereotypical symmetric warm-core (tropical cyclone) and asymmetric cold-core (extratropical cyclone) life cycles are illustrated using 1° Navy Operational Global Atmospheric Prediction System (NOGAPS) operational analyses and 2.5° NCEP–NCAR reanalyses. The transitions between cyclone phases are clearly illustrated within the phase space, including extratro...

A Climatology of the Extratropical Transition of Atlantic Tropical Cyclones

Abstract A comprehensive climatology of extratropically transitioning tropical cyclones in the Atlantic basin is presented. Storm tracks and intensities over a period from 1899 to 1996 are examined. More detailed statistics are presented only for the most reliable period of record, beginning in 1950. Since 1950, 46% of Atlantic tropical cyclones transitioned to the extratropical phase. The coastal Atlantic areas most likely to be impacted by a transitioning tropical cyclone are the northeast United States and the Canadian Maritimes (1–2 storms per year), and western Europe (once every 1–2 yr). Extratropically transitioning tropical cyclones represent 50% of landfalling tropical cyclones on the east coasts of the United States and Canada, and the west coast of Europe, combined. The likelihood that a tropical cyclone will transition increases toward the second half of the tropical season, with October having the highest probability (50%) of transition. Atlantic transition occurs from 24° to 55°N, with a muc...

Objective indicators of the life cycle evolution of extratropical transition for Atlantic tropical cyclones

Abstract Forty-six percent of Atlantic tropical storms undergo a process of extratropical transition (ET) in which the storm evolves from a tropical cyclone to a baroclinic system. In this paper, the structural evolution of a base set of 61 Atlantic tropical cyclones that underwent extratropical transition between 1979 and 1993 is examined. Objective indicators for the onset and completion of transition are empirically determined using National Hurricane Center (NHC) best-track data, ECMWF 1.125° × 1.125° reanalyses, and operational NCEP Aviation Model (AVN) and U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) numerical analyses. An independent set of storms from 1998 to 2001 are used to provide a preliminary evaluation of the proposed onset and completion diagnostics. Extratropical transition onset is declared when the storm becomes consistently asymmetric, as measured by the 900–600-hPa thickness asymmetry centered on the storm track. Completion of the ET process is identified using a...

Using Normalized Climatological Anomalies to Rank Synoptic-Scale Events Objectively

Abstract A method for ranking synoptic-scale events objectively is presented. NCEP 12-h reanalysis fields from 1948 to 2000 are compared to a 30-yr (1961–90) reanalysis climatology. The rarity of an event is the number of standard deviations 1000–200-hPa height, temperature, wind, and moisture fields depart from this climatology. The top 20 synoptic-scale events from 1948 to 2000 for the eastern United States, southeast Canada, and adjacent coastal waters are presented. These events include the “The Great Atlantic Low” of 1956 (ranked 1st), the “superstorm” of 1993 (ranked 3d), the historic New England/Quebec ice storm of 1998 (ranked 5th), extratropical storm Hazel of 1954 (ranked 9th), a catastrophic Florida freeze and snow in 1977 (ranked 11th), and the great Northeast snowmelt and flood of 1996 (ranked 12th). During the 53-yr analysis period, only 33 events had a total normalized anomaly (MTOTAL) of 4 standard deviations or more. An MTOTAL of 5 or more standard deviations has not been observed during ...

An Examination of Tropical Cyclone Position, Intensity, and Intensity Life Cycle within Atmospheric Reanalysis Datasets

AbstractThe following study examines the position and intensity differences of tropical cyclones (TCs) among the Best-Track and five atmospheric reanalysis datasets to evaluate the degree to which reanalyses are appropriate for studying TCs. While significant differences are found in both reanalysis TC intensity and position, the representation of TC intensity within reanalyses is found to be most problematic owing to its underestimation beyond what can be attributed solely to the coarse grid resolution. Moreover, the mean life cycle of normalized TC intensity within reanalyses reveals an underestimation of both prepeak intensification rates as well as a delay in peak intensity relative to the Best-Track. These discrepancies between Best-Track and reanalysis TC intensity and position can further be described through correlations with such parameters as Best-Track TC age, Best-Track TC intensity, Best-Track TC location, and the extended Best-Track TC size. Specifically, TC position differences within the 4...

Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings

Abstract Forecasting significant weather events, such as floods, heat waves, arctic outbreaks, ice storms, large severe weather outbreaks, and major winter storms, is a critical function for all weather services. However, conventional pressure level geopotential and temperature fields often are insufficient to determine whether an event represents a large departure from normal. This is largely due to the variability that exists throughout the year and regionally throughout the world. What represents an unusual departure from average conditions in fall may not be as unusual in winter. What is an unusual departure from average conditions in California may be normal in New England. This paper presents a method, normalized field departures from local climatology, that gives forecasters guidance on the relative rarity of events. Thus, in this paper a method is presented to help forecasters identify potentially significant weather events. The focus of this paper is on significant winter storms. However, a recor...

Estimating Local Memory of Tropical Cyclones through MPI Anomaly Evolution

This study examines the local memory of atmospheric and oceanic changes associated with a tropical cyclone (TC). The memory is quantified through anomalous maximum potential intensity (MPI) evolution for 20 days prior to the arrival of a TC through 60 days after the TC passage. The local MPI weakens and is not restored to the evolving climatology until well after the TC has departed. Stabilization occurs through warming of the atmosphere and cooling of the ocean surface on different time scales. The time scale of MPI stabilization following TC passage is approximately 30–35 days for a tropical storm to 50–60 days for a category 3–5 hurricane, with significant storm-specific and basin-specific variability. The atmospheric stabilization (warming with respect to SST) begins with TC arrival and continues for approximately 7–10 days after passage, when the troposphere cools below normal. The rewarming of SST and the subsequent rewarming of the atmosphere occurs within approximately 35 days for all intensities, despite a positive (weakened) MPI anomaly through two months. This suggests that the atmosphere retains anomalous warmth beyond what can be attributable to sensible heating from the rewarmed SST. The maintenance of a positive MPI anomaly beyond 35 days is thus attributed to a feedback on larger scales that requires considerable further research. A TC’s passage through a region does not always lead to a weakening of the MPI. In regions poleward of the sharp SST gradient, the MPI one month after TC passage is often several millibars stronger than climatology. There are also mesoscale regions of destabilization one month after TC passage that may result partially from salinity changes driven by oceanic mixing as well as changes in precipitation and evaporation.

Synoptic Composites of the Extratropical Transition Life Cycle of North Atlantic Tropical Cyclones: Factors Determining Posttransition Evolution

Abstract A 34-member ensemble-mean trajectory through the cyclone phase space (CPS) is calculated using Navy Operational Global Atmospheric Prediction System (NOGAPS) analyses for North Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET). Synoptic composites at four ET milestones are examined: 24 h prior to the beginning of ET (TB − 24), the beginning of ET (TB), the end of ET (TE), and 24 h after the end of ET (TE + 24). While the extratropically transitioning TC structure is tightly constrained in its tropical phase, it has a variety of evolutions after TE. Partitioning the ensemble based upon post-ET intensity change or structure discriminates among statistically significant ET precursor conditions. Compositing the various post-ET intensity regimes provides insight into the important environmental factors governing post-ET development. A TC that intensifies (weakens) after TE begins transition (t = TB) with a negatively (positively) tilted trough 1000 km (1500 km) upstream. The ne...

Use of Normalized Anomaly Fields to Anticipate Extreme Rainfall in the Mountains of Northern California

Abstract Extreme rainfall events contribute a large portion of wintertime precipitation to northern California. The motivations of this paper were to study the observed differences in the patterns between extreme and more commonly occurring lighter rainfall events, and to study whether anomaly fields might be used to discriminate between them. Daily (1200–1200 UTC) precipitation amounts were binned into three progressively heavier categories (12.5–50.0 mm, light; 50–100 mm, moderate; and >100 mm, heavy) in order to help identify the physical processes responsible for extreme precipitation in the Sierra Nevada range between 37.5° and 41.0°N. The composite fields revealed marked differences between the synoptic patterns associated with the three different groups. The heavy composites showed a much stronger, larger-scale, and slower-moving negative geopotential height anomaly off the Pacific coast of Oregon and Washington than was revealed in either of the other two composites. The heavy rainfall events were...