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Clark Evans and Robert E. Hart

Abstract

Extratropical transition brings about a number of environmentally induced structural changes within a transitioning tropical cyclone. Of particular interest among these changes is the acceleration of the wind field away from the cyclone’s center of circulation along with the outward movement of the radial wind maximum, together termed wind field expansion. Previous informal hypotheses aimed at understanding this evolution do not entirely capture the observed expansion, while a review of the literature shows no formal work done upon the topic beyond analyzing its occurrence. This study seeks to analyze the physical and dynamical mechanisms behind the wind field expansion using model simulations of a representative transition case, North Atlantic Tropical Cyclone Bonnie of 1998. The acceleration of the wind field along the outer periphery of the cyclone is found to be a function of the net import of absolute angular momentum within the cyclone’s environment along inflowing trajectories. This evolution is shown to be a natural outgrowth of the development of isentropic conveyor belts and asymmetries associated with extratropical cyclones. Asymmetries in the outer-core wind field manifest themselves via the tightening and development of height and temperature gradients within the cyclone’s environment. Outward movement of the radial wind maximum occurs coincident with integrated net cooling found inside the radius of maximum winds. Tests using a secondary circulation balance model show the radial wind maximum evolution to be similar yet opposite to the response noted for intensifying tropical cyclones with contracting eyewalls.

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Robert Pincus and K. Franklin Evans

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This paper examines the tradeoffs between computational cost and accuracy for two new state-of-the-art codes for computing three-dimensional radiative transfer: a community Monte Carlo model and a parallel implementation of the Spherical Harmonics Discrete Ordinate Method (SHDOM). Both codes are described and algorithmic choices are elaborated. Two prototype problems are considered: a domain filled with stratocumulus clouds and another containing scattered shallow cumulus, absorbing aerosols, and molecular scatterers. Calculations are performed for a range of resolutions and the relationships between accuracy and computational cost, measured by memory use and time to solution, are compared.

Monte Carlo accuracy depends primarily on the number of trajectories used in the integration. Monte Carlo estimates of intensity are computationally expensive and may be subject to large sampling noise from highly peaked phase functions. This noise can be decreased using a range of variance reduction techniques, but these techniques can compromise the excellent agreement between the true error and estimates obtained from unbiased calculations. SHDOM accuracy is controlled by both spatial and angular resolution; different output fields are sensitive to different aspects of this resolution, so the optimum accuracy parameters depend on which quantities are desired as well as on the characteristics of the problem being solved. The accuracy of SHDOM must be assessed through convergence tests and all results from unconverged solutions may be biased.

SHDOM is more efficient (i.e., has lower error for a given computational cost) than Monte Carlo when computing pixel-by-pixel upwelling fluxes in the cumulus scene, whereas Monte Carlo is more efficient in computing flux divergence and downwelling flux in the stratocumulus scene, especially at higher accuracies. The two models are comparable for downwelling flux and flux divergence in cumulus and upwelling flux in stratocumulus. SHDOM is substantially more efficient when computing pixel-by-pixel intensity in multiple directions; the models are comparable when computing domain-average intensities. In some cases memory use, rather than computation time, may limit the resolution of SHDOM calculations.

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Robert E. Hart, Jenni L. Evans, and Clark Evans

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 negative tilt permits a contraction and intensification of the eddy potential vorticity (PV) flux, while the positive trough tilt prevents contraction and intensification of the forcing. In 6 of the 34 cases, the posttropical cold-core cyclone develops a warm-seclusion structure, rather than remaining cold core. Anticipation of this warm-seclusion evolution is critical since it represents a dramatically increased risk of middle- to high-latitude wind and wave damage. The warm-seclusion evolution is most favored when the scale of the interacting trough closely matches the scale of the transitioned TC, focusing the eddy PV flux in the outflow layer of the transitioning TC. The sensitivity of structural evolution prior to and after TE illustrated here gives insight into the degradation of global model midlatitude forecast accuracy during a pending ET event.

Eliassen–Palm flux cross sections suggest that ET is primarily driven by the eddy angular momentum flux of the trough, rather than the eddy heat flux associated with the trough. The response of the transitioning TC to the eddy angular momentum forcing is to produce adiabatic ascent and cooling radially inward and beneath the region of the forcing to restore thermal wind balance. In the case of ET, the forcing is maximized lower in the atmosphere, and spread over much greater depth, than in the case of trough-induced TC intensification. Only after TE is the eddy heat flux forcing as significant as the eddy angular momentum forcing, further supporting a physical foundation for the CPS description of cyclone evolution.

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Robert E. Hart and Jenni L. Evans

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 much higher frequency between the latitudes of 35° and 45°N. Transition occurs at lower latitudes at the beginning and end of the season, and at higher latitudes during the season peak (August–September). This seasonal cycle of transition location is the result of competing factors. The delayed warming of the Atlantic Ocean forces the location of transition northward late in the season, since the critical threshold for tropical development is pushed northward. Conversely, the climatologically favored region for baroclinic development expands southward late in the season, pinching off the oceanic surface area over which tropical development can occur. The relative positions of these two areas define the typical life cycle of a transitioning tropical cyclone: tropical intensification, tropical decay, extratropical transition and intensification, occlusion.

Using a synthesis of National Hurricane Center Best-Track data and European Centre for Medium-Range Weather Forecasts reanalyses data, the intensity changes during and after transition are evaluated. It is extremely rare for a transitioning tropical cyclone to regain (in the extratropical phase) its peak (tropical phase) intensity. However, of the 61 transitioning tropical storms during the period 1979–93, 51% underwent post-transition intensification. Over 60% of cyclones that underwent post-transition intensification originated south of 20°N. In contrast, 90% of tropical cyclones that underwent post-transition decay originated north of 20°N. This suggests that strong baroclinic characteristics during formation are not necessary for strong post-transition development;in fact, they appear to hinder post-transition intensification and, therefore, the post-transition life span of the cyclone itself.

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Robert Pincus, Cécile Hannay, and K. Franklin Evans

Abstract

Three-dimensional radiative transfer calculations are accurate, though computationally expensive, if the spatial distribution of cloud properties is known. The difference between these calculations and those using the much less expensive independent column approximation is called the 3D radiative transfer effect. Assessing the magnitude of this effect in the real atmosphere requires that many realistic cloud fields be obtained, and profiling instruments such as ground-based radars may provide the best long-term observations of cloud structure. Cloud morphology can be inferred from a time series of vertical profiles obtained from profilers by converting time to horizontal distance with an advection velocity, although this restricts variability to two dimensions. This paper assesses the accuracy of estimates of the 3D effect in shallow cumulus clouds when cloud structure is inferred in this way. Large-eddy simulations provide full three-dimensional, time-evolving cloud fields, which are sampled every 10 s to provide a “radar’s eye view” of the same cloud fields. The 3D effect for shortwave surface fluxes is computed for both sets of fields using a broadband Monte Carlo radiative transfer model, and intermediate calculations are made to identify reasons why estimates of the 3D effect differ in these fields. The magnitude of the 3D effect is systematically underestimated in the two-dimensional cloud fields because there are fewer cloud edges that cause the effect, while the random error in hourly estimates is driven by the limited sample observed by the profiling instrument.

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Jenni L. Evans and Robert E. Shemo

Abstract

A fully automated, objective classification system has been developed to analyze infrared satellite imagery. This automated system facilitates tracking and categorization of convective weather systems into various classes. The classes chosen reflect the maximum degree of organization attained by each weather system. Four classes of convective weather system are defined; tropical cyclones (TS; including prestorm clusters through to decaying storms), mesoscale convective complexes (MCC), convective cloud clusters (CCC), and disorganized shortlived convection (DSL). Systems are identified, tracked, and then classified. If a system satisfies the criteria for any of the organized convection classes (TS, MCC, or CCC) for at least two time periods, the entire track is allocated to that class. In cases where a system satisfies the criteria for more than one type of organized convection (commonly MCC and CCC), it is assigned to the “most organized” class (in this case, MCC). Thus, the characteristics of each class incorporate the life cycles of systems that satisfy the imposed criteria for at least a 6-h period.

Two satellite infrared-based (IR) rain-rate algorithms are applied to the convective areas in order to obtain precipitation amounts for the various classes of convection. The domain of interest extends from the eastern Pacific margin to the African coast (15°W) and 40°N–40°S.

In addition to the IR data, rain rates derived from Special Sensor Microwave/Imager data are compared with the infrared retrieved rain rates at available times for a subset of each of the three organized convection classes. Rainfall amounts obtained from these infrared algorithms are also compared with ground-based station observations over Florida. Comparison of the inferred rainfall with station data reveals that the TS precipitation is in approximate agreement (in the mean), whereas the precipitation contributions from the other forms of convection are somewhat overestimated. DSL is overestimated the most and CCCs are overestimated the least.

According to the infrared-based rain-rate algorithms, DSLs (short-lived systems) contribute the most total (basinwide, annual) precipitation, CCCs contribute the second largest amount, MCCs are third in the contribution of precipitation, and TSs contribute the least to the total precipitation.

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Robert X. Black and Katherine J. Evans

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The statistics, horizontal structure, and linear barotropic dynamics of anomalous weather regimes are evaluated in a 15-winter integration of the NCAR Community Climate Model (CCM2). Statistical and ensemble analyses of simulated regimes are contrasted with parallel analyses derived from NCEP–NCAR reanalyses. The CCM2 replicates much of the structure of observed frequency distributions for anomalous weather regimes over the North Pacific and North Atlantic regions. The main differences are a northward shift and longitudinal broadening of the North Pacific frequency maximum and a weakening and southward shift of the North Atlantic maximum.

Ensemble analyses reveal that simulated North Pacific regimes attain a more isotropic horizontal anomaly structure than observed cases, which are zonally elongated. The E-vector diagnoses indicate that North Pacific cases in the CCM2 are also associated with much weaker local barotropic energy conversions from the climatological-mean flow. This is partly due to the relatively weak climatological-mean diffluence simulated by the CCM2 in the jet exit region over the eastern North Pacific. The model’s North Atlantic regimes have horizontal anomaly patterns quite similar to observed cases, except for a southwestward shift relative to observations. Both simulated and observed North Atlantic cases exhibit robust local barotropic interactions with the climatological-mean flow, with the strongest conversions shifted southwestward in the model.

The results suggest a larger role for mechanisms besides barotropic instability in maintaining anomalous weather regimes over the North Pacific in the CCM2. The model’s North Atlantic events occur southwest of observed cases apparently in order to more efficiently utilize the available “barotropic energy reservoir” in the model climatology. The authors conclude that for GCMs to properly represent important dynamical characteristics of anomalous weather regimes, it is paramount that the model accurately depict the climatological-mean stationary wave field.

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Robert E. Hart and Jenni L. Evans

Abstract

For over a century it has been known that each vortex in a multiple vortex configuration will move in response to the other vortices. However, despite advances since that time, the complexities of multiple vortex scenarios when sheared environments are present are still not completely understood. The interaction of binary vortices within horizontal environmental shear is explored here through shallow water simulations on a β plane. Due to nonlinear feedbacks, the combination of environmental vorticity (or vorticity gradient) and shear, as well as the multiple vortex situation, results in a more complicated track than for a storm experiencing any individual component. Despite the complexity of these vortex–environment interactions, the use of previous single-vortex studies greatly aids interpretation. Centroid-relative motion of the individual vortices is considered, as well as the propagation of the vortex pair centroid, to understand motion effects of the different vortex–environment combinations.

As the vortices interact, vortex Rossby waves are generated through distortion of the symmetric vorticity field by the opposing vortex. Initially, the high-frequency waves have an insignificant effect upon vortex intensity or propagation, and β-induced wavenumber one asymmetry dominates as expected. However, as the waves approach a critical radius (ζ = 0), wave potential vorticity filamentation and stretching by the circulation of the adjacent vortex leads to a coupling of the two vortices. This vortex coupling results in enhanced propagation speeds of the two vortices proportional to the effective size of the dual-vortex system.

The sign of vorticity of the environmental flow can act to enhance or negate β-drift such that single- or dual-vortex propagation is altered. Further, when environmental vorticity is present, the rate of mutual orbit from Fujiwhara rotation is altered. When the environmental flow is cyclonic, the cyclonic mutual rotation of the vortices is accelerated. Conversely, when the environmental flow is anticyclonic, the mutual rotation of the vortices is substantially decelerated, but remains cyclonic.

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Katherine J. Evans and Robert X. Black

Abstract

Piecewise tendency diagnosis (PTD) is extended and employed to study the dynamics of weather regime transitions. Originally developed for adiabatic and inviscid quasigeostrophic flow on a beta plane, PTD partitions local geopotential tendencies into a linear combination of dynamically meaningful source terms within a potential vorticity (PV) framework. Here PTD is amended to account for spherical geometry, diabatic heating, and ageostrophic processes, and is then used to identify the primary mechanisms responsible for Northern Hemisphere weather regime transitions.

Height tendency patterns obtained by summing the contributions of individual PTD forcing terms correspond very well to actual height tendencies. Composite PTD analyses reveal that linear PV advections provide the largest dynamical forcing for the weather regime development over the North Pacific. Specifically, linear baroclinic growth provides the primary forcing while barotropic deformation of PV anomalies provides a secondary contribution. North Atlantic anticyclonic anomalies develop from the combined effects of barotropic deformation, baroclinic growth, and nonlinear eddy feedback. The Atlantic cyclonic cases develop primarily from barotropic deformation and nonlinear eddy feedback. All four weather regime types decay primarily due to enhanced wave energy propagation away from the primary circulation anomaly. In some cases, regime decay is aided by decreasing positive contributions from barotropic deformation as the circulation anomaly attains a deformed horizontal shape. The current results 1) provide quantitative measures of the primary mechanisms responsible for weather regime transition and 2) demonstrate the utility of the extended PTD as a concise diagnostic paradigm for studying large-scale dynamical processes in the midlatitude troposphere.

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Jenni L. Evans and Robert E. Hart

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 measure of the thermal wind over the same layer. These diagnostics are consistent with the definitions of tropical and baroclinic cyclones and are readily calculable using operational analyses. Comparisons of these objective measures of ET timing with more detailed three-dimensional analyses and NHC classifications show good agreement.

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