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W. Edward Bracken and Lance F. Bosart

Abstract

The synoptic-scale flow during tropical cyclogenesis and cyclolysis over the North Atlantic Ocean is investigated using compositing methods. Genesis and lysis are defined using the National Hurricane Center (NHC, now known as the Tropical Prediction Center) best-track data. Genesis (lysis) occurs when NHC first (last) identifies and tracks a tropical depression in the final best track dataset. Storm-centered composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ∼900 hPa) and 200-hPa winds for June–November produced by NHC for the years 1975–93. Results show that significant regional differences exist in 200-hPa flow during genesis across the Atlantic basin. Composites of genesis in the western part of the basin show a 200-hPa trough (ridge) located to the west (east) of the ATOLL disturbance. In the eastern half of the basin composites of genesis show a sprawling 200-hPa ridge centered northeast of the ATOLL disturbance. The major axis of this elliptically shaped 200-hPa anticyclone extends zonally slightly poleward of the ATOLL level disturbance. Another composite of relatively rare genesis events that are associated with the equatorward end of frontal boundaries show that they generally occur in the equatorward entrance region of a jet streak in conjunction with an ATOLL cyclonic vorticity maximum in a region where vertical shear is minimized.

An approximation of the Sutcliffe–Trenberth form of the quasigeostrophic omega equation is used to estimate the forcing for vertical motion in the vicinity of developing tropical cyclones. Forcing for ascent is found in all three genesis composites and is accompanied by a nonzero minimum in vertical shear directly above the ATOLL cyclonic vorticity maximum. Vertical shear over developing depressions is found to be near 10 m s−1, suggestive that weak shear is necessary during tropical cyclogenesis to help force synoptic-scale ascent. Composites of tropical cyclone lysis show much weaker ATOLL cyclonic vorticity when compared to the genesis composites. The magnitude of the vertical shear and the forcing for ascent above the lysis ATOLL disturbance are stronger and weaker, respectively, than in the genesis composites. These differences arise due to the presence of a jet-streak and a longer half-wavelength between the trough and ridge axes in the lysis 200-hPa flow composite.

The genesis flow patterns are decomposed by crudely removing the signature of the developing cyclone and its associated convection. Two separate and very different flow patterns commonly observed during genesis over the eastern and western Atlantic Ocean are found to be very similar once the flows are decomposed. Both flows are characterized by strong deformation at low levels and at 200 hPa with an upper-level jet exit region near the developing depression.

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Len G. Keshishian, Lance F. Bosart, and W. Edward Bracken

Abstract

A limited regional climatology of cyclones with and without inverted troughs that form in the Colorado region is presented along with case study results from two major cyclone events in which an inverted trough plays a prominent role in the life cycle of the storm. Typically, the inverted trough separates a polar or arctic air mass dammed up along the eastern foothills of the Rockies from an older modified polar air mass over the plains. Inverted troughs are favored when anticyclonic conditions prevail at the surface across south-central Canada and the northern plains states beneath a confluent flow aloft. Although both types of cyclones form in response to a progressive trough crossing the Rockies, a composite analysis shows that an inverted trough is most likely when a band of meridionally oriented ascent in the lower and middle troposphere persists along the eastern slopes of the Rockies beneath confluent flow aloft. Cyclones without an inverted trough tend to occur when the synoptic-scale ascent region moves rapidly eastward away from the mountains, so that surface pressure falls immediately to the east of the mountains with attendant cold-air damming cannot be sustained.

The life cycle of both cyclones departs significantly from the simple conceptual ideas illustrated in the Norwegian cyclone model. Four principal air masses are estimated to be involved in the cyclone evolution: 1) warm moist air from the Gulf of Mexico, 2) older modified polar air returning poleward behind a retreating surface anticyclone, 3) subsided Pacific air crossing the southern Rockies, and 4) a new polar or arctic air mass moving southward east of the Rockies. The inverted trough separates 2 from 4, and a weak warm front delineates 1 from 2. The primary cold front marks the boundary between 1 and 3, while a secondary cold front, originating as a northerly wind surge along the eastern slopes of the Rockies and appearing as a bent-back cold front, separates 3 from 4. The secondary cold front eventually becomes the dominant cold front as the primary front weakens. In the January 1975 case a third cold front, marking the leading edge of arctic air, eventually overtakes the second cold front. Although the “catch-up” of the warm front by the cold front as envisioned in the Norwegian cyclone model occurs in both storms, the results depict a rich life cycle tapestry that depends upon the interaction of orographically induced mesoscale circulations with synoptic-scale transient disturbances. For example, in the April 1986 case the older modified polar air mass wraps cyclonically westward and then southward against the colder air to the west (the inverted trough is acting as the primary warm front) creating a warm-air extrusion near the cyclone center between the highly baroclinic inverted trough and the much weaker occluded front to the east. The conventional surface warm front plays only a secondary role in the storm life cycle.

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Lance F. Bosart, W. Edward Bracken, and Anton Seimon

Abstract

An analysis is presented of prominent mesoscale structure in a moderately intense cyclone with emphasis on a long-lived, large-amplitude inertia–gravity wave (IGW) that moved through the northeastern United States on 4 January 1994. Available National Weather Service WSR-88D Doppler radar and wind profiler observations are employed to illustrate the rich, time-dependent, three-dimensional structure of the IGW. As the IGW amplified [peak crest-to-trough pressure falls exceeded 13 hPa (30 min)−1], it also accelerated away from the cyclone, reaching a peak forward speed of 35–40 m s−1 across eastern New England. The IGW was one of three prominent mesoscale features associated with the cyclone, the others being a weak offshore precursor warm-frontal wave and an onshore band of heavy snow (“snow bomb”) in which peak hourly snowfalls of 10–15 cm were observed. None of these three prominent mesoscale features were well forecast by existing operational prediction models, particularly with regard to precipitation amount, onset, and duration. The observed precipitation discrepancies illustrate the subtle but important effects of subsynoptic-scale disturbances embedded within the larger-scale cyclonic circulation. The precursor offshore warm-frontal wave was instrumental in reinforcing the wave duct preceding the IGW. The snow bomb was an indication of vigorous ascent, large upper- (lower-) level divergence (convergence), unbalanced flow, and associated large parcel accelerations, environmental conditions known to be favorable for IGW formation.

Small-amplitude IGWs (<1 hPa) are first detected over the southeastern United States from surface microbarogram records and are confirmed independently by the presence of organized and persistent mesoscale cloud bands oriented approximately along the wave fronts. The area of IGW genesis is situated poleward of a weak surface frontal boundary where there is a weak wave duct (stable layer) present in the lower troposphere. In the upper troposphere the region of IGW genesis is situated on the forward side of a deep trough where there is significant cyclonic vorticity advection by the thermal wind. Diagnostic evidence supports the importance of shearing instability and/or unbalanced flow in IGW genesis.

The large-amplitude IGW originates on the downstream edge of the northeastward-advancing packet of small-amplitude IGWs. Wave amplification occurs near the upshear edge of a high, cold cloud shield that generally marks the warm conveyor belt. Although it is not possible to conclusively state whether the amplifying IGW forms in situ or grows from a predecessor weaker (<1 hPa) disturbance, rapid amplification occurs 1) as the wave encounters an increasingly deeper and stronger wave duct, possibly permitting wave overreflection, in the cold air damming region east of the Appalachians, and 2) downshear of an area of significantly positive unbalanced divergence and parcel divergence tendency. The authors raise the possibility that IGW amplification can be associated with the penetration and perturbation of the wave duct by vigorous subsynoptic-scale vertical motions whose vigor is increased by wave-induced latent heat release.

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David M. Schultz, W. Edward Bracken, and Lance F. Bosart

Abstract

Motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected Mexico and Central America, the climatology of Central American cold surges is examined in this paper. An independently derived listing of 177 cold-surge events is employed for which the following properties are tabulated: onset date, duration, time between cold-surge events, latitude of maximum equatorward penetration (ϕ min), and 48-h maximum surface temperature change at Merida, Mexico (ΔT). These data show that 75% of the cold surges have durations of 2–6 days, the same timescale as mobile disturbances in the westerlies. Also, there does not appear to be any relationship between ΔT and the duration of the event, although cold surges that penetrate to low latitudes (ϕ min = 7°–10°N) have a weak tendency to persist longer than those that do not penetrate to low latitudes (ϕ min = 15°–20°N). In addition, the Reding data indicate that the cold surges tend to reach their most equatorward extent where topographic features impede the progress of equatorward-moving cold air; the temperature decrease in the postsurge air (as measured by ΔT) does not appear to be related to the most equatorward extent.

To examine the planetary- and synoptic-scale patterns associated with different categories of cold surges, events with similar characteristics from this database were composited: COLD (ϕ min ≤ 10°N and ΔT ≥ 9°C), COOL (ϕ min ≤ 10°N and ΔT = 4°–5°C), and LONG (events lasting at least 8 days). COLD surges are characterized by a persistent upper-level ridge over the western United States, 200-hPa confluence over the Gulf of Mexico, and the migration of a Canadian lower-tropospheric anticyclone equatorward along the Rocky Mountains and the Sierra Madre. In contrast, COOL surges are associated with a progressive, upper-level ridge over the western United States, weak 200-hPa confluence over the Gulf of Mexico, and the migration of a North Pacific anticyclone over the intermountain west and into the southeast United States. LONG surges are associated with a slower- moving planetary-scale pattern; 200-hPa confluence over the Gulf of Mexico; the occurrence of multiple cold surges, which reinforce the anticyclone over Mexico; and the absence of low-latitude, upper-tropospheric, mobile short-wave troughs to prematurely weaken the anticyclone. Cold surges (especially COLD) can be associated with an acceleration of the trade winds over the eastern North Pacific Ocean and play a role in El Niño–Southern Oscillation. The results in this paper are compared to the results of previous studies of North American, Central American, and east Asian cold surges.

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Lance F. Bosart, W. Edward Bracken, John Molinari, Christopher S. Velden, and Peter G. Black

Abstract

Hurricane Opal intensified rapidly and unexpectedly over the Gulf of Mexico between 1800 UTC 3 October and 1000 UTC 4 October 1995. During this period the storm central pressure decreased from 963 to 916 hPa and sustained winds reached 68 m s−1. Analyses that include high-resolution GOES-8 water vapor winds and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) gridded datasets are employed to examine the rapid intensification phase of Opal.

Opal first reached tropical storm strength on 29–30 September 1995 as it interacted with a trough while situated over the Yucatan Peninsula. Opal deepened moderately (∼20 hPa) in the 24 h ending 1200 UTC 2 October as it achieved minimal hurricane strength and as it turned northeastward. The deepening occurred in conjunction with an environmental flow interaction as determined by an Eliassen balanced vortex outflow calculation.

As Opal accelerated toward the Gulf coast by 1200 UTC 3 October, it approached the equatorward jet-entrance region of a progressive synoptic-scale trough. The trough tail extended southwestward toward the lower Texas coast. As the poleward portion of the trough moved eastward, the equatorward end of the trough lagged behind, stretched meridionally, and partially fractured as it encountered a deformation region over the northwest Gulf. Enhanced outflow and increased divergence in the upper troposphere poleward of Opal was associated with the deformation zone and the partially fractured trough tail.

An analysis of the 300–200-hPa layer-averaged divergence and 6-h divergence change based on an analysis of the water vapor winds shows a significant increase in the magnitude and equatorward extension of the divergence core toward Opal that begins at 1200 UTC 3 October and is most apparent by 1800 UTC 3 October and 0000 UTC 4 October. This divergence increase is shown to precede convective growth in the eyewall and the onset of rapid intensification and is attributed to a jet–trough–hurricane interaction in a low-shear environment. Calculations of balanced vortex outflow based on the ECMWF and NCEP gridded datasets confirms this interpretation.

A crucial finding of this work is that the jet–trough–hurricane interaction and explosive intensification of Opal begins near 0000 UTC 4 October when the storm is far from its maximum potential intensity (MPI), and the 850–200-hPa shear within 500 km of the center is weak (2–3 m s−1). In this first stage of rapid intensification the winds increase by almost 15 m s−1 to 52 m s−1 prior to the storm reaching an oceanic warm-core eddy. The second stage of rapid intensification occurs between 0600 and 1000 UTC 4 October when Opal is over the warm-core eddy and sustained winds increase to 68 m s−1. During this second stage conditions are still favorable for a jet–trough–hurricane interaction as demonstrated by the balanced vortex outflow calculation. Opal weakens rapidly after 1200 UTC 4 October when the storm is near its MPI, the shear is increasing, and the eye is leaving the warm-core eddy. This weakening occurs as Opal moves closer to the trough. It is suggested that an important factor in determining whether a storm–trough interaction is favorable or unfavorable for intensification is how far a storm is from its MPI. The results suggest that a favorable storm–trough interaction (“good trough”) can occur when a storm is far from its MPI.

It is suggested that although the ECMWF (and to lesser extent NCEP) analyses reveal the trough–jet–hurricane interaction through the balanced vortex outflow calculation, that the failure of the same models to predict the rapid intensification of Opal can be attributed to the inability of the model to resolve the eye and internal strorm structure and the associated influence of the trough–jet–hurricane interaction on the diabatically driven storm secondary circulation. The analyses also indicate that the high spatial and temporal resolution of the GOES-8 water vapor winds reveal important mesoscale details of the trough–jet–hurricane interaction that would otherwise be hidden.

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Lance F. Bosart, Gregory J. Hakim, Kevin R. Tyle, Mary A. Bedrick, W. Edward Bracken, Michael J. Dickinson, and David M. Schultz

Abstract

The results of a multiscale analysis of the 12–14 March 1993 superstonn (SS93) over eastern North America are presented. A time sequence of overlapping 10-day time-mean 5OO-hPa geopotential height and anomaly composites shows that the Northern Hemisphere (NH) flow pattern from 18 February to 15 March 1993 is characterized by 1) three persistent troughs situated over eastern Asia and the northwestern Pacific, over eastern North America, and over northwestern Africa and southwestern Europe eastward to central Russia; and 2) a massive blocking anticyclone located over the central and eastern Atlantic. Beginning 8–9 March 1993 the planetary-scale flow amplifies substantially. The explosive SS93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the NH available potential energy content, after peaking on 9 March 1993, decreases by about 6%–7%.

A dynamical tropopause analysis is used to track coherent transient potential vorticity (PV) anomalies and show their qualitative interaction with the planetary-scale flow. SS93 is attributed to the interaction and eventual merger of strong PV anomalies embedded in the northern and southern branches of the westerlies in a background confluent northwesterly flow associated with an amplifying positive-phase Pacific–North American flow pattern. The northern PV anomaly originates in southwestern Canada on 18 February and circumnavigates the NH at relatively high latitudes, a track that permits it to maintain arctic characteristics prior to merger. The southern PV anomalies, tracked from Europe and western Asia eastward across the Pacific, reach North America by 11 March 1993 where they become associated with widespread convention over southern Texas and the northwestern Gulf of Mexico beginning 12 March 1993.

The unique aspects of SS93 are attributed to 1) the near simultaneous amplification of the planetary-scale flow and the lateral and vertical interaction of individual PV anomalies cast of the Rockies during the merger process, and 2) the lag of the northern PV anomaly relative to the southern anomaly so that a baroclinic zone containing lower-tropospheric air of significant conditional instability is allowed to remain in place over southern Texas and the northwest Gulf of Mexico in the cyclogenetic environment ahead of the northern PV anomaly.

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Michael J. Dickinson, Lance F. Bosart, W. Edward Bracken, Gregory J. Hakim, David M. Schultz, Mary A. Bedrick, and Kevin R. Tyle

Abstract

The incipient stages of the 12–14 March 1993 “superstorm” (SS93) cyclogenesis over the Gulf of Mexico are examined. Noteworthy aspects of SS93 include 1) it is the deepest extratropical cyclone ever observed over the Gulf of Mexico during the 1957–96 period, and 2) existing operational prediction models performed poorly in simulating the incipient cyclogenesis over the northwestern Gulf of Mexico. A dynamic-tropopause (DT) analysis shows that SS93 is triggered by a potent potential vorticity (PV) anomaly as it crosses extreme northern Mexico and approaches the Gulf of Mexico. The low-level environment over the western Gulf of Mexico is warmed, moistened, and destabilized by a persistent southerly flow ahead of the approaching PV anomaly. Ascent and a lowering of the DT (associated with a lowering of the potential temperature) ahead of the PV anomaly contributes to further destabilization that is realized in the form of a massive convective outbreak.

An examination of the National Centers for Environmental Prediction (NCEP) Medium Range Forecast (MRF) model-initialized fields after convection begins shows that the MRF does not fully resolve important features of the potential temperature, pressure, and wind fields on the DT in the incipient SS93 environment. Similarly, the NCEP MRF 12-h/24-h forecasts verifying 1200 UTC 12 March and 0000 UTC 13 March are unable to simulate sufficient deep convection over the Gulf of Mexico, low-level PV growth in the incipient storm environment, high-level PV destruction and the associated warming and lifting of the DT over and downshear of the developing storm. Given that the MRF-initialized fields possess sufficient conditional instability, moisture, and ascent to trigger widespread deep convection, the poorly forecast incipient SS93 development appears to be associated with the failure of the model cumulus parameterization scheme. A comparison of the MRF forecasts with selected forecast fields derived from the European Centre for Medium-Range Weather Forecasts operational model supports this interpretation.

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David M. Schultz, W. Edward Bracken, Lance F. Bosart, Gregory J. Hakim, Mary A. Bedrick, Michael J. Dickinson, and Kevin R. Tyle

Abstract

In the wake of the eastern United States cyclone of 12–14 March 1993, a cold surge, originating over Alaska and western Canada, brought northerlies exceeding 20 m s−1 and temperature decreases up to 15°C over 24 h into Mexico and Central America. This paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale, focusing on 1) the structure and evolution of the leading edge of the cold surge, 2) the reasons for its extraordinary intensity and equatorward extent, and 3) the impact of the surge on the Tropics, specifically, on the strength of the trade winds and on the sea surface temperature in the eastern Pacific.

The cold surge was initiated as a developing cyclone over the Gulf of Mexico, and an upstream anticyclone east of the Rockies caused an along-barrier pressure gradient to form, forcing topographically channeled northerlies along the Rocky and Sierra Madre Mountains to transport cold air equatorward. On the mesoscale, the leading edge of the cold surge possessed nonclassical frontal structure. For example, as the cold surge entered Mexico, the coldest air and the strongest wind arrived at about 900 hPa before affecting the surface, suggestive of a tipped-forward leading edge to the surge. Also, satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air. The cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a gravity current, but none of these conceptual/dynamical models was fully applicable. Associated with the cold surge, gap winds up to 25 m s−1 were observed in the Gulfs of Tehuantepec (a tehuantepecer), Fonseca, Papagayo, and Panama, owing to the strong cross-mountain pressure gradient. In the case of the tehuantepecer, a rope cloud emanated from the Isthmus of Tehuantepec and turned anticyclonically, consistent with an inertial oscillation.

On the synoptic and planetary scales, the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold-air damming, by a low-latitude upper-tropospheric trough, and by the lower branch of the secondary circulation associated with a confluent jet-entrance region aloft. The cold surge also impacted the tropical atmosphere and ocean, by contributing to the strengthening of the northeast trade winds over the eastern Pacific Ocean and by inducing local cooling of the sea surface temperature in the Gulfs of Tehuantepec and Papagayo by about 4°–8°C.

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Rainer Bleck, Howard Bluestein, Lance Bosart, W. Edward Bracken, Toby Carlson, Jeffrey Chapman, Michael Dickinson, John R. Gyakum, Gregory Hakim, Eric Hoffman, Haig lskenderian, Daniel Keyser, Gary Lackmann, Wendell Nuss, Paul Roebber, Frederick Sanders, David Schultz, Kevin Tyle, and Peter Zwack

The Eighth Cyclone Workshop was held at the Far Hills Inn and Conference Center in Val Morin, Quebec, Canada, 12–16 October 1992. The workshop was arranged around several scientific themes of current research interest. The most widely debated theme was the applicability of “potential vorticity thinking” to theoretical, observational, and numerical studies of the life cycle of cyclones and the interaction of these cyclones with their environment on all spatial and temporal scales. A combination of invited and contributed talks, with preference given to younger scientists, made up the workshop.

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