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Gerald D. Bell
and
Lance F. Bosart

Abstract

Observational composites of midtropospheric closed cyclone formation are constructed and diagnosed for three regions: the southwestern United States, the eastern United States, and the southern lee of the Alps. The spatial scales upon which closed cyclone formation occurs are then examined by zonally decomposing the composite 500-hPa height fields into three distinct wave groups: the planetary scale (zonal waves 1–3), the large synoptic scale (zonal waves 4-9), and the small synoptic scale (zonal waves 10-25). This analysis leads to a description of closed cyclogenesis as a combined wave interaction and wave superposition process involving both wave groups 4–9 and 10–25, which is intimately linked to preexisting along-stream speed variations and flow curvature. This description is inconsistent with modal and nonmodal analytical instability theories of cyclogenesis.

The essence of the closed cyclogenesis process is contained in the relative positioning of, and interaction between, preexisting jets and waves. In all regions the precursor wave pattern is characterized by a broad trough over the impending cyclone region, with the strongest meridional flow and implied geostrophic vorticity maximum located upstream of this trough axis. This flow configuration is associated with sustained cyclonic vorticity advection into the amplifying trough axis, and also provides a conduit by which intensifying transient short-wave trough-jet streak features can propagate into the downstream trough. A closed circulation then develops as the geostrophic wind speed maximum moves into the base of the trough and cyclonic vorticity becomes concentrated within the trough axis. This evolution also occurs coincident with the movement of the transient trough feature directly into the amplifying long-wave trough axis.

In the southwestern United States and Alps cases, the favorable northwesterly flow configuration is initiated two days prior to closed cyclone formation by vigorous upstream wave amplification and by the rapid eastward movement of the upstream ridge axis relative to the downstream trough axis. Downstream of the cyclogenesis region, relatively modest anticyclogenesis, and modest mid- and lower-tropospheric thermal advection, is observed in these cases. In contrast, the favorable northwesterly flow configuration in the eastern United States cases is already established two days prior to closed cyclone formation. These cases are also characterized by vigorous downstream planetary-scale ridge amplification and a well-defined pattern of mid- and lower-tropospheric thermal advection.

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Gerald D. Bell
and
Lance F. Bosart

Abstract

Appalachian cold-air damming is investigated by means of 1) a 50-yr monthly climatology, 2) a synoptic case study of the event of 21–23 March 1985 and 3) an investigation of the flow structure and force balance within the cold dome.

The climatology reveals cold-air damming is a year-round phenomenon in the southern Appalachians with the most frequent, prolonged and intense events occurring in winter (particularly December and March) when three-five events per month can be expected. Cold-air damming is least frequent and intense in July.

The synoptic case study reveals that cold-air damming is critically dependent upon the configuration of the synoptic-scale flow. The cold dome can be identified by a “U” shaped ridge (trough) in the sea level isobar (thermal) patterns and the 930-mb height (temperature) fields representative of conditions at the base of the inversion overlying the cold dome. Differential horizontal and vertical thermal advection, as well as adiabatic and evaporative cooling, are responsible for the configuration of a strongly sloping inversion at the top of the cold dome and the pronounced baroclinic zone along the eastern edge of the cold dome. Evaporative cooling accounts for roughly 30% of the total cooling in parts of the dome, while adiabatic cooling explains a similar percentage of the cooling adjacent to the mountain slopes.

An accelerated flow nearly parallel to the mountains within the cold dome is identified and shown to be linked to the evolution of the synoptic-scale pressure field. The mountain-parallel component of the pressure gradient force is the primary acceleration source. The force balance on the accelerated flow after cold dome formation is geostrophic in the cross-mountain direction and antitriptic in the along-mountain direction. A geostrophic adjustment process is triggered from the formation of a region of small-scale ridging against the mountain slopes as cold air is constrained by the mountains to remain along the eastern slopes. The tendency for the Coriolis force to turn the flow toward the mountain is negated and the flow within the cold dome is directed ready parallel to the mountains and down the large-scale pressure gradient. Cold dome drainage occurs with the advection of the cold air toward the coast in response to synoptic-scale pressure falls accompanying coastal cyclogenesis.

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Gerald D. Bell
and
Lance F. Bosart

Abstract

A 15-year (1963–77) Northern Hemisphere 2 × 5 degree latitude-longitude gridded dataset of 500 mb geopotential heights has been used to construct a climatology of 500 mb closed circulation centers. These centers, defined by at least one closed 30 m contour around a central minimum or maximum geopotential height value, were identified objectively between 24° and 82°N from the twice-daily analysis grids. Tracks for all closed circulation centers were computed to establish genesis and lysis distributions, and to examine the monthly, seasonal and interannual variability characteristics of the closed circulation center distributions for specified regions.

The occurrence of closed cyclone centers is maximized north of and within the main belt of westerlies extending from northeast Asia to the Gulf of Alaska near 50°N, and extending from eastern Canada and the extreme northeast United States to southeast of Greenland and west of the United Kingdom. Their occurrence is also maximized south of the main belt of westerlies in a band extending from the east-central Atlantic Ocean across southern Europe to the Caspian Sea and central Asia. Important regional features include cool season maxima over the Mediterranean basin and the southwestern United States, and year-round maxima south of the jet stream extending from the eastern Pacific into the southwestern United States.

The occurrence of closed anticyclone centers is maximized over the subtropical oceans in all seasons and over the subtropical continents during the summer. These centers show a tendency to be displaced substantially northward along the west coasts of both North America and Europe. Closed anticyclone center maxima are also noted north of the main belt of westerlies in the North Pacific and Atlantic ocean basins, and in a band extending from northwestern Europe to central Asia. The anticyclone centers north of the jet stream coincide with the locations of some of the persistent geopotential height anomalies, low pass geopotential height anomalies and low pass geopotential height variance maxima reported elsewhere in the literature.

Genesis and lysis regions tend to coincide for both cyclone and anticyclone centers south of the jet stream, indicative of quasi-stationary, equivalent barotropic disturbances. In some of the more baroclinically active regions of higher latitudes the closed cyclone center genesis regions tend to be found somewhat upstream of lysis regions, suggestive of propagating disturbances. Likewise, anticyclone genesis centers tend to be somewhat to the east of the anticyclone frequency maxima (especially over high latitude ocean basins), suggesting retrogression. Finally, there is considerable monthly, seasonal and interannual variability for both closed cyclone and anticyclone centers in our sample period.

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Gerald D. Bell
and
Daniel Keyser

Abstract

Equations are presented for the evolution of isobaric shear and curvature vorticity and for isentropic shear and curvature potential vorticity in natural (streamline-following) coordinates, in the case of adiabatic, frictionless flow. In isobaric coordinates, two terms of equal magnitude and opposite sign arise in the respective tendency equations for shear and curvature vorticity; these terms represent conversions between shear and curvature vorticity in the sense that their sum does not alter the total tendency of absolute vorticity. In isentropic coordinates, only the conversion terms remain in the tendency equations for shear and curvature potential vorticity, consistent with potential-vorticity conservation. The vorticity and potential-vorticity conversions arise from (i) along-stream variations in wind speed in the presence of Lagrangian changes in wind direction and (ii) flow-normal gradients of Lagrangian changes in wind speed. The assumption of horizontal nondivergence simplifies the interpretation of the vorticity-interchange process by relating the conversion terms directly to flow curvature. Schematics are developed in order to illustrate the conversion terms in idealized representations of jet-entrance and jet-exit regions and curved flow patterns; these schematics provide the basis for understanding vorticity interchanges in realistic flow regimes.

The evolution of the midtropospheric shear- and curvature-potential-vorticity fields is described for a jet- trough interaction event in northwesterly flow, leading to the formation of a well-defined midtropospheric cutoff cyclone over the eastern United States between [8 and 20 January 1986. This time period coincides with the first intensive observing period of the Genesis of Atlantic Lows Experiment. Major midtropospheric cyclogenesis begins as a jet embedded in northwesterly flow, identified as a maximum of cyclonic shear potential vorticity, propagates toward the base of a diffluent trough, identified as a maximum of cyclonic curvature potential vorticity. The potential-vorticity tendency equations reveal that for this particular stage, the interchange terms contribute both to the amplification of the trough and to the formation of a maximum of cyclonic shear potential vorticity on the downstream side of the trough. The potential-vorticity interchange process is shown to play a key role in transforming the asymmetric configuration of shear and curvature potential vorticity characteristic of the diffluent trough stage, where the cyclonic shear maximum lags the cyclonic curvature maximum, to the relatively symmetric configuration characteristic of the cutoff stage. At the culmination of the cutoff stage, the shear- and curvature-potential-vorticity maxima overlap substantially. This overlap is a consequence of the presence of a single, cyclonically curved jet within the base of the cutoff cyclone.

A second important structural change occurring during midtropospheric cyclogenesis is the transformation of the potential-vorticity anomaly corresponding to the cutoff cyclone into a circularly symmetric configuration, which is accomplished by the contraction of the northwestern extension of the potential-vorticity anomaly toward the cyclone center. This contraction process, which is shown to involve significant interchanges between shear and curvature potential vorticity, results in the detachment of the potential-vorticity anomaly from the “stratospheric reservoir” of potential vorticity located north of the cyclone.

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Gerald D. Bell
and
Lance F. Bosart

Abstract

The synoptic-scale evolution during the formation phase of a midtropospheric cutoff cyclonic circulation over the eastern United States is diagnosed within the potential vorticity framework using the GALE (Genesis of Atlantic Lows Experiment) case of 18–19 January 1986. The study examines 1) the precursor flow evolution prior to cutoff cyclone formation; 2) the wind, mass, and potential vorticity evolution during the 2-day period encompassing cutoff formation; and 3) the relative contribution of upper-versus lower-tropospheric forcing on the quasigeostrophic height tendency field prior to and during cutoff formation.

The primary large-scale features prior to cutoff cyclone formation are an amplifying ridge over the western United States and eastern North Pacific and a diffluent trough over the central United States. The primary smaller-scale feature prior to cutoff formation is a short-wave trough-jet streak system that propagates through the longer-wave-amplifying ridge, and then intensifies upon arriving in northwesterly flow downstream of the ridge axis. The intensification of this shorter-wavelength system is associated with increases in stratospheric potential vorticity at levels considered to be well within the middle and upper troposphere. Major midtropospheric cyclogenesis then ensues as the jet propagates toward the base of the diffluent trough while further intensifying. The circulation then “closes off” at 500 hPa within the base of the amplifying trough as stratospheric potential vorticity values descend to near 620 hPa, and become increasingly confined to the base of the trough.

The subsequent intensification of the cutoff circulation is accompanied by sustained potential vorticity and temperature increases well above the level of the extruded tropopause. This intensification phase is also accompanied by an increasingly isolated distribution of stratospheric potential vorticity, and by the formation of an isolated warm pool, in the mid-and upper troposphere above the circulation center. These features are consistent with calculations showing that the primary mass loss required to support the formation and subsequent intensification of the cutoff circulation is confined to the upper troposphere.

A quasigeostrophic height tendency diagnosis suggests that the advection of potential vorticity at and above the 500-hPa level drives the process of upper-level trough amplification and cutoff cyclogenesis in this case. The quasigeostrophic height tendency patterns are also entirely consistent with the observed mass and wind-field tendencies, and with previous observational and theoretical analyses regarding the invertibility principle of potential vorticity.

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Christopher W. Landsea
,
Gerald D. Bell
,
William M. Gray
, and
Stanley B. Goldenberg

Abstract

The 1995 Atlantic hurricane season was a year of near-record hurricane activity with a total of 19 named storms (average is 9.3 for the base period 1950–90) and 11 hurricanes (average is 5.8), which persisted for a total of 121 named storm days (average is 46.6) and 60 hurricane days (average is 23.9), respectively. There were five intense (or major) Saffir–Simpson category 3, 4, or 5 hurricanes (average is 2.3 intense hurricanes) with 11.75 intense hurricane days (average is 4.7). The net tropical cyclone activity, based upon the combined values of named storms, hurricanes, intense hurricanes, and their days present, was 229% of the average. Additionally, 1995 saw the return of hurricane activity to the deep tropical latitudes: seven hurricanes developed south of 25°N (excluding all of the Gulf of Mexico) compared with just one during all of 1991–94. Interestingly, all seven storms that formed south of 20°N in August and September recurved to the northeast without making landfall in the United States.

The sharply increased hurricane activity during 1995 is attributed to the juxtaposition of virtually all of the large-scale features over the tropical North Atlantic that favor tropical cyclogenesis and development. These include extremely low vertical wind shear, below-normal sea level pressure, abnormally warm ocean waters, higher than average amounts of total precipitable water, and a strong west phase of the stratospheric quasi-biennial oscillation. These various environmental factors were in strong contrast to those of the very unfavorable conditions that accompanied the extremely quiet 1994 hurricane season.

The favorable conditions for the 1995 hurricane season began to develop as far back as late in the previous winter. Their onset well ahead of the start of the hurricane season indicates that they are a cause of the increased hurricane activity, and not an effect. The extreme duration of the atmospheric circulation anomalies over the tropical North Atlantic is partly attributed to a transition in the equatorial Pacific from warm episode conditions (El Niño) to cold episode conditions (La Niña) prior to the onset of the hurricane season.

Though the season as a whole was extremely active, 1995’s Atlantic tropical cyclogenesis showed a strong intraseasonal variability with above-normal storm frequency during August and October and below normal for September. This variability is likely attributed to changes in the upper-tropospheric circulation across the tropical North Atlantic, which resulted in a return to near-normal vertical shear during September. Another contributing factor to the reduction in tropical cyclogenesis during September may have been a temporary return to near-normal SSTs across the tropical and subtropical North Atlantic, caused by the enhanced tropical cyclone activity during August.

Seasonal hurricane forecasts for 1995 issued at Colorado State University on 30 November 1994, 5 June 1995, and 4 August 1995 correctly anticipated an above-average season, but underforecast the extent of the extreme hurricane activity.

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