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winter cyclone frequencies over the eastern United States and adjacent western Atlantic, 1964–1973

Student paper—First place winner of The Father James B. Macelwane Annual Award in Meteorology, announced at the Annual Meeting of the AMS, Philadelphia, Pa., 21 January 1976

Stephen J. Colucci

Analyses of winter cyclone frequency and deepening rates are presented for a 10-year period over the eastern United States and western Atlantic Ocean. Results are presented for 1° latitude-longitude quadrangles. The data source was microfilmed copies of NOAA's North American Surface Charts series routinely available over facsimile every 3 h.

The analyses reveal a concentration of storms in a band from Cape Hatteras to New England, over the northern edge of the Gulf Stream current, and over the eastern Great Lakes. In addition, distinct minimums of winter cyclones are evident over the Appalachian Mountain range and, to a lesser degree, over the Florida peninsula. Analysis on a similar scale of 3 h pressure changes in these cyclones indicates that deepening was most favorable over the southern Appalachians, immediate Carolina coastal strip, the northern edge of the Gulf Stream, and the eastern Great Lakes. Significant positive or negative departures from normal winter precipitation along the East Coast of the United States may be attributed to anomalies in adjacent sea surface temperatures, as evidenced by investigation into precipitation data and offshore sea surface temperatures of three regions exhibiting such departures.

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Stephen J. Colucci

Abstract

The local preconditioning of the midtropospheric planetary-scale flow prior to the onset of a blocking episode during January 1985 is investigated. The preconditioned flow is anomalously diffluent, or characterized by anomalously negative planetary-scale, geostrophic stretching deformation. This deformation increases in magnitude with time during the transition to blocking; this tendency in turn is quasigeostrophically forced by the shape of the planetary-scale component of potential vorticity transports. In particular, the planetary-scale component of potential vorticity advection that became increasingly anticyclonic with eastward distance at a rate that increased northward near the block-onset region forced the local planetary-scale flow to become more diffluent prior to blocking. Self-interactions among the synoptic-scale waves and synoptic-to-planetary-scale interactions contributed more importantly than self-interactions among planetary-scale waves to this preconditioning. In the frequency domain, the preconditioning is primarily attributable to the interactions between low- and high-frequency components of the flow, notably to the advection of slowly varying, low potential vorticity by the high-frequency flow.

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Stephen J. Colucci

Abstract

Large-scale circulation changes attending explosive surface cyclogenesis are quantitatively examined in two cases selected from recent winter seasons. Both cases feature a rapidly deepening surface cyclone over the western Atlantic Ocean, but changes in the 500 mb geopotential height field near the cyclone differ in each case. One event, during January 1977, is characterized by the retrogression of an anticyclonic vortex (blocking high) in the 500 mb height field downstream of the surface cyclone. The second case, in February 1978, is distinguished by the formation of a 500 mb cyclonic vortex (cutoff low) upstream of the surface cyclone, but no downstream anticyclonic vortex is observed. The retrogression of the January 1977 block over the Atlantic Ocean coincides with the migration of a 500 mb synoptic-scale perturbation (associated with the surface cyclone) from a planetary- scale trough over North America toward a planetary-scale ridge over Europe. In the February 1978 case, the blocking cyclonic vortex evolves out of a synoptic-scale perturbation migrating from a long-wave ridge over western North America toward a long-wave trough over eastern North America, initiating the oceanic surface cyclone event.

Quasi-geostrophic model diagnosis of atmospheric data during these cases reveals that the middle tropospheric geopotential height tendencies in the blocking systems are forced by the superposition of thermal and vorticity advections. Thermodynamically, the observed temperature increase in the blocking anticyclone case is forced both by warm air advection and subsidence warming, while the temperature decrease observed in the blocking cyclone case is forced by adiabatic cooling attending strong ascent.

These and other case studies are consistent with the results of previous theoretical and observational work which have shown that atmospheric blocking patterns can arise due to the interaction of transient, synoptic-scale perturbations with the planetary-scale environment. In this context, blocking may be understood as a response of the planetary waves to synoptic-scale perturbations, which act as sources of energy and vorticity for the incipient blocks. This paper shows, however, that the type of response may depend critically on the location of the synoptic scale perturbation relative to the planetary waves. Specifically, synoptic-scale waves migrating from long-wave ridge (trough) to long-wave trough (ridge) can, in certain instances, favor blocking cyclonic (anticyclonic) vortices.

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Stephen J. Colucci

Abstract

Four cases of tropospheric weather systems (two sea level cyclones, one sea level anticyclone, and one blocking midtropospheric anticyclone) are investigated with the goal of understanding the role of stratospheric versus tropospheric processes in their developments. The relative contributions of the stratosphere and troposphere to geopotential height tendency fields (1000 mb for the sea level systems, 500 mb for the midtropospheric system) are quantified through vertical integration of thermodynamic processes (advective, adiabatic, and diabatic) over and following tendency centers associated with these systems. Previously known or suspected tropospheric contributions to system development, as well as the influence of stratospheric warm-air advection in the sea level cyclogenesis cases, are confirmed by the diagnoses. New findings include identification of the influence of stratospheric adiabatic cooling in the sea level and midtropospheric anticyclogenesis cases. It is further found that the stratospheric contribution to tropospheric development can be larger in magnitude than the tropospheric contribution and can even overwhelm an opposing tropospheric effect. In particular, it is shown that the midtropospheric anticyclogenesis associated with the blocking case critically depended on the stratospheric contribution and could not be solely explained by tropospheric processes. The 500-mb height tendencies associated with quasigeostrophic potential vorticity (QGPV) changes above the 500-mb level in this case were twice as large those associated with QGPV changes at and below 500 mb during blocking onset.

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Stephen J. Colucci

Abstract

A 17-day period during November 1980 is investigated to obtain insight into differing large-scale 500 mb circulation changes during three consecutive synoptic-scale cyclone “events” Each event is defined by at least one rapidly intensifying surface cyclone over eastern North America or the western Atlantic Ocean. The first event is followed in time by the retrogression of a downstream split-flow type of block at 500 mb. The second event is characterized by the intensification of a large-scale 500 mb trough into a blocking cyclonic vortex. During the third event, no blocking systems are established or intensified; instead, the blocking cyclonic vortex is ejected downstream. Quasi-geostrophic model diagnosis reveals that during the first two (blocking) events the large-scale waves are reinforced by relatively large and spatially and temporally persistent transports of potential vorticity associated with 500 mb synoptic-scale waves linked with the surface cyclones. In the third (nonblocking)event, during which the planetary waves have lower amplitude than before, the potential vorticity transports at 500 mb near the surface cyclones are relatively large but neither spatially nor temporally persistent. It is suggested that, on the basis of these analyses, whether a 500 mb blocking structure occurs and what type of structure (cyclonic or anticyclonic vortex) follows an intense surface cyclone event may depend critically upon the amplitude of existing planetary waves and the phase of these waves relative to the surface cyclones and attendant 500 mb potential vorticity transports.

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Li Dong and Stephen J. Colucci

Abstract

The opposition between two block-onset forcing mechanisms, previously identified in midtropospheric analyses over the Southern Hemisphere midlatitudes, is analytically interpreted with an idealized model. These mechanisms are the interaction (F inter) between deformation and potential vorticity and the advection (F adv) of meridionally varying potential vorticity. Weather systems of concern, primarily consisting of planetary- and synoptic-scale waves, mostly fall into two regimes of zonal and meridional wavenumber space in which the opposition between the two block-onset forcing mechanisms is analytically derived. A synoptic interpretation of this opposition is schematically presented within the framework of barotropic dynamics. It is found that whether blocking occurs in diffluent or confluent flow depends upon the critical wavelength associated with the geostrophic flow. Blocking tends to take place in the diffluent flow of long waves in which F inter dominates over F adv. In addition, blocking also tends to occur in the confluent flow of relative short waves in which F adv prevails over F inter. An investigation of Rossby wave phase speeds in one diagnosed case reveals a lengthening with time of the dominant wave until it reaches the stationary wavelength on the block-onset day. In this context blocking may be understood as a stationarity and thus persistence of one of the two block-onset forcing mechanisms.

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Li Dong and Stephen J. Colucci

Abstract

The relative importance of interactions between deformation and potential vorticity (PV) as a block-onset mechanism is examined in 30 cases of atmospheric blocking over the Southern Hemisphere (SH). The blocking cases are diagnosed with a quasigeostrophic model for the u component of the geostrophic wind tendency. In this model, two mechanisms, the advection of the meridional gradient of PV and interactions between deformation and PV, can force the weakening of westerly flow or increasing easterly flow associated with blocking. The first forcing mechanism, which does not directly include deformation, indicates that the advection of equatorward increasing cyclonic PV (or equatorward decreasing anticyclonic PV) could force a local weakening of geostrophic westerlies or increasing easterlies. The second forcing mechanism, which represents the net effect of interactions between deformation and PV, indicates that eastward increasing PV embedded in a cyclonically sheared flow or equatorward increasing PV coincident with a stretching (diffluent) flow could each force a weakening in the westerlies.

While deformation is a distinct signature of blocking, it may not always actively participate in the formation of blocking. Advection and interaction contributions generally opposed each other in both the diagnosed blocking and nonblocking cases. Weakening westerlies associated with block onset would occur when one effect (usually the advection effect) contributes more negatively to the wind tendency than the opposing, positive contribution from the other effect. When deformation is actively involved in the formation of blocking, self-interactions between synoptic-scale PV and deformation and self-interactions between planetary-scale PV and deformation contribute more importantly than synoptic-to-planetary-scale interactions between PV and deformation fields to the weakening of westerlies associated with block onsets.

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Li Dong and Stephen J. Colucci

Abstract

A generalized frictionless, adiabatic geostrophic zonal wind tendency equation is derived to diagnose the nonquasigeostrophic forcings to blocking onset in the Southern Hemisphere through case study and composite analysis. In general, the quasigeostrophic model is capable of representing the key physical processes associated with blocking onset in the troposphere reasonably well in most blocking cases. The consideration of nonquasigeostrophic forcings moderately improves the quasigeostrophic representation in a majority of the blocking events selected for this study, but not all events. This suggests that the nonquasigeostrophic terms could be important in a specific blocking event but not in a composite meaning. Furthermore, the nonquasigeostrophic forcing of geostrophic advection of ageostrophic relative vorticity term, , is extensively examined in this study. This forcing is found to be the leading nonquasigeostrophic forcing term among all nonquasigeostrophic forcings. In a composite sense, the forcing appears to have an alternative contribution that is dependent upon the curvature of the geostrophic flow within the blocking structure. In general, the southwesterly flow is likely associated with the -favoring effect to blocking onset whereas northwesterly flow is associated with the -opposing effect. Therefore, it is important to use the geostrophic flow pattern prior to blocking onset to foresee this ageostrophic-related nonquasigeostrophic forcing to blocking onset. Finally, a pronounced overestimation of geostrophic zonal wind tendency by the quasigeostrophic model is commonly found for selected blocking events within the stratosphere, in comparison to the nonquasigeostrophic model. This overestimation is essentially caused by geostrophic wind approximation.

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Stephen M. Jessup and Stephen J. Colucci

Abstract

Heavy precipitation and flash flooding have been extensively studied in the central United States, but less so in the Northeast. This study examines 187 warm-season flash flood events identified in Storm Data to better understand the structure of the precipitation systems that cause flash flooding in the Northeast. Based on the organization and movement of these systems on radar, the events are classified into one of four categories—back-building, linear, multiple, and other/size—and then further classified into subtypes for each category. Eight of these subtypes were not previously recognized in the literature. The back-building events were the most common, followed by the multiple, other/size, and linear types. The linear event types appear to produce flash flooding less commonly in the Northeast than in other regions. In general, the subtypes producing the highest precipitation estimates are those whose structures are most conducive to a long duration of sustained moderate to heavy rainfall. The event types were found to differ from those in the central United States in that the events were more often found to be more disorganized in the Northeast. One event type in particular, back-building with merging features, while not more disorganized than the previously recognized event types, offers promise for improved forecasting because its radar signature makes the duration of sustained heavy precipitation potentially easier to predict.

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J. Todd Hawes and Stephen J. Colucci

Abstract

The National Meteorological Center's 72-b spectral model forecasts for the 1983–84 cool season are examined in an appraisal of the model's ability to simulate 500-mb cyclones and anticyclones, defined by the existence of at least one (60-m interval) closed contour. Position and intensity errors we determined from comparison between forecast and observed 500-mb height fields. On the basis of this sample it is concluded that there is a tendency, with some geographical exceptions, for the model to overpredict the heights in these systems. This is particularly true of high latitude anticyclones. One noteworthy error characteristic in the model is a recurring failure to predict closed 500-mb cyclonic circulations which evolve from troughs crossing western North America. This suggests either initial data problems with troughs originating over the Pacific or that the model does not simulate the troughs' interaction with the Rocky Mountains very well on some occasions. Selected examples of model error are presented, accompanied by corresponding 48-h forecasts from the operational, limited-area, fine-mesh, primative equation model in one example. Computations of 850-mb geostrophic temperature advection from this latter model are compared with observed computations in this example and suggest that model errors in surface weather systems may he coupled with model errors in 500-mb systems, in this case through incorrect prediction of lower tropospheric temperature advection.

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