ALBERT

Description: The year 1919 was important in meteorology, not only because it was the year that the American Meteorological Society was founded, but also for two other reasons. One of the foundational papers in extratropical cyclone structure by Jakob Bjerknes was published in 1919, leading to what is now known as the Norwegian cyclone model. Also that year, a series of meetings was held that led to the formation of organizations that promoted the international collaboration and scientific exchange required for extratropical cyclone research, which by necessity involves spatial scales spanning national borders. This chapter describes the history of scientific inquiry into the structure, evolution, and dynamics of extratropical cyclones, their constituent fronts, and their attendant jet streams and storm tracks. We refer to these phenomena collectively as the centerpiece of meteorology because of their central role in fostering meteorological research during this century. This extremely productive period in extratropical cyclone research has been possible because of 1) the need to address practical challenges of poor forecasts that had large socioeconomic consequences, 2) the intermingling of theory, observations, and diagnosis (including dynamical modeling) to provide improved physical understanding and conceptual models, and 3) strong international cooperation. Conceptual frameworks for cyclones arise from a desire to classify and understand cyclones; they include the Norwegian cyclone model and its sister the Shapiro–Keyser cyclone model. The challenge of understanding the dynamics of cyclones led to such theoretical frameworks as quasigeostrophy, baroclinic instability, semigeostrophy, and frontogenesis. The challenge of predicting explosive extratropical cyclones in particular led to new theoretical developments such as potential-vorticity thinking and downstream development. Deeper appreciation of the limits of predictability has resulted from an evolution from determinism to chaos. Last, observational insights led to detailed cyclone and frontal structure, storm tracks, and rainbands.

Description: The polar jet (PJ) and subtropical jet (STJ) often reside in different climatological latitude bands. On occasion, the meridional separation between the two jets can vanish, resulting in a relatively rare vertical superposition of the PJ and STJ. A large-scale environment conducive to jet superposition can be conceptualized as one that facilitates the simultaneous advection of tropopause-level potential vorticity (PV) perturbations along the polar and subtropical waveguides toward midlatitudes. Once these PV perturbations are transported into close proximity to one another, interactions between tropopause-level, lower-tropospheric, and diabatically generated PV perturbations work to restructure the tropopause into the two-step, pole-to-equator tropopause structure characteristic of a jet superposition. This study employs piecewise PV inversion to diagnose the interactions between large-scale PV perturbations throughout the development of a jet superposition during the 18–20 December 2009 mid-Atlantic blizzard. While the influence of PV perturbations in the lower troposphere as well as those generated via diabatic processes were notable in this case, tropopause-level PV perturbations played the most substantial role in restructuring the tropopause prior to jet superposition. A novel PV partitioning scheme is presented that isolates PV perturbations associated with the PJ and STJ, respectively. Inversion of the jet-specific PV perturbations suggests that these separate features make distinct contributions to the restructuring of the tropopause that characterizes the development of a jet superposition.

Description: A polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events.

Description: This study considers the development of continental U.S. extreme temperature events (ETEs) during the cool season (September–May), where extreme temperatures are defined in terms of percentiles and events are defined in terms of the spatial coverage of extreme temperatures. Following their identification, ETEs are classified into geographic clusters and stratified based on the state of the North Pacific jet (NPJ) stream prior to ETE initiation using an NPJ phase diagram. The NPJ phase diagram is developed from the two leading modes of NPJ variability during the cool season. The first mode corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second mode corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. The projection of 250-hPa zonal wind anomalies onto the NPJ phase diagram prior to ETEs demonstrates that the preferred state and evolution of the NPJ prior to ETEs varies considerably based on the geographic location of ETE initiation and the season. Southern plains extreme warm events are an exception, however, since extreme warm events in that location most frequently initiate following a retracted NPJ during all seasons. The NPJ phase diagram is subsequently utilized to examine a synoptic-scale flow evolution highly conducive to the initiation of southern plains extreme warm events via composite analysis. The composite analysis demonstrates that a retracted NPJ supports an amplification of the upper-tropospheric flow pattern over North America, which then induces persistent lower-tropospheric warm-air advection over the southern plains prior to ETE initiation.

Description: Previous studies employing empirical orthogonal function (EOF) analyses of upper-tropospheric zonal wind anomalies have identified the leading modes of North Pacific jet (NPJ) variability that prevail on synoptic time scales. The first mode corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second mode corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. These NPJ regimes can strongly influence the character of the large-scale flow pattern over North America. Consequently, knowledge of the prevailing NPJ regime and the forecast skill associated with each NPJ regime can add considerable value to operational medium-range (6–10-day) forecasts over North America. This study documents the development of an NPJ phase diagram, which is constructed from the two leading EOFs of 250-hPa zonal wind anomalies during 1979–2014 excluding the summer months (June–August). The projection of 250-hPa zonal wind anomalies at one or multiple times onto the NPJ phase diagram provides an objective characterization of the state or evolution of the upper-tropospheric flow pattern over the North Pacific with respect to the two leading EOFs. A 30-yr analysis of GEFS reforecasts with respect to the NPJ phase diagram demonstrates that forecasts verified during jet retraction and equatorward shift regimes are associated with significantly larger average errors than jet extension and poleward shift regimes. An examination of the best and worst forecasts further suggests that periods characterized by rapid NPJ regime transition and the development and maintenance of North Pacific blocking events exhibit reduced forecast skill.

Description: A severe thunderstorm wind gust climatology spanning 2003–09 for the contiguous United States is developed using measured Automated Surface Observing System (ASOS) and Automated Weather Observing System (AWOS) wind gusts. Archived severe report information from the National Climatic Data Center publication Storm Data and single-site volumetric radar data are used to identify severe wind gust observations [≥50 kt (25.7 m s−1)] associated with thunderstorms and to classify the convective mode of the storms. The measured severe wind gust distribution, comprising only 2% of all severe gusts, is examined with respect to radar-based convective modes. The convective mode scheme presented herein focuses on three primary radar-based storm categories: supercell, quasi-linear convective systems (QLCSs), and disorganized. Measured severe gust frequency revealed distinct spatial patterns, where the high plains received the greatest number of gusts and occurred most often in the late spring and summer months. Severe wind gusts produced by supercells were most frequent over the plains, while those from QLCS gusts were most frequent in the plains and Midwest. Meanwhile, disorganized storms produced most of their severe gusts in the plains and Intermountain West. A reverse spatial distribution signal exists in the location between the maximum measured severe wind gust corridor located over the high plains and the maximum in all severe thunderstorm wind reports from Storm Data, located near and west of the southern Appalachians.

Description: Contributions to the increased poleward moisture flux that characterized the second day of the 1–3 May Nashville, Tennessee, flood of 2010 are examined from the perspective of polar and subtropical jet superposition and its influence on the secondary ageostrophic circulation. Employing the Sawyer–Eliassen circulation equation, the analysis reveals that the poleward moisture flux attributed to the jet increased nearly 120% prior to the second day of the event in response to the superposed jet’s ageostrophic circulation, helping to further fuel the production of heavy rainfall. The full Sawyer–Eliassen circulation associated with the superposed jet is further partitioned into its geostrophic and diabatic components. The geostrophic forcing drove midtropospheric ascent that fueled the production of deep convection and the record rainfall. The diabatic component, through forcing lower-tropospheric ascent and vigorous lower-tropospheric poleward moisture flux, provided the link between the tropical moisture and the deep convective environment. Since superposed jets, by their nature, develop on the poleward edge of the tropical or subtropical air, it is suggested that such a mutually reinforcing interaction between these two component forcings of the secondary circulation may routinely characterize the involvement of superposed jet structures in high-impact weather events.

Description: A polar–subtropical jet superposition represents a dynamical and thermodynamic environment conducive to the production of high-impact weather. Prior work indicates that the synoptic-scale environments that support the development of North American jet superpositions vary depending on the case under consideration. This variability motivates an analysis of the range of synoptic–dynamic mechanisms that operate within a double-jet environment to produce North American jet superpositions. This study identifies North American jet superposition events during November–March 1979–2010 and subsequently classifies those events into three characteristic event types. “Polar dominant” events are those during which only the polar jet is characterized by a substantial excursion from its climatological latitude band, “subtropical dominant” events are those during which only the subtropical jet is characterized by a substantial excursion from its climatological latitude band, and “hybrid” events are those characterized by a mutual excursion of both jets from their respective climatological latitude bands. The analysis indicates that North American jet superposition events occur most often during November and December, and subtropical dominant events are the most frequent event type for all months considered. Composite analyses constructed for each event type reveal the consistent role that descent plays in restructuring the tropopause beneath the jet-entrance region prior to jet superposition. The composite analyses further show that surface cyclogenesis and widespread precipitation lead the development of subtropical dominant events and contribute to jet superposition via their associated divergent circulations and diabatic heating, whereas surface cyclogenesis and widespread precipitation tend to peak at the time of superposition and well downstream of polar dominant events.