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.

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.

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.

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.

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.

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.

Abstract

The synoptic climatology of rapid surface anticyclogenesis, defined by a surface anticyclone pressure increase of at least 5 mb per 24 h, is investigated for the 1984 calendar year over the western portion of the Northern Hemisphere. In this sample, the phenomenon occurs preferentially during the cool season and over land, especially over northwestern North America and southeastern Canada. The northwestern North American events are associated with cold anticyclones downstream of amplifying 500-mb ridges, and most are followed by 500-mb trough amplification and cold air outbreaks over North America. Most of the southeastern Canadian events are each linked with a relatively warm anticyclone intensifying between a mobile upstream 500-mb trough and a stationary downstream 500-mb cutoff low, which is displaced downstream during local warming. A diagnosis of one such event reveals that large quasi-geostrophic height rises are observed at 500 mb near the rapidly intensifying surface anticyclone as the 500-mb cutoff low is ejected downstream. Comparison of this case with a similar example during which large height rises and rapid surface anticyclogenesis are not observed suggests the approaching trough may have to be at the same latitude and nearly the same size as the cutoff low in order for rapid surface anticyclogenesis and attendant cutoff displacement to be observed.

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.

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.

Abstract

Motivated by the success of ensemble forecasting at the medium range, the performance of a prototype short-range ensemble forecast system is examined. The ensemble dataset consists of 15 case days from September 1995 through January 1996. There are 15 members of the ensemble, 10 from an 80-km version of the eta model and five from the regional spectral model. Initial conditions include various in-house analyses available at the National Centers for Environmental Prediction as well as bred initial conditions interpolated from the medium-range forecast ensemble. Forecasts from the 29-km mesoeta model were archived as well for comparison.

The performance of the ensemble is first evaluated by the criterion of “uniformity of verification rank.” Assuming a perfect forecast model, equally plausible initial conditions, and the verification is a plausible member of the ensemble, these imply the verification when pooled with the 15 ensemble forecasts and sorted is equally likely to occur in each of the 16 ranks. Hence, over many independent samples, a histogram of the rank distribution should be nearly uniform. Using data from the ensemble forecasts, rank distributions were populated and found to be nonuniform. This was determined to be largely a result of model and initial condition deficiencies and not problems with the verification data. The uniformity of rank distributions varied with atmospheric baroclinicity for midtropospheric forecast variables but not for precipitation forecasts.

Examination of the error characteristics of individual ensemble members showed that the assumption of identical errors for each member is not met with this particular ensemble configuration, primarily because of the use of both bred and nonbred initial conditions in this test. Further, there were both differences in the accuracy of eta and regional spectral model bred member forecasts.

The performance of various summary forecasts from the ensemble such as its mean showed that the ensemble can generate forecasts that have similar or lower error than forecasts from the 29-km mesoeta, which was approximately equivalent in computational expense. Also, by combining the ensemble forecasts with rank information from other cases, reliable ensemble precipitation forecasts could be created, indicating the potential for useful probabilistic forecasts of quantitative precipitation from the ensemble.