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Marco L. Carrera and John R. Gyakum

Abstract

The various modes of atmospheric mass redistribution characterize the principal variations of the general circulation of the atmosphere. Interhemispheric exchanges of atmospheric mass occur with considerable regularity on subseasonal time scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions (e.g., atmospheric blocking) may often be related to interhemispheric atmospheric mass exchange.

Using the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis surface pressure, significant events when the Northern Hemisphere (NH) loses dry atmospheric mass on subseasonal time scales during the boreal winter from 1968 to 1997 are identified. A total of 25 events is found, with a preferred time scale of 9 days from the time of maximum to minimum NH dry atmospheric mass. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and Southern Hemisphere (SH) is −0.91 for the 25 events, indicating very strong interhemispheric compensation and increasing confidence in the suitability of the NCEP–NCAR reanalysis dataset for the study of interhemispheric dry atmospheric mass exchange.

Positive sea level pressure anomalies are found over northern Eurasia, the North Pacific, and the North Atlantic prior to the onset of the composite NH dry atmospheric mass collapse event. Over northern Eurasia the building of the Siberian high is found to be a statistically significant precursor to the events. The breakdown of NH dry atmospheric mass occurs in association with the decay of the positive atmospheric mass anomaly in the North Pacific as a cyclone deepens explosively in the Gulf of Alaska. Pressure surges over Southeast Asia and North America, associated with statistically significant positive atmospheric mass anomalies, are mechanisms that act to channel the atmospheric mass equatorward out of the NH extratropics on a rapid time scale (∼4 days). The dry atmospheric mass increase in the SH is manifested as enhanced surface ridging over the South Pacific and south Indian Oceans, two noted regions of atmospheric blocking.

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John R. Gyakum and Richard E. Danielson

Abstract

Thirty-five cases of cyclogenesis that occurred during the cold seasons from 1975 to 1995 in the western North Pacific Ocean are studied to determine common and disparate dynamic and thermodynamic structures in both the ordinary and rapid developments. An analysis of 1000-hPa height and 1000–500-hPa thickness anomalies with respect to the 20-yr climatology reveals the following results. Though each sample of cyclogenesis is characterized by a favorable-appearing thickness trough–ridge structure, important differences are found. Both the upstream surface anticyclone and the downstream precedent cyclone are preferentially stronger at the beginning of the most rapid cyclogenesis in the strong sample. Because of the consequently stronger equatorward flow, the 1000–500-hPa thickness anomaly in the strong sample is colder by approximately 40 m (∼2°C) in the region of incipient cyclogenesis and eastward by 1500 km.

A harmonic time series analysis of NCEP gridded fields partitions the geopotential height fields into high- (corresponding to synoptic-scale waves) and low-frequency wave components. This analysis shows the 500-hPa synoptic-scale disturbances that trigger both ordinary and rapid cyclogenesis are easily tracked as early as 72 h prior to the event. These triggering disturbances, 72 h prior to the most rapid cyclogenesis, are found most typically in central Siberia. Additionally, the synoptic-scale trough–ridge couplet is stronger at the onset of development for the explosive sample, suggesting a stronger large-scale forcing for cyclogenesis.

To gain insight into possible physical mechanisms associated with these structural differences, the SST anomalies (with respect to a 30-yr climate) in the rapid developments are compared with those of the weaker systems. Though there is no statistically significant difference in SST anomalies, the preferentially colder tropospheric air mass in the strong sample suggests this sample to be characterized by stronger surface fluxes. Indeed, the NCEP reanalyses reveal both the sensible and latent heat fluxes to be 50–75 W m−2 greater in the rapid development cases in the region along their subsequent cyclone tracks. These statistically significant differences are also reflected in moisture budget analyses, which reveal surface evaporation to be larger in the explosive cases. This evaporation component contributes importantly to the computed precipitation in each class of cyclogenesis.

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Marco L. Carrera and John R. Gyakum

Abstract

A recent study of significant events of atmospheric mass depletion from the Northern Hemisphere (NH) during the extended boreal winter indicated that Southeast Asian pressure surges were an important physical mechanism that acted to channel the atmospheric mass equatorward out of the NH on a rapid time scale. This study builds upon this finding and examines both the direct and indirect roles of Southeast Asian pressure surges for a particular event of dry atmospheric mass depletion from the NH. The focus of this study is on the enhanced interhemispheric interactions and associated Southern Hemisphere (SH) tropical and extratropical responses resulting from the pressure surges.

First, this study examines the conservation of dry atmospheric mass (i.e., the relationship between the dry meridional winds and the area-integrated dry air surface pressure) in the NCEP reanalysis for the 25 significant events of dry atmospheric mass depletion from the NH. Results indicate that the NCEP dry meridional winds are able to qualitatively capture the dry atmospheric mass evacuation from the NH. In a quantitative sense there is very good agreement between the wind and pressure data in the extratropics of both hemispheres. A distinct negative or southward bias in the NCEP vertically and zonally integrated dry meridional winds is apparent between 5° and 17.5°N. This southward bias was not present in the ECMWF Re-Analysis. The source of the southward bias in NCEP appears to result from a weaker analyzed ITCZ.

The particular case of dry atmospheric mass depletion from the NH examined in detail is associated with an intense pressure surge over Southeast Asia. A significant enhancement of convection in the monsoon trough region of northern Australia occurs roughly 4 days after the peak intensity of the Siberian high. A low-level westerly wind burst develops in response to this enhanced zonal pressure gradient caused by the pressure surge as part of the onset of an active phase of the Australian summer monsoon. This study shows that three prominent anticyclonic circulations intensify in the SH extratropics, stretching from the south Indian Ocean to the South Pacific, beneath regions of upper-tropospheric dry atmospheric mass convergence, originating partly from the monsoon convection outflow. These anticyclonic circulations are regional manifestations of the dry atmospheric mass increase in the SH.

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John M. Henderson, Gary M. Lackmann, and John R. Gyakum

Abstract

Hurricane Opal’s landfall in October 1995 forms the basis of a serious hurricane forecast problem—the potential for hurricane conditions over land with insufficient warning time. Official National Hurricane Center (NHC, a division of the Tropical Prediction Center) forecasts predicted landfall and passage inland over the eastern United States at a later time than observed because of underestimation of the northward component of the steering flow by the National Centers for Environmental Prediction’s (NCEP) operational models and other hurricane track models. The goal of this paper is to isolate the cause of the poor forecast of meridional storm motion in NCEP’s early Eta Model by using quasigeostrophic potential vorticity (QGPV) inversion. QGPV inversion permits decomposition of the steering flow into contributions from different synoptic-scale features.

The inversion procedure is applied to the Eta analysis and 48-h Eta forecast valid at 1200 UTC 5 October 1995. Analyses from the European Centre for Medium-Range Weather Forecasts form an independent comparison for the Eta Model forecasts and analyses. An extratropical cyclone to the northwest of Opal and a synoptic-scale ridge to the east are identified as being major contributors to the steering flow. The Eta Model underpredicted the intensity of the ridge positioned immediately downstream of the storm, resulting in a corresponding underprediction of the meridional steering flow by 5 m s−1.

It is hypothesized that the Eta Model underforecasted the magnitude and extent of Opal’s outflow, and subsequent interaction with the downstream ridge, largely due to the model’s inability to correctly represent the convection associated with the hurricane in both the analyses and forecasts. Underforecasted upper-tropospheric temperatures downstream of Opal are consistent with this hypothesis. Accurate initialization of the model in the region containing Opal may have been hampered by the dearth of upper-air data over the Gulf of Mexico. Failure to properly resolve the hurricane is hypothesized to have resulted in the underforecasting of the downstream ridge and its associated steering flow.

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Richard E. Danielson, John R. Gyakum, and David N. Straub

Abstract

The impact of eddy energy growth and radiation from a western North Pacific cyclone on the intensity of an eastern North Pacific cyclone a few days later is examined. Associated with the western cyclone is an upstream ridge and trough couplet, initially over Siberia on 8 March 1977. The amplitude of this couplet is perturbed in 5-day numerical simulations of the two marine cyclones. Balanced initial conditions are created by potential vorticity inversion. The magnitude of the upper-level couplet governs much of the subsequent growth of eddy energy in the western cyclone as well as the propagation of eddy energy between the two cyclones. This culminates in measurable changes in the maximum intensity of the eastern surface cyclone. The broader question of the sensitivity of this cyclone to upstream perturbations is also briefly addressed.

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John R. Gyakum, Paul J. Roebber, and Timothy A. Bullock

Abstract

We examine the idea that antecedent vorticity development, defined as the surface vorticity spinup in the period prior to a cyclone's maximum intensification, is an important dynamical conditioning process for explosive cyclogenesis. Previous suggestions from case study research that subsequent intensification may be proportional to the intensity of the preexisting circulation are supported through the systematic study of a large sample of weakly and explosively developing cyclones in the North Pacific and North Atlantic basins.

Additional support for this concept is found with an examination of composite weakly and strongly developing cyclones at the onset of their most rapid intensification period. At this onset, the strongly developing cyclone composite has substantially stronger surface circulation and vorticity than is found in a composite of the weak cases. Ensembles of successive forecasts of an explosive cyclogenesis case during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) suggest similar dynamical behavior, in that small errors in the surface intensity subsequently amplify into larger errors only 12 h later under predominantly similar upper-level conditions.

The temporal evolution of large-scale geostrophic vorticity for 62 cases of cyclogenesis shows that stretching in the presence of relative vorticity is present throughout the life cycle of both the weakly and rapidly developing cases. An examination of 794 cyclones in the North Pacific basin reveals a general trend of increased maximum development as the antecedent deepening increases. Explosively developing cyclones are preferentially characterized by at least 12 h of antecedent development.

We investigate the relationship between the amplitude of the 500-mb quasigeostrophic-ascent forcing and maximum surface cyclone intensification and find a significant positive correlation, as previous studies have shown. However, computations with model-based surface convergence suggest that the response to the upper-level forcing is conditioned by the low-level antecedent vorticity development. Furthermore, variations in successive numerical weather prediction model forecasts of maximum cyclone intensification are well correlated with variations in the initial surface vorticity as well as variations in the 500-mb forcing.

This study suggests that explosive development is typically characterized by a nonlinear interaction between two cyclonic disturbances in the lower and upper troposphere. These disturbances, in some cases, may have formed independently of one another. Thus, the correct simulation of the full life cycle of these cyclones, including the antecedent phase, may be crucial for accurate numerical forecasts.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

St. John’s, Newfoundland, Canada (CYYT), is frequently affected by extreme precipitation events, particularly in the cool season (October–April). Previous work classified precipitation events at CYYT into categories by precipitation amount and a manual synoptic typing was performed on the 50 median extreme precipitation events, using two separate methods. Here, consecutive extreme precipitation events in December 2008 are analyzed. These events occurred over a 6-day period and produced over 125 mm of precipitation at CYYT. The first manual typing method, using a backward-trajectory analysis, results in both events being classified as “southwest,” which were previously defined as the majority of the backward trajectories originating in the Gulf of Mexico. The second method of manual synoptic typing finds that the first event is classified as a “cyclone,” while the second is a “frontal” event. A synoptic analysis of both events is conducted, highlighting important dynamic and thermodynamic structures. The first event was characterized by strong quasigeostrophic forcing for ascent in a weakly stable atmosphere in association with a rapidly intensifying extratropical cyclone off the coast of North America and transient high values of subtropical moisture. The second event was characterized by primarily frontogenetical forcing for ascent in a weakly stable atmosphere in the presence of quasi-stationary high values of subtropical moisture, in association with a northeast–southwest-oriented baroclinic zone situated near CYYT. In sum, the synoptic structures responsible for the two events highlight rather disparate means to produce an extreme precipitation event at CYYT.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

The St. Lawrence River valley (SLRV) is an important orographic feature in eastern Canada that can affect surface wind patterns and contribute to locally higher amounts of precipitation. The impact of the SLRV on precipitation distributions associated with transitioning, or transitioned, tropical cyclones that approached the region is assessed. Such cases can result in heavy precipitation during the warm season, as during the transition of Hurricane Ike (2008). Thirty-eight tropical cyclones tracked within 500 km of the SLRV from 1979 to 2011. Utilizing the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR), 19 of the 38 cases (group A) had large values of ageostrophic frontogenesis within and parallel to the SLRV, in a region of northeasterly surface winds associated with pressure-driven wind channeling. Using composite and case analyses, results show that the heaviest precipitation is often located within the SLRV, regardless of the location of large-scale forcing for ascent, and is concomitant with ageostrophic frontogenesis. The suggested physical pathway for precipitation modulation in the SLRV is as follows. Valley-induced near-surface ageostrophic frontogenesis is due to pressure-driven wind channeling as a result of the along-valley pressure gradient [typically exceeding 0.4 hPa (100 km)−1] established by the approaching cyclone. Near-surface cold-air advection as a result of the northeasterly pressure-driven channeling results in a temperature inversion, similar to what is observed in cool-season wind-channeling cases. The ageostrophic frontogenesis, acting as a mesoscale ascent-focusing mechanism, helps air parcels to rise above the temperature inversion into a conditionally unstable atmosphere, which results in enhanced precipitation focused along the SLRV.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

Quantitative precipitation forecasting (QPF) continues to be a significant challenge in operational forecasting, particularly in regions susceptible to extreme precipitation events. St. John’s, Newfoundland, Canada (CYYT), is affected frequently by such events, particularly in the cool season (October–April).

The 50 median events in the extreme (>33.78 mm during a 48-h period) precipitation event category are selected for further analysis. A manual synoptic typing is performed on these 50 events, using two separate methodologies to partition events. The first method utilizes a Lagrangian backward air parcel trajectory analysis and the second method utilizes the evolution of dynamically relevant variables, including 1000–700-hPa horizontal temperature advection, 1000–700-hPa (vector) geostrophic frontogenesis, and 700–400-hPa absolute vorticity advection.

Utilizing the first partitioning method, it is found that south cases are characterized by a strong anticyclone downstream of St. John’s, southwest events are synoptically similar to the overall extreme composite and are marked by a strong cyclone that develops in the Gulf of Mexico, while west events are characterized by a weak Alberta clipper system that intensifies rapidly upon reaching the Atlantic Ocean. The second partitioning method suggests that while cyclone events are dominated by the presence of a rapidly developing cyclone moving northeastward toward St. John’s, frontal events are characterized by the presence of a strong downstream anticyclone and deformation zone at St. John’s.

It is the hope of the authors that the unique methodology and results of the synoptic typing in this paper will aid forecasters in identifying certain characteristics of future precipitation events at St. John’s and similar stations.

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Ying-Hwa Kuo, John R. Gyakum, and Zitian Guo

Abstract

A rapid mesoscale cyclogenesis event took place over the southeastern United States during 28–29 March 1984. This small-scale cyclone, whose initial radius of circulation was approximately 120 km, was associated with a 3-h pressure fall of 11 mb, rainfall exceeding 60 mm, and numerous tornadoes. The development of this mesoscale cyclone was poorly forecasted by the operational Limited-Area Fine-Mesh Model (LFM). Later experiments with the Nested Grid Model (NGM) and eta model also experienced similar failure. In this paper, the authors present a series of numerical experiments using The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 4 (MM4) with the goat of determining factors crucial to a successful prediction of the surface cyclogenesis.

The control experiment simulated the rapid mesoscale cyclogenesis by using a 40-km grid spacing; explicit prediction of cloud water, rainwater, and cloud ice; subgrid cumulus parameterization developed by Grell; and the planetary boundary layer scheme developed by Blackadar. The simulated cyclone track and intensity followed the observations reasonably. The model also produced a precipitation distribution superior to that of the operational forecasts. However, the timing of rapid cyclogenesis and heavy precipitation lagged behind the observations by approximately 6 h.

Additional experiments were performed to test the sensitivity of the simulations to latent heat release, precipitation parameterization, surface energy fluxes, horizontal grid resolution, the time of initialization, and the treatment of topography. The authors find that the mesoscale cyclogenesis is the result of interaction between an upper-level disturbance and latent heat release. The occurrence of heavy precipitation is strongly influenced by the supply of warm, moist air in the boundary layer, which is in turn affected by the surface energy fluxes. The treatment of precipitation parameterization and the horizontal grid resolution also exert an influence on the accuracy of the simulation. The mesoscale cyclogenesis is not affected significantly by the Appalachian Mountains during the 24-h simulation period. Because of the diabatic nature of the mesoscale cyclone, this cyclogenesis event is found to be highly sensitive to the quality of initial conditions and, therefore, has limited predictability.

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