Zwack Okossi Research Articles

Variations in Key Factors at Different Explosive Development Stages of an Extreme Explosive Cyclone over the Japan Sea

Explosive cyclones (ECs) occur frequently over the Japan Sea. The most rapidly intensifying EC over the Japan Sea during the 44-year period 1979–2022, in the cold season (October–April), was examined to reveal the variations in the key factors at different explosive development stages. The EC deepened at a maximum deepening rate of 3.07 bergerons and explosive development lasted for 15 h. At the initial moment of explosive development, the EC had distinctive low-level baroclinicity, the low-level water vapor convergence was weak, and mid-level cyclonic vorticity advection was far away from the EC’s center. At the moment at which the EC reached the maximum deepening rate, the low-level water vapor convergence and mid-level cyclonic vorticity advection increased distinctly and approached the EC’s center. A diagnostic analysis using the Zwack–Okossi equation showed that the main contributor to the initial explosive development was warm-air advection. Through the evolutionary process of the explosive development, the non-key factors of the cyclonic vorticity advection and diabatic heating at the initial explosive development stage increased quickly and became key factors contributing to the maximum explosive development. The key factors contributing to the explosive development varied with the stage of explosive development. The cross-section and vertical profile of each term suggested that the cyclonic vorticity advection was enhanced in the upper troposphere and diabatic heating increased in the middle troposphere.

Kinetic Energy and Vorticity Perspectives of the Rapid Development of an Explosive Extratropical Cyclone Over the Northwest Pacific Ocean in February 2018

Explosive extratropical cyclones (EECs) have long been a research focus for the meteorological society as they often cause serious economic losses and casualties. However, after a long period of research, there still remain some knowledge gaps about their rapid development. In this article, we conducted the first study by using both vorticity and kinetic energy (KE) budgets simultaneously on a typical EEC, which was the strongest EEC that affected the coastal areas of China in the last 3 years, to further the understanding of the mechanisms governing its rapid enhancement in rotation and wind speed. The vorticity budget shows that the lower-level convergence-related vertical stretching and the vertical transport of vorticity acted as the most and second most favorable factors for the increase in the cyclone's cyclonic vorticity, respectively, which were different from those findings based on the Zwack–Okossi vorticity budget. In contrast, the horizontal transport of vorticity and tilting mainly decelerated the EEC's development. Energetics features governing the rapid wind enhancement of the EEC were shown for the first time. It is found that the work on the rotational wind by the pressure gradient force and the net import transport of KE by the rotational wind contributed the largest and second largest to the cyclone's increase in wind speed. In contrast, the upward transport of KE and the cyclone's displacement mainly acted in an opposite way. Analysis based on the Green's theorem and rotational wind shows that the enhancement of the EEC's rotation and wind speed were linked to each other solidly.

OZO v.1.0: software for solving a generalised omega equation and the Zwack–Okossi height tendency equation using WRF model output

Abstract. A software package (OZO, Omega–Zwack–Okossi) was developed to diagnose the processes that affect vertical motions and geopotential height tendencies in weather systems simulated by the Weather Research and Forecasting (WRF) model. First, this software solves a generalised omega equation to calculate the vertical motions associated with different physical forcings: vorticity advection, thermal advection, friction, diabatic heating, and an imbalance term between vorticity and temperature tendencies. After this, the corresponding height tendencies are calculated with the Zwack–Okossi tendency equation. The resulting height tendency components thus contain both the direct effect from the forcing itself and the indirect effects (related to the vertical motion induced by the same forcing) of each physical mechanism. This approach has an advantage compared with previous studies with the Zwack–Okossi equation, in which vertical motions were used as an independent forcing but were typically found to compensate the effects of other forcings.The software is currently tailored to use input from WRF simulations with Cartesian geometry. As an illustration, results for an idealised 10-day baroclinic wave simulation are presented. An excellent agreement is found between OZO and the direct WRF output for both the vertical motion and the height tendency fields. The individual vertical motion and height tendency components demonstrate the importance of both adiabatic and diabatic processes for the simulated cyclone. OZO is an open-source tool for both research and education, and the distribution of the software will be supported by the authors.

The Vorticity Budgets of North Atlantic Winter Extratropical Cyclone Life Cycles in MERRA Reanalysis. Part II: Decaying Phase*

Abstract The role of physical forcing mechanisms that contribute to the decay of winter North Atlantic extratropical cyclones during the period 1979–2009 are examined using the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The paired Zwack–Okossi tendency equation and omega equation explained in part I of this paper is employed to investigate the total effects of forcing processes (the direct effect of the forcing mechanisms and the indirect effect of the induced adiabatic cooling) that dissipate the 950-hPa cyclonic geostrophic vorticity at the cyclone center. Composite analyses reveal that the commencement of the decay is associated mainly with the upper-level anticyclonic vorticity advection, cold-air advection, and positive ageostrophic vorticity tendency. The secondary contributor to the dissipation of cyclonic circulation is the lower-tropospheric adiabatic cooling induced mainly by friction and positive ageostrophic vorticity tendency. The dynamics is found to be different at the beginning of the decay than in the later stages. While the negative tilt of troughs aloft and the surface cyclone is required for cyclolysis to occur, low air processes show a greater effect in the termination of the low pressure systems. Further, the total effect of the vorticity advection and temperature advection terms are modest, while the ageostrophic vorticity tendency and friction terms show a greater total negative contribution. This is because the two latter terms decrease the cyclonic geostrophic vorticity at the low center through both their direct and indirect effects. The latent heat release maximizing at 800–700 hPa produces cyclonic circulation, thus reducing the spindown of decaying cyclones.

The Vorticity Budgets of North Atlantic Winter Extratropical Cyclone Life Cycles in MERRA Reanalysis. Part I: Development Phase*

Abstract This series of papers (parts I and II) examines the vorticity budgets of winter North Atlantic extratropical cyclones during the period 1979–2009 using the Modern-Era Retrospective Analysis for Research and Application (MERRA). The authors use a new partitioning technique to combine the Zwack–Okossi (Z–O) equation with the omega equation. The combination provides a possibility to partition the adiabatic term in the Z–O equation into its different forcing mechanisms. Thus, both the direct effect of the dynamic and thermodynamic forcings and their indirect effect on the adiabatic term can be calculated to provide the total effect (direct plus indirect) on the 950-hPa geostrophic vorticity tendency. It is demonstrated that the total-effect diagnostic is a suitable tool to identify the dynamically consistent characteristics of cyclone development in midlatitudes because it possesses less case-to-case variability. The authors found that the vorticity advection is the major forcing process, the tendencies attributed to the ageostrophic vorticity tendency term are considerable, and the opposing effect of the friction term in moderating the deepening is significant. In general, the upper-level dynamics drive the deepening of the cyclones, except at the end of development, where a combination of midlevel latent heating, positive ageostrophic vorticity tendency, and positive indirect effect of vorticity advection contribute to the development. Additionally, the total effects of temperature advection and latent heating on the intensification of cyclones are reduced because of the inclusion of counteractive indirect effects, as are their variabilities within the cyclone composite.

Predictability of Explosive Cyclogenesis over the Northwestern Pacific Region Using Ensemble Reanalysis

Abstract The predictability of explosive cyclones over the northwestern Pacific region is investigated using an ensemble reanalysis dataset. Explosive cyclones are categorized into two types according to whether the region of the most rapid development is in the Sea of Okhotsk or Sea of Japan (OJ) or in the northwestern Pacific Ocean (PO). Cyclone-relative composite analyses are performed for analysis increments (the differences between the analysis and the 6-h forecast) and ensemble spreads (the standard deviations of ensemble members of the analysis or first guess) at the time of the maximum deepening rate. The increment composite shows that the OJ explosive cyclone center is forecast too far north compared to the analyzed center, whereas the PO explosive cyclone is forecast shallower than the analyzed center. To understand the cause of these biases, a diagnosis of the increment using the Zwack–Okossi (Z-O) development equation is conducted. The results suggest that the increment characteristics of both the OJ and PO explosive cyclones are associated with the most important cyclone development mechanisms. The OJ explosive cyclone forecast error is related to a deeper upper trough, whereas the PO explosive cyclone error is related to weaker latent heat release in the model. A diagnosis of the spread utilizing the Z-O development equation clarifies the mechanism underlying the uncertainty in the modeled sea level pressure. For OJ explosive cyclones, the spread of adiabatic warming causes substantial sea level pressure spreading southwest of the center of the cyclones. For PO explosive cyclones, the latent heat release causes substantial sea level pressure spreading around the cyclone center.

The Impact of the Eastward Propagation of Convective Systems over the Tibetan Plateau on the Southwest Vortex Formation in Summer

AbstractBased on the temperature of the black body (TBB), station observed and NCEP reanalysis data, the impacts of the eastward propagation of convective cloud systems over the Tibetan Plateau on the southwest vortex (SWV) formation that occurred at 1800 UTC on 29 June 2003 are analyzed by using the Zwack-Okossi (Z-O) equation to diagnose the thermal and dynamic processes. It is found that, in summer, severe convective activities often occur over the Tibetan Plateau due to the abundant supply of moisture. The convective cloud near the east edge of the plateau could move eastward with a short-wave trough in the westerly. The divergent center that is induced by latent heat release, which is associated with severe convective activities, moves out with the convective cloud and contributes to the low level decompression which is favorable for the formation of plateau edge cyclogenesis (PEC). The Z-O equation indicates that, in this case, the latent heat release and convergence are the two most important facto...

A diagnosis of warm‐core and cold‐core extratropical cyclone development using the Zwack–Okossi equation

AbstractIn this study, the development of a warm‐core and cold‐core extratropical cyclone over North Atlantic is examined. The geostrophic relative vorticity tendency used to diagnose the development is calculated utilizing the so‐called extended form of the Zwack–Okossi development equation. In both cases, the cyclonic vorticity advection acted to develop the system, but warm‐air advection (diabatic heating) made the largest contribution to explosive development in the cold‐core (warm‐core) case. Further, a vertical cross section of the temperature advection in the warm‐core case reveals that the largest values of this contributor are located far and ahead of the cyclone center. Copyright © 2009 Royal Meteorological Society

Calculated Height Tendencies in Two Southern Hemisphere Blocking and Cyclone Events: The Contribution of Diabatic Heating to Block Intensification

Abstract The Zwack–Okossi vorticity tendency equation was used to calculate 500-hPa height tendencies in two intensifying Southern Hemisphere blocking events. The National Centers for Environmental Prediction–National Center for Atmospheric Research gridded reanalyses were used to make each of these calculations. The block intensification period for each event was associated with a deepening surface cyclone during a 48-h period beginning at 1200 UTC 28 July and 1200 UTC 8 August 1986, respectively. These results demonstrate that the diabatic heating forces height rises through the sensible and latent heating terms in these two Southern Hemisphere blocking events. The sensible heating was the larger contributor, second only to (about the same as) the vorticity advection term in the first (second) event. The vorticity advection term has been shown by several studies to be associated with block intensification.

An Investigation of Extratropical Cyclone Development Using a Scale-Separation Technique

The explosive development phase of an extratropical cyclone (ETC) is examined using output generated by the fifth-generation PSU‐NCAR Mesoscale Model (MM5). A full-physics run of MM5 with 60-km grid spacing was used to simulate the intensive observation period (IOP)-4 storm of 4‐5 January 1989 from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). A diagnosis of the simulated ETC is performed using the Zwack‐Okossi (Z‐O) equation to examine the forcing mechanisms influencing development. A secondorder Shapiro filter is used to partition the terms in the Z‐O equation into synoptic-scale and subsynoptic-scale contributions to the near-surface synoptic-scale geostrophic vorticity tendency. Results confirm that previous work using the Z‐O equation at coarser resolutions correctly identified synopticscale processes as the most important cyclone development mechanisms. However, the results also show that both adiabatic and diabatic subsynoptic thermal processes can make important contributions to synoptic-scale ETC development.

The Importance of the Horizontal Distribution of Heating during Extratropical Cyclone Development

Abstract Diagnostic and modeling results reveal that atmospheric heating typically acts to intensify extratropical cyclones. In addition, both the Petterssen–Sutcliffe and Zwack–Okossi development equations reveal that this relationship depends on the proportionality that exists between surface geostrophic vorticity tendency and the negative of the horizontal Laplacian of atmospheric heating. Because of this Laplacian relationship, the impact of a heating field with a given magnitude and vertical distribution depends on its horizontal distribution. This paper will show how horizontal heating distributions that differ by relatively small amounts over their entire extent can yield vorticity tendency responses that could contribute to either development or decay of an underlying cyclone.

An analysis of a relatively rare case of continental blocking

Planetary- and synoptic-scale analyses of a relatively rare continental blocking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its counterparts which occur over oceanic regions. the planetary-scale analysis demonstrates that during the spring season a ridge was located further inland over the North American continent and amplified with respect to climatology. the position of this ridge may have been linked to a broad region of colder-than-normal sea surface temperatures found over the north central Pacific during the spring season and much of the previous winter. Simple ‘Sutcliffe-type’ and thermodynamic analyses of the accompanying lower-tropospheric warm anomaly associated with the ridging show that lower-tropospheric temperature advection and subsidence associated with anticyclonic-vorticity advection by the time-mean thermal wind produced much of the anomalous warmth. A simple synoptic-scale analysis was performed using both the Zwack-Okossi (ZO) equation and potential vorticity (PV) thinking approaches. These complementary analyses demonstrated that synoptic-scale cyclones were instrumental in the formation and maintenance (and/or intensification) of this blocking event. the PV analysis demonstrated that low-PV air was swept polewards and then was advected over the blocking region sustaining the broad region of low potential vorticity associated with the block over North America. the ZO analysis showed that the advection of anticyclonic vorticity was the most important mechanism forcing geopotential-height rises at 500 hPa over the block centre. the region of low PV and ZO height rises could be associated with the anticyclonic-shear side of an upstream jet maximum typically found in association with developing and/or intensifying blocking events. Thus, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship between the upstream cyclones and the large-scale ridge is necessary for block development or intensification.

An analysis of a relatively rare case of continental blocking

AbstractPlanetary‐ and synoptic‐scale analyses of a relatively rare continental blocking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its counterparts which occur over oceanic regions. the planetary‐scale analysis demonstrates that during the spring season a ridge was located further inland over the North American continent and amplified with respect to climatology. the position of this ridge may have been linked to a broad region of colder‐than‐normal sea surface temperatures found over the north central Pacific during the spring season and much of the previous winter. Simple ‘Sutcliffe‐type’ and thermodynamic analyses of the accompanying lower‐tropospheric warm anomaly associated with the ridging show that lower‐tropospheric temperature advection and subsidence associated with anticyclonic‐vorticity advection by the time‐mean thermal wind produced much of the anomalous warmth.A simple synoptic‐scale analysis was performed using both the Zwack‐Okossi (ZO) equation and potential vorticity (PV) thinking approaches. These complementary analyses demonstrated that synoptic‐scale cyclones were instrumental in the formation and maintenance (and/or intensification) of this blocking event. the PV analysis demonstrated that low‐PV air was swept polewards and then was advected over the blocking region sustaining the broad region of low potential vorticity associated with the block over North America. the ZO analysis showed that the advection of anticyclonic vorticity was the most important mechanism forcing geopotential‐height rises at 500 hPa over the block centre. the region of low PV and ZO height rises could be associated with the anticyclonic‐shear side of an upstream jet maximum typically found in association with developing and/or intensifying blocking events. Thus, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship between the upstream cyclones and the large‐scale ridge is necessary for block development or intensification.

The Dynamics of an Explosively Developing Cyclone Simulated by a General Circulation Model

Abstract The synoptic structure and dynamics of an explosively developing winter extratropical cyclone simulated by NCAR’s CCM2 general circulation model is examined and compared with cyclones that have developed explosively in nature. The primary diagnostic tool utilized in this analysis is the Zwack–Okossi equation. Synoptic results show that in addition to yielding an unusually large cyclone, the development initially occurred in the absence of a prominent upper-level trough. Rather, the surface disturbance was accompanied by an unusually strong upstream jet streak. Despite these departures from normally observed synoptic structure, the dynamics of the cyclone development and decay conformed to the dynamics found in observational cases. In particular, the cyclone developed in response to cyclonic vorticity and warm air advection, both of which maximized in the upper troposphere, and latent heat release, which were sufficient to overcome the cyclolytic effects of adiabatic cooling, friction, and sensibl...

The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season

Abstract Using the Goddard Laboratory for Atmospheres Goddard Earth Observing System 5-yr analyses and the Zwack–Okossi equation as the diagnostic tool, the horizontal distribution of the dynamic and thermodynamic forcing processes contributing to the maintenance of a Northern Hemisphere midlatitude blocking anticyclone that occurred during the summer season were examined. During the development of this blocking anticyclone, vorticity advection, supported by temperature advection, forced 500-hPa height rises at the block center. Vorticity advection and vorticity tilting were also consistent contributors to height rises during the entire life cycle. Boundary layer friction, vertical advection of vorticity, and ageostrophic vorticity tendencies (during decay) consistently opposed block development. Additionally, an analysis of this blocking event also showed that upstream precursor surface cyclones were not only important in block development but in block maintenance as well. In partitioning the basic data ...

A Comparison of Extended and Quasigeostrophic Dynamics for a Case of Small–Rossby Number Extratropical Cyclone Development

Abstract This note compares the extended (EXT) and quasigeostrophic (QG) dynamics of a small–Rossby number extratropical cyclone using the Zwack–Okossi (ZO) equation. Applied to a cyclone that occurred on 8–9 November 1985 over the North Atlantic Ocean, results show that although differences exist, both the EXT and QG forms of the ZO equation provide very adequate estimates of the large-scale forcing processes associated with this case.

A Composite Diagnosis of Synoptic-Scale Extratropical Cyclone Development over the United States

This paper presents a composite diagnosis of synoptic-scale forcing mechanisms associated with extratropical cyclone evolution. Drawn from 12 cyclone cases that occurred over the continental United States during the cool season months, the diagnosis provides a “climatology” of development mechanisms for difference categories of cyclone evolution ranging from cyclone weakening through three stages of cyclone intensification. Computational results were obtained using an “extended” form of the Zwack–Okossi equation applied to routine upper-air and surface data analyzed on a 230 km × 230 km grid. Results show that cyclonic vorticity advection, which maximizes in the upper troposphere, was the primary contributor to cyclone development regardless of the stage of development. A second consistent contributor to development was latent heat release. Horizontal temperature advection, often acknowledged as a development mechanism, was found to contribute to development only during more intense stages. During weakening and weaker development stages, temperature advection opposed development, as the warm-air advection invariably found at upper levels was dominated by cold air advection in the lower half of the sphere. In the more intense stages, development was moderated by dry-adiabatic cooling associated with the ascending vertical motions.

A Diagnosis of a Model-Simulated Explosively Developing Extratropical Cyclone

Abstract This paper examines a simulation of the explosive development phase of the ERICA IOP 4 extratropical cyclone case obtained from the 70-km Limited Area Mesoscale Prediction System. A detailed diagnosis of the simulated cyclone is performed using the Zwack–Okossi equation to examine the forcing mechanisms influencing the near-surface synoptic-scale pressure tendency. The diagnosis includes two special features, the inclusion of forcing terms representing synoptic-subsynoptic exchange processes and a unique approach to evaluating the contribution of individual levels to the vertically integrated forcing. Explosive cyclogenesis was initiated by the combined effects of warm-air advection, cyclonic vorticity advection, and latent heat release, the first two maximizing above 400 mb and the latter below 750 mb. At the cyclone center the temperature advection profiles also featured a secondary warm-air advection maximum below 800 mb whose impact on surface pressure tendency was often equal to or greater t...

A Diagnosis of the Development of a Winter Anticyclone over North America

This paper examines the 48-h life cycle of a winter anticyclone occurring over North America from 18 to 20 January 1979 using Goddard Laboratory for Atmospheres FGGE level IIIb (SOP 1) global analyses on a 4° latitude by 5° longitude grid. Applying the relatively new methodology of the Zwack–Okossi equation, results show that anticyclonic vorticity advection and cold-air advection acted to develop the anticyclone, while adiabatic warming in the descending air opposed development. Other forcing processes made only small contributions to anticyclone changes. Vertical profiles of the development quantities reveal that vorticity and temperature advections, as well as the adiabatic warming, maximized in the 200–300-mb layer.

Planetary and synoptic-scale interactions during the life cycle of a mid-latitude blocking anticyclone over the North Atlantic

The formation of a blocking anticyclone over the North Atlantic has been examined over its entire life-CyCle using the Zwack-Okossi (Z-O) equation as the diagnostic tool. This blocking anticyclone occurred in late October and early November of 1985. The data used were provided by the NASA Goddard Laboratory for Atmospheres on a global 2.O degree latitude by 2.5 degree longitudinal grid. The horizontal distribution of the atmospheric forcing mechanisms that were important to 500 mb block formation, maintenance and decay were examined. A scale-partitioned form of the Z-O equation was then used to examine the relative importance of forcing on the planetary and synoptic scales, and their interactions. As seen in previous studies, the results presented here show that upper tropospheric anticyclonic vorticity advection was the most important contributor to block formation and maintenance. However, adiabatic warming, and vorticity tilting were also important at various times during the block lifetime. In association with precursor surface cyclogenesis, the 300 mb jet streak in the downstream (upstream) from a long-wave trough (ridge) amplified significantly. This strengthening of the jet streak enhanced the anti-cyclonic vorticity advection field that aided the amplification of a 500 mb short-wave ridge. Tile partitioned height tendency results demonstrate that the interactions between the planetary and sn,noptic-scale through vorticity advection was the most important contributor to block formation. Planetary-scale, synoptic-scale. and their interactions contributed weakly to the maintenance of the blocking anticyclone with the advection of synoptic-scale vorticity by the planetary-scale flow playing a more important role. Planetary-scale decay ofthe long-wave ridge contributed to the demise of this blocking event.