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Phillip J. Smith

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.

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Phillip J. Smith

Abstract

No abstract available.

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Phillip J. Smith

Abstract

A procedure is introduced which allows estimation of the net influence of subgrid-scale thermodynamic processes on large-scale available potential energy. A numerical example representing the average energetics over North America for March, 1962 suggests that “net” subgrid-scale generation greatly exceeds the average grid-scale generation, with positive contributions occurring in the low and middle troposphere.

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PHILLIP J. SMITH

Abstract

Several techniques exist for computing vertical motions. In this paper, radiosonde wind observations are used to compute vertical motions by the kinematic method. The presence of cumulative bias errors necessitates adjustment techniques. Simple tests of two techniques indicate that, for the period of this study, a divergence adjustment that is a function of pressure yields the best adjusted vertical motion fields. Further analysis shows that the adjusted estimates correlate well with observed synoptic features. Finally, comparison with estimates by the numerical method indicates that adjusted kinematic vertical motion fields are comparable.

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PHILLIP J. SMITH

Abstract

The kinetic energy budgets of several examples of synoptic scale systems are reviewed. Included are systems containing a major cyclone development, the immediate cyclone vicinity, and the anticyclone preceding the cyclone development. These are then considered in terms of their role in the general circulation of the middle latitudes.

Results show that the cyclone system and cyclone vicinity are respectively about two and five times more active energetically than the general circulation. Further, when averaged with results of Petterssen and Smehye, the resulting mean cyclone system accounts for about one-third of the energetic activity of the middle latitudes. On the other hand, circulations associated with the anticyclone case exhibit much less intense energetic properties.

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James M. Vasilj
and
Phillip J. Smith

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.

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Karen D. Walthorn
and
Phillip J. Smith

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 sensible heating. Decay ensued when a sufficiently deep layer of cold air was advected into the center of the cyclone. Thus, even though CCM2 simulations may sometimes misrepresent eddy structures, these same simulations are capable of representing the dynamics of the growth of such structures in an essentially correct fashion.

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Anthony R. Lupo
and
Phillip J. Smith

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 fields into their planetary-scale (P) and synoptic-scale (S) components, 500-hPa height tendencies forced by processes on each scale, as well as by interactions (I) between each scale, were also calculated. Over the lifetime of this blocking event, the S and P processes were most prominent in the blocked region. During the formation of this block, the I component was the largest and most consistent contributor to height rises at the center point. It was also shown that the height-rise regions located on the anticyclonic side of the jet maxima associated with block development and intensification were primarily composed of the S and I components. Also, the precursor cyclones were associated with S or S and I height rises that contributed to the formation of this block. Finally, the results of this paper show that the forcing associated with summer-season blocking events are similar to that of their winter-season counterparts neglecting the natural case-to-case variability. In comparing these results to the results of other papers in this series, however, it is suggested that there may be two models for block development.

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Gregory L. Lamberty
and
Phillip J. Smith

Abstract

This Paper Presents a statistical diagnosis of the differences between two Goddard Laboratory for Atmospheres analysis representations of temperature and wind speed, one containing satellite data influences (SAT analyses) and one without (NOSAT analyses), for a case of oceanic blocking anticyclone development. Results, obtained using area means and standard deviations of the two fields and correlation coefficients and root-mean-square differences between the two fields, indicate that the inclusion of satellite data can have significant impact on ocean-domain analyses. This is especially evident if one examines higher-order fields that represent gradients in the basic variable fields or contain covariances between two different variables (e.g., advections). More specifically, the inclusion of satellite data resulted in a cold (warm) temperature bias at low (high) levels and weaker temperature gradients, stronger winds, weaker vertical wind shears, and a warm-air advection bias at most levels. The SAT analyses also exhibited larger standard deviations than the NOSAT for wind speed, relative vorticity, vorticity advection, temperature advection, and 500-mb height tendencies, suggesting that for this case the spatial variability of the circulation features were enhanced by the inclusion of satellite data. Even so, since height tendencies forced by vorticity and temperature advection were loss sensitive to the addition of satellite data than were the advection quantities themselves, the dynamics of the blocking system was apparently less influenced by the presence of satellite data than was the structure of the system.

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Kenneth E. Parsons
and
Phillip J. Smith

Abstract

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 second- order 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 synoptic- scale 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.

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