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  • Author or Editor: I. Orlanski x
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I. Orlanski and J. Sheldon

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Numerical simulations have been made of the initiation of a strong ridge-trough system over western North America and the eastern Pacific (the terminus of the Pacific storm track), with the objective of determining the extent to which downstream development contributed to its growth, and the possible influence of topography on the energetics of the storm. While a control simulation demonstrated considerable skill in reproducing the storm, a “simplified” simulation in which topography, surface that fluxes, and latent heating were removed not only reproduced the primary features of the ridge-trough system—permitting a clearer interpretation of the factors contributing to its growth—but actually generated a stronger system, suggesting that these effects as a whole inhibited storm development. Application of an energy budget that distinguishes between energy generation via baroclinic processes and generation via the convergence of geopotential fluxes revealed that early growth of studies that have shown that eddies near the downstream end of a storm track grow, at least initially, primarily through the convergence of downstream energy fluxes. Baroclinic conversion, mostly in the form of cold advection, became the primary energy source only after the development was well under way. This sequence of initial energy growth via flux convergence followed by additional contributions by lower-level baroclinic conversion comprise a process designated “downstream baroclinic development” (DBD). A similar analysis of the control simulation showed that the energy budget was essentially the same, with the exception of baroclinic conversion, which was more significant early budget was essentially the same, with the exception of baroclinic conversion, which was more significant early in the eddy's development due to orographic lifting of warm westerly flow. The decay of the storm in both simulations was mainly the result of flux divergence after the storm reached the dispersion of additional kinetic energy generated by latent heat release upstream from the system. It is believed that the techniques employed here could represent a valuable new tool in the study of the development of such baroclinic systems and the diagnosis of model deficiencies.

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G. Rivière and I. Orlanski

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This study focuses on feedbacks of the high-frequency eddy activity onto the quasi-stationary circulation, particularly with regard to the North Atlantic Oscillation (NAO). The methodology consists of analyzing NCEP–NCAR reanalysis data and sensitivity runs from a high-resolution nonhydrostatic regional model. Consistent with recent studies, results show that the jet displacement characteristic of the NAO phenomenon depends strongly on the dynamics of the synoptic-scale waves and the way they break. Positive and negative phases of the NAO are closely related to anticyclonic and cyclonic wave breaking, respectively. Indeed, the high-frequency momentum flux whose sign is directly related to the type of wave breaking is correlated with the NAO index over the Atlantic. The peak of the momentum flux signal precedes that of the NAO by a few days suggesting that wave breaking is triggering NAO events. Two examples illustrate the significant impact of single storms, in particular those occurring in the east coast of the United States. The wave breaking at the end of their life cycle can suddenly change the NAO index in few days, and as the return to equilibrium takes generally a longer time, it can even affect the sign of the NAO during an entire month.

An important issue determining the NAO phase is related to upstream effects. By considering a domain extending from the eastern Pacific to western Europe and by forcing the regional model with real data at the western boundary, sensitivity runs show that the right sign of the NAO index can be recovered. It indicates that waves coming from the eastern Pacific are crucial for determining the NAO phase. According to their spatial scales and frequencies when they reach the Atlantic domain, they can break one way or another and push the Atlantic jet equatorward or poleward. Synoptic waves with periods between 5 and 12 days break anticyclonically whereas those with periods between 2 and 5 days break both anticyclonically and cyclonically with a predominance for cyclonic wave breaking. Another crucial factor concerns surface effects. Cyclonic wave breaking in the upper levels is strongly connected with an explosive cyclonic development at the surface accompanied by strong surface moisture fluxes whereas such an explosive growth is not present in the anticyclonic wave breaking case. Finally, it is proposed that these results are not only useful for explaining the intraseasonal variations of the NAO but would serve also as a basis for understanding its interannual and interdecadal variations.

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I. Orlanski and J. Katzfey

Abstract

The energetics of a Southern Hemisphere cyclone wave have been analyzed using ECMWF data and the results of a limited-area model simulation. An analysis of the energy budget for a storm that developed in the eastern Pacific on 4–6 September 1987 showed the advection of the geopotential height field by the ageostrophic wind to be both a significant source and the primary sink of eddy kinetic energy. Air flowing through the wave gained kinetic energy via this term as it approached the energy maximum and then lost it upon exiting. Energy removal by diffusion, friction, and Reynolds stresses was found to be small. The most important conclusion was that, while the wave grew initially by poleward advection of heat as expected from baroclinic theory, the system evolved only up to the point where this source of eddy energy and the conversion of eddy potential to eddy kinetic energy (typically denoted “ωα”) was compensated for by energy flux divergence (dispersion of energy), mainly of the ageostrophic geopotential flux, vaϕ. Energy exported in this fashion was then available for the downstream development of a secondary system. This finding seems to differ from the results of studies of the life cycle of normal-mode-type waves in zonal flows, which have been shown to decay primarily through transfer of energy to the mean flow via Reynolds stresses. However, this apparent inconsistency can be explained by the fact that while ageostrophic geopotential fluxes can also be very large in the case of individual normal modes, the waves export energy downstream at exactly the same rate as they gain from upstream. The group velocity of the 4–6 September storm, calculated from the ageostrophic geopotential height fluxes, showed that the energy packet comprising the system had an eastward group velocity slightly larger than the time-mean flow.

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I. Orlanski and B. B. Ross

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I. Orlanski and B. B. Ross

Abstract

The transient behavior of an idealized dry frontal system is investigated using a two-dimensional numerical model. The development of a cross-stream circulation within stationary and moving cold fronts is determined for various frontal and synoptic conditions. In the stationary front, a circulation is generated by symmetric baroclinic instability, but nonlinear effects restrict this circulation to remain very weak. In the moving cold front, the vertical shear of the synoptic wind which advects the front produces an ageostrophic residue as a result of the differential advection of the vertical shear of the frontal jet and the horizontal temperature gradient across the front. This residue, which depends upon the vertical synoptic shear and the thermal wind structure of the frontal system, will generate a cross-stream circulation which maintains the cold front in a quasi-steady state. The resulting motion field is described well by the streamfunction balance equation. The lifting produced by the cross-stream circulation in the moving cold front system may be sufficient to trigger deep convection under favorable conditions in the moisture and synoptic wind fields.

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B. B. Ross and I. Orlanski

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The effect of moisture upon the dynamics of mature idealized cold front systems is investigated using a two-dimensional numerical model. Lifting produced by the initial cross-stream frontal circulation studied by Orlanski and Ross (1977) is shown to saturate the warm moist air above the nose of the front when initial humidity levels are sufficiently high. If the atmosphere is convectively unstable, this saturated air will develop into deep convection with the convection-induced circulation overwhelming the initial frontal circulation. The initial development of convection is also shown to produce a gravity wave exhibiting similar scales to those of the convective zone. This wave propagates into the warm air at a much faster speed than the moving front-cloud system. Comparisons are made of the intensity of convection for different initial humidity and temperature conditions and when a low-level capping inversion is present. Also a comparison is made of cloud development caused by a combination of frontal lifting and surface heating when temperature inversions of different intensifies are present. The stronger inversion is shown to suppress convection produced by surface heating alone with the combined effect of frontal lifting and surface heating required to release the convective instability.

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I. Orlanski, J. Katzfey, C. Menendez, and M. Marino

Abstract

A rapidly deepening cyclone that occurred over the South Pacific on 5 September 1987 was investigated in order to assess the possible factors contributing to its development. Cyclogenesis took place when a disturbance in the subtropics merged with a wave in the polar westerlies. Analysis revealed that the evolution of the cyclone system was associated with the interaction of a potential vorticity anomaly from the subpolar region with a subtropical surface disturbance in a manner typical of “Class B” cyclogenesis. As the storm intensified, the subtropical jet merged with the polar jet, producing a strong poleward heat transport characteristic of baroclinic systems. However, the absence of tilt to the frontal zone, together with weak vertical wind shear, was suggestive of a significant barotropic component to the storm. The zonal average of potential vorticity over the storm displayed large regions where the meridional gradients have different signs, indicating that the system could have developed initially by internal instabilities (barotropic and/or baroclinic) without significant external forcings.

Sensitivity experiments were conducted to determine the role of surface processes in the development of the storm. It was found that development was insensitive to both surface heat fluxes and the presence of South American topography, with little change in either the circulation or kinetic energy of the storm. Intensification of the storm was substantially affected by surface frictional effects, as indicated by significant increases in the vertically averaged kinetic energy when the surface roughness was reduced. The results suggest a need to reduce the roughness heights not only over sea ice, but over the ocean in areas of strong winds as well.

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I. Orlanski, B. B. Ross, and L. J. Polinsky

Abstract

A two-dimensional mesoscale atmospheric model is presented and used to study unsteady dynamic processes occurring in the planetary boundary layer (PBL) driven by diurnal heating at the ground. The model reproduces turbulent fluxes of heat and momentum both by explicitly modeling resolvable eddies and by employing a single parameterization at all levels of the model to represent vertical fluxes caused by subgrid-scale eddies. The unsteady behavior of horizontally-averaged profiles of temperature and velocity respond quite realistically to the diurnally-varying heat flux at the ground, particularly with regard to the time variation of lapse rates and the occurrence times of maximum and minimum temperatures at various levels in the lower boundary layer. The spatial variation of predicted atmospheric quantities shows a great deal of resolved eddy activity during the day with a significant remnant persisting through the night at higher levels of the PBL. These eddies account for the predominant means of vertical heat and momentum transfer away from the surfaces with the model realistically reproducing the unsteady behavior of heat fluxes in the PBL. Temporal variation of vertical heat and momentum profiles shows boundary layer activity to be confined to a few hundred meters at night while extending up to a kilometer during the day. A weak heat flux source at the ground with an amplitude of 10% of the maximum daytime heating produced a nocturnal heat island some 60 m high with a maximum city-country temperature contrast of ∼1C.

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I. Orlanski, B. B. Ross, and L. J. Polinsky

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No abstract available.

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