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Isidoro Orlanski

Abstract

We can identify the diurnal oscillation of the atmospheric boundary layer as an important source of mesoscale internal gravity waves in the lower atmosphere.

The oscillation period of these waves is a function of latitude. A definitive two-day period may be found in the equatorial regions with scales on the order of a few hundred kilometers. In particular, for a situation in which the mean stratification at any time of the day is unstable, the wavelength could he on the order of 100 km. This result suggests that some cloud clusters may be originated by this process.

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Isidoro Orlanski

Abstract

From preliminary reports of the GATE experiment it was concluded that there is some strong evidence that mesoscale waves with a periodicity close to 2 days exist in the equatorial regions. The dynamics of unstable internal gravity waves due to trapeze instability was discussed by means of a two-dimensional, β-plane numerical model. It was concluded that the trapeze instability may be the means by which the observed 2-day waves are excited.

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Isidoro Orlanski

Abstract

The stability of a two-layer model is analyzed using a numerical method taking into account the effect of bottom topography. A jet in geostropic equilibrium exists in the upper layer and baroclinic instability may occur. It is found if the bottom topography has a large amplitude relative to the total depth, that it has a destabilizing rather than a stabilizing influence. Applying the model to the Gulf Stream, it is found that the most unstable disturbances, corresponding to the basic flow upstream from Cape Hatteras, are markedly different in wavelength and period from those corresponding to the basic flow downstream from Hatteras. The baroclinic disturbances in the model are consistent with the limited observational evidence on momentum transfer by Gulf Stream eddies.

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Isidoro Orlanski

Abstract

The stability of the classical Norwegian polar front model is investigated, using a numerical technique to supplement the more precise conclusions which are possible in the limiting cases of zero density difference or zero wavenumber. The feasibility of the numerical technique depends on a careful formulation of boundary conditions at the limits of the frontal zone. The numerical results cover the region of Rossby number (Ro) ≤ 3 and Richardson number(Ri) ≤ 5, but their interpretation is unclear at Ri > 2 and Ro > 1. Unstable waves exist at all wavelengths; Rayleigh shear instability at small Ri, Helmholtz shear instability at large Ro and small Ri, shear instability and geostrophic baroclinic instability simultaneously at small Ro and Ri > 2, and a combination of geostrophic and Helmholtz instability when Ri > 2 and Ro < l (but not too small). The previous conclusion of Kotschin that this frontal model is stable for Ri < 2 is therefore incorrect.

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Isidoro Orlanski

Abstract

Second-order expansion of the aspect ratio gives rise to simple equations with a quasi-hydrostatic approximation that perform far better than the classical hydrostatic system in the simulation of moist convection in a mesoscale model. It also suggests that a simple modification to this system may extend the validity of schemes for aspect ratios larger than 1.

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Isidoro Orlanski

Abstract

There is a fairly well defined stationary wave and storm track response to El Niño SSTs over the Pacific. In this paper, the case is made that this response is a direct result of increased baroclinicity in the central Pacific and that changes in the stationary wave pattern farther east are primarily forced by changes in these transient eddies. There is also a lot of natural variability that is not associated with El Niño. The paper also stresses the point that much of the variability can be understood as forced by variations in the upstream seeding of the storm track. The question of whether these seeding variations should be thought of as chaotic noise or forced by identifiable mechanisms is not addressed. Thus, the claim is that the storm track variability and its feedback to the quasi-stationary circulation depends on two key parameters: mid-Pacific baroclinicity, controlled by SSTs, and the strength of the upstream seeding.

The approach is to first examine the effect of storm track seeding by waves entering from the Asian continent during normal years (non-ENSO years). The results show that two mechanisms operate to distribute eddy energy along the storm track: downstream development and baroclinic development. The large effect on baroclinic development at the storm track entrance results from a combination of factors: surface baroclinicity, land–sea contrast, and strong moist fluxes from the western subtropics. Experiments show that sensitivity to the seeding amplitude is large. The larger the seeding amplitude, the closer the more intense baroclinic waves flux energy downstream to upper-level waves. These barotropic waves tend to break anticyclonically and produce a ridge in the eastern Pacific.

Sensitivity to SST anomalies shows qualitative and quantitative similarity with the observed anomalies. Simulations show increased mid-Pacific baroclinicity because stronger convection in the midtropical Pacific enhances a large pool of warm air over the entire mideastern subtropical ocean. Waves with sources at the storm track entrance break anticyclonically and produce the ridge in the eastern Pacific. On the other hand, baroclinic waves generated or regenerated in the mid-Pacific tend to break cyclonically, produce a trough tendency, and reduce the eastern ridge amplitude in the Pacific–North American (PNA) sector.

These results strongly suggest that

  1. the variability of the quasi-permanent circulation indeed could be produced by the high-frequency eddy feedback, and
  2. two mechanisms are primarily responsible for the forcing of the quasi-permanent circulation: downstream development from the western Pacific and the anomalous baroclinicity in the mideastern Pacific.

The intensity of these counteracting forcings gives the different flavors of the El Niño response over the PNA region. Regardless of the SST anomaly strength, the PNA patterns seem unique but obviously have different intensities.

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Isidoro Orlanski

Abstract

By analyzing a number of very high resolution, nonhydrostatic experiments of baroclinic lifecycles, it was concluded that the intensity of the near-surface baroclinic development influences the upper-level wave to such an extent that it could produce cyclonic or anticyclonic wave breaking. Since the final jet position is equatorward or poleward, the position depends on whether the waves break cyclonically or anticyclonically, respectively. The low-level baroclinicity plays a very important role in the outcome of the wave and feedback to the mean circulation. Using a shallow water model the hypothesis that the intensity of the eddy forcing from the lower layers of the atmosphere can have a profound effect on the disturbances of the upper layers is tested. From these experiments the following is concluded.

For weak intensities, the strong effective beta asymmetries due to the earth's sphericity produce anticyclonic wave breaking and a poleward shift of the zonal jet will occur. For moderate forcing, anticyclonic wave breaking occurs and consequently, as before, a poleward shift of the zonal jet will occur. However, there is an important distinction between weak and moderate forcing. In the latter case, the eddy anticyclonic centers are very intense. The influence of the two anticyclones produces a difluence field that will strain the cyclonic vortex along the SW–NE direction. Consequently, the meridional vorticity flux υζ′ is positive in the north and negative in the south. This process has two effects: thinning the cyclone and producing positive vorticity fluxes on the north, negative fluxes on the south and moving the jet poleward. By increasing the forcing, the cyclone centers become considerably more intense than the anticyclones (CVC) and they are able to deform and thin the anticyclones, thus moving the jet equatorward. This transition is very abrupt; above a threshold amplitude, the life cycle bifurcates to a cyclonic wave breaking.

The implications for storm track variability are quite direct. In normal years, at the entrance of the storm track, intense baroclinicity produces CVCs with a slight shift of the jet equatorward. At the last half of the storm track, due to much weaker baroclinicity, anticyclonic wave breaking occurs (AVCs) displacing the jet poleward. The eddies at the entrance of the storm track develop from the baroclinicity of the subtropical jet. Downstream fluxing and weaker surface baroclinicity make the upper-level waves more aloft and barotropic by the middle of the storm track. These waves normally break anticyclonically, enhancing the subpolar eddy-driven jet. In the warm phase of ENSO, more baroclinicity (and subtropical moisture flux) is present in the eastern Pacific Ocean. This enhanced baroclinicity could support more CVCs in the eastern basin, maintaining the subtropical jet further east.

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Isidoro Orlanski

Abstract

An analysis of 11 years of European Centre for Medium-Range Weather Forecasts data focuses primarily on the vertically averaged high-frequency transients. The conclusions are discussed in the context of (a) the winter storm track, (b) monthly variability, and (c) interannual variability. (a) Winter storm track: Results show that the pattern of the forcing by the high-frequency eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primarily responsible for the location of the trough–ridge system associated with the stationary flow. The results also clarify the role of wind component covariance terms uυ and (υ2u2) in the column-averaged vorticity forcing. The simpler term uυ has the well-known effect of intensifying the anticyclonic (cyclonic) tendencies on the southern (northern) side of the jet, thereby producing an increase in the barotropic component of the zonal jet. The (υ2u2) term displays a quadrupole pattern, which is also approximately in phase with the trough–ridge system associated with the stationary flow. (b) Monthly variability: Eddy activity has been shown to possess a seasonal life cycle, increasing during the early fall and reaching a maximum around the month of November, then decaying for most of the winter months. Month-to-month variations in eddy activity over the Pacific Ocean show that energy levels increase up through November, decreasing thereafter, at the same time the trough–ridge circulation pattern is intensifying. By December, baroclinicity in the western Pacific has increased substantially, and low-level eddies are found to break by the middle of the ocean. Upper-level eddies start to break well before reaching the west coast of North America, resulting in a displacement of the maximum in (υ2u2) westward from its November position and increasing the trough–ridge forcing by the high-frequency eddies. (c) Interannual variability: Wintertime eddy kinetic energy is seen to extend further eastward through the Pacific Ocean during the warm phase but displays an abrupt termination during the cold phase. Anomalies in the eddy transient forcing tend to be quite similar to that of the Pacific–North American pattern itself. The extension of the storm track during the warm phase resembles that of fall conditions and is present in the winter season because the source of low-level baroclinicity is extended well into the eastern Pacific for this El Niño–Southern Oscillation phase.

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Isidoro Orlanski and Brian Gross

Abstract

The life cycle of baroclinic eddies in a controlled storm track environment has been examined by means of long model integrations on a hemisphere. A time-lagged regression that captures disturbances with large meridional velocities has been applied to the meteorological variables. This regressed solution is used to describe the life cycle of the baroclinic eddies. The eddies grow as expected by strong poleward heat fluxes at low levels in regions of strong surface baroclinicity at the entrance of the storm track, in a manner similar to that of Charney modes. As the eddies evolve into a nonlinear regime, they grow deeper by fluxing energy upward, and the characteristic westward tilt exhibited in the vorticity vanishes by rotating into a meridional tilt, in which the lower-level cyclonic vorticity center moves poleward and the upper-level center moves equatorward.

This rather classical picture of baroclinic evolution is radically modified by the simultaneous development of an upper-level eddy downstream of the principal eddy. The results suggest that this eddy is an integral part of a self-sustained system here named as a couplet, such that the upstream principal eddy in its evolution fluxes energy to the upper-level downstream eddy, whereas at lower levels the principal eddy receives energy fluxes from its downstream companion but grows primarily from baroclinic sources. This structure is critically dependent on the strong zonal variations in baroclinicity encountered within the storm track environment.

A second important result revealed by this analysis is the fact that the low-level vorticity centers that migrate poleward tend to follow isotachs that closely correspond to the phase speed of the eddies. It is suggested that the maximum westward momentum that the eddies deposit at lower levels corresponds to the phase velocity, a quantity that can be estimated just from the upstream conditions. The intensity and direction of propagation of these waves will determine the overall structure of the storm track.

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Bin Wang and Isidoro Orlanski

Abstract

A case of the heavy rain vortex which occurred during the period 14–15 July 1979 is studied using a limited-area mesoscale numerical model. This is a representative example of a group of warm southwest vortices that often form over the eastern flank of the Tibetan Plateau after the onset of the summer Indian monsoon.

Some common features of the dynamic structures exhibited both by the simulation and by observations are discussed. The developing vortex is noticeably detached from the polar frontal zone. A 180° phase shift exists between the upper and lower layer vorticity fields. In the boundary layer, a pronounced northward transport of mass and moisture is connected with an intense upward motion near and to the east of the 700-mb vortex center, whole the southward cold advection is insignificant.

The vortex originated and rapidly developed in a stagnation region on the lee side of the plateau. The presence of the stagnation region not only removes local dynamical energy sources from the environmental flow, but also diminishes topographic generation of vorticity by reducing the vortex stretching in the wind component flowing over the plateau and the horizontal convergence in the component moving around the plateau. Without latent heating, dynamic instability and/or forcing of the large-scale flow interacting with the Tibetan Plateau is not sufficient to generate the observed disturbance.

On the other hand, the plateau blocking effect favors the establishment of a conditionally unstable environment. The simulation indicates that a sudden onset of vigorous deep convection, following by a rapid growth of relative vorticity in the lower troposphere, takes place once the dynamic forcing associated with a mesoscale plateau disturbance was positioned over the western stagnation region. Our principle result is that the warm heavy rain vortex in this can study is triggered by a migratory plateau boundary layer disturbance and basically driven by cumulus convective heating. The thermal influence of the elevated plateau topography may appreciably affect the vortex initiation through changing the intensity of the forcing associated with the triggering mechanism.

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