Bibliography - Isidoro Orlanski

1. Orlanski, Isidoro, 2007: The rationale for why climate models should adequately resolve the mesoscale In High Resolution Numerical Modelling of the Atmosphere and Ocean, New York, NY, Springer-Verlag, 29-44.
   [ Abstract ]

   A review of the importance of the cyclone-frontal scale system in climate variability and the ability of present climate models to simulate them has been presented. The analysis of three different Climate models, GISS, the NCAR community climate model CCM3, and the GFDL Finite volume AM2 (M90), have been discussed. The intention here was not to determine which one is better but rather to indicate what deficiency may be common to all oif them. Evidence shows that the three models tend to be deficient in the generation of cyclone wave activity with the consequences that heat, momentum, and moisture may be deficient in the extratropical and subpolar regions. This will affect cloudiness, wind stress, and precipitation. Bauer and Del Genio (2005) have shown that the deficiency of moisture and cloudiness over the subpolar regions was due to the lack of cyclone waves to transport moisture and clouds to these regions. A discussion of complementary work done on clustering of cyclone trajectories by Gaffney et al. (2005) was also presented. Consistent with the present analysis, this study also showed that differences in trajectories between reanalysis and model simulation for each cluster of trajectories were here interpreted to be related to the lack of intense high frequency eddies of the GCM. The previous two studies depend on the surface characteristics based on trajectories of the high frequency eddies. The present analysis on the GFDL-GCM is totally eulerian and based on the upper level eddy activities (300 mb). However, a similar conclusion has been drawn from the analysis of the band pass frequency of energy and momentum for the GFDL AM2.M90 17 year runs, where it is quite clear that the momentum and energy of the very high frequency is much lower in the model simulation than in the reanalysis. The variance of meridional velocity also shows that the deficiency of the high frequency is in the latitude area where the reanalysis shows it to be positioned in the storm track: the model displaces it south of that. There is also a suggestion that to achieve the correct intensity of the high frequency baroclinic eddies, models should have enough resolution to resolve them, since this intensity depends on the lower level circulation of the frontal circulation system. The mesoscale circulation associated with cyclones could be adequately represented in models with resolution equal or superior to 1/4° resolution. It is clear that to adequately resolve the mesoscale, it is necessary to not only improve the resolution but also to improve the boundary layer and surface fluxes. Clearly, at the present low resolution of climate models, this improvement is probably unattainable. However, if the cloudiness and sea ice over the subpolar regions are important to the overall climate, this should be an attainable goal because no sophistication in the moist convection or sea-ice model could correct those deficiencies due to the unresolved dynamics.

2. Orlanski, Isidoro, and C Kerr, 2007: Project TERRA: A glimpse into the future of weather and climate modeling In High Resolution Numerical Modelling of the Atmosphere and Ocean, New York, NY, Springer-Verlag, 45-50.
   [ Abstract ]

   One major challenge in obtaining useful numerical simulations of weather and climate is addressing the sensitivity of these simulations to the characteristics and distribution of clouds in the model(s). Latent heat release produced in clouds as a consequence of moist convection can dramatically affect the dynamics that govern the development of larger scale weather systems and storm tracks. Also, given the profound effects of cloud distribution on the radiative characteristics of the atmosphere, these interactions critically affect the models' climate and thus our conclusions regarding climate change. A very high resolution global model has recently been run at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) to investigate the potential value of cloud-resolving numerical models to weather forecasts and climate simulations. Dubbed "Project TERRA", this experiment was conceived as an experimental 1-day simulation with GFDL's ZETAC model.

3. Rivière, G, and Isidoro Orlanski, January 2007: Characteristics of the Atlantic storm-track eddy activity and its relation with the North Atlantic Oscillation. Journal of the Atmospheric Sciences, 64(2), doi:10.1175/JAS3850.1.
   [ Abstract ]

   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.

4. Orlanski, Isidoro, 2005: A new look at the Pacific storm track variability: sensitivity to tropical SST's and to upstream seeding. Journal of the Atmospheric Sciences, 62(5), 1367-1390.
   [ Abstract PDF ]

   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 contrrast, 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.

5. Horowitz, Larry, S Walters, D L Mauzerall, L K Emmons, P J Rasch, C Granier, X Tie, J F Lamarque, M Schulz, G S Tyndall, Isidoro Orlanski, and G P Brasseur, 2003: A global simulation of tropospheric ozone and related tracers: description and evaluation of MOZART, version 2. Journal of Geophysical Research, 108(D24), 4784, doi:10.1029/2002JD002853.
   [ Abstract PDF ]

   We have developed a global three-dimensional chemical transport model called Model of Ozone and Related Chemical Tracers (MOZART), version 2. This model, which will be made available to the community, is built on the framework of the National Center for Atmospheric Research (NCAR) Model of Atmospheric Transport and Chemistry (MATCH) and can easily be driven with various meteorological inputs and model resolutions. In this work, we describe the standard configuration of the model, in which the model is driven by meteorological inputs every 3 hours from the middle atmosphere version of the NCAR Community Climate Model (MACCM3) and uses a 20-min time step and a horizontal resolution of 2.8° latitude × 2.8° longitude with 34 vertical levels extending up to approximately 40 km. The model includes a detailed chemistry scheme for tropospheric ozone, nitrogen oxides, and hydrocarbon chemistry, with 63 chemical species. Tracer advection is performed using a flux-form semi-Lagrangian scheme with a pressure fixer. Subgrid-scale convective and boundary layer parameterizations are included in the model. Surface emissions include sources from fossil fuel combustion, biofuel and biomass burning, biogenic and soil emissions, and oceanic emissions. Parameterizations of dry and wet deposition are included. Stratospheric concentrations of several long-lived species (including ozone) are constrained by relaxation toward climatological values. The distribution of tropospheric ozone is well simulated in the model, including seasonality and horizontal and vertical gradients. However, the model tends to overestimate ozone near the tropopause at high northern latitudes. Concentrations of nitrogen oxides (NOx) and nitric acid (HNO3) agree well with observed values, but peroxyacetylnitrate (PAN) is overestimated by the model in the upper troposphere at several locations. Carbon monoxide (CO) is simulated well at most locations, but the seasonal cycle is underestimated at some sites in the Northern Hemisphere. We find that in situ photochemical production and loss dominate the tropospheric ozone budget, over input from the stratosphere and dry deposition. Approximately 75% of the tropospheric production and loss of ozone occurs within the tropics, with large net production in the tropical upper troposphere. Tropospheric production and loss of ozone are three to four times greater in the northern extratropics than the southern extratropics. The global sources of CO consist of photochemical production (55%) and direct emissions (45%). The tropics dominate the chemistry of CO, accounting for about 75% of the tropospheric production and loss. The global budgets of tropospheric ozone and CO are generally consistent with the range found in recent studies. The lifetime of methane (9.5 years) and methylchloroform (5.7 years) versus oxidation by tropospheric hydroxyl radical (OH), two useful measures of the global abundance of OH, agree well with recent estimates. Concentrations of nonmethane hydrocarbons and oxygenated intermediates (carbonyls and peroxides) generally agree well with observations.

6. Orlanski, Isidoro, 2003: Bifurcation in eddy life cycles: Implications for storm track variability. Journal of the Atmospheric Sciences, 60(8), 993-1023.
   [ Abstract PDF ]

   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 ν' ξ' 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.

7. Orlanski, Isidoro, and Brian D Gross, 2000: The life cycle of baroclinic eddies in a storm track environment. Journal of the Atmospheric Sciences, 57(21), 3498-3513.
   [ Abstract PDF ]

   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 simultaneious 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.

8. Orlanski, Isidoro, 1998: Poleward deflection of storm tracks. Journal of the Atmospheric Sciences, 55(16), 2577-2602.
   [ Abstract PDF ]

   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 u'v' and (v'2 - u'2) in the column-averaged vorticity forcing. The simpler term u'v' 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 (v'2 - u'2) 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 (v'2 - u'2) 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.

9. Chang, E K., and Isidoro Orlanski, 1994: On energy flux and group velocity of waves in baroclinic flows. Journal of the Atmospheric Sciences, 51(24), 3823-3828.
   [ Abstract PDF ]

   A modified energy flux is defined by adding a nondivergent term that involves beta to the traditional energy flux. The resultant flux, when normalized by the total eddy energy, is exactly equal to the group velocity of Rossby waves on a beta plane with constant zonal flow. In this study, we computed the normalized energy flux for linear wave packets in baroclinic basic states with different vertical profiles. The results show that the normalized energy flux is a good approximation to the group velocity of all parts of the wave packet for the basic states examined. The extension to the nonlinear case is briefly discussed. The magnitude of the fluxes of a downstream developing wave group over the wintertime northern Pacific storm track defined by a regression analysis is computed, and the group velocity defined by the energy fluxes is found to be comparable to the group velocity of propagation of the observed wave packet. The results indicate a very strong component of downstream energy radiation, suggesting that downstream energy dispersion is very important to the evolution of waves in the storm track.

10. Orlanski, Isidoro, and Brian D Gross, 1994: Orographic modification of cyclone development. Journal of the Atmospheric Sciences, 51, 589-611.
    [ Abstract PDF ]

    The orographic modification of cyclone development is examined by means of primitive equation model simulations. When a mature baroclinic wave impinges on the east-west oriented mountain ridge, a relatively intense cyclone forms on the south side of the ridge. This cyclone extends throughout the depth of the troposphere and possesses relatively small vertical tilts, large velocities, and strong temperature perturbations compared to classical baroclinic eddies. The vorticity growth in the orographic cyclone center is larger than that of baroclinic eddies that grow over flat terrain. However, there is no absolute instability associated with this orographic enhancement. A longer ridge produces a more intense eddy The behavior of small-amplitude normal modes on a zonally symmetric mountain ridge shows that baroclinic development is enhanced where the topography slopes in the same direction as the isentropes. This is consistent with earlier studies using uniform slopes that show that the heat flux forced by this terrain enhances the conversion of available potential energy. It is shown that the structure of nonlinear waves is similar to that of linear modes over a mountain ridge with steep slopes, in which the cross-ridge flow and the associated heat flux are partially blocked by the mountain. Simulations of a stationary cold front interacting with a mountain ridge suggest that orographic cyclogenesis is triggered when the mountain ridge locally modifies the frontal circulation as it impinges on the ridge. Warm southerly flow in the front is diverted westward by the mountain ridge, intensifying the strong hydrostatic pressure gradient between the mountain anticyclone and the developing cyclone to the south. In contrast, cold northerly flow is diverted eastward as it approaches the mountain and effectively broadens the mountain anticyclone toward the north. This produces the characteristic pressure dipole observed in orographic cyclogenesis. It is concluded that mature baroclinic eddies approaching the mountain ridge should have a strong frontal zone with a considerable temperature contrast and strong circulation for an intense response.

11. Orlanski, Isidoro, and E K M Chang, 1993: Ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves. Journal of the Atmospheric Sciences, 50(2), 212-225.
    [ Abstract PDF ]

    With the use of a simple primitive equation model, it is demonstrated that the convergence/divergence of ageostrophic geopotential fluxes can be a major source/sink of kinetic energy for both downstream and upstream development of baroclinic waves, and can play a dominant role during the early stages of wave development. It is also shown that both surface friction and beta effects lead to an asymmetry in the upstream versus downstream development, with downstream development much stronger. A total group velocity is defined based on ageostrophic fluxes, and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed.

12. Orlanski, Isidoro, and John P Sheldon, 1993: A case of downstream baroclinic development over western North America. Monthly Weather Review, 121, 2929-2950.
    [ Abstract PDF ]

    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 heat 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 the convergence of geopotential fluxes revealed that early growth of the system was dominated by flux convergence. These findings are in agreement with the results of previous 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 by lower-level baroclinic conversion comprise a 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 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 maturity, although this process was somewhat delayed in the control case because of larger fluxes resulting from 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.

13. Orlanski, Isidoro, 1992: Atmospheric fronts In Encyclopedia of Earth System Science, Vol. 1, San Diego, CA, Academic Press, 201-216.
14. Orlanski, Isidoro, and J J Katzfey, 1991: The life cycle of a cyclone wave in the Southern Hemisphere. Part I: Eddy energy budget. Journal of the Atmospheric Sciences, 48(17), 1972-1998.
    [ Abstract PDF ]

    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 was compensated for by energy flux divergence (dispersion of energy), mainly of the ageostrophic geopotential flux. 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.

15. Orlanski, Isidoro, J J Katzfey, C Menendez, and M Marino, 1991: Simulation of an extratropical cyclone in the Southern Hemisphere: model sensitivity. Journal of the Atmospheric Sciences, 48(22), 2292-2311.
    [ Abstract PDF ]

    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.

16. Orlanski, Isidoro, M Marino, C Menendez, and J J Katzfey, 1989: The role of cyclones in the daily variability of Antarctic ozone In Third International Conference on Southern Hemisphere Meteorology and Oceanography, Boston, MA, American Meteorological Society, 416-420.
17. Orlanski, Isidoro, and J J Katzfey, 1987: Sensitivity of model simulations for a coastal cyclone. Monthly Weather Review, 115(11), 2792-2821.
    [ Abstract PDF ]

    A nested global, limited-area model was used to predict the Presidents' Day cyclone of 18-19 February 1979. Both a low (~150 km) and a high (~50 km) horizontal resolution version were used. The model has full physics with a planetary boundary layer, moisture, moist convective adjustment, and radiation. The low-resolution model, using a global analysis for initial and boundary conditions (termed a simulation), was able to capture the general development and movement of the cyclone. Some discrepancies were noted for the intensity of upper-air features between the analyses and the model solution during the first 24 hours. The primary focus of this paper is to determine the effect of initial and boundary conditions, as well as model parameterizations on the accuracy of the predictions. The evolution of the storm is discussed with an emphasis on the quality of the numerical simulation. The impact of the initial conditions on the model solution was tested by using four different global analyses. It was found that the variability between the solutions was less than the variability between the analyses. Varying the horizontal diffusion in the model produced stronger development with weaker diffusion, but the character of the development did not change significantly. The sensitivity of the simulation to latent heat was tested by running the model without latent heating. A low did develop in this model solution, although it was much weaker and it did not develop vertically as in the cases with latent heating. The most significant improvement in accuracy in this sensitivity study occurred when the horizontal resolution was increased from 1.25° x 1.0° (~150 km) to 0.4° x 0.32° (~50 km). The position and intensity of the surface low were much closer to reality, as indicated by comparison with a mesoanalysis and to satellite pictures. The nested model was also run in forecast mode with boundary conditions for the limited-area model supplied by the (Geophysical Fluid Dynamics Laboratory) GFDL global spectral model forecast. In general, the quality of the limited-area forecast compared very well with the simulations. The overall character and intensity of the development were similar. The role of lateral boundary conditions was demonstrated by comparing forecasts and simulations with identical initial conditions. The results suggest the increasing importance of the boundary data with time in the limited-area forecast and show high correlation between the errors in the limited-area forecast and the global forecast within the limited-area domain.

18. Wang, B, and Isidoro Orlanski, 1987: Study of a heavy rain vortex formed over the eastern flank of the Tibetan Plateau. Monthly Weather Review, 115(7), 1370-1393.
    [ Abstract PDF ]

    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 degree 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, while 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, followed 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 case 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.

19. Orlanski, Isidoro, 1986: Localized baroclinicity: a source for meso-a cyclones. Journal of the Atmospheric Sciences, 43(23), 2857-2885.
    [ Abstract ]

    An investigation has been made using a two-dimensional model to solve the initial value problem describing the evolution of disturbances on a mean baroclinic state. Three main problems are considered: the effect of static stability on meso-baroclinic waves in a periodic domain; downstream instability in an open domain and the effect of surface sensible heat; and the effect of moisture on these unstable waves. It was found that a flow can be unstable to mesoscale baroclinic waves. The requirement for instability of wavelengths less than 1000 km is similar to that for the planetary quasi-geostrophic baroclinic waves. The Rossby penetration height can be derived from the solution of the unstable waves as these unstable waves will only be sensitive to the baroclinicity of the atmosphere in a layer with a depth delta. The characteristics of the finite-amplitude unstable waves suggest that the limiting amplitude for the baroclinic waves is achieved by an energy cascade to frontal scales. Perhaps the most signicant finding of this study has been to demonstrate the importance of localized surface heating in producing the more intense development of short baroclinic waves. It was found that waves in the presence of surface heating grew twice as fast as those without. These waves, having a depth on the order of the boundary layer and horizontal scales of a few hundred kilometers, can organize convergence of surface moisture on these scales. With the addition of moisture, the shallow meso-baroclinic wave will explosively develop into a deep intense system.

20. Orlanski, Isidoro, and Bruce B Ross, 1986: Low-level updrafts in stable layers forced by convection. Journal of the Atmospheric Sciences, 43(10), 997-1005.
    [ Abstract PDF ]

    An investigation is made of the stability of a convectively unstable atmosphere in the presence of a stably stratified layer beneath, which is moving with a constant velocity relative to the upper air. This work is an extension of the linear model presented as part of the recent study of Orlanski and Ross in which they sought to explain the structure of their simulated squall line. A stability analysis shows that two modes are possible: 1) the gravitational or convective mode due to the unstable stratification in the upper layer which modifies the stable region below and 2) the classical Kelvin-Helmholtz mode due to shear across the interface. The Kelvin-Helmholtz mode is of limited physical interest in this case. On the other hand, the gravitational mode produces an updraft structure similar to updrafts in the stable lower layer of a convective system. Analysis of the horizontal displacement of the surface convergence for this mode relative to the convergence in the convective zone shows this displacement to depend primarily on the wind, stratification, and depth of the stable lower layer. The resulting relationship provides a method for determining whether a dual or single updraft will occur in a convective system.

21. Orlanski, Isidoro, Bruce B Ross, Larry J Polinsky, and R Shaginaw, 1985: Advances in the theory of atmospheric fronts. Advances in Geophysics, 28B, 223-252.
    [ PDF ]
22. Orlanski, Isidoro, and Bruce B Ross, 1984: The evolution of an observed cold front. Part II: mesoscale dynamics. Journal of the Atmospheric Sciences, 41(10), 1669-1703.
    [ Abstract PDF ]

    A detailed analysis is made of a three-dimensional numerical simulation of the evolution of an observed moist frontal system over a 48 h period. The simulated front undergoes an initial period of frontogenetic growth, characterized by an alignment of vertical vorticity and horizontal convergence near the surface. The front then evolves to a mature, quasi-steady state as the line of maximum convergence moves ahead of the maximum vorticity. This phase shift is shown to be the result of a negative feedback mechanism which inhibits further vorticity growth while reducing the amount of viscous damping required to achieve a steady state. The influence of viscosity and surface drag upon this mechanism is also assessed. When moisture is included in the numerical solution, the squall line which develops along the front exhibits a dual updraft structure with low-level convergence near the nose of the front and midlevel convergence located 100 km to the rear at a height of 3 km. This configuration is very similar to that found by Ogura and Liou in their analysis of an Oklahoma squall line not associated with a cold front. Analysis of the equations of motion within the convective zone of the mature squall line shows the diabatic heating to be closely balanced by adiabatic cooling due to vertical temperature advection. As a result, the only net warming within this region occurs as adiabatic warming in the clear air outside of the cloud zone. A linear, two-layer, dry model containing stable lower and unstable upper layers is shown to reproduce the dual updraft structure for certain low-level wind intensities without requiring microphysics. Also, for all wind conditions, this simple model produces strong convergence at the interface between the two layers. This suggests that the occurrence of a convergence maximum at the level of free convection should be a common feature of convectively unstable cloud systems.

23. Orlanski, Isidoro, and Larry J Polinsky, 1984: Predictability of mesoscale phenomena In International Symposium on Nowcasting II: Mesoscale Observations and Very-Short-Range Forecasting, Noordwijk, Netherlands, ESA Scientific & Technical Publications Branch, 271-280.
    [ Abstract ]

    A number of simulations with a high-resolution three-dimensional primitive equation model were conducted to assess the impact of initial and boundary data inaccuracies for the simulation of mesoconvective systems and their environments. Attention has been given to the simulations of Pacific comma clouds, frontal squall lines, mesoconvective complexes, and coastal cyclogenesis; all these cases are from FGGE year 1979. The sensitivity of these simulations to surface boundary layer variables and the cloud fraction (latent heat parameterization) has been investigated. Considerable success has been achieved in those simulations. It has been found that, in most mesoconvective systems, the environmental convergence of a preexisting front is responsible for the growth of the storm's vorticity, whereas moisture is essential for its explosive evolution.

24. Orlanski, Isidoro, and Larry J Polinsky, 1983: Ocean response to mesoscale atmospheric forcing. Tellus A, 35A(4), 296-323.
    [ Abstract PDF ]

    Many processes have been proposed as possible forcing mechanisms for mesoscale oceanic variability. The present study shows that atmospheric forcing can be an important source of mesoscale variability in the ocean. We show that the response is linearly proportional to the product of the time scale of the storm and its intensity. We clarify the point that for storms with scales considerably smaller than the barotropic Rossby radius of deformation, the oceanic stratification and the horizontal extent of the storm are the only factors determining the penetration depth of the response, implying that it is not the Rossby radius of deformation but rather the scale of penetration depth (h = (f/N)L) that characterizes the response. In exploring the effect of differing eddy-viscosity parameterization on oceanic-response, we find no significant qualitative differences, although as one might expect we find quantitative differences in the results. The role of the mixed layer is considered very important in the transfer of surface stresses down into the system. The mixed layer does not seem to be important in determining the characteristic lengths of the problem, however, at least for storms that give a penetration depth considerably larger than the mixed layer (for a mixed layer on the order of 20 m, the storm should be larger than a few kilometers). The non-linear advection terms seem to affect the adjustment process more by reducing the associated wave energy than by modifying the characteristics of the geostrophic response. Finally, making the stratification more realistic has no significant impact on the resulting oceanic response.

25. Cerasoli, C P., and Isidoro Orlanski, 1982: Resonant and nonresonant wave-wave interactions for internal gravity waves In Computational Methods and Experimental Measurements: Proceedings of the International Conference, New York, NY, Springer-Verlag, 228-239.
26. Ross, Bruce B., and Isidoro Orlanski, 1982: The evolution of an observed cold front. Part I: Numerical simulation. Journal of the Atmospheric Sciences, 39(3), 296-327.
    [ Abstract PDF ]

    The 48 h evolution of an observed cold front is simulated by a three-dimensional mesoscale-numerical model for a typical springtime synoptic situation over the southeastern United States. The model used in this study employs anelastic equations of motion on a limited-area domain with locally determined inflow/outflow side boundaries. Both the observed and simulated characteristics of the weather system indicate a mature front which intensifies and then weakens over the 48 h period. Moist convection occurs in the form of intermittent squall lines in the observed case; in the numerical simulation, convection develops above and somewhat ahead of the surface front after 24 h as an ensemble of convective cells. An investigation is made of the mesoscale and subsynoptic-scale features of this solution to determine their sensitivity to the inclusion of moisture and to the magnitude of the eddy viscosity used in the numerical simulation. The primary effect of increased eddy viscosity is to reduce somewhat the propagation speed of the front. The major changes due to moisture inclusion occur when convection develops along the cold front; these convective effects, which are apparent in the subsynoptic as well as the mesoscale features of the solution, include increased low-level convergence, reduced surface pressure due to diabatic heating, and the deflection of winds due to upper-level divergence. In addition, small temperature changes occur in the middle troposphere between the jet stream and the surface front when either viscosity or moisture is varied; these disturbances are a clear manifestation of the effect which changes in the cross-stream circulation intensity have upon the frontal system. A fundamental feature of the mesoscale structure of the front in all cases is the tendency of the line of maximum horizontal convergence at the surface to move ahead of the line of maximum vertical vorticity. This phase shift appears to be related to the propagation characteristics of the frontal system. Also, the mesoscale moist convection develops a cellular structure throughout the convective zone in the low-viscosity solution; the use of higher viscosity tends to suppress these cells, particularly near the surface.

27. Orlanski, Isidoro, 1981: The quasi-hydrostatic approximation. Journal of the Atmospheric Sciences, 38(3), 572-582.
    [ Abstract PDF ]

    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.

28. Orlanski, Isidoro, and C P Cerasoli, 1981: Energy transfer among internal gravity modes: weak and strong interactions. Journal of Geophysical Research, 86(C5), 4103-4124.
    [ Abstract ]

    The general characteristics of the energy spectrum for internal gravity waves in the ocean are well known from the large body of recent experimental observations. The theoretical understanding has not developed at the same rate, perhaps due to the limitation of linear or quasi-linear theories, which can cope only with weak interaction processes and are inadequate for representing the more violent and sporadic wave breaking processes present in nature. A detailed study of energy transfer among two-dimensional internal gravity modes in a fully nonlinear regime was performed. Wave-wave interactions and overturning were included in the solutions of a two-dimensional numerical model, and the results are presented here. A background spectrum of finite amplitude, random internal gravity wave field was generated by a long-time integration of a two-dimensional model with random body forcing. Over this background field, two sets of experiments were performed: spike-random, where energy at low, medium, and high wave numbers were introduced and integrated in time, and band-random, where energy was introduced over a band of wave numbers instead of introducing only discrete modes. The results can be summarized as follows. Multiple triad interactions will result in a filling of the energy spectrum when energy is introduced in a particular band of wave numbers. For bands where the energy level is high enough to result in nonlinear time scales of only a few intrinsic periods, wave-wave interactions (resonant and nonresonant) provide the mechanism for filling the spectrum. The energy transfer becomes more and more rapid with increasing energy, and no universal spectrum appears to result from these processes. As the energy input increases, energy will accumulate in high wave numbers until localized instabilities (overturning) occur. From that point on, these high wave numbers will remain at a saturation such that any additional energy input at the saturated band, either directly or via wave-wave interactions, will result in localized mixing. On the other hand, additional energy input at bands other than the saturated band will result in an increase of low and medium wave band energy (via wave-wave interactions) until an equilibrium level is achieved. The equilibrium level of any particular band will depend on the high wave number bands being saturated. For instance, any energy above the equilibrium at low wave numbers will produce localized mixing in physical space almost instantaneously. This does not mean that the low wave numbers are saturated, as their energy levels can be much lower than a saturation level. What takes place at or near an equilibrium level is that the contributions from high and low wave numbers result in localized regions in physical space where the criterion for instability is almost met. In fact, this superposition effect means that low and medium wave numbers are far from meeting any breaking criterion when taken individually, yet cannot tolerate any additional input energy when in the presence of a saturated band of high wave numbers. It was found also that the dissipation is approximately constant over the wave numbers and small compared with the large transfer of energy between neighboring waves. However, if bands of waves are considered, very little energy is transferred between neighboring bands above the equilibrium level. Rather, a direct cascade of energy from low to high wave numbers occurs due to localized instabilities which result in overturning, and it is this amount of energy flux which is dissipated by the high wave numbers.

29. Sun, W-Y, and Isidoro Orlanski, 1981: Large mesoscale convection and sea breeze circulation. Part I: linear stability analysis. Journal of the Atmospheric Sciences, 38(8), 1675-1693.
    [ Abstract PDF ]

    The interactions between the sea breeze circulation and trapeze instability are investigated using a set of linearized equations. The results show that mesoscale waves associated with trapeze instability can be easily triggered by the sea breeze circulation and can propagate far inland, but no mesoscale waves are observed over the ocean. The wavelength is a few hundred kilometers. The period of the waves, which depends on the Coriolus parameter, eddy viscosity and the strength of land-sea contrast, can be either one day or two days, or a combination of one and two days. The waves obtained here are similar to the cloud bands observed in West Africa and South America.

30. Sun, W-Y, and Isidoro Orlanski, 1981: Large mesoscale convection and sea breeze circulation. Part II: Nonlinear numerical model. Journal of the Atmospheric Sciences, 38(8), 1694-1706.
    [ Abstract PDF ]

    The nonlinear equations are applied to study the interactions between the sea breeze circulation and trapeze instability in low latitudes. The well-developed sea breeze circulation produced by the nonlinear model at coast is not so affected by the trapeze instability as shown in Part I of this study. However, the waves over the land are quite similar to those of Part I, buth with a smaller growth rate. This study suggests that a strong diurnal temperature variation over the land and/or the latent heat may be required to produce the observed cloud bands in the tropical continental region.

31. Orlanski, Isidoro, and C P Cerasoli, 1980: Resonant and non-resonant wave-wave interactions for internal gravity waves In Marine Turbulence, Elsevier Oceanography Series, 28, Amsterdam, Elsevier Science Publishers, 65-100.
    [ Abstract ]

    A detailed study of energy transfer among two-dimensional internal gravity modes in a fully non-linear regime was performed. A number of techniques were used: They were (i) solutions of the gyroscopic equations with three and four waves. (ii) integration of a finite difference numerical model, and (iii) laboratory experiments. The solution of the four wave gyroscopic equations differed dramatically from the three wave case, and the four wave solutions were aperiodic. In both cases, the non-linear interaction time scale, was found to be inversely proportional to the square root of the total wave energy, even when the weak interaction assumption was violated. Integration of a finite difference numerical model showed that triad evolution was greatly affected when many waves other than the primary triad components could be excited. Initial condition experiments for triad evolution were performed with either a quiescent background state or a random field of waves, and the final states were similar, although the time to reach steady state was short when a background field was present. The numerical model was used to simulate surface forced, resonant modes and results were compared to laboratory experiments. Good agreement was found, not only in the initial wave evolution but also in energy level of the final states. An equilibrium state was achieved in both types of experiments, and wave-wave interactions and wave breaking were important in the energy distribution. The numerical model was used to create a random, finite amplitude internal wave field, and a set of experiments whereby this basic state initially perutrbed was performed. In these experiments energy was introduced over bands of low, medium and high wavenumbers. The results show that when the basic state energy is low and non-linear time scales are much greater than intrinsic wave periods, multiple triad interactions account for the distribution of any input energy. As the energy level increases, the high wavenumbers become saturated and localized overturning provides the dissipation mechanism. Additional energy input to low and medium wavenumbers will eventually result in an equilibrium state, whereby any extra energy input will result in very rapid, localized overturning. This equilibrium level depends on the presence of saturated high wavenumbers and once achieved, the system is very inefficient at transferring energy via wave-wave interactions while very efficient at dissipating energy via localized overturning.

32. Orlanski, Isidoro, and Bruce B Ross, 1980: Reply. Journal of the Atmospheric Sciences, 37(4), 909-911.
    [ PDF ]
33. Orlanski, Isidoro, and Bruce B Ross, 1979: 3-D simulation of a frontal squall line: Sensitivity studies In Eleventh Conference on Severe Local Storms, Boston, MA, American Meteorological Society, 557-558.
34. Ross, Bruce B., and Isidoro Orlanski, 1978: The circulation associated with a cold front. Part II: Moist case. Journal of the Atmospheric Sciences, 35(3), 445-465.
    [ Abstract PDF ]

    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 conditons 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 intensities are present. The stonger 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.

35. Orlanski, Isidoro, and Larry J Polinsky, 1977: Spectral distribution of cloud cover over Africa. Journal of the Meteorological Society of Japan, 55(5), 483-493.
    [ Abstract PDF ]

    An analysis is made of the spectral characteristics of the cloud cover observed over Africa for a period of three months. The results indicate the predominance of a 2-2.7 day spectral peak within the vicinity of the Equator (10 degrees N - 10 degrees S) with the intensity of this peak much stronger over land than over the ocean. The peak itself may not be detected if the smallest resolved area used in the data analysis is too large. Coherence was found to be maximum in belt-like configurations along certain latitude bands. The phase difference, although very noisy, indicates a horizontal scale on the order of 2000 kilometers.

36. Orlanski, Isidoro, and Bruce B Ross, 1977: The circulation associated with a cold front. Part I: Dry case. Journal of the Atmospheric Sciences, 34(10), 1619-1633.
    [ Abstract PDF ]

    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.

37. Orlanski, Isidoro, 1976: A simple boundary condition for unbounded hyperbolic flows. Journal of Computational Physics, 21, 251-269.
    [ Abstract PDF ]

    A Sommerfeld radiation condition is proposed for problems requiring a prescribed open boundary. The equations must be hyperbolic in nature (although the author believes that they may also be good for some elliptic and parabolic problems). It is proven that the proposed condition was shown to be free of reflection for single wave propagation. Two severe tests were used to demonstrate the applicability of the open boundary condition: (i) the collapsing bubble, a dynamic event which excites many different internal gravity waves. The results show minimum distortion. (ii) The spatially growing K-H instability. This test differs from the previous one in that the only waves excited are those corresponding to the maximum unstable wavelengths. In this case, the maximum amplitude is reached at the open boundary. As it has been shown, the open boundary condition produces minimum distortion.

38. Orlanski, Isidoro, 1976: The trapeze instability in an equatorial b-plane. Journal of the Atmospheric Sciences, 33(5), 745-763.
    [ Abstract PDF ]

    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, b-plane numerical model. It was concluded that the trapeze instability may be the means by which the observed 2-day waves are excited.

39. Delisi, D P., and Isidoro Orlanski, 1975: On the role of density jumps in the reflexion and breaking of internal gravity waves. Journal of Fluid Mechanics, 69(Part 3), 445-464.
    [ Abstract ]

    A laboratory experiment is presented which examines the role of density jumps in the reflexion and breaking of internal gravity waves. It is found that the measured phase shift of the reflected wave and the measured amplitude of the density jump are in good agreement with linear theory. Local overturning occurs when wave amplitude becomes large, and there appears to be a critical amplitutde above which overturning will occur and below which it will not. The overturning seems to be due to local gravitational instability, caused by the horizontal advection of density. Overturning changes the basic flow field in the region of interaction; and it results in smaller-scale motions.

40. Orlanski, Isidoro, 1975: A rational subdivision of scales for atmospheric processes. Bulletin of the American Meteorological Society, 56(5), 527-530.
    [ Abstract PDF ]

    Some atmospheric scale definitions are reviewed and a proposed new subdivision of scales that covers the entire spectrum is described.

41. Orlanski, Isidoro, Bruce B Ross, and Larry J Polinsky, 1975: Reply. Journal of the Atmospheric Sciences, 32(4), 842.
    [ PDF ]
42. Orlanski, Isidoro, Bruce B Ross, and Larry J Polinsky, 1974: Diurnal variation of the planetary boundary layer in a mesoscale model. Journal of the Atmospheric Sciences, 31(4), 965-989.
    [ 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.

43. Orlanski, Isidoro, 1973: Trapeze instability as a source of internal gravity waves. Part I. Journal of the Atmospheric Sciences, 30(6), 1007-1016.
    [ Abstract PDF ]

    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 be on the order of 100 km. This result suggests that some cloud clusters may be originated by this process.

44. Orlanski, Isidoro, and M D Cox, 1973: Baroclinic instability in ocean currents. Geophysical Fluid Dynamics, 4, 297-332.
    [ Abstract PDF ]

    Unstable waves in a western boundary current are investigated in a full three-dimensional, numerical model. A numerical integration is carried out which traces the evolution of a growing wave on an initially uniform current with vertical shear. As indicated in earlier analytic studies based on simpler 2-layer models (Orlanski, 1969) the current is baroclinically unstable for the observed parameter range of the Gulf Stream. Large meanders of the jet in the western boundary current are noticeable within 10 days. Finite amplitude effects, which can be investigated by the numerical model, reduce the growth rate of the disturbance by nearly an order of magnitude compared to linear theory. Comparison with observations indicate that the meanders of the Florida Current between Miami and Hatteras are probably baroclinically unstable waves.

45. Orlanski, Isidoro, and Bruce B Ross, 1973: Numerical simulation of the generation and breaking of internal gravity waves. Journal of Geophysical Research, 78(36), 8808-8826.
    [ Abstract ]

    A numerical model is presented that permits the simulation of stratified fluid phenomena in which gravitationally unstable regions are present. The influence of subgrid scale turbulence generation due to convective instability is parameterized by relating eddy viscosity coefficients to the local Rayleigh number in unstable regions. The model is used to study three different laboratory scale flow problems involving gravity wave generation, wave breaking, and penetrative convection. The numerical solutions show good agreement with available experimental and analytic results as well as with a numerical solution obtained by other investigators.

46. Orlanski, Isidoro, 1972: On the breaking of standing internal gravity waves. Journal of Fluid Mechanics, 54(Part 4), 577-598.
    [ Abstract ]

    A solution has been found for the transient behaviour of resonant growing standing waves by using a perutrbation expansion. Comparison with laboratory experiments as well as a numerical nonlinear solution of the same problem leads to the conclusion that: (i) the transient behaviour and the nonlinear tendency of the standing waves are described well by the analytic expression; (ii) the numerical results describe the solution very well until the wave starts to break; (iii) from the laboratory experiments and the numerical results, the standing internal gravity waves break owing to local gravitational instability at a critical amplitude which is similar to the one predicted by the expansion theory; (iv) the critical amplitude seems to be the maximum amplitude that a wave can reach; (v) when the generation of turbulence is violent, the small eddies begin forcing a secondary flow characterized by layers of strong jets separated by patches of turbulence.

47. Orlanski, Isidoro, 1971: On the breaking of standing internal gravity waves. Journal of Fluid Mechanics, 54(Part 4), 577-598.
    [ Abstract ]

    A solution has been found for the transient behaviour of resonant growing standing waves by using a perutrbation expansion. Comparison with laboratory experiments as well as a numerical nonlinear solution of the same problem leads to the conclusion that: (i) the transient behaviour and the nonlinear tendency of the standing waves are described well by the analytic expression; (ii) the numerical results describe the solution very well until the wave starts to break; (iii) from the laboratory experiments and the numerical results, the standing internal gravity waves break owing to local gravitational instability at a critical amplitude which is similar to the one predicted by the expansion theory; (iv) the critical amplitude seems to be the maximum amplitude that a wave can reach; (v) when the generation of turbulence is violent, the small eddies begin forcing a secondary flow characterized by layers of strong jets separated by patches of turbulence.

48. Orlanski, Isidoro, 1969: The influence fo bottom topography on the stability of jets in a baroclinic fluid. Journal of the Atmospheric Sciences, 26, 1216-1232.
    [ Abstract PDF ]

    The stability of a two-layer model is analyzed using a numerical method taking into account the effect of bottom topography. A jet in geostrophic 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.

49. Orlanski, Isidoro, and Kirk Bryan, 1969: Formation of the thermocline step structure by large-amplitude internal gravity waves. Journal of Geophysical Research, 74(28), 6975-6983.
    [ Abstract ]

    It is suggested that a possible mechanism for the formation of the thermocline step structure is a sporadic overturning by rotors associated with finite-amplitude internal waves. A criterion for the required critical amplitude of an internal wave is derived, and good agreement is found with numerical experiments illustrating the mechanism. A scale analysis for the ocean shows that downward-propagating waves with a vertical wavelength of 10-20 meters would be most favored to 'break' by the convective instability mechanism. Examiniation of velocity spectra measured in the North Atlantic shows that more than enough energy exists in the internal wave frequency range for this type of instability to occur.

50. Orlanski, Isidoro, 1968: Instability of frontal waves. Journal of the Atmospheric Sciences, 25(2), 178-200.
    [ Abstract PDF ]

    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) less than or equal to 3 and Richardson number (Ri) less than or equal to 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 > 1 (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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