GFDL - Geophysical Fluid Dynamics Laboratory

Dynamics of Winter Storms

The Blizzard of 1993

I. Orlanski and J. Sheldon: Stages in the Energetics of Baroclinic Systems; Tellus, 1993

Advanced storm diagnostics developed at GFDL provide important insights into the evolution of intense storms, such as the so-called “Blizzard of ’93”, which roared up the east coast of the U.S. on 13-14 March 1993, setting new records in terms of snowfall, temperatures, and sea-level pressures. The figure bellow illustrates some of these diagnostics as applied to a simulation of the Blizzard using the GFDL mesoscale dynamical model. The lower surface shows the model terrain, and the red contours depict sea level pressure. The color-shaded plane above shows a measure of the energy of the storm at a level approximately 9 km above ground. The vectors in the plane indicates a transfer of energy from upstream disturbances to the energy center associated with the develop ment of the surface cyclone.

Abstract:

The results from several idealized and case studies are drawn together to form a comprehensive picture of “downstream baroclinic evolution” using local energetics. This new viewpoint offers a complementary alternative to the more conventional descriptions of cyclone development. The development of a system’s energetics is divided into three stages. In Stage 1, a pre-existing disturbance well upstream of an incipient trough loses energy via ageostrophic geopotential fluxes directed downstream through the intervening ridge, generating a new energy center there. In Stage 2, this new energy center grows vigorously, at first due to the convergence of these fluxes, and later by baroclinic conversion as well. As the center matures, it begins to export energy via geopotential fluxes to the eastern side of the trough, initiating yet another energy center there. In Stage 3, this new energy center continues to grow while that on the western side decays due to a dwindling supply of energy via fluxes from the older upstream system and also as a consequence of its own export of energy downstream. As the eastern energy center matures, it exports energy further downstream, and the sequence begins anew. These insights are made possible largely because the local energetics approach permits one to define an energy flux vector which accurately describes the direction of energy dispersion and quantifies the role of neighboring systems in local development.

The U.S. “Blizzard of ’93” is used as a new case study to test the limits to which this conceptual sequence might apply, as well as to augment the limited set of case studies. It is shown that, despite the extraordinary magnitude of the event, the evolution of the trough associated with the Blizzard fits the conceptual picture of downstream baroclinic evolution quite well, with geopotential fluxes playing a critical role in three respects. First, fluxes from an old, decaying system in the Pacific were convergent over the west coast of North America, creating a kinetic energy center there and modifying the jet, resulting in a large extension of the overall kinetic energy center well into Mexico. Second, energy fluxes from this extension of the northwesterly flow were strongly convergent east of the trough, producing explosive growth of kinetic energy over the northwestern Gulf of Mexico, with baroclinic conversion following shortly thereafter. Lastly, the kinetic energy generated by the vigorous baroclinic conversion in the cold advection on the west side of the trough was very effectively transferred to the energy center on the east side of the trough via geopotential fluxes

Mountains

A Cold Front Interacting with a Mountain Ridge

I. Orlanski and B. Gross: Orographic Modification of Cyclone Development; J. A. S 1994

Simulations of a stationary cold front interacting with a mountain ridge suggest that oro graphic 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.

Abstract:

The orographic modification of cyclone development is examined by means of primitive equation model simulations. When a mature baroclinic wave impinges on an east-west oriented mountain ridge. This cyclone extends throughout the depth of the tropopause 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.

Storm Tracks

Animation of Time Lag Regression from -48h to +48h

I. Orlanski and B. Gross: Baroclinic lifecycles in a Storm track environment; J.A.S 2000

Relative Vorticity as inferred from Regression Analysis (-48h to +48h lag). A series of numerical experiments being used to analyze the observed interaction between high-frequency eddies and the mean flow in idealized storm tracks. In these simulations, baroclinic eddies evolve within a baroclinic jet that is forcibly maintained within a small zonally confined region. A time-lag regression has been applied to the last 200 days of the model integrations in order to establish a composite view of the wave breaking process.

An analysis of the cyclone environment at zero lag is shown in the figure, in which isosurfaces of cyclonic relative vorticity, the surface potential temperature and wind vectors, and upper level energy fluxes are displayed.

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.

Seasons

Storm Track of the Northern Hemisphere

I. Orlanski: Poleward Deflectin of Storm tracks; Journal of Atmospheric Sciences 1998 .

An analysis of eleven years of ECMWF 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.

a) Winter Storm Track; Results show that pattern of the forcing by the high frequency (HF) eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primary responsible for the location of the trough-ridge system associated with the stationary flow. The results also clarify the role of u’v’ and 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 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 posses 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 west ward from its November position and increasing the trough-ridge forcing by the HF eddies.

El Nino/La Nina

The effect of El Nino and La Nina on cyclone trajectories

I. Orlanski: Poleward Deflectin of Storm tracks; Journal of Atmospheric Sciences 1998.

Wintertime cyclonic energy defines the Storm tracks and is seen to extend farther eastward through the Pacific Ocean during Warm Events (El Nino years), but display an abrupt termination during Cold Events (La Nina years). For warm events, the storm trajectories are from west to east over the Southern US. However, in cold events the trajectories are from Western Canada in a south-easterly direction.

Abstract:

An analysis of eleven years of ECMWF 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 pattern of the forcing by the high frequency (HF) eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primary responsible for the location of the trough-ridge system associated with the stationary flow. The results also clarify the role of u’v’ and 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 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 posses 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(Figure). 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 west ward from its November position and increasing the trough-ridge forcing by the HF 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 PNA 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 ENSO phase.