Bibliography - Brian D Gross

1. 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.

2. Gross, Brian D., 1997: The effect of compressibility on barotropic and baroclinic instability. Journal of the Atmospheric Sciences, 54(1), 24-31.
   Abstract PDF

   The effect of compressibility on two-dimensional barotropic and baroclinic growth rates is examined by means of a linearized nonhydrostatic compressible model. It is shown that the growth rates are diminished when compressibility is included because perturbation internal energy represents a sink of basic-state kinetic energy when work is done to compress the medium. Nonlinear simulations provided by compressible and incompressible versions of the ZETA model show that the solutions are nearly identical, but the compressible solution develops more slowly than the incompressible one, consistent with the linear analysis

3. Gross, Brian D., 1996: The effects of compressibility on barotropic and baroclinic instability In Research Activities in Atmospheric and Oceanic Modelling, CAS/JSC Working Group on Numerical Experimentation, Report No. 23 WMO/TD No. 734, World Meteorological Organization, 2.10-2.11.
4. Gross, Brian D., 1994: Frontal interaction with isolated orography. Journal of the Atmospheric Sciences, 51(11), 1480-1496.
   Abstract PDF

   The interaction of a three-dimensional cold front and an isolated orographic ridge is examined by means of primitive equation model simulations. The front evolves as part of a developing nonlinear baroclinic wave and propagates southward toward the ridge. Many of the features in this interaction, such as the anticyclonic distortion of the front, divergence and fontolysis on the windward slope, convergence and frontogenesis in the lee, and the frontogenetical forcing associated with tilting, have previously been captured by simulations of a passive scalar traversing a ridge. It is shown that the ridge decelerates the cold postfrontal air and creates a high pressure anomaly on the windward slope. If this anomaly is strong enough, it accelerates air over the ridge peak in a shallow ageostrophic flow that possesses many features found in a gravity current. This current provides relatively strong surface frontogenesis through the convergence term, but cannot transport enough mass across the peak to weaken the anomalous high pressure. The cold air and pressure anomaly propagate eastward in a manner similar to a topographic Rossby wave. When the east ridge end is reached, the anomalous pressure gradient accelerates the flow into the lee, where frontogenesis occurs from shearing. The motion behind the front as it propagates over and around the ridge is distinctly unbalanced. Blocking, as measured by the ratio of the mass flux around the ridge end to that over the peak, is determined by a Froude number that depends on the propagation speed of the front (i.e., the strength of the baroclinic wave) and the mountain height. Higher mountains or weaker waves tend to produce total blocking of the front, resulting in flow only around the east ridge end. Lower mountains and stronger waves produce frontogenesis patterns and frontal distortions that more closely resemble the passive scalar simulations.

5. 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.

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