ALBERT

Notes: We have applied the model and technique previously applied to the change of Curie temperature with pressure for correlated-electron uranium systems to predict the change in Curie temperature and ordered moment with dilution alloying. The theory is remarkably successful in its predictions, and this success has important implications for the overall understanding of magnetic ordering in correlated-electron systems including heavy fermion systems. For US, the dilution alloying behavior found experimentally is dramatic. In UxLa1−xS, the magnetic ordering abruptly disappears at about 55% uranium. Our ab initio-based theory quantitatively predicts this abrupt disappearance while also quantitatively predicting the monotonic decrease of Curie temperature with pressure for undiluted US. In addition, in agreement with experiment, the theory predicts the correct trend for the magnetic ordering to disappear with dilution, while at the same time absolutely and quantitatively predicting the nonmonotonic variation of Curie temperature with pressure, for USe and UTe. The ab initio-based model gives absolute material-specific predictions using input from the local density approximation paramagnetic uranium f-electron-projected density of states plus ab initio calculated values of the correlation energy U. The key physics of the model is the recognition and quantification of the concept that the f spectral density in the vicinity of a specific uranium nucleus (so-to-speak in the muffin-tin sphere) can either be in a stable f3 configuration for a long enough period of time that, through coupling to other such stable f3 sites, it can magnetically order, or can be in a situation such that the configuration fluctuates rapidly between f3 and f2, and for purposes of magnetic ordering acts like a hole in the f-electron lattice in the same way that substitution of lanthanum for uranium creates a hole. Both pressure and dilution alloying, by causing an increase in f-delocalization, increase the fraction of uranium sites in the rapidly fluctuating, magnetically ineffective, condition. For dilution alloying, at a certain point this increase in fluctuations causes the stable f3 component to fall abruptly below a critical value necessary to sustain any magnetic ordering, and hence brings about a catastrophic collapse in magnetic ordering. © 1998 American Institute of Physics.

Notes: Summary Numerical experiments are performed for inviscid flow past an idealized topography to investigate the formation and development of lee mesolows, mesovortices and mesocyclones. For a nonrotating, low-Froude number flow over a bell-shaped moutain, a pair of mesovortices form on the lee slope move downstream and weaken at later times. The advection speed of the lee vortices is found to be about two-thirds of the basic wind velocity, which is due to the existence of a reversed pressure gradient just upstream of the vortices. The lee vortices do not concur with the upstream stagnation point in time, but rather form at a later time. It is found that a pair of lee vortices form for a flow withFr=0.66, but take a longer time to form than in lower-Froude number flows. Since the lee vortices are formed rather progressively, their formation may be explained by the baroclinically-induced vorticity tilting as the mountain waves become more and more nonlinear. A stationary mesohigh and mesolow pressure couplet forms across the mountain and is produced in both high and low-Froude number flows. The results of the high Froude number simulations agree well with the classical results predicted by linear, hydrostatic mountain wave theory. It is found that the lee mesolow is not necessarily colocated with the lee vortices. The mesolow is formed by the downslope wind associated with the orographically forced gravity waves through adiabatic warming. The earth's rotation acts to strengthen (weaken) the cyclonic (anticyclonic) vortex and shifts the lee mesolow to the right for an observer facing downstream. The cyclonic vortex then develops into a mesocyclone with the addition of planetary vorticity at later times. For a flow over a steeper mountain, the disturbance is stronger even though the Froude number is kept the same. For a southwesterly flow past the real topography of Taiwan, there is no stagnation point or lee vortices formed because the impinging angle of the flow is small. A major mesoscale low forms to the southeast of the Central Mountain Range (CMR), while a mesohigh forms upstream. For a westerly flow past Taiwan, a stagnation point forms upstream of the mountain and a pair of vortices form on the lee and move downstream at later times. The cyclonic vortex then develops into a mesocyclone. A mesolow also forms to the southeast of Taiwan. For a northeasterly flow past Taiwan, the mesolow forms to the northwest of the mountain. Similar to flows over idealized topographies, the Taiwan mesolow is formed by the downslope wind associated with mountain waves through adiabatic warming. A conceptual model of the Taiwan southeast mesolow and mesocyclone is proposed.

Notes: Summary The nonlinear response of a dynamically unstable shear flow with critical level to an initial temperature anomaly is investigated using a nonlinear numerical model. Both nonconstant and constant shear profiles of the basic flow are considered. Effects of the solid lower boundary on the dynamically unstable, nonlinear flow are also studied. It is found that in a dynamically unstable, linear flow with a hyperbolic tangent wind profile, the updraft is tilted upshear. The result in consistent with that of a linear stability model (LC). The upshear tilt can be explained by the Orr mechanism (1907) and the energy argument proposed by LC. In a dynamically unstable, nonlinear flow, the updrafts produced by a sinusoidal initial temperature perturbation are stronger in the lower layer and are more compact and located further apart compared to the corresponding linear flow. In addition, the perturbed wave energy is slightly smaller than the linear case. It is found that the growth rate is smaller during the early stage and much larger during the later stage. For a localized initial temperature perturbation in a dynamically unstable flow, a stronger updraft with two compensated downdrafts are produced. Gravity waves are produced in a dynamically stable flow with both a hyperbolic tangent wind profile and a linear wind profile. For a linear shear flow with Richardson number less than 1/4, the disturbance grows in the early stage and then decays algebraically at later times, similar to that found in other linear theoretical studies. The influence of the solid lower boundary is to suppress the shear instability in a nonlinear flow with a hyperbolic tangent wind profile ofRi〈1/4.

Notes: Summary Using observational analysis and mesoscale numerical simulations we investigate the subtropical jet (STJ) and its effects on the lower environment (associated mass and momentum adjustments, development of a low-level jet (LLJ), and low-level PV) 48 to 6 hours before the Raleight tornado outbreak (1988). We also compare the environment to a synoptically similar event in which severe weather forecasted but did not develop over central North Carolina. In the severe weather case a self-maintaining. low-level circulation originated over Mexico, propagated across the Gulf Coast and moved over the Piedmont at the time of the tornado. It is characterized by a surface trough, low-level PV maximum, mid-level jet, a warm Mexican airmass and STJ exit region that was co-located and moved across the Gulf Coast States as a coupled system. Initially, a STJ exit region (with thermally indirect ageostrophic circulation) approached the Gulf Coast creating upper-level divergence and ascent, which helped to maintain a low-level trough. A warm Mexican airmass was located over the Gulf Coast (southeast of the surface trough) creating a northwestward-directed PGF, which created a mid-level jet. The right entrance region of the mid-level jet and its associated thermally direct circulation (ascent) was over the low-level trough. These features created an environment favorable to deep convection and the release of latent heat that generated low-level PV. In the non-event case, these features (low-level warm Mexican airmass, mid-level jet, deep convection, low-level PV and low-level trough) were absent over the Gulf Coast States.

Notes: Summary Observational analysis and mesoscale numerical simulations are in agreement concerning key dynamical processes which occurred over Mexico and the Gulf of Mexico 84 hours prior to the 1988 Raleigh (RDU), NC tornado outbreak. The subtropical jet (STJ) over northern Mexico and its associated transverse ageostrophic circulation forced air down the eastern side of the Sierra Madre Mountains creating adiabatic warming due to compressional heating. Along with this warm air, a low-level trough of low pressure formed and a low-level jet (LLJ) developed over the western Gulf of Mexico. This LLJ began the process that transported very warm and potential vorticity (PV) rich air from the Mexican plateau to the Carolina Piedmont. The low-level PV maximum over central NC at the time of the tornado was a coherent entity traceable back 84 hours to the Mexican plateau. Over the Mexican plateau, the STJ transported the PV rich air southward then down to the midlevels. There was substantial heating over the plateau producing a deep well-mixed layer and a mountain-plains solenoid. An area of strong vertical convergence developed in the 500–600 hPa layer which increased the thermal gradient and maintained the PV. This mid-level PV was transported to the low-levels by a hydrostatic mountain wave. As the PV maxima moved down the lee of the mountains it increased due to strong static stability, tilting and frictional effects. Finally, the PV maxima moved along the Gulf Coast and up the East Coast to central NC.