B.S. (Honors Physics), 1965, University of Montreal
Ph.D. (Geophysical Fluid Dynamics), 1973, The University of Chicago
NCAR postdoctoral Fellow, 1972-73
Research Interests:
Atmospheric dynamics with an emphasis on nonlinear processes and
interactions. Topics of current and anticipated research include: stability
of spatially varying tropospheric jets; role of mountains and thermal contrasts
in tropospheric dynamics; combined forced-free midlatitude dynamics;
interactions between tropical and midlatitude disturbances; climate dynamics.
Classic explanations of various atmospheric phenomena are based on linearized models of the atmospheric. By ignoring the nonlinear effects, such explanations cannot account for the complex interactions among the various scales of atmospheric motion, or the interactions between the atmosphere and the surface forcings, both of which are responsible for the unpredictable nature of the atmosphere. To gain a better understanding of even an isolated atmospheric phenomenon, the pertinent nonlinear effects must be retained in the equations modeling this phenomenon and their effect on the solution must be carefully examined.
The nonlinear dynamics of the midlatitude troposphere are governed simultaneously by internal mechanisms and external forcings. The important internal mechanisms are baroclinic and barotropic instabilities and their associated wave-mean flow and wave-wave interactions. The important surface forcings arise from the mechanical form drag associated with the interactions between the flow and surface topography or from thermal and moisture contrasts between land and sea or other surface inhomogeneities.
Through the internal instabilities and their associated nonlinear dynamics, tropospheric disturbances grow and act to redistribute heat, momentum, and potential vorticity across latitude circles. For example, the observed poleward transport of heat is due to the westerly vertical tilt of growing baroclinic waves, its strength increases in proportion of the disturbances size and the degree of the tilt. Similarly, the rectified momentum flux is a result of the north-south tilt of both the cyclone and the stationary waves and acts to maintain the observed midlatitude westerly jet against dissipation. Topographic and thermal forcings give rise to stationary and quasistationary and tropospheric wave features which introduce zonal asymmetries in the flow. Under near resonant conditions, a forced feature can acquire a large amplitude and persist for an extended period of time as a "blocking" pattern in the flow.
In the atmosphere all of the above mechanisms are present to some degree at all times. Irrespective of which mechanism is dominant in a given physical situation, the others can still distort, or, if a nonlinearly induced bifurcation of solution occurs, completely alter the flow. Thus, a fundamental question arises: What is the evolution of atmospheric disturbances when the various internal and forced mechanisms coexist in certain realistic proportions? Although a substantial amount of published research exists which examines either the individual internal or the individual forced mechanisms and the structure and evolution of disturbances each produces, little research exists on the combined role of these mechanisms in tropospheric dynamics, our current research goals are to investigate the dynamics of the tropospheric motions in the combined presence of internal instabilities and interactions as well as external forcings.
Selected Publications
Chou, S.-H., and A.Z. Loesch, 1986a: Supercritical dynamics of baroclinic disturbances in a free-surface model. J. Atmospheric Science, 43, 285-301.
Chou, S.-H., and A.Z. Loesch, 1986b: Supercritical dynamics of baroclinic disturbances in the presence of asymmetric Ekman dissipation. J. Atmospheric Science, 43, 1781-1795.
Chou, S.-H., and A.Z. Loesch, 1991: Supercritical baroclinic disturbances under the influence of topography. J. Atmospheric Science, 48, 2461-2475.
Nathan, T.R., 1988: Finite amplitude interactions between unstable baroclinic waves and resonant topographic waves. J. Atmospheric Sciencee, 45, 1052-1071.
Nathan, T.R. and A.Z. Loesch, 1987: Finite amplitude stability of a topographically forced wave with Ekman dissipation, Tellus, 39A, 95-109.
Peng, M., and A.Z. Loesch, 1989: Spectral evolution of baroclinic waves in continuously stratified Eady model with Ekman dissipation. Eur. J. Mech., B/Fluids, 8, 1-15.