Highly Nonlinear Solitary Waves in Compressible Fluids

Principal Investigator

Douglas R. Christie

Research School of Earth Sciences

It has become increasingly clear that the observed evolution properties of highly nonlinear solitary waves in the lower atmosphere are strongly influenced by variations in waveguide structure along the path of the disturbance. This project is therefore concerned with the numerical simulation of large amplitude trapped wave motions in inhomogeneous waveguides. A very high resolution numerical model has been developed to study solitary wave propagation in realistic temporally and spatially varying atmospheric boundary layer waveguides. Boundary conditions have been developed which prevent reflections of both sound and gravity waves at all lateral boundaries and also at the upper boundary of the computational domain. This model is being used initially to study the formation, evolution and decay of solitary waves on a time-varying density current. Further studies of nonlinear atmospheric wave motions in other evolving waveguide systems, including flow configurations which allow long wave propagation into the upper atmosphere, will be carried out during the coming year.

Determining:

a) How the propagation properties of internal solitary waves depend on variations in waveguide structure,

b) How energy loss due to radiation into the upper atmosphere influences the morphology of highly nonlinear trapped waves in the atmospheric boundary layer,

c) If there is an amplitude threshold below which coherent trapped wave propagation is not possible, and

d) How large scale turbulent motions in the waveguide layer influence the propagation of highly nonlinear internal waves,

are the main aims of this project.

Co-Investigators

Damien L. Bright

Research School of Earth Sciences

Projects

s52 - VPP




What are the results to date and the future of the work?

A preliminary study of solitary wave generation by the interaction of a descending thunderstorm microburst with a time varying stable layer created in the outflow from a preceding storm has been carried out. The results of this simulation provide a model for the formation of intense propagating windshear disturbances in the atmospheric boundary layer. A number of aviation disasters in recent years have been attributed to encounters with disturbances of this type. The results of this simulation show that the resulting solitary waves are not created in the transformation of the descending microburst vortex pair; propagating windshear disturbances in the form of highly nonlinear solitary waves are created directly by the impulsive disturbance of the waveguide. The initial microburst vortices dissipate rapidly and do not propagate away from the point of impact. Further studies are being carried out to determine how these intense quasi-stable propagating disturbances adapt to the ever-changing waveguide conditions. An investigation of the evolution and stability properties of solitary waves created in the collision of two opposing density currents is also being carried out. Both of these studies will provide valuable insight into the processes which lead to hazardous windshear disturbances in the thunderstorm environment.

What computational techniques are used?

The properties of evolving large amplitude wave phenomena are determined from a series of numerical simulations which are based on the integration of the fully nonlinear nonhydrostatic primitive ensemble-averaged equations for a compressible fluid.

- Appendix A