Michael T. Montgomery
Department of Atmospheric Science, Colorado State University
1. INTRODUCTION
To me, the interaction between small-scale cumulus convection and the larger-scale hurricane vortex lies at the heart of the hurricane problem. Without any doubt, useful information about the hurricane's structure has been obtained from ground-based and airborne observations. Considerable insight into the physics of hurricanes has also been acquired using axisymmetric theory and models. But many questions remain unanswered concerning the role of non-axisymmetric processes in the tropical cyclone life cycle. Listed below are some of my favorites.
(i) Can the physics of tropical cyclone genesis be understood from the perspective of axisymmetric dynamics, or are eddy processes essential? If eddy processes are essential, then what is the role of large-scale eddies (migratory upper-level troughs, anticyclones, etc.) and small-scale eddies (cyclonic vorticity anomalies generated by penetrative convection, mesoscale convective vortices generated by stratiform convection, warm core eddies in the ocean, etc.) in the initiation process? Are both large- and small-scale eddies essential, or will either one suffice?
(ii) How do vortex asymmetries affect cyclone intensity? Vortex asymmetries come in many flavors: stationary and propagating bands, SST anomalies (ocean eddies, cold wakes, etc.), upper-level troughs, an approaching coastline, increased vertical shear of the environmental flow, and even tornados. Must each of these problems be studied separately with complex mesoscale models or is there an underlying theoretical framework that can be usefully applied to all of these problems?
While some researchers may claim to have the definitive answer to each of these questions, I think the jury is still out on many of them.
2. SPIN UP AND VORTEX EVOLUTION BY VORTEX ROSSBY WAVES
In this talk I will concentrate on the problem of vortex development by asymmetric convective forcing. Theoretical predictions and numerical simulations based on the work of Montgomery and Kallenbach (1997), Montgomery and Enagonio (1997), Moeller and Montgomery (1997) and Shapiro (1997) will be presented in order to illustrate the manner in which non-axisymmetric processes can promote the upscale transfer of convective-scale energy to vortex-scale energy. These studies examine both small and large Rossby number flow regimes using balance and primitive equation models. The results suggest a different paradigm of convective vortex development than either CISK or WISHE that are essentially axisymmetric intensification mechanisms in which angular momentum tends to be conserved on fluid parcels. The development mechanism is analogous to the problem of spin up and maintenance of a large-scale zonal jet by forced planetary Rossby waves (Shepherd 1987), except here the pertinent vorticity gradient is that of the basic state vortex and the pertinent Rossby waves are those that reside on the vortex.
3. CONCLUSIONS
Although the results presented are based on highly idealized models, I believe they point us in a fruitful direction for obtaining a more complete understanding of how cumulus convection interacts with the larger-scale vortex.
4. ACKNOWLEDGMENT
This research is supported by the Office of Naval Research Grant N00014-93-1-0456, National Science Foundation Grant ATM-9529295, and Colorado State University.
5. REFERENCES
Moeller, J.D., and M.T. Montgomery, 1997: Hurricane intensification via vortex Rossby-waves. 78th AMS Annual Meeting, 11-16 January 1998, Phoenix, Arizona.
Montgomery, M.T., and J.Enagonio, 1997: Tropical cyclogenesis via convectively forced vortex Rossby waves in a three-dimensional quasigeostrophic model. J. Atmos. Sci., accepted with revision.
Montgomery, M.T., and R.J. Kallenbach, 1997: A theory for vortex Rossby waves and its application to spiral bands and intensity changes in hurricanes. Quart. J. Roy. Meteor. Soc., 123, 435-465.
Shapiro, L.J., 1997: Convective asymmetries and tropical cyclone evolution. 78th AMS Annual Meeting, 11-16 January 1998, Phoenix, Arizona.
Shepherd, T.G., 1987: Rossby waves and two-dimensional turbulence in a large-scale zonal jet. J. Fluid Mech., 183, 467-509.