MMM SEMINAR NCAR
NCAR/MMM Boulder Colorado
It is well known that there is a set of broad-scale
parameters that must be satisfied to enable tropical cyclone development (Gray,
1968). However, these only provide necessary conditions, they say nothing about
sufficiency. Actual development of a tropical cyclone requires some other
processes to occur within these broad conditions. We suggest that these
processes can be isolated by breaking cyclone genesis down into two main types:
Subtropical Genesis (which is influenced by baroclinic systems and
extratropical interactions) and Tropical Genesis. The Tropical Genesis class
can further be broken down into monsoon trough development, shear zone or ITCZ
development, and easterly wave development. Whilst more than one of these can
apply at the same time (e.g. an easterly wave family propagating into and along
an ITCZ), isolated cases of each are common. In this talk, I shall address the
manner in which easterly waves can focus energy and vorticity down to small
scales. A series of seminal papers by Webster and Chang have shown how
Rossby-mode waves in the equatorial duct can accumulate energy in regions where
there the zonal winds are increasingly easterly towards the east. In essence,
given the right environmental conditions, divergent Rossby waves can focus both
energy and vorticity down to smaller scales, without any requirement for moist
processes. This downscale transfer then establishes enhanced conditions for
ongoing cyclogenesis by moist processes. Since this process occurs primarily at
the larger scales, it provides one explanation for the success of global
forecast models in predicting genesis. I will first briefly outline these
processes, and then move on to describe their applicability to tropical cyclone
formation in the eastern North Atlantic and eastern North Pacific Oceans. This research has highlighted data
limitations that are restricting further understanding. I will conclude by
briefly describing how we plan to work with field experiments in the eastern
Pacific and eastern Atlantic to further our understanding of these fundamental
processes.
Friday, 4 March 2005, 3:30 PM
Refreshments 3:15 PM
NCAR-Foothills Laboratory
3450 Mitchell Lane, Bldg 2 Auditorium (Rm1022)