A new method for evaluating the favorability of observed and idealized
environments for tropical cyclogenesis: Point-Downscaling

David S. Nolan
Rosenstiel School of Marine and Atmospheric Science
University of Miami
Miami, Florida, USA


Determining the atmospheric and oceanic conditions that are favorable or
unfavorable for tropical cyclone (TC) genesis has long been a matter of great
interest. This interest has increased further due to the possibility that TC
activity may increase (or decrease) due to global climate change. Until
recently, determining the favorability of a particular climate for TC genesis
has been achieved through three methods: 1) combining large-scale evironmental
parameters such as sea surface temperature, shear, and stability into a single
"genesis parameter" that can vary in space and time; 2) counting the number of
tropical cyclone-like vortices simulated in global climate models; 3) counting
the number of tropical cyclones in a higher-resolution, regional model with
boundary conditions from a global model.

We present a new method which allows for much higher resolution
simulations and more direct control over the surrounding environment. A
doubly-periodic domain is initialized with pre-defined profiles of
temperature, humidity, and wind as a function of height. These profiles may be
idealized, may come from observations, or from future climate scenarios. With
small modifications to the equations of motion, the winds can be balanced so
that the wind profiles remain nearly constant across the domain as the
simulation proceeds. The development, or decline, of a pre-cursor tropical
cyclone disturbance embededded in this environment is then simulated. The rate
of development (or failure) is an indicator of the favorability of that
particular sounding and wind profile for TC genesis. Along with TC genesis,
the point-downscaling technique can be used to evaluate favorability of
particular soundings for rapid intensification and other structural changes.