a particular form of the circulation theorem. The development or
non-development of a cyclone results from the interplay between the
convergence of vorticity, which tends to spin up the cyclone, and
surface friction, which tends to spin it down. The theory provides a
quantitative estimate all of the qualitative environmental factors
which are commonly thought to control cyclone spinup.
The vorticity convergence depends on the mass convergence and the
vorticity of the surrounding environment. The mass convergence is in
turn driven primarily by deep convection within the cyclone. Rainfall
from convection in the tropics tends to be closely related to
low-level convergence and plays an important role in spinup. Recent
work by Bretherton and others shows that the production of rainfall by
convection is a steeply increasing function of the column relative
humidity or saturation fraction of the surrounding air, i. e., the
precipitable water divided by the saturated precipitable water.
Factors affecting the humidity in the cyclone core, such as wind
shear, which can transport dry environmental air into the core, thus
become key to cyclone development. Our theory provides a quantitative
measure of this ventilation. Another factor relevant to cyclone core
thermodynamics is the surface moist entropy flux, which is a strong
function of sea surface temperature, wind and ocean heat content.
The classical picture of surface friction in tropical cyclones has the
surface stress distributed through the boundary layer. However, the
transport of momentum through the top of the boundary layer due either
to entrainment processes or to deep convection is bound to have an
important effect on frictional spindown as well. This is an area
needing extensive investigation, as its strength and even sign have
not been measured in the context of tropical cyclones.
This paper will serve as an introduction to two other papers: (1)
evaluation of the theory from IFEX dropsonde observations (Cisneros);
and (2) the workings of the theory in the NCEP GFS model, which has
shown some success in predicting tropical cyclogenesis (Marin).