Environmental influences on the spinup of tropical cyclones

We outline here a theory for the spinup of tropical cyclones, based on

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).