How Does a Tropical Cyclone Intensify?

Abstract:

The increase in the lower tropospheric circulation of a tropical
cyclone is marked by the deep inflow of air above the boundary layer.
This increase is associated with a decrease in surface pressure and a
corresponding increase in tangential winds. The inflow is driven via
mass continuity with an upward convective mass flux in the cyclone
core that increases with height. The factors governing this mass flux
thus become central to cyclone intensification. We hypothesize that
deep layer mass fluxes are controlled by certain characteristics of
thermodynamic profiles in the core of the cyclone. In particular,
high column relative humidity and low (but positive) values of lower
tropospheric moist convective instability are needed. This differs
from Ooyama's classical picture in that the convective mass flux is
independent of the flux that would be produced in the boundary layer
by frictional convergence. Spinup occurs only if the former exceeds
the latter. Our hypothesis is bolstered by analyses of idealized
high-resolution simulations of tropical cyclone formation as well as a
wealth of other work on tropical convection, of which key results are
presented here.