Dry and Semi-Dry Tropical Cyclones

Previous work has shown that dry axisymmetric tropical cyclones (TCs) can form in a state of dry radiative-convective equilibrium. What can such “dry hurricanes” teach us about intensity, structure, and size of real moist tropical cyclones in nature? Are dry TCs even stable in 3D? What about surfaces that are nearly dry but have some latent heat flux – can they also support TCs?

To address these questions, we use the SAM cloud-system resolving model to simulate radiative-convective equilibrium on a rapidly rotating f-plane, subject to constant tropospheric radiative cooling. We use a homogeneous surface with fixed temperature and with surface saturation vapor pressure scaled by a factor 0-1 relative to that over pure water – allowing for continuous variation between moist and dry limits.

A perfectly moist surface yields a classic TC-world where multiple vortices form spontaneously and persist for tens of days. A completely dry surface can also yield a parallel dry TC-world with stable persistent vortices – but if the rotation rate is too large, vortices become unstable. Over a large range of the semi-dry parameter space, however, spontaneous cyclogenesis fails to occur at all. We hypothesize that it is much harder to form cyclones over a semi-dry surface because even a small amount of rain evaporation in a deep dry boundary layer impedes the formation of a surface low beneath a region of large-scale ascent. We search for upper and lower limits of surface saturation vapor pressure scale factor that allow for spontaneous cyclogenesis, and for bounds on the rotation rate that allow for a TC-world with stable vortices.