Dry and Semi-Dry Tropical Cyclones in Radiative-Convective Equilibrium

Our understanding of dynamics in our real moist atmosphere is strongly informed by idealized dry models. It is widely believed that tropical cyclones (TCs) are an intrinsically moist phenomenon – relying fundamentally on evaporation and latent heat release – yet recent work has shown formation of dry axisymmetric tropical cyclones from 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? What does the moist-dry transition look like? And how can dry radiative-convective equilibrium – both rotating and non-rotating – serve as a useful idealized model of atmospheric convection?

To address these questions, we use the SAM cloud-system resolving model to simulate radiative-convective equilibrium in both non-rotating and rapidly rotating f-plane configurations, 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. We also explore cases with surface enthalpy fluxes that are uniform in space and time, where partitioning between latent and sensible heat fluxes is specified directly.

We find that a completely moist surface yields a 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 many vortices that are even more stable and persistent. Spontaneous cyclogenesis, however, is impeded for a range of low to intermediate surface wetness values, or by the combination of large rotation rates and a dry surface. We discuss whether these constraints on spontaneous cyclogenesis might arise from: 1) rain evaporation in the subcloud layer limiting the range of viable surface wetness values, and 2) a natural convective Rossby number limiting the range of viable rotation rates. Finally, we discuss simulations with uniform surface enthalpy fluxes, which suggest that wind-induced surface heat exchange may differ in its importance for dry and moist cyclones.