Dry and Semi-Dry Tropical Cyclones

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 numerical modeling work has found 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? 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. As a second way of investigating the moist-dry transition, we also conduct simulations across a wide range of surface temperatures (between 240 K and 300 K), since very low surface temperatures decrease the surface latent heat flux to near zero and also reach a nearly-dry dynamical limit.

We find that a completely moist surface, and sea-surface temperatures above 280 K, both yield “normal” TC-world states where multiple vortices form spontaneously and persist for tens of days. A completely dry surface, or the lowest sea-surface temperature of 240 K, both also spontaneously yield similar dry TC-world states with many vortices that are even more stable and persistent. Dry cyclones are weaker and smaller than their normal moist counterparts, but have a larger radius of maximum winds compared to their outer radius, and prominent eyewall asymmetries. For both intermediate surface wetness values, and intermediate surface temperatures of 250-270 K, we find that spontaneous cyclogenesis strikingly fails to occur. Simulations with time-varying surface moisture and sea-surface temperatures are used to explore whether these constraints on spontaneous cyclogenesis also limit the survival of existing storms as ambient conditions slowly change.