Microphysics of air-sea interface in tropical cyclones

Tropical cyclone track prediction is steadily improving, while storm intensity prediction has seen little progress in the last quarter century. The primary elements contributing to numerical tropical cyclone forecast success are physics, computational power, and observations. Discretized physics are essential for numerical models of tropical cyclones, along with parameterization of processes occurring on unresolved spatial and temporal scales. Computational performance is important for improved numerical grid resolution, more sophisticated physics, timely operational data assimilation, and multi-ensemble forecasting. Observations contribute to specification of the initial vortex and the evolving ocean-atmosphere environment, and are essential for testing predictions. During the last quarter-century, computational power increased by orders of magnitude; in addition, more extensive and intensive tropical cyclone observations are now made. Substantial improvement in computations and observations suggest that poorly parameterized or missing physics are the weakest component in tropical cyclone prediction systems. Missing and unresolved physics, including the air-sea interface, are limiting storm predictions. We bring forth the concept that the air-sea interaction regime, under very high wind speed conditions, is associated with the widespread disruptions of the interface and extensive generation of sea spray and air-bubbles. The resulting two-phase environment suppresses short gravity-capillary waves, affecting the aerodynamic drag of the sea surface. Direct disruption of the interface, bypassing development of energy contacting waves, can occur through the Kelvin-Helmholtz (KH) shear instability. KH waves are not able to disrupt the interface under moderate winds due to stabilizing gravity and surface tension forces. Microscale wave breaking occurs but does not disrupt the interface very much. Under strong winds the growing KH waves are able to overcome gravity and surface tension forces result in disruptions of the air-sea interface and formation of large droplets–spume. The KH instability of the interface for fluids with very large density difference, such as water and air, typically organizes in the form of projections. We reproduced this type of instability in a laboratory experiment at the UM RSMAS Air-Sea Interaction Salt Water Tank, coordinated with the Volume of Fluid Large Eddy Simulation. The local conditions near the wave crest are more favorable for KH instability development because the instantaneous interfacial shear near wave crests is higher than the time-averaged shear. The characteristic time scale of the KH instability is much shorter than the periods of energy containing wind waves; as a result, the KH instability develops within a relatively short time period and locally disrupts the interface. In the more general case of the turbulent atmospheric boundary layer above the wavy sea surface, wind gusts interacting with the waves result in stochastic shear intensifications triggering local KH instabilities at the air-sea interface resulting in almost complete “white out” of the see surface under tropical cyclones. To smoothly connect the two-phase regime with the well-known “Charnock” regime where wave-form induced turbulent drag is most important, we rely on the fact that the two-phase layer cannot support surface gravity-capillary waves whose wavelengths are shorter than or comparable to the thickness of this layer. This property is not taken into account in classic wave theory (for good reasons), and thus we parameterize the effects of two-phase mixture on the gravity-capillary wave spectrum. As wind speed increases, the thickness of the two-phase layer increases, eliminating successively longer waves in the high wavenumber range of the wave spectrum with consequent diminishment of the air-sea drag coefficient. However, in major tropical cyclones, the two-phase layer may exceed the partially suppressed wave-form drag, resulting in an increase of the drag coefficient with wind and development of a local minimum of the drag coefficient. Unfortunately, calculations of wave-form stress with existing models of wind-wave interaction have an order of magnitude uncertainty. In operational wave models, this uncertainty is customarily compensated by introducing empirical coefficients, which are determined from field and laboratory experiments. It is, however, not clear how representative these models are under extreme wind speed conditions. Our calculations of the wave-form stress are based on two different models of wind-wave interaction. Unifying both wave-form and two-phase effects in the wind stress parameterization shows the well-known increase of the drag coefficient with wind speed up to ~30 m/s, and it explains the observed diminished dependence on wind speed above that threshold. However, around 60 m/s the new parameterization predicts a previously unknown feature—the local minimum of the drag coefficient and maximum of the enthalpy to drag coefficient ratio. This feature may help explain rapid intensification of some storms to major hurricanes and the bi-modal distribution of tropical cyclone maximum intensity observed in the best-track data. We expect that implementation of the realistic sea surface microphysics in tropical cyclone forecasting models will improve predictions of tropical cyclone intensity, storm surges, and the associated wave field.