Explicit Air-Sea Momentum Exchange in Coupled Atmosphere-Wave-Ocean Modeling of Tropical Cyclones

Atmosphere and ocean are coupled through momentum, enthalpy, and mass fluxes on all spatial and temporal scales. Accurate representation of these fluxes in numerical models is essential for prediction of global weather and climate systems. Current physical parameterizations of the surface fluxes were developed based on observations in low-to-moderate wind speeds. They are not suited for high wind conditions, especially in extreme weather conditions such as tropical cyclones (TC) and mid-latitude winter storms. In high winds, ocean surface waves control most of the air-sea momentum transfer. While there has been some progress in representation of atmosphere-wave-ocean momentum exchange in coupled models, explicit and conservative air-sea momentum exchange has not been accomplished to date. In this study, we have developed an explicit air-sea momentum exchange through surface waves, namely the Unified Wave INterface (UWIN) for coupled models, which is physically based and computationally efficient. UWIN has been implemented and tested in a fully coupled atmosphere- wave-ocean model (UWIN-CM). The goal of this study is to better understand air-sea momentum exchange in high winds and its impact on TC prediction using UWIN-CM and observations. To address the complexity of the fully-coupled physical processes, we conducted UWIN-CM simulations of five TCs with a wide range of storm intensity over the Atlantic and Pacific basins, including Ike (2008), Earl (2010), Fanapi (2010), Isaac (2012), and Sandy (2012). A set of uncoupled and coupled numerical experiments is done for each TC case to investigate the impacts of explicit wave-based momentum exchange on the TC track, intensity, wind speed structure, and ocean feedback processes. Model results are evaluated using a comprehensive set of atmospheric and oceanic measurements from the Impact of Typhoons on the Ocean in the Pacific (ITOP) and the Grand Lagrangian Deployment (GLAD) field campaigns. Surface waves in TCs vary with storm size and intensity, storm-relative asymmetry, and between deep and shallow water. UWIN-CM produces the observed wind, wave, and upper-ocean structures in most cases. Based on wind speed measurements from 32 flights in Ike, Earl, Fanapi, and Isaac, we find that coupling with waves improves the prediction of storm size and asymmetry compared to drag coefficient-based coupling and uncoupled modeling. One of the most important capabilities of UWIN is its treatment of the air-sea momentum exchange through surface waves, which allows the wind-wave and wave-current stresses to be computed explicitly through wave growth and dissipation tendencies in the wave energy balance equation. The ocean surface currents are largely driven by dissipation of steep waves and to a lesser extent by surface wind. The largest difference between atmospheric and oceanic stress is found on the left-hand side of the storm due to complex wind-wave interactions. Waves that propagate against wind increase atmospheric stress while dissipating energy. The ratio between the oceanic and atmospheric stress is typically between 0.85 and 1 depending on the wave state. Wave momentum budget calculations indicate that approximately 10% of wave momentum leaks from the storm into the environment. Explicit stress treatment affects the amount of momentum delivered to subsurface currents, impacting upper-ocean mixing and sea surface temeperature response. Forcing the ocean with atmospheric stress leads to an overprediction of surface temperature cooling in the wake of the storm by up to 1 degree C. Through ocean feedback processes, TC winds and subsequent evolution of the storm are impacted. Besides governing the atmospheric and oceanic stress, waves also induce mass transport in the direction of their propagation. The velocity associated with this transport, Stokes drift, is strongly sheared near the surface and interacts with subsurface Eulerian circulation. Based on UWIN-CM simulation and Lagrangian velocity estimates from nearly 200 surface drifters deployed in the path of Hurricane Isaac (2012), we find that Stokes drift contributes up to 20% of material surface transport. It induces structured, basin-scale pattern of surface trajectories that are cyclonic on the left-hand side of the storm and anti-cyclonic on the right-hand side. Waves significantly enhance cross-track and shoreward transport within the storm, and to a lesser extent, relative dispersion of surface material