Seastate-Dependent Sea Spray Heat Fluxes and Impacts on Tropical Cyclone Structure and Intensity Using Fully Coupled Atmosphere-Wave-Ocean Model Simulations

Thesis (Ph.D.)--University of Washington, 2023Air-sea fluxes of sensible and latent heat are fundamental to the energetics of tropical cyclones (TCs) and their intensity. The contributions of sea spray to air-sea heat fluxes and their impacts on TC structure and intensity change are not well understood, due partly to the extreme difficulty of taking measurements of sea spray and air-sea heat fluxes at the high-wind air-sea interface, to the complexity of the physical processes governing spray and its impacts, and to the difficulty of representing spray physics and processes in numerical TC forecast models. Previous studies have made simple but unrealistic assumptions to represent sea spray using windspeed-dependent methods. Addressing the role of ocean surface waves in spray production and heat transfer is challenging and remains largely unaddressed in models. Consequently, the interactions connecting seastate, spray generation, surface heat fluxes, the TC surface layer (SL) and boundary layer (BL), eyewall deep convection, and TC structure and intensity change remain unclear.This dissertation aims to address deficiencies in both understanding and modeling of seastate-dependent sea spray heat fluxes and their interactions with TCs by 1) developing an improved parameterization for seastate-dependent air-sea heat fluxes with spray, 2) implementing it in a fully coupled atmosphere-wave-ocean (AWO) modeling framework, the Unified Wave Interface-Coupled Model (UWIN-CM), and 3) performing UWIN-CM model experiments with spray for a diverse set of TCs to provide insight into the complex interactions between waves, spray, heat fluxes, the hurricane SL and BL, and TC structure and intensity change. In Chapter 2, we present a new seastate-dependent parameterization for air-sea heat fluxes with spray for use in fully coupled AWO models. We exercise the new parameterization across a wide range of conditions provided by output from a diverse set of UWIN-CM simulations of TCs to broadly characterize seastate-dependent heat fluxes with spray. In Chapter 3, we implement the new heat flux parameterization with spray into the UWIN-CM and perform coupled model simulations to understand the interactions connecting spray to TC structure and intensity change in the fully coupled AWO modeling framework. In Chapter 4, we test the sensitivity of our findings to the large uncertainty remaining in spray generation by performing a sensitivity study varying the total mass and droplet size distribution of spray in fully coupled AWO simulations. The new parameterization presented in Chapter 2 explicitly represents seastate-dependent spray generation with a variable droplet size distribution using surface wave properties such as wave dissipation, significant wave height, and dominant phase speed, which may be uncorrelated with local winds. The parameterization also presents a new physics-based treatment of near-surface (i.e., subgrid) feedback between spray heat fluxes and near-surface temperature and humidity profiles, addressing interactions and feedbacks between spray and turbulent heat fluxes in a physically consistent way. Exercising the new model across output from UWIN-CM simulations of five TCs to characterize its behavior, we find that seastate-dependent spray mass flux is substantially different than a wind-dependent model, especially when wave shoaling occurs with enhanced wave dissipation near the coast during TC landfall. Spray increases the air-sea enthalpy flux near the radius of maximum wind (RMW) by approximately 5–20% when mean 10-m windspeed at the RMW reaches 40–50 m s-1. These values can be amplified significantly by coastal wave shoaling. Spray latent heat fluxes may be dampened in the eyewall due to high saturation ratio, and they consistently produce a moistening and cooling effect outside the eyewall. Spray strongly modifies the total sensible heat flux and can cause either a warming or cooling effect at the RMW depending on eyewall surface relative humidity. Traditional scaling used for bulk turbulent heat fluxes fails for spray heat fluxes because spray introduces physical interactions that cannot be characterized by traditional thermodynamic variables. This includes the dependence of spray generation on wave properties, the dependence of spray evaporation on relative humidity rather than the air-sea specific humidity difference, and the coupling of spray latent and sensible heat fluxes through evaporative cooling. Implementing our new parameterization into the UWIN-CM in Chapter 3 and testing it using simulations of four TCs, we find that spray evaporation outside the eyewall consistently cools and moistens the SL (i.e., the lowest few levels of the atmospheric model that directly interact with surface heat fluxes) and BL (i.e., the atmospheric mixed layer), reducing the surface buoyancy flux and vertical mixing, producing a shallower atmospheric mixed layer. Under the eyewall, spray may either increase or decrease the surface buoyancy flux, depending on whether the net sensible heat flux due to spray produces a warming or cooling effect, respectively. Spray causes changes to BL winds, producing a shallower inflow layer and a downward shift of the top-of-BL outflow layer. Our results show spray’s impact on our intensifying model TCs as progressing in three stages. These are: 1) When maximum azimuthal-mean 10-m windspeed U10 is below 30–40 m s-1, spray evaporation cools BL inflow, suppressing eyewall deep convection and causing spray to weaken intensification. 2) Intensification beyond this U10 threshold produces surface warming due to spray under the eyewall, which should promote intensification; however, spray-induced changes to the BL and secondary circulation reduce the efficiency of moist static energy (MSE) transport into eyewall deep convection, and spray continues to oppose intensification overall. Further intensification may strengthen spray surface warming under the eyewall enough to overwhelm the influence of the lower structural efficiency, and a transition occurs towards spray promoting intensification. 3) In the presence of strong warming due to spray under the eyewall, the eyewall contracts and spray promotes intensification. Coastal interaction involving wave shoaling amplifies spray enthalpy fluxes and surface warming during landfall. Our results highlight the important point that spray’s impact on intensity change is not determined solely by the magnitude of the spray enthalpy flux but also by the ways in which spray modifies the transport of MSE within the TC vortex and the development of eyewall deep convection. Overall, our results suggest that spray plays a modulating role, rather than a dominating role, in TC intensity change, with the range of spray impact on minimum sea level pressure in our simulations lying within approximately +/- 5 mb. In Chapter 4, we investigate the sensitivity of TC intensity to the total generated mass of sea spray and the shape of the droplet size distribution by conducting UWIN-CM simulations of Hurricane Florence (2018). We find that TC intensity appears more sensitive to the total spray mass flux than to the shape of the droplet size distribution. This finding has important implications for the future study of sea spray, especially on observing system design to focus on measuring the spray mass flux rather than the shape of the droplet size distribution in high-wind conditions. The maximum differences in minimum sea level pressure and maximum azimuthal-mean 10-m windspeed among our sensitivity simulations are approximately 10 mb and 8 m s-1, respectively. These ranges are meaningful in terms of both forecast intensity (i.e., storm Category) and the destructive potential of winds (i.e., surface wind kinetic energy)