Sea Spray and Its Feedback Effects: Assessing Bulk Algorithms of Air–Sea Heat Fluxes via Direct Numerical Simulations

Abstract

Sea spray exchanging momentum, heat, and moisture is one of the major uncertainties in modeling air–sea surface heat fluxes under high wind speeds. As a result of several untested assumptions in existing models and low fidelity in the measurements, questions regarding the appropriate method for modeling the effects of spray on air–sea fluxes still exist. In this study, we implement idealized direct numerical simulations (DNS) via an Eulerian–Lagrangian model to simulate spray droplets in turbulent flows. Then, we verify the bulk spray models of Fairall et al. and Andreas et al. with the detailed physics from DNS. We find that the quality of the underlying assumptions of bulk models is sensitive to the time scales governing spray microphysics and lifetime. While both models assume that spray experiences a uniform and steady ambient condition, our results show that this assumption only works well for droplets with long thermodynamic time scales and relatively short lifetime. When the thermodynamic time scales are short, the models fail to predict the correct temperature and radius change of spray (e.g., condensation), thus spray-mediated heat fluxes, which in turn overestimates the total heat fluxes. Moreover, using our two-way coupled simulations, we find a negative feedback induced by the spray evaporation that may be missing in the bulk models, which could lead to further overestimates of the total heat flux when the spray-mediated flux is treated as an add-on to the corresponding interfacial flux. We further illustrate that the feedback effects are consistent under different flow Reynolds numbers, which suggests that the findings are relevant at practical scales.

Effects of the Sea Surface Roughness and Sea Spray‐Induced Flux Parameterization on the Simulations of a Tropical Cyclone

“…The droplets of sea spray release water vapor to the near‐surface of the atmosphere by evaporation and loses sensible heat to the atmosphere. The studies by Richter and Stern () and Peng and Richter () discussed the role of the sea spray on the enthalpy flux under the strong wind (typhoon wind).…”

“…The logarithmic formula of wind profile was modified by introducing the influence of sea spray droplets on the dynamic sea surface roughness length in the strong wind conditions (Makin, ). While a few studies (Wu et al, ; Garg et al, ) reported existing uncertainty in the impact of sea spray on the intensity of the storms, a recent study by Peng and Richter () stated that the contribution of sea spray to the enthalpy flux in hurricane conditions might be greatly overestimated. Hence, the role of sea spray on storms is not properly understood.…”

Predicting Vertical Concentration Profiles in the Marine Atmospheric Boundary Layer With a Markov Chain Random Walk Model

“…In the LES considered here, the simulated Lagrangian sea spray aerosol particles maintain a constant size, meaning that hygroscopicity and aerosol swell are not considered (Winkler, 1988). The changing atmospheric conditions due to the diurnal cycle have been neglected, as have momentum and energy exchange between the aerosol particles and the air (e.g., neglecting the effects of spray modifying heat and moisture in the surface layer Peng & Richter, 2019). Lastly, the LES assumes a flat surface with a prescribed aerodynamic roughness length, although in the open ocean the moving surface waves may play a substantial role in the transport and fate of sea spray aerosol particles (Richter et al, 2019).…”

A Review of Parameterizations for Enthalpy and Momentum Fluxes from Sea Spray in Tropical Cyclones

“…The three studies Peng andRichter (2017, 2019) and Peng and Richter (2020) used direct numerical simulations (DNSs) with a Lagrangian-Eulerian framework and focused on whether common assumptions made by many bulk flux algorithms, including neglecting interactions between spray drops and neglecting feedbacks between spray fluxes and the ambient conditions, are valid assumptions. The 3D domains used throughout these experiments were all on the order of 0.01 m 3 , had a no slip lower boundary condition, and a stress-free upper boundary condition.…”

“…Improving the representation of air-sea enthalpy and momentum fluxes in particular is key to improving intensity forecast accuracy. Since sea spray has been shown to mediate a significant percentage of the total enthalpy and momentum fluxes in TCs (Andreas and Emanuel 2001, hereafter AE1;Andreas 2004;Zhao et al 2006;Andreas 2010;Richter and Stern 2014;Mueller and Veron 2014b;Troitskaya et al 2018b;Peng and Richter 2017, 2020, bulk parameterizations have begun to incorporate the microphysics associated with sea spray-mediated fluxes. In particular, many of the investigations specifically focus on the way sea spray affects the bulk surface exchange coefficients for enthalpy C K and momentum C D .…”