Spray-Mediated Enthalpy Flux to the Atmosphere and Salt Flux to the Ocean in High Winds

Abstract

Forecasts for the intensity and intensity changes of tropical cyclones have not improved as much as track forecasts. In high winds, two routes exist by which air and sea exchange heat and momentum: by spraymediated processes and by interfacial transfer right at the air-sea interface, the only exchange route currently parameterized in most storm models. This manuscript quantifies two processes mediated by sea spray that could affect predictions of storm intensity when included in coupled ocean-atmosphere models. Because newly formed spray droplets cool rapidly to an equilibrium temperature that is lower than the air temperature, they cool the ocean when they reenter it, clearly transferring enthalpy from sea to air. These reentrant droplets proliferate in storm winds and are predicted to transfer enthalpy at a rate comparable to interfacial processes when the near-surface wind speed reaches 30 m s 21 . Because reentrant spray droplets give up pure water to the atmosphere during their brief lifetime, they return to the sea saltier than the surface ocean water and thus also constitute an effective salt flux to the ocean (also related to a freshwater flux and a buoyancy flux). That is, reentrant spray droplets add enthalpy to the atmosphere to power storms and destabilize the ocean by increasing the salinity at the surface. Both processes can affect storm intensity. This manuscript demonstrates the magnitudes of the spray enthalpy and salt fluxes by combining a sophisticated microphysical model and data from the study of Humidity Exchange over the Sea (HEXOS) and the Fronts and Atlantic Storm-Tracks Experiment (FASTEX). It goes on to develop a fast algorithm for predicting these two fluxes in large-scale models.

Predicting the Turbulent Air-Sea Surface Fluxes, Including Spray Effects, from Weak to Strong Winds

“…As with most such algorithms, inputs are sea surface temperature and salinity; barometric pressure; and surface-level values of wind speed, air temperature, and humidity. Besides the heat fluxes plotted in Figure 9, our algorithm also predicts the surface stress or friction velocity (u * ,B ) and both the interfacial and spray components of the enthalpy, salt, and freshwater fluxes (Andreas 2010). Our work on the vertical structure of the coastal zone boundary layer off of the East Coast of the U.S. reveals a different type of low-level wind maximum than for the extensively studied jets on the West Coast.…”

“…Meanwhile, the spray terms (H L,sp , H s,sp , Q en,sp ) in our full flux model require extensive microphysically based calculations, as described by Andreas and DeCosmo (2002), Andreas et al (2008), and Andreas (2010). Using the datasets we have assembled (summarized in , we can evaluate this hypothesized flux algorithm.…”

“…Andreas has also developed a software "kit" that contains instructions and the Fortran programs necessary to implement the bulk air-sea flux algorithm described by Andreas et al (2008) and Andreas (2010). The current version of that code is 3.4, and the kit is posted at http://www.nwra.com/resumes/andreas/software.php, where it can be freely downloaded.…”

Spray-Mediated Air-Sea Gas Exchange: The Governing Time Scales

“…We therefore begin this formal treatment of how and whether sea spray might influence the rate of air-sea gas transfer following the approach that Andreas and colleagues used for assessing how sea spray affects air-sea heat and moisture transfer [9,[27][28][29][30][31]. Microphysical modeling of the temperature and radius evolution of individual spray droplets underlies most of this work [32][33][34][35].…”

A Numerical Study on Physical Characterizations of Microwave Scattering and Emission From Ocean Foam Layer

“…Foam layers on ocean surfaces due to breaking waves have attracted interest in the past decades for its various applications in geophysical parametric retrievals [1], climate modeling [2], and ocean wind sensing, among several others. Studies have shown that a foam layer profoundly affects the microwave emission from a sea surface; its presence increases the effective emissivity of the sea surface [3].…”