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Edgar L. Andreas

Abstract

The sea spray generation function quantifies the rate at which spray droplets of a given size are produced at the sea surface. As such, it is important in studies of the marine aerosol and its optical properties and in understanding the role that sea spray plays in transferring heat and moisture across the air–sea interface. The emphasis here is on this latter topic, where uncertainty over the spray generation function, especially in high winds, is a major obstacle. This paper surveys the spray generation functions available in the literature and, on theoretical grounds, focuses on one by M. H. Smith et al. that has some desirable properties but does not cover a wide enough droplet size range to be immediately useful for quantifying spray heat transfer. With reasonable modifications and extrapolations, however, the paper casts the Smith function into a new form that can be used to predict the production of sea spray droplets with radii from 2 to 500 μm for 10-m winds from 0 to 32.5 m s−1. The paper closes with sample calculations of the sensible and latent heat fluxes carried by spray that are based on this new spray generation function.

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Edgar L. Andreas

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Surface-level meteorological observations and upper-air soundings in the Weddell Sea provide the first in situ look at conditions over the deep Antarctic ice pack in the spring. The surface‐level temperature and humidity were relatively high, and both were positively correlated with the northerly component of the 850 mb wind vector as far as 600 km from the ice edge. Since even at its maximum extent at least 60% of the Antarctic ice pack is within 600 km of the open ocean, long‐range atmospheric transport of heat and moisture from the ocean must play a key part in Antarctic sea ice heat and mass budgets. From one case study, the magnitude of the ocean's role is inferred: at this time of year the total turbulent surface heat loss can be 100 W m−2 greater under southerly winds than under northerly ones.

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Edgar L. Andreas

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Edgar L. Andreas

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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 spray-mediated 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−1. 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.

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Edgar L. Andreas

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Edgar L Andreas

Abstract

With sea ice in the Arctic continuing to shrink, the Arctic Ocean and the surrounding marginal seas will become more like the ocean at lower latitudes. In particular, with more open water, air–sea exchange will be more intense and storms will be stronger and more frequent. The longer fetches over open water and the more energetic storms will combine to produce higher waves and more sea spray. Offshore structures—such as oil drilling, exploration, and production platforms—will face increased hazards from freezing sea spray. On the basis of sea spray observations made with a cloud-imaging probe at Mount Desert Rock (an island off the coast of Maine), the spray that artificial islands built in the Arctic might experience is quantified. Mount Desert Rock is small, low, and unvegetated and has an abrupt, rocky shoreline like these artificial islands might have. Many of the observations were at air temperatures below freezing. This paper reports the near-surface spray concentration and the rate of spray production at this rocky shoreline for spray droplets with radii from 6.25 to 143.75 μm and for wind speeds from 5 to 17 m s−1. Spray concentration increases as the cube of the wind speed, but the shape of the concentration spectrum with respect to radius does not change with wind speed. Both near-surface spray concentration and the spray-production rate are three orders of magnitude higher at this rocky shoreline than over the open ocean because of the high energy and resulting continuous white water in the surf zone.

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Edgar L. Andreas
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Edgar L Andreas

Abstract

A traditional use of scintillometry is to infer path-averaged values of the turbulent surface fluxes of sensible heat Hs and momentum τ ( , where ρ is air density and u * is the friction velocity). Many scintillometer setups, however, measure only the path-averaged refractive-index structure parameter ; the wind information necessary for inferring u * and Hs comes from point measurements or is absent. The Scintec AG SLS20 surface-layer scintillometer system, however, measures both and the inner scale of turbulence 0, where 0 is related to the dissipation rate of turbulent kinetic energy ɛ. The SLS20 is thus presumed to provide path-averaged estimates of both u * and Hs . This paper describes comparisons between SLS20-derived estimates of u * and Hs and simultaneous eddy-covariance measurements of these quantities during two experiments: one, over Arctic sea ice; and a second, over a midlatitude land site during spring. For both experiments, the correlation between scintillometer and eddy-covariance fluxes is reasonable: correlation coefficients are typically above 0.7 for the better-quality data. For both experiments, though, the scintillometer usually underestimates u * and underestimates the magnitude of Hs when compared with the corresponding eddy-covariance values. The data also tend to be more scattered when < 10−14 m−2/3: the signal-to-noise ratio for scintillometer-derived fluxes decreases as decreases. An essential question that arises during these comparisons is what similarity functions to use for inferring fluxes from the scintillometer and 0 measurements. The paper thus closes by evaluating whether any of four candidate sets of similarity functions is consistent with the scintillometer data.

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Edgar L. Andreas

Abstract

In winds approaching hurricane strength, spray droplets proliferate. Once created, these droplets accelerate to the local wind speed in 1 s or less and thereby extract momentum from the wind. Because these droplets have substantial mass, they eventually plunge back into the ocean, delivering their horizontal momentum to the surface in the form of a spray stress. Inadequate information on the production rate and size distribution of spray droplets, however, hampered previous attempts to estimate the magnitude of this spray-mediated momentum exchange. This paper therefore uses recent estimates of the spray generation function to reconsider spray's ability to alter air–sea momentum exchange. Conservation of momentum requires that spray cannot enhance the air– sea stress beyond what the large-scale flow dictates. However, spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate. For a wind of 30 m s−1, spray supports about 10% of the surface stress; for a wind of about 60 m s−1, spray supports all of the surface stress. The paper goes on to show how this partitioning affects the near-surface wind speed profile. Last, the paper reviews evidence that suggests the sea surface undergoes a transition in its aerodynamic behavior in the wind speed range 30–40 m s−1. The fact that whitecap coverage extrapolates to 100% in this range may be one cause. Also in this range, the “rain” of spray droplets back onto the sea surface creates a mass flux with a magnitude that has been shown to damp the short waves that sustain most of the atmospheric drag on the sea surface. As a consequence, spray may play a key role in a negative feedback loop that limits air–sea momentum transfer.

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Edgar L. Andreas

Abstract

Several electro-optical methods exist for measuring a path-averaged value of the inner scale of turbulence l 0. By virtue of Monin–Obukhov similarity, in the atmospheric surface layer such l 0 measurements are related to the friction velocity u * or to the surface stress τ = &rhou * 2, where ρ is the air density. Because l 0 is a path-averaged quantity, u * is too. Here the question of how precisely u * can be measured is investigated by combining these inner-scale measurements with two-wavelength scintillation measurements that yield the sensible and latent heat fluxes and, thereby, facilitate stability collections. The analysis suggests that current path-averaging instruments can generally measure u * to ± 20%–30%.

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