A sensor which detects ice accretion on ground-based structures (IMIOI Ice Monitor) uses surface

It is not known whether sea spray droplets can act as agents that influence air-sea gas exchange. We begin to address that question here by evaluating the time scales that govern spray-mediated air-sea gas transfer. To move between the interior of a spray droplet and the atmospheric gas reservoir, gas molecules must complete three distinct steps: (1) Gas molecules must mix between the interior surface and the deep interior of the aqueous solution droplet; time scale τaq estimates the rate of this transfer; (2) Molecules must cross the droplet’s interface; time scale τint parameterizes this transfer; and (3) The molecules must transit a “jump” layer between a spray droplet’s exterior surface and the atmospheric gas reservoir; time scale τair dictates the rate of this transfer. The same steps, in reverse order, pertain to gas molecules moving from an atmospheric reservoir to a drop’s interior. For the six most plentiful gases, excluding water vapor, in the atmosphere—helium, neon, argon, oxygen, nitrogen, and carbon dioxide—τair, τint, and τaq are shorter than the time scales that quantify the rate at which a newly formed spray droplet’s temperature, radius, and salinity evolve. We therefore conclude that, following the assumptions herein, a model for spray-mediated air-sea gas exchange can assume that the gas concentration in spray droplets is always in instantaneous equilibrium with the local atmospheric gas concentration.

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 mu m 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.

AbstractSummertime eddy correlation measurements from an offshore tower are analyzed to investigate the dependence of the friction velocity for stable conditions on the mean wind speed V, air–sea difference of virtual potential temperature δθυ, and nonstationary submeso motions. The quantity δθυ sometimes exceeds 3°C, usually because of the advection of warm air from land over cooler water at this site. Thin stable boundary layers result. Unexpectedly, does not depend systematically on the stratification δθυ even for weak winds. For weak winds, increases systematically with increasing submeso variations of the wind. The relationship for a given V is greater in nonstationary conditions. Additionally, this study examines as a function of wind direction. The relationship appears to be affected by swell direction for weak winds and advection from land for short fetches.

For over 30 years, air-sea interaction specialists have been evaluating and parameterizing the role of whitecap bubbles in air-sea gas exchange. To our knowledge, no one, however, has studied the mirror image process of whether sea spray droplets can facilitate air-sea gas exchange. We are therefore using theory, data analysis, and numerical modeling to quantify the role of spray on air-sea gas transfer. In this, our first formal work on this subject, we seek the rate-limiting step in spray-mediated gas transfer by evaluating the three time scales that govern the exchange: τair, which quantifies the rate of transfer between the atmospheric gas reservoir and the surface of the droplet; τint, which quantifies the exchange rate across the air-droplet interface; and τaq, which quantifies gas mixing within the aqueous solution droplet.

Nature is wild, unconstrained, and often dangerous. In particular, studying air-sea interaction in winds typical of tropical cyclones can place researchers, their instruments, and even their research platforms in jeopardy. As an alternative, laboratory wind-water tunnels can probe 10-m equivalent winds of hurricane strength under conditions that are well constrained and place no personnel or equipment at risk. Wind-water tunnels, however, cannot simulate all aspects of air-sea interaction in high winds. The authors use here the comprehensive data from the Air-Sea Interaction Salt Water Tank (ASIST) wind-water tunnel at the University of Miami that Jeong, Haus, and Donelan published in this journal to demonstrate how spray-mediated processes are different over the open ocean and in wind tunnels. A key result is that, at all high-wind speeds, the ASIST tunnel was able to quantify the so-called interfacial air-sea enthalpy fluxthe flux controlled by molecular processes right at the air-water interface. This flux cannot be measured in high winds over the open ocean because the ubiquitous spray-mediated enthalpy transfer confounds the measurements. The resulting parameterization for this interfacial flux has implications for modeling air-sea heat fluxes from moderate winds to winds of hurricane strength.

Numerical models of the atmosphere, oceans, and sea ice are divided into horizontal grid cells that can range in size from a few kilometers to hundreds of kilometers. In these models, many surface-level variables are assumed to be uniform over a grid cell. Using a year of in situ data from the experiment to study the Surface Heat Budget of the Arctic Ocean (SHEBA), the authors investigate the accuracy of this assumption of gridcell uniformity for the surface-level variables pressure, air temperature, wind speed, humidity, and incoming longwave radiation. The paper bases its analysis on three statistics: the monthly average and, for each season, the spatial correlation function and the spatial bias. For five SHEBA sites, which had a maximum separation of 12 km, the analysis supports the assumption of gridcell uniformity in pressure, air temperature, wind speed, and humidity in all seasons. In winter, when the incidence of fractional cloudiness is largest, the incoming longwave radiation may not be uniform over a grid cell. In other seasons, the bimodal distribution in cloud cover-either clear skies or total cloud cover-tends to homogenize the incoming radiation at scales of 12 km and less.

This article describes a local similarity theory for developed turbulence in the stably stratified boundary layer that is based on the Brunt–Väisälä frequency and the dissipation rate of turbulent kinetic energy instead of the turbulent fluxes used in the traditional Monin–Obukhov similarity theory. Based on dimensional analysis (Pi theorem), it is shown that any properly scaled statistics of the small‐scale turbulence are universal functions of a stability parameter defined as the ratio of a reference height z and the Dougherty–Ozmidov length scale, which in the limit of z ‐less stratification is linearly proportional to the Obukhov length scale. Measurements of atmospheric turbulence made at five levels on a 20 m tower over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to examine the behaviour of different similarity functions in the stable boundary layer. In the framework of this approach the non‐dimensional turbulent viscosity is equal to the gradient Richardson number, whereas the non‐dimensional turbulent thermal diffusivity is equal to the flux Richardson number. These results are a consequence of the approximate local balance between production of turbulence by shear in the mean flow and viscous dissipation. The turbulence framework based on the Brunt–Väisälä frequency and the dissipation rate of turbulent kinetic energy may have practical advantages for estimating turbulence when the fluxes are not directly available.

Over 5000 aircraft eddy-covariance measurements from four different aircraft in nine different experiments are used to develop a simple model for the friction velocity over the sea. Unlike the widely used Coupled Ocean-Atmosphere Response Experiment (COARE) bulk flux scheme, the simple model (i) does not use Monin-Obukhov similarity theory (MOST) and therefore does not require an estimate of the Obukhov length, (ii) does not require a correction to the wind speed for height or stability, (iii) does not require an estimate of the aerodynamic roughness length, and (iv) does not require iteration. In comparing the model estimates developed in this work and those of the COARE algorithm, comparable fitting metrics for the two modeling schemes are found. That is, using Monin-Obukhov similarity theory and the Charnock relationship did not significantly improve the predictions. It is not clear how general the simple model proposed here is, but the same model with the same coefficients based on the combined dataset does a reasonable job of describing the datasets both individually and collectively. In addition, the simple model was generally able to predict the observed friction velocities for three independent datasets that were not used in tuning the model coefficients. Motivation for the simple model comes from the fact that physical interpretation of MOST can be ambiguous because of circular dependencies and self-correlation. Additional motivation comes from the large uncertainty associated with estimating the Obukhov length and, especially, the aerodynamic roughness length.

A bulk air–sea flux algorithm couples the ocean and the lower atmosphere through flux boundary conditions and can be used in various analyses and in numerical models. The algorithm described here has two features not present in any other existing bulk flux algorithm. First, it has a new air–sea drag relation. Here, for wind speeds above about 9 m s−1, the friction velocity u✻, which is related to the square root of the surface stress, is linearly related to UN10, the neutral‐stability 10 m wind speed. When extrapolated to hurricane‐strength winds, this drag relation has better properties than relations formulated in terms of a drag coefficient or a roughness length. The second unique feature of this flux algorithm is that it recognizes two routes by which heat and moisture cross the air–sea interface: one is the interfacial route, which is controlled by molecular processes right at the air–sea interface; the second is the spray‐mediated route, which is governed by microphysical processes at the surface of sea spray droplets. Through microphysical theory and our analysis of 4000 sets of eddy‐covariance measurements of the scalar fluxes, we separate the measured fluxes into the interfacial and spray contributions and thereby produce the only spray flux algorithm tested and validated against oceanic data. Because all components of our flux algorithm are physics‐based and validated with data for winds up to 25 m s−1, one application is extrapolating this algorithm to hurricane‐strength winds, where sea spray plays a dominant role in scalar transfer.

The Bowen ratio, the ratio of the turbulent surface fluxes of sensible (Hs) and latent (HL) heat, Bo ≡ Hs/HL, occurs throughout micrometeorology. It finds application in the Bowen ratio and energy budget method, where it provides both turbulent heat fluxes when only the available energy at the surface is known. It can yield an estimate of a missing Hs or HL if the other flux is known. We also suggest that the Bowen ratio may provide the missing piece needed to infer the surface sensible heat flux from satellite data. For this study, we analyze almost 9000 eddy‐covariance measurements of Hs and HL. About half were made over sea ice; the other half, over the open ocean. These are saturated surfaces where the surface specific humidity is the saturation value at the surface temperature. Surface temperatures ranged from −44°C to 32°C and predict the Bowen ratio through the Bowen ratio indicator, . Here cp is the specific heat of air at constant pressure, Lv is the latent heat of sublimation or vaporization, and is the derivative of the saturation specific humidity (Qsat) with temperature (Θ). All quantities are evaluated at the surface temperature, Θs. Although Hs and HL can occur in nine possible combinations, in our data set, three combinations represent over 90% of the cases: Hs > 0 and HL > 0, Hs < 0 and HL < 0, and Hs < 0 and HL > 0. In each of these three cases, the data suggest , where a is 0.40, 3.27, and −0.65, respectively.

: The goal is to investigate, through theory and by analyzing existing data, sea surface physics and air-sea exchange in winds that range from weak to hurricane-strength. Ultimately, we want to develop unified parameterizations for the fluxes of momentum, sensible and latent heat, and enthalpy across the air-sea interface. These flux parameterizations will provide improved model coupling between the ocean and the atmosphere and, in essence, set the lower flux boundary conditions on atmospheric models and the upper flux boundary conditions on ocean models.

Measurements of atmospheric turbulence made over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to determine the limits of applicability of Monin-Obukhov similarity theory (in the local scaling formulation) in the stable atmospheric boundary layer. Based on the spectral analysis of wind velocity and air temperature fluctuations, it is shown that, when both of the gradient Richardson number, Ri, and the flux Richardson number, Rf, exceed a 'critical value' of about 0.20 - 0.25, the inertial subrange associated with the Richardson-Kolmogorov cascade dies out and vertical turbulent fluxes become small. Some small-scale turbulence survives even in this supercritical regime, but this is non-Kolmogorov turbulence, and it decays rapidly with further increasing stability. Similarity theory is based on the turbulent fluxes in the high-frequency part of the spectra that are associated with energy-containing/flux-carrying eddies. Spectral densities in this high-frequency band diminish as the Richardson-Kolmogorov energy cascade weakens; therefore, the applicability of local Monin-Obukhov similarity theory in stable conditions is limited by the inequalities Ri < Ri_cr and Rf < Rf_cr. However, it is found that Rf_cr = 0.20 - 0.25 is a primary threshold for applicability. Applying this prerequisite shows that the data follow classical Monin-Obukhov local z-less predictions after the irrelevant cases (turbulence without the Richardson-Kolmogorov cascade) have been filtered out.

The diurnal cycles of near-surface meteorological parameters over Antarctic sea ice in six widely used atmospheric reanalyses are validated against observations from Ice Station Weddell. The station drifted from February through May 1992 and provided the most extensive set of meteorological observations ever collected in the Antarctic sea ice zone. For the radiative and turbulent surface fluxes, both the amplitude and shape of the diurnal cycles vary considerably among different reanalyses. Near-surface temperature, specific humidity, and wind speed in the reanalyses all feature small diurnal ranges, which, in most cases, fall within the uncertainties of the observed cycle. A skill score approach revealed the superiority of the ERA-Interim reanalysis in reproducing the observed diurnal cycles. An explanation for the shortcomings in the reanalyses is their failure to capture the diurnal cycle in cloud cover fraction, which leads to errors in other quantities as well. Apart from the diurnal cycles, NCEP-CFSR gave the best error statistics.

A Rutan Aircraft Factory Long-EZ aircraft flew numerous low-level slant soundings on two summer days in 2001 off the northeastern coast of the United States. The soundings are analyzed here to study the nonstationary vertical structure of the wind, temperature, and turbulence. An error analysis indicates that fluxes computed from the aircraft slant soundings are unreliable. The first day is characterized by a weakly stable boundary layer in onshore flow capped by an inversion. A low-level wind maximum formed at about 100 m above the sea surface. The second day is characterized by stronger stability due to advection of warm air from the upwind land surface. On this more stable day, the wind maxima are very sharp and the speed and height of the wind maxima increase with distance from the coast. Although trends in the vertical structure are weak, variations between subsequent soundings are large on time scales of tens of minutes or less. The vertical structure of the wind and turbulence is considerably more nonstationary than the temperature structure, although the existence of the wind maximum is persistent. Causes of the wind maxima and their variability are examined but are not completely resolved.

From almost 7000 near-surface eddy-covariance flux measurements over the sea, the authors deduce a new air-sea drag relation for aerodynamically rough flow:u(*) = 0.0583U(N10) - 0.243.Here u(*) is the measured friction velocity, and U-N10 is the neutral-stability wind speed at a reference height of 10 m. This relation is fitted to U-N10 values between 9 and 24 m s(-1). A drag relation formulated as u(*) versus U-N10 has several advantages over one formulated in terms of C-DN10 = (u(*)/U-N10)(2). First, the multiplicative coefficient on U-N10 has smaller experimental uncertainty than do determinations of C-DN10. Second, scatterplots of u(*) versus U-N10 are not ill posed when U-N10 is small, as plots of C-DN10 are; u(*)-U-N10 plots presented here suggest aerodynamically smooth scaling for small U-N10. Third, this relation depends only weakly on Monin-Obukhov similarity theory and, consequently, reduces the confounding effects of artificial correlation. Finally, with its negative intercept, the linear relation produces a C-DN10 function that naturally rolls off at high wind speed and asymptotically approaches a constant value of 3.40 X 10(-3). Hurricane modelers and the air-sea interaction community have been trying to rationalize such behavior in the drag coefficient for at least 15 years. This paper suggests that this rolloff in C-DN10 results simply from known processes that influence wind-wave coupling.

A hierarchy of parametrizations of the neutral 10 m drag coefficients over polar sea ice with different morphology regimes is derived on the basis of a partitioning concept that splits the total surface drag into contributions of skin drag and form drag. The new derivation, which provides drag coefficients as a function of sea ice concentration and characteristic length scales of roughness elements, needs fewer assumptions than previous similar approaches. It is shown that form drag variability can explain the variability of surface drag in the marginal sea ice zone (MIZ) and in the summertime inner Arctic regions. In the MIZ, form drag is generated by floe edges; in the inner Arctic, it is generated by edges at melt ponds and leads due to the elevation of the ice surface relative to the open water surface. It is shown that an earlier fit of observed neutral drag coefficients is obtained as a special case within the new concept when specific simplifications are made which concern the floe and melt pond geometry. Due to the different surface morphologies in the MIZ and summertime Arctic, different functional dependencies of the drag coefficients on the sea ice concentration result. These differences cause only minor differences between the MIZ and summertime drag coefficients in average conditions, but they might be locally important for atmospheric momentum transport to sea ice. The new parametrization formulae can be used for present conditions but also for future climate scenarios with changing sea ice conditions.

The variation of the sea surface sensible heat flux is investigated using data from the Gulf of Tehuantepec Experiment (GOTEX) and from eight additional aircraft datasets representing a variety of surface conditions. This analysis focuses on near-neutral conditions because these conditions are common over the sea and are normally neglected, partly because of uncertain reliability of measurements of the small air sea temperature difference. For all of the datasets, upward heat flux is observed for slightly stable conditions. The frequency of this "countergradient" heat flux increases with increasing wind speed and is possibly related to sea spray or microscale variations of surface temperature on the wave scale. Upward area-averaged sensible heat flux for slightly stable conditions can also be generated by mesoscale heterogeneity of the sea surface temperature (SST). Significant measurement errors cannot be ruled out.The countergradient heat flux for weakly stable conditions is least systematic for weaker winds, even though it occurs with weak winds in all of the datasets. In an effort to reduce offset errors and different SST processing and calibration procedures among field programs, the authors adjusted the SST in each field program to minimize the countergradient flux for weak winds. With or without this adjustment for the combined dataset, the extent of the upward heat flux for weakly stable conditions increases with increasing wind speed.