Air-Sea Interaction Experiment Research Articles

Evidence of Sea-State Dependence of Aerosol Concentration in the Marine Atmospheric Boundary Layer

AbstractSea spray aerosols represent a large fraction of the aerosols present in the maritime environment. Despite evidence of the importance of surface wave– and wave breaking–related processes in coupling the ocean with the atmosphere, sea spray source generation functions are traditionally parameterized by the 10-m wind speed U10 alone. It is clear that unless the wind and wave field are fully developed, the source function will be a function of both wind and wave parameters. This study reports primarily on the aerosol component of an air–sea interaction experiment, the phased-resolved High-Resolution Air–Sea Interaction Experiment (HIRES), conducted off the coast of northern California in June 2010. Detailed measurements of aerosol number concentration in the marine atmospheric boundary layer (MABL) at altitudes ranging from as low as 30 m up to 800 m above mean sea level (MSL) over a broad range of environmental conditions (significant wave height Hs of 2 to 4.5 m and U10 from 10 to 18 m s−1) collected from an instrumented research aircraft are presented. Aerosol number densities and volume are computed over a range of particle diameters from 0.1 to 200 μm, while the sea surface conditions, including Hs, moments of the breaker length distribution Λ(c), and wave breaking dissipation, were measured by a suite of electro-optical sensors that included the NASA Airborne Topographic Mapper (ATM). The sea-state dependence of the aerosol concentration in the MABL is evident, stressing the need to incorporate wave parameters in the spray source generation functions that are traditionally parameterized by surface winds alone.

Air–Sea Interaction in the Ligurian Sea: Assessment of a Coupled Ocean–Atmosphere Model Using In Situ Data from LASIE07

Abstract In situ experimental data and numerical model results are presented for the Ligurian Sea in the northwestern Mediterranean. The Ligurian Sea Air–Sea Interaction Experiment (LASIE07) and LIGURE2007 experiments took place in June 2007. The LASIE07 and LIGURE2007 data are used to validate the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)1 developed at the Naval Research Laboratory. This system includes an atmospheric sigma coordinate, nonhydrostatic model, coupled to a hydrostatic sigma-z-level ocean model (Navy Coastal Ocean Model), using the Earth System Modeling Framework (ESMF). A month-long simulation, which includes data assimilation in the atmosphere and full coupling, is compared against an uncoupled run where analysis SST is used for computation of the bulk fluxes. This reveals that COAMPS has reasonable skill in predicting the wind stress and surface heat fluxes at LASIE07 mooring locations in shallow and deep water. At the LASIE07 coastal site (but not at the deep site) the validation shows that the coupled model has a much smaller bias in latent heat flux, because of improvements in the SST field relative to the uncoupled model. This in turn leads to large differences in upper-ocean temperature between the coupled model and an uncoupled ocean model run.

The intOA Experiment: A Study of Ocean-Atmosphere Interactions Under Moderate to Strong Offshore Winds and Opposing Swell Conditions in the Gulf of Tehuantepec, Mexico

The Gulf of Tehuantepec air–sea interaction experiment (intOA) took place from February to April 2005, under the Programme for the Study of the Gulf of Tehuantepec (PEGoT, Spanish acronym for Programa para el Estudio del Golfo de Tehuantepec). PEGoT is underway aiming for better knowledge of the effect of strong and persistent offshore winds on coastal waters and their natural resources, as well as performing advanced numerical modelling of the wave and surface current fields. One of the goals of the intOA experiment is to improve our knowledge on air–sea interaction processes with particular emphasis on the effect of surface waves on the momentum flux for the characteristic and unique conditions that occur when strong Tehuano winds blow offshore against the Pacific Ocean long period swell. For the field campaign, an air–sea interaction spar (ASIS) buoy was deployed in the Gulf of Tehuantepec to measure surface waves and the momentum flux between the ocean and the atmosphere. High frequency radar systems (phase array type) were in operation from two coastal sites and three acoustic Doppler current profilers were deployed near-shore. Synthetic aperture radar images were also acquired as part of the remote sensing component of the experiment. The present paper provides the main results on the wave and wind fields, addressing the direct calculation of the momentum flux and the drag coefficient, and gives an overview of the intOA experiment. Although the effect of swell has been described in recent studies, this is the first time for the very specific conditions encountered, such as swell persistently opposing offshore winds and locally generated waves, to show a clear evidence of the influence on the wind stress of the significant steepness of swell waves.

Observed development of the vertical structure of the marine boundary layer during the LASIE experiment in the Ligurian Sea

Abstract. In the marine environment, complete datasets describing the surface layer and the vertical structure of the Marine Atmospheric Boundary Layer (MABL), through its entire depth, are less frequent than over land, due to the high cost of measuring campaigns. During the seven days of the Ligurian Air-Sea Interaction Experiment (LASIE), organized by the NATO Undersea Research Centre (NURC) in the Mediterranean Sea, extensive in situ and remote sensing measurements were collected from instruments placed on a spar buoy and a ship. Standard surface meteorological measurements were collected by meteorological sensors mounted on the buoy ODAS Italia1 located in the centre of the Gulf of Genoa. The evolution of the height (zi) of the MABL was monitored using radiosondes and a ceilometer on board of the N/O Urania. Here, we present the database and an uncommon case study of the evolution of the vertical structure of the MABL, observed by two independent measuring systems: the ceilometer and radiosondes. Following the changes of surface flow conditions, in a sequence of onshore – offshore – onshore wind direction shifting episodes, during the mid part of the campaign, the overall structure of the MABL changed. Warm and dry air from land advected over a colder sea, induced a stably stratified Internal Boundary Layer (IBL) and a consequent change in the structure of the vertical profiles of potential temperature and relative humidity.

Effects of air-sea interaction parameters on ocean surface microwave emission at 10 and 37 GHz

WindSat, the first polarimetric radiometer on orbit, launched in January 2003, provides the promise of passive ocean wind vector retrievals on a continuous basis, simultaneous with the retrieval of many other geophysical variables such as sea surface temperature, atmospheric water vapor, cloud liquid water, and sea ice extent and concentration. WindSat also serves as risk reduction for the upcoming National Polar-orbiting Operational Environmental Satellite System (NPOESS) Conical Scanning Microwave Imager/Sounder (CMIS). Since the dependence of microwave brightness temperatures on wind direction is small relative to that of other parameters such as wind speed, wind direction retrieval relies on increasingly accurate knowledge of the ocean surface microwave emission, which depends upon surface properties such as roughness and foam due to wave breaking. Coordinated near-surface measurements of ocean surface microwave emission and air-sea interaction parameters are needed to quantify the effects of the processes mentioned above in surface emission models to improve the accuracy of wind vector retrievals. Such coordinated observations were performed during the Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) experiment conducted on the R/P Floating Instrument Platform (FLIP) in the northeastern Pacific Ocean during the Fall of 2000. X- and Ka-band partially polarimetric radiometers were mounted at the end of the port boom of R/P FLIP to measure ocean surface emission at incidence angles of 45/spl deg/, 53/spl deg/, and 65/spl deg/. A bore-sighted video camera recorded the fractional area of foam in the field of view of the radiometers. Air-sea interaction parameters that were measured concurrently include wind speed, friction velocity, heat fluxes, and significant wave height. The measured dependence of ocean surface emissivity on wind speed and friction velocity is in good agreement with, and extends, earlier observations and empirical models based on satellite data. Concurrent radiometric measurements and fractional area foam coverage data strengthen the possibility of retrieval of sea surface foam coverage using airborne or spaceborne radiometry. The dependence of emissivity on atmospheric stability is shown to be much smaller than the dependence of emissivity on wind speed. Analysis of emissivity dependence on atmospheric stability alone was inconclusive, due to the variation in atmospheric stability with wind speed. The effect of long-wave incidence angle modulation on sea surface emissivity for near-surface measurements was found to be negligible when emissivity measurements were averaged over tens to hundreds of long waves.

The Southern Ocean Waves Experiment. Part II: Sea Surface Response to Wind Speed and Wind Stress Variations

Abstract The Southern Ocean Waves Experiment (SOWEX) was an international collaborative air–sea interaction experiment in which a specially instrumented meteorological research aircraft simultaneously gathered marine boundary-layer atmospheric turbulence data and sea surface roughness data over the Southern Ocean, particularly for gale-force wind conditions. In this paper analysis and findings are presented on key aspects of the coupled variability of the wind field, the wind stress, and the underlying sea surface roughness. This study complements the overview, methodology, and mean results published in Part I. Weakly unstable atmospheric stratification conditions prevailed during SOWEX, with wind speeds ranging from gale force to light and variable. Throughout the SOWEX observational period, the wind field was dominated by large-scale atmospheric roll-cell structures, whose height scale was comparable with the thickness of the marine atmospheric boundary layer (MABL). Well above the sea surface, these co...

Wind Sea Growth and Dissipation in the Open Ocean

Abstract Wind sea growth and dissipation in a swell-dominated, open ocean environment is investigated to explore the use of wave parameters in air–sea process modeling. Wind, wave, and whitecap observations are used from the Gulf of Alaska surface scatter and air–sea interaction experiment (Critical Sea Test-7, Phase 2), conducted 24 February through 1 March 1992. Wind sea components are extracted from buoy directional wave spectra using an inverted catchment area approach for peak isolation with both wave age criteria and an equilibrium range threshold used to classify the wind sea spectral domain. Dimensionless wind sea energy is found to scale with inverse wave age independently of swell. However, wind trend causes significant variations, such as underdeveloped seas during rising winds. These important effects are neglected in wind-forced air–sea process models. The total rate of wave energy dissipation is conveniently estimated using concepts from the Phillips equilibrium range theory. Replacing wind ...

Near‐inertial internal wave interactions with mesoscale fronts: Observations and models

Moored observations of near‐inertial currents and buoyancy fluctuations from the Frontal Air Sea Interaction Experiment (FASINEX) may be understood as nearinertial internal waves reflecting off sheared background currents. The FASINEX moored array measured upper ocean currents, temperature, and conductivity between January and June 1986 in a region of the Sargasso Sea heavily populated by strong upper ocean fronts. The background jets associated with these features have typical cross‐front scales of 20km, maximum currents of 0.5 m s−1, and relative vorticities of ±0.3ƒ. In the vicinity of fronts, observed near‐inertial currents have short horizontal scales, show no clear horizontal phase structure, and trace out elongated current ellipses with elevated across‐front over along‐front variance. During these periods, inertial band‐pass buoyancy fluctuations often correlate with major‐axis currents. WKB ray‐tracing models of propagating, monochromatic plane waves exhibit horizontal phase propagation and correlations between minor‐axis currents and buoyancy and thus cannot explain these results. Models of near‐inertial internal waves reflected or trapped by a barotropic frontal jet produce mode‐like structures with short horizontal scales, little horizontal phase propagation, and anisotropic current ellipses aligned primarily cross front. Model buoyancy may correlate with one or both current ellipse components, depending on the incidence angle of the wave and the degree to which it reflects. Completely reflected waves with horizontal scales similar to those of the front and near‐normal incidence angle produce current ellipses and correlation patterns consistent with the observations.

The Subinertial Momentum Balance of the North Atlantic Subtropical Convergence Zone

Abstract The upper-ocean to forcing by pressure gradients and wind stress is examined using observations from the Frontal Air–Sea Interaction Experiment. A moored way acquired time series of winds. upper-ocean currents, temperatures, and salinities between winter and late spring in a region of the Sargasso Sea known for the presence of upper-ocean fronts. These fronts have timescales of 10 days and dominate current variance, while winds varied with the 4-day timescale of passing weather systems. Employing a frequency domain regression model. it is found that gestrophy accounts for most of the low-frequency (>100 h)current variance in the seasonal pycnoclino, but wind-forced shear becomes important nearer the surface. In particular, currents oriented in the typical NE–SW alonfront direction display geostrophic balance, while those perpendicular to them do not. Wind forcing can produce geostrophic currents indirectly through Ekman pumping. and knowledge of the geostrophic shear is required to distinguish be...

The Upper-Ocean Response to Surface Heating

Abstract Moored observations of atmospheric variables and upper-ocean temperatures from the Long-Term Upper-Ocean Study (LOTUS) and the Frontal Air-Sea Interaction Experiment (FASINEX) are used to examine the upper-ocean response to surface heating. FASINEX took place between January and June 1986 at 27°N, 7°M while LOTUS took place between May and October 1982 at 34°N, 7°W. The frequency-domain transfer function between rate of change of heat and the net surface heat flux is consistent with a one-dimensional heat balance between heating and convergence of vertical turbulent heat flux at timescales longer than the inertial. The observations satisfy the vertically integrated one-dimensional heat equation and indicate that the response to surface heating has been successfully isolated. Within the internal waveband, upward phase propagation in the response is inconsistent with a one-dimensional balance and the vertically integrated heat balance fails. The internal waveband response is explained as a balance ...

Wind Stress from Wave Slopes Using Phillips Equilibrium Theory

An open ocean, deep water air–sea interaction experiment was conducted in the Gulf of Alaska. Wave amplitude and slope data were measured using a WAVEC heave, pitch, and roll buoy that was let drift in the Alaska gyre. Wind stress estimates were obtained from a fast-sample anemometer using the dissipation technique and from synoptic measurements through a boundary-layer model. The fundamental correlation and predictive relationships between wind friction velocity and wave spectral properties were established. A comparison of the slope spectrum to simultaneous wind stresses allowed us to estimate the Phillips proposed universal constant β. Reintroducing this constant β into the Phillips slope spectrum and using measured slope spectral characteristics, an inferred wind stress was calculated that was shown to agree well with both the dissipation and model stresses thereby validating both Phillips theory and the boundary-layer model. Any discrepancies with the model stresses were attributed to second-order wave age effects. The roughness length z0, nondimensionalized by the sea rms wave height, was shown to decrease with wave age in a manner consistent with Kitaigorodskii's functional form. A general expression for Cd as a function of wind speed or friction velocity and wave age was proposed and verified with independent data.

Observations of Superinertial and Near-Inertial Wind-driven Flow

Abstract The superinertial and near-inertial wind-driven flow in the western North Atlantic is examined using data from two recent experiments. The Frontal Air–Sea Interaction Experiment (FASINEX) took place at 27°N, 70°W during 1986. The Long-Term Upper-Ocean Study (LOTUS) took place at 34°N, 70°W during 1982. Each experiment included moored measurements of meteorological variables that allowed estimation of the wind stress and oceanic currents. The directly wind-driven flow is isolated from other sources of variability, such as internal waves and mooring motion, using a transfer function between geocentric acceleration and wind stress. The transfer function is examined in rotary spectral bands bounded by periods of 36 and 12 h, and 12 and 2 h. For surface-moored observations, wind-driven mooring motion is found to cause a response that extends at least to 1000 m (much deeper than the frictional layer of direct wind forcing). Once this artifact is removed, the directly wind-driven flow is identified. Thi...

Linear instability of a mixed‐layer front

The linear instabilities of a geostrophically balanced frontal jet are computed numerically in an effort to understand the origin of intense small‐scale cold‐core features observed in the North Atlantic subtropical convergence zones during the Frontal Air‐Sea Interaction Experiment (FASINEX). A simple analytic continuously‐stratified basic state is used to represent the observed frontal structure. The most unstable linear modes have e‐folding timescales of 2–3 days, alongfront wavelengths near 70 km, and significant asymmetry across the front. The results provide indirect evidence that baroclinic instability of the frontal jet is a possible source of the cold‐core features observed during FASINEX.

Estudio de las características oceanográficas del Caribe colombiano. Región III, Zona I, PDCTM

El estudio de la variación estacional y espacial de las condiciones oceanográficas del Mar Caribe constituye un factor importante en el desarrollo de los Planes Nacionales de Ciencias y Tecnología del Mar en Colombia (PDCTM, 1990-2000) en cuyo contexto se enmarca el presente documento.
 En este trabajo se integran los resultados preliminares de observaciones tomadas durante el crucero Caribe I realizado en el Caribe colombiano (Región III, Zona 1, PDCTM) en la época de transición entre las estaciones seca-lluviosa de 1992.
 Se analizan datos registrados en 24 estaciones oceanográficas desde la superficie hasta 1000 metros de profundidad, obteniendo la distribución de las propiedades físico-químicas salinidad, oxígeno, temperatura y nutrientes, e identificando las masas de agua que conforman la región estudiada. Se demarca el área de influencia del sistema de surgencia.
 Como resultado del experimento de interacción océano-atmósfera, se analiza el comportamiento de la capa de mezcla en la región. Los requisitos de la variación diaria de la capa de mezcla se utilizan para validar un modelo numérico unidimensional que simula la evolución térmica del estrato superior del océano.
 Las conclusiones presentadas amplían el conocimiento actual en torno a la oceanografía del Mar Caribe y a la vez sirven de base para estudios futuros de la circulación general del Mediterráneo Americano.

Estudio de las características oceanográficas de Caribe colombiano. Región III, Zona I, PDCTM

The study of the seasonal and spatial variability of the oceanographic conditions of the Caribbean sea represents an important factor in the development of national plans in technology and science of the sea in Colombia. In this context, the following document is presented. Preliminary results are den ved from the measurements taken during transition period between the dry and wet seasons of 1992. The data registered in 24 stations are analysed for the upper 1000m. The distribution of the physico-chemical properties salinity, oxigen content, temperature and nutrients are evaluated. The water masses within the study region are identified. The area of influence of upwelling is infered. As a result of the air-sea interaction experiment, the variability of the mixed layer was analyzed. The obtained daily variations of the mixed la yen were used to val/date a one-dimension numerical model that simulates the evolution of the thermal structure of the upper layer of the ocean. The outcome of this study will contribute to a better understanding of the different oceanographic characteristics of the Caribbean sea. The present work will serve as a departure point for more general circulation studies of the American mediterranean.

Gulf of Alaska air-deployed SUS surface reverberation measurements

During the February/March 1992 Gulf of Alaska surface scatter and air–sea interaction experiment, air-deployed explosive charges (SUS) and sonobuoys were used to obtain surface scattering and reverberation data during three flights. At each of the surface scattering measurements sites, sonobuoys of different acoustic sensitivities were dropped within a 200-m circle to cover the wide dynamic range of the received SUS signals. SUS charges were then dropped at the location of these sonobuoys, providing monostatic reverberation measurements. On two of the flights, separated by 3 days in time, environmental measurements made from surface ships involved in the experiment indicates similar wind speeds. However, the measurement time periods were preceded by very high winds on one day and lower winds on the other day. Reverberation levels and surface scattering strength are addressed using the data from these two flights. [Work supported by the AEAS Program of the Office of Naval Research.]

Low-frequency distant surface/near-surface reverberation measurements in the Gulf of Alaska

Distant reverberation measurements in the surface/near-surface and air–sea interaction experiment in the Gulf of Alaska (February/March 1992) were designed to assess the Doppler and spatial characters of primarily low grazing angle reverberation as functions of frequency environmental conditions, and source steering angle. Twenty-two tests using cw and HFM pulses of 8- to 12-s duration were conducted during a variety of sea conditions, at wind speeds ranging from 4 to 19 m/s. These data were collected simultaneously at two frequencies (usually 250 and 1000 Hz). The data illustrate the contrast in the spectral and temporal characteristics of long-range reverberation (20 to 30 km) between low sea states where fish scattering is important and higher sea states where subsurface bubbles are important.

Oceanic whitecap coverage in the Gulf of Alaska during the February–March 1992 surface scattering and air–sea interaction experiment

The fraction of the sea surface covered by stage-A whitecaps, i.e., spilling wave crests, was determined for some 300 ten-minute daylight intervals between 19 February and 1 March 1992 from the analysis of video records collected from U. S. and Canadian research vessels in the Gulf of Alaska. These average stage-A whitecap coverage values were related to the relevant 10-m elevation wind speed values using the approach of Monahan and O’Muircheartaigh [Int. J. Remote Sensing 7, 627–642 (1986)], and the resulting functional relationship was compared to similar expressions derived from the analysis of stage-A whitecap records obtained during earlier open ocean experiments. The possible relationships between stage-A whitecap coverage, rate of bubble injection into the oceanic mixed layer, and the production of sound at discrete positions on the sea surface are described. [Work supported by the Office of Naval Research via Grants N00014-92-J-1167 and N00014-90-J-1538.]

Small scale inhomogeneity and intermittency of surface scattering during the 1992 Gulf of Alaska surface scatter and air–sea interaction experiment

An experimental and processing technique is described that provides high resolution, broadband estimates of near-surface acoustic scattering variation in time and space. Application of the technique in the 1992 Gulf of Alaska surface scatter and air–sea interaction experiment achieved approximately 1-m×5-m range and cross-range scattering resolution over the frequency range of 100–1400 Hz, with temporal changes measured over intervals of 7 s to 30 min. Results from this experiment clearly show that during high wind, high sea state conditions, the surface scattering was dominated by intense, highly localized events with spatial densities on the order of 10–100 per sq km. A significant fraction of these events endured for time scales of a few minutes only. Some of these discrete scatterers were sufficiently removed from the surface (20–30 m) to generate resolvable multipaths. Also described is the intensity and spatial density of scatterers as a function of the environmental conditions over the several day period of the experiment.

Broadband measurements of sea surface backscattering strength in the Gulf of Alaska

Measurements of low-frequency (70–1000 Hz) sea surface backscattering strength were made in the Gulf of Alaska during February–March 1992 as part of a surface backscatter and air–sea interaction experiment. The measurements described here used explosive charges (SUS MK-59 MOD1a) as sources and a towed horizontal line array of hydrophones as receiver. The experiment objective was to measure surface backscattering strength as a function of frequency, grazing angle, and environmental conditions of the air–sea boundary. Scattering strengths were obtained during 18 tests (each of duration approximately 30 min) conducted from 24 February to 1 March for wind speeds that varied from approximately 4 to 18 m/s. The experiment uses a direct path test geometry for which the measured surface backscatter originated from surface/near-surface regions at ranges of several km or less. Surface grazing angles for these measurements varied from about 5 to 30 deg. Results are compared to earlier surface scatter measurements below 1000 Hz and are interpreted relative to concurrent air–sea boundary environmental measurements.