Andrew T. Jessup

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Defining and quantifying microscale wave breaking with infrared imagery

Breaking without air entrainment of very short wind-forced waves, or microscale wave breaking, is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air-sea interface. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. In this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air-water interface, producing signatures that can be detected with infrared imagery. Simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process. The infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage, which is substantial under the moderate wind speed conditions investigated. The results imply that ”bursting“ phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts. Oceanic measurements show the ability to quantify microscale wave breaking in the field. Our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air-sea heat and gas fluxes, as well as unprecedented details on the origin and evolution of microscale wave breaking.

Microscale wave breaking and air‐water gas transfer

Laboratory results showing that the air-water gas transfer velocity k is correlated with mean square wave slope have been cited as evidence that a wave-related mechanism regulates k at low to moderate wind speeds [Jahne et al., 1987; Bock et al., 1999]. Csanady [1990] has modeled the effect of microscale wave breaking on air-water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and AB, the fractional area coverage of microscale breaking waves. Simultaneous, colocated infrared (IR) and wave slope imagery is used to verify that AB detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and AB made at the University of Washington wind-wave tank at wind speeds from 4.6 to 10.7 m s−1, we show that k is linearly correlated with AB, regardless of the presence of surfactants. This result is consistent with Csanadys [1990] model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.

Microbreaking and the enhancement of air‐water transfer velocity

[1] The role of microscale wave breaking in controlling the air-water transfer of heat and gas is investigated in a laboratory wind-wave tank. The local heat transfer velocity, kH, is measured using an active infrared technique and the tank-averaged gas transfer velocity, kG, is measured using conservative mass balances. Simultaneous, colocated infrared and wave slope imagery show that wave-related areas of thermal boundary layer disruption and renewal are the turbulent wakes of microscale breaking waves, or microbreakers. The fractional area coverage of microbreakers, AB, is found to be 0.1–0.4 in the wind speed range 4.2–9.3 m s � 1 for cleaned and surfactant-influenced surfaces, and kH and kG are correlated with AB. The correlation of kH with AB is independent of fetch and the presence of surfactants, while that for kG with AB depends on surfactants. Additionally, AB is correlated with the mean square wave slope, hS 2 i, which has shown promise as a correlate for kG in previous studies. The ratio of kH measured inside and outside the microbreaker wakes is 3.4, demonstrating that at these wind speeds, up to 75% of the transfer is the direct result of microbreaking. These results provide quantitative evidence that microbreaking is the dominant mechanism contributing to air-water heat and gas transfer at low to moderate wind speeds. INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4506 Oceanography: Physical: Capillary waves; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: microbreaking, gas transfer, waves

Simultaneous particle image velocimetry and infrared imagery of microscale breaking waves

We report the results from a laboratory investigation in which microscale breaking waves were detected using an infrared (IR) imager and two-dimensional (2-D) velocity fields were simultaneously measured using particle image velocimetry (PIV). In addition, the local heat transfer velocity was measured using the controlled flux technique. To the best of our knowledge these are the first measurements of the instantaneous 2-D velocity fields generated beneath microscale breaking waves. Careful measurements of the water surface profile enabled us to make accurate estimates of the near-surface velocities using PIV. Previous experiments have shown that behind the leading edge of a microscale breaker the cool skin layer is disrupted creating a thermal signature in the IR image [Jessup et al., J. Geophys. Res. 102, 23145 (1997)]. The simultaneously sampled IR images and PIV data enabled us to show that these disruptions or wakes are typically produced by a series of vortices that form behind the leading edge of the breaker. When the vortices are first formed they are very strong and coherent but as time passes, and they move from the crest region to the back face of the wave, they become weaker and less coherent. The near-surface vorticity was correlated with both the fractional area coverage of microscale breaking waves and the local heat transfer velocity. The strong correlations provide convincing evidence that the wakes produced by microscale breaking waves are regions of high near-surface vorticity that are in turn responsible for enhancing air–water heat transfer rates.

The Miami2001 Infrared Radiometer Calibration and Intercomparison. Part II: Shipboard Results

The second calibration and intercomparison of infrared radiometers (Miami2001) was held at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS) during a workshop held from May to June 2001. The radiometers targeted in these two campaigns (laboratory-based and at-sea measurements) are those used to validate the skin sea surface temperatures and land surface temperatures derived from the measurements of imaging radiometers on earth observation satellites. These satellite instruments include those on currently operational satellites and others that will be launched within two years following the workshop. The experimental campaigns were completed in one week and included laboratory measurements using blackbody calibration targets characterized by the National Institute of Standards and Technology (NIST), and an intercomparison of the radiometers on a short cruise on board the R/V F. G. Walton Smith in Gulf Stream waters off the eastern coast of Florida. This paper reports on the results obtained from the shipborne measurements. Seven radiometers were mounted alongside each other on the R/V Walton Smith for an intercomparison under seagoing conditions. The ship results confirm that all radiometers are suitable for the validation of land surface temperature, and the majority are able to provide high quality data for the more difficult validation of satellitederived sea surface temperature, contributing less than 0.1 K to the error budget of the validation. The measurements provided by two prototype instruments developed for ship-of-opportunity use confirmed their potential to provide regular reliable data for satellite-derived SST validation. Four high quality radiometers showed agreements within 0.05 K confirming that these instruments are suitable for detailed studies of the dynamics of air‐sea interaction at the ocean surface as well as providing high quality validation data. The data analysis confirms the importance of including an accurate correction for reflected sky radiance when using infrared radiometers to measure SST. The results presented here also show the value of regular intercomparisons of ground-based instruments that are to be used for the validation of satellite-derived data products—products that will be an essential component of future assessments of climate change and variability.

Infrared-Based Measurements of Velocity, Turbulent Kinetic Energy, and Dissipation at the Water Surface in a Tidal River

Thermal infrared (IR)-based particle image ve locimetry (PIV) is used to measure the evolution of velocity, turbulent kinetic energy (TKE), and the TKE dissipation rate at the water surface in the tidally influenced Snohomish River. Patterns of temperature variability in the IR imagery arise from disruption of the cool-skin layer and are used to estimate the 2-D velocity field. Comparisons of IR-based PIV mean velocity made with a colocated acoustic velocimeter demonstrate high cor relation (r² >; 0.9). Over a tidal period, surface TKE computed from the IR velocity varies from 10^-4 to 3 × 10^-3 J · kg^-1, with an average difference from the in situ measurements of 8%. IR-derived TKE dissipation rates vary from approximately 3 × 10^-6 to 2 × 10^-4 W · kg^-1 at peak ebb, agreeing on average to within 7% of the in situ velocimeter results. IR-based PIV provides detailed measurements of previously inaccessible surface velocities and turbulence statistics.

Measurement of the geometric and kinematic properties of microscale breaking waves from infrared imagery using a PIV algorithm

Infrared techniques have been shown to be uniquely capable of detecting and quantifying microscale breaking waves at an air–water interface. Here we extend current capabilities by developing image processing algorithms to measure the crest lengths and velocities of microbreaking waves in a laboratory wind–wave tank. The measurements are used to compute the distribution of crest lengths as a function of speed, Λ(c), introduced by Phillips [1] as a formulation for the distribution of breaking waves. Two methods to determine the crest velocity by applying a particle imaging velocimetry (PIV) algorithm to the infrared imagery are developed and compared to a method based on tracking the centroid of the crest. The crest-PIV method is based on estimation of the velocity of crests identified using a temperature threshold. The image-PIV method is based on a velocity threshold applied to a surface velocity map obtained by using the PIV algorithm over the entire image. Both methods are used to compute the surface turnover rate, which is compared to the frequency of breaking. The methods developed demonstrate the potential for infrared imaging techniques to measure the geometric and kinematic properties of microbreaking waves and are relevant to air–sea flux studies.

Skin layer recovery of free-surface wakes: Relationship to surface renewal and dependence on heat flux and background turbulence

The thermal signatures of free-surface wakes observed in the open ocean show that the recovery of the cool skin layer is related to the degree of surface mixing and to ambient environmental conditions. Wakes produced by two surface-piercing cables of O(10 -2 m) in diameter are analyzed using infrared imagery. Under low-wind-speed conditions when the swell and surface current were aligned, the wakes exhibited distinctive patchlike features of O(1 m) in diameter that were generated by the passage of individual waves. The time t * required by the skin layer to recover from these disturbances is compared to the surface-renewal timescale τ used in heat and gas flux models. At low wind speeds, t * is comparable to τ, but at moderate wind speeds the agreement is poor. The spatial and temporal variations in the skin temperature of these wakes are related to a wave Reynolds number used to characterize the strength of the disturbance due to the waves. The recovery process is characterized in terms of the restoring internal energy flux J r which is proportional to both the initial thickness and the thermal recovery rate of the skin layer and was found to be directly related to the strength of the surface disruption. Comparison of the wake results with laboratory and other field measurements of breaking waves implies that J r is also a strong function of the net heat flux and background turbulence, which relate directly to the existing environmental conditions such as wind stress and sea state. Our results demonstrate that J r may vary by several orders of magnitude, depending on the environmental conditions.

Observations of rain-induced near-surface salinity anomalies

Vertical salinity gradients in the top few meters of the ocean surface can exist due to the freshwater input from rain. If present, surface gradients complicate comparing salinity measured at depths of a few meters to salinities retrieved using L-band microwave radiometers such as SMOS and Aquarius. Therefore, understanding the spatial scales and the frequency of occurrence of these vertical gradients and the conditions under which they form will be important in understanding sea surface salinity maps provided by microwave radiometers. Salinity gradients in the near-surface ocean were measured using a towed profiler that profiled salinity in the top 2 m of the ocean with a minimum measurement depth of 0.1 m. In addition, an Underway Salinity Profiling System was installed on the R/V Thomas G. Thompson. This measured near-surface salinity at depths of 1 and 2 m. Both the towed profiler and the underway system found the occurrence of negative salinity anomalies (i.e., salinity decreasing toward the surface) was correlated with the presence of rain. The magnitude of the anomaly (i.e., the difference between salinity at 0.1 m and the salinity at 0.26 m) was proportional to the cube of the rain rate for rain rate, R, greater than 6 mm h−1. From this, for R > 15–22 mm h−1, depending on the areal extent of the salinity anomalies, rain can cause scene-averaged salinity offsets that are as large as the accuracy goal for Aquarius of 0.1‰.