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Jim Thomson and Andrew T. Jessup

Abstract

A Fourier-based method is presented to process video observations of water waves and calculate the speed distribution of breaking crest lengths. The method has increased efficiency and robust statistics compared with conventional algorithms that assemble distributions from tracking individual crests in the time domain. The method is tested using field observations (video images of whitecaps) of fetch-limited breaking waves during case studies with low (6.7 m s−1), moderate (8.5 m s−1), and high (12.6 m s−1) wind speeds. The method gives distributions consistent with conventional algorithms, including breaking rates that are consistent with direct observations. Results are applied to obtain remote estimates of the energy dissipation associated with wave breaking.

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Gary A. Wick, J. Carter Ohlmann, Christopher W. Fairall, and Andrew T. Jessup

Abstract

The oceanic near-surface temperature profile must be accurately characterized to enable precise determination of air–sea heat exchange and satellite retrievals of sea surface temperature. An improved solar transmission parameterization is integrated into existing models for the oceanic warm layer and cool skin within the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux model to improve the accuracy of predictions of the temperature profile and corresponding heat flux components. Application of the revised bulk flux model to data from 12 diverse cruises demonstrates that the improved parameterization results in significant changes to the predicted cool-skin effect and latent heat fluxes at low wind speeds with high solar radiation due to reduced absorption of solar radiation just below the surface. Daytime skin-layer cooling is predicted to increase by 0.03 K on average but by more than 0.25 K for winds below 1 m s−1 and surface irradiance exceeding 900 W m−2. Predicted changes to the warm-layer correction were smaller but exceeded 0.1 K below 1 m s−1. Average latent and sensible heat fluxes changed by 1 W m−2, but the latent flux decreased by 5 W m−2 near winds of 0.5 m s−1 and surface irradiance of 950 W m−2. Comparison with direct observations of skin-layer cooling demonstrated, in particular, that use of the improved solar transmission model resulted in the reduction of previous systematic overestimates of diurnal skin-layer warming. Similar results can be achieved using a simplified treatment of solar absorption with an appropriate estimate of the fraction of incident solar radiation absorbed within the skin layer.

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Austin S. Hudson, Stefan A. Talke, Ruth Branch, Chris Chickadel, Gordon Farquharson, and Andrew Jessup

Abstract

Tides and river slope are fundamental characteristics of estuaries, but they are usually undersampled due to deficiencies in the spatial coverage of water level measurements. This study aims to address this issue by investigating the use of airborne lidar measurements to study tidal statistics and river slope in the Columbia River estuary. Eight plane transects over a 12-h period yield at least eight independent measurements of water level at 2.5-km increments over a 65-km stretch of the estuary. These data are fit to a sinusoidal curve and the results are compared to seven in situ gauges. In situ– and lidar-based tide curves agree to within a root-mean-square error of 0.21 m, and the lidar-based river slope estimate of 1.8 × 10−5 agrees well with the in situ–based estimate of 1.4 × 10−5 (4 mm km−1 difference). Lidar-based amplitude and phase estimates are within 10% and 8°, respectively, of their in situ counterparts throughout most of the estuary. Error analysis suggests that increased measurement accuracy and more transects are required to reduce the errors in estimates of tidal amplitude and phase. However, the results validate the use of airborne remote sensing to measure tides and suggest this approach can be used to systematically study water levels at a spatial density not possible with in situ gauges.

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James Edson, Timothy Crawford, Jerry Crescenti, Tom Farrar, Nelson Frew, Greg Gerbi, Costas Helmis, Tihomir Hristov, Djamal Khelif, Andrew Jessup, Haf Jonsson, Ming Li, Larry Mahrt, Wade McGillis, Albert Plueddemann, Lian Shen, Eric Skyllingstad, Tim Stanton, Peter Sullivan, Jielun Sun, John Trowbridge, Dean Vickers, Shouping Wang, Qing Wang, Robert Weller, John Wilkin, Albert J. Williams III, D. K. P. Yue, and Chris Zappa

The Office of Naval Research's 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 Martha's 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 scales of O(1–104 mm) using a mesoscale mooring array, aircraft-borne remote sensors, drifters, and ship surveys. To our knowledge, the former represents the first successful attempt to directly and simultaneously measure the heat and momentum exchange on both sides of the air–sea interface. The latter provided a 3D picture of the oceanic boundary layer during the month-long main experiment. These observations have been combined with numerical models and direct numerical and large-eddy simulations to investigate the processes that couple the atmosphere and ocean under these conditions. For example, the oceanic measurements have been used in the Regional Ocean Modeling System (ROMS) to investigate the 3D evolution of regional ocean thermal stratification. The ultimate goal of these investigations is to incorporate improved parameterizations of these processes in coupled models such as the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to improve marine forecasts of wind, waves, and currents.

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