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Peter G. Black, Eric A. D'Asaro, William M. Drennan, Jeffrey R. French, Pearn P. Niiler, Thomas B. Sanford, Eric J. Terrill, Edward J. Walsh, and Jun A. Zhang

The Coupled Boundary Layer Air–Sea Transfer (CBLAST) field program, conducted from 2002 to 2004, has provided a wealth of new air–sea interaction observations in hurricanes. The wind speed range for which turbulent momentum and moisture exchange coefficients have been derived based upon direct flux measurements has been extended by 30% and 60%, respectively, from airborne observations in Hurricanes Fabian and Isabel in 2003. The drag coefficient (C D) values derived from CBLAST momentum flux measurements show C D becoming invariant with wind speed near a 23 m s−1 threshold rather than a hurricane-force threshold near 33 m s−1 . Values above 23 m s−1 are lower than previous open-ocean measurements.

The Dalton number estimates (C E) derived from CBLAST moisture flux measurements are shown to be invariant with wind speeds up to 30 m s −1 which is in approximate agreement with previous measurements at lower winds. These observations imply a C E/C D ratio of approximately 0.7, suggesting that additional energy sources are necessary for hurricanes to achieve their maximum potential intensity. One such additional mechanism for augmented moisture flux in the boundary layer might be “roll vortex” or linear coherent features, observed by CBLAST 2002 measurements to have wavelengths of 0.9–1.2 km. Linear features of the same wavelength range were observed in nearly concurrent RADARSAT Synthetic Aperture Radar (SAR) imagery.

As a complement to the aircraft measurement program, arrays of drifting buoys and subsurface floats were successfully deployed ahead of Hurricanes Fabian (2003) and Frances (2004) [16 (6) and 38 (14) drifters (floats), respectively, in the two storms]. An unprecedented set of observations was obtained, providing a four-dimensional view of the ocean response to a hurricane for the first time ever. Two types of surface drifters and three types of floats provided observations of surface and subsurface oceanic currents, temperature, salinity, gas exchange, bubble concentrations, and surface wave spectra to a depth of 200 m on a continuous basis before, during, and after storm passage, as well as surface atmospheric observations of wind speed (via acoustic hydrophone) and direction, rain rate, and pressure. Float observations in Frances (2004) indicated a deepening of the mixed layer from 40 to 120 m in approximately 8 h, with a corresponding decrease in SST in the right-rear quadrant of 3.2°C in 11 h, roughly one-third of an inertial period. Strong inertial currents with a peak amplitude of 1.5 m s−1 were observed. Vertical structure showed that the critical Richardson number was reached sporadically during the mixed-layer deepening event, suggesting shear-induced mixing as a prominent mechanism during storm passage. Peak significant waves of 11 m were observed from the floats to complement the aircraft-measured directional wave spectra.

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D. R. Smith, M. A. Rosenthal, J. P. Mulvany, W. Sanford, W. R. Krayer, B. L. Smith, B. S. Palko, J. J. Matkins, G. J. Koester, R. L. Lees, and P. A. McKinstry

For the third consecutive year mid-Atlantic Atmospheric Education Resource Agents (AERAs) conducted a regional workshop for educators on hazardous weather. This workshop attracted teachers from New York to Georgia for sessions by Project ATMOSPHERE AERAs, meteorologists from the National Weather Service, universities, the media, and private industry, who addressed a variety of topics pertaining to the impact of severe weather. As has been the case with the previous workshops, this event represents a partnership of individuals from schools, government agencies, and the private sector that enhances science education and increases public awareness of hazardous weather conditions.

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E.A. D'Asaro, P. G. Black, L. R. Centurioni, Y.-T. Chang, S. S. Chen, R. C. Foster, H. C. Graber, P. Harr, V. Hormann, R.-C. Lien, I.-I. Lin, T. B. Sanford, T.-Y. Tang, and C.-C. Wu

Tropical cyclones (TCs) change the ocean by mixing deeper water into the surface layers, by the direct air–sea exchange of moisture and heat from the sea surface, and by inducing currents, surface waves, and waves internal to the ocean. In turn, the changed ocean influences the intensity of the TC, primarily through the action of surface waves and of cooler surface temperatures that modify the air–sea fluxes. The Impact of Typhoons on the Ocean in the Pacific (ITOP) program made detailed measurements of three different TCs (i.e., typhoons) and their interaction with the ocean in the western Pacific. ITOP coordinated meteorological and oceanic observations from aircraft and satellites with deployments of autonomous oceanographic instruments from the aircraft and from ships. These platforms and instruments measured typhoon intensity and structure, the underlying ocean structure, and the long-term recovery of the ocean from the storms' effects with a particular emphasis on the cooling of the ocean beneath the storm and the resulting cold wake. Initial results show how different TCs create very different wakes, whose strength and properties depend most heavily on the nondimensional storm speed. The degree to which air–sea fluxes in the TC core were reduced by ocean cooling varied greatly. A warm layer formed over and capped the cold wakes within a few days, but a residual cold subsurface layer persisted for 10–30 days.

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Shelley D. Crausbay, Aaron R. Ramirez, Shawn L. Carter, Molly S. Cross, Kimberly R. Hall, Deborah J. Bathke, Julio L. Betancourt, Steve Colt, Amanda E. Cravens, Melinda S. Dalton, Jason B. Dunham, Lauren E. Hay, Michael J. Hayes, Jamie McEvoy, Chad A. McNutt, Max A. Moritz, Keith H. Nislow, Nejem Raheem, and Todd Sanford
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Andrey Y. Shcherbina, Miles A. Sundermeyer, Eric Kunze, Eric D’Asaro, Gualtiero Badin, Daniel Birch, Anne-Marie E. G. Brunner-Suzuki, Jörn Callies, Brandy T. Kuebel Cervantes, Mariona Claret, Brian Concannon, Jeffrey Early, Raffaele Ferrari, Louis Goodman, Ramsey R. Harcourt, Jody M. Klymak, Craig M. Lee, M.-Pascale Lelong, Murray D. Levine, Ren-Chieh Lien, Amala Mahadevan, James C. McWilliams, M. Jeroen Molemaker, Sonaljit Mukherjee, Jonathan D. Nash, Tamay Özgökmen, Stephen D. Pierce, Sanjiv Ramachandran, Roger M. Samelson, Thomas B. Sanford, R. Kipp Shearman, Eric D. Skyllingstad, K. Shafer Smith, Amit Tandon, John R. Taylor, Eugene A. Terray, Leif N. Thomas, and James R. Ledwell

Abstract

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.

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