Coupled Boundary Layer Air-Sea Transfer Experiment for Low to Moderate Winds

Collaborators: James B. Edson (University of Connecticut)

J. Thomas Farrar (Woods Hole Oceanographic Institution)

Andrew Jessup (Applied Physics Laboratory, University of Washington)

Robert A. Weller (Woods Hole Oceanographic Institution)

Accurate knowledge of the skin temperature has been shown to be critical to estimating surface fluxes and as a result its spatial variability influences the small-scale distribution of those fluxes. Evidence also exists that the horizontal variability of surface temperature may be related to subsurface circulation. Airborne infrared imagery observations produce instantaneous snapshots of 2-D skin temperature that characterize the processes that promote surface heat and gas exchange as well as quantify the spatial and temporal spectra of these processes. C. Zappa Principal Investigator in the Office of Naval Research program Coupled Boundary Layer Air-Sea Transfer Experiment for Low to Moderate Winds (CBLAST-LOW) in collaboration with A. Jessup of APL-UW and J. Edson of the University of Connecticut, Avery Point [Edson et al., 2007].

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We have made ocean skin temperature measurements during the CBLAST-LOW Experiments off the south coast of Martha's Vineyard from the NOAA LongEZ and a Cessna Skymaster aircraft. The high spatial coverage and fine spatial and temperature resolution of the infrared imaging system aboard the LongEZ allowed for the investigation of spatial scales in skin temperature from 1 km (processes that span the atmospheric boundary layer) down to 1 m (surface ocean wave-related processes). CBLAST Airplane Picture The surveys in 2002 and 2003 quantified the horizontal mesoscale variability in the domain around the CBLAST-LOW site near the offshore tower and the horizontal ocean mooring/buoy CBLAST Picture 3 array throughout the region extending 40-50 km offshore. Temperature maps from 2003 can be found on the CBLAST-Low website . The myriad of processes observed include breaking waves, sharp temperature fronts, ramping features related to stratificaiton breakdown, and distinctive band-like streaks that are the surface manifestation of internal waves propagating shoreward [ Zappa and Jessup, 2005]. Airborne infrared measurements constitute a significant advance in the scientific understanding of upper-ocean processes, and therefore will have an impact on the larger issues of the uptake of carbon dioxide by the ocean, the horizontal patchiness of organisms and nutrients in the photic zone, and the transport and fate of slightly buoyant material. A greater description of the airborne IR imagery system can be found on the CBLAST-Low website.

Field campaigns implemented an infrared imaging system developed to use up- and down-looking cameras that will allow us to discriminate between real skin temperature variations and apparent variations caused by reflection from clouds. Our airborne measurements 43 provided the variability that occurs within satellite pixel scales to compare directly with the measurements of the atmospheric and oceanic boundary layer processes important in near-surface mixing and air-sea fluxes made at the Air-Sea Interaction Tower (ASIT) off the South coast of Martha's Vineyard. Fine-scale snapshot imagery of ocean skin temperature elucidate a variety of mechanisms related to atmospheric and sub-surface phenomena that produce horizontal variability over a wide range of scales that decreased with increasing wind speed. The distribution of length scales are dominated by different mechanisms including coherent ramping structures coherent ramping structures CBLAST Picturewithin an active internal wave field, and Langmuir circulation [Zappa et al., 2006]. Moreover, comparisons of the IR measurements to in-situ sea-surface microlayer characteristics and a suite of moored, drifting, and towed ocean measurements are used to investigate the mechanisms that affect the spatial and temporal scales of ocean skin temperature variability. Results are used to evaluate the ability of a warm-layer model to describe the observed phenomena.

In collaboration with R. Weller and J. T. Farrar of WHOI, we used to test two previously hypothesized mechanisms for SST signatures of oceanic internal waves: a modulation of the cool-skin effect, and a modulation of vertical mixing within the diurnal warm layer [Farrar et al., 2007]. Under conditions of weak winds and strong insolation (which favor formation of a diurnal warm layer), the data reveal a link between the spatially periodic SST fluctuations and sub-surface temperature and velocity fluctuations associated with oceanic internal waves, suggesting that some mechanism involving the diurnal warm layer is responsible for the observed signal. Internal-wave signals in skin temperature very closely resemble temperature signals measured at a depth of about 20 cm, indicating that the observed internal-wave SST signal is not a result of modulation of the cool-skin effect. Numerical experiments using a one-dimensional upper-ocean model support the notion that internal-wave heaving of the warm-layer base can produce alternating bands of relatively warm and cool SST through the combined effects of surface heating and modulation of wind-driven vertical shear.