{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,12]],"date-time":"2025-10-12T04:32:07Z","timestamp":1760243527541,"version":"build-2065373602"},"reference-count":34,"publisher":"MDPI AG","issue":"7","license":[{"start":{"date-parts":[[2013,7,4]],"date-time":"2013-07-04T00:00:00Z","timestamp":1372896000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/3.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>Turbulent mixing in the ocean can, in some cases, be so intense as to leave surface imprints, or \u201cboils\u201d, that are detectable from space. Examples include turbulent flow over a submerged obstacle and instability of large-amplitude internal waves. In this paper we examine the particular case of tidal flow over a ~60-m-deep sill, which forms a barrier for the flow of dense water from the Pacific Ocean into the Strait of Georgia. The flow response during flood tide is illustrated using visible and thermal-band satellite and airborne imagery, the latter having high-resolution multi-looks that capture the formative stage of the boils. The image examples capture aspects of the expected flow response based on in situ measurements reported in the literature, but they also suggest differences, and they reveal the level of complexity of the surface structure. A new result is that, after the front is pushed well off the sill, boils emerge several hundred meters downstream from the sill crest, grow at a rate of ~60 m2\/s, and attain a size of 3,800 m2 (an equivalent diameter of 70 m) after one minute. These boils appear to arise from vorticity generated by vertical shear at the sill crest, and provide an additional source of vertical mixing and (through wave breaking) air-sea gas exchange.<\/jats:p>","DOI":"10.3390\/rs5073239","type":"journal-article","created":{"date-parts":[[2013,7,5]],"date-time":"2013-07-05T03:53:40Z","timestamp":1372996420000},"page":"3239-3258","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["Surface Imprints of Water-Column Turbulence: A Case Study of Tidal Flow over an Estuarine Sill"],"prefix":"10.3390","volume":"5","author":[{"given":"George","family":"Marmorino","sequence":"first","affiliation":[{"name":"Remote Sensing Division, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington,  DC 20375, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4570-2092","authenticated-orcid":false,"given":"Geoffrey","family":"Smith","sequence":"additional","affiliation":[{"name":"Remote Sensing Division, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington,  DC 20375, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4940-5987","authenticated-orcid":false,"given":"W.","family":"Miller","sequence":"additional","affiliation":[{"name":"Remote Sensing Division, Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington,  DC 20375, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2013,7,4]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"91","DOI":"10.1146\/annurev.earth.32.071603.152635","article-title":"Recent developments in the study of ocean turbulence","volume":"32","author":"Thorpe","year":"2004","journal-title":"Annu. Rev. Earth Planet. Sci"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"364","DOI":"10.1109\/LGRS.2007.895702","article-title":"Ocean mixed-layer depth and current variation estimated from imagery of surfactant streaks","volume":"4","author":"Marmorino","year":"2007","journal-title":"IEEE Geosci. Remote Sens. Lett"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"251","DOI":"10.1038\/22295","article-title":"Surface effects of bottom-generated turbulence in a shallow tidal sea","volume":"400","author":"Thorpe","year":"1999","journal-title":"Nature"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"1711","DOI":"10.1175\/2008JPO3931.1","article-title":"Boils and turbulence in a weakly stratified shallow tidal sea","volume":"38","author":"Thorpe","year":"2008","journal-title":"J. Phys. Oceanogr"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"699","DOI":"10.1109\/JOE.2003.819310","article-title":"Quantitative visualization of geophysical flows using digital oblique time-lapse imaging","volume":"28","author":"Pawlowicz","year":"2003","journal-title":"IEEE J. Ocean. Eng"},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Hennings, I., and Herbers, D (2006). Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions. J. Geophys. Res., 111.","DOI":"10.1029\/2005JC003302"},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Chickadel, C.C., Horner-Devine, A.R., Talke, S.A., and Jessup, A.T. (2009). Vertical boil propagation from a submerged estuarine sill. Geophys. Res. Lett., 36.","DOI":"10.1029\/2009GL037278"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"587","DOI":"10.1016\/j.ecss.2008.01.015","article-title":"Thermal remote sensing of estuarine spatial dynamics: Effects of bottom-generated vertical mixing","volume":"78","author":"Marmorino","year":"2008","journal-title":"Estuar. Coast. Shelf Sci"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1649","DOI":"10.1016\/0278-4343(94)00084-Z","article-title":"Three-dimensional structure in a tidal convergence front","volume":"15","author":"Farmer","year":"1995","journal-title":"Cont. Shelf Res"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"43","DOI":"10.1016\/S0377-0265(02)00024-6","article-title":"Tilting separation flows: A mechanism for intense vertical mixing in the coastal ocean","volume":"36","author":"Farmer","year":"2002","journal-title":"Dyn. Atmos. Ocean"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1218","DOI":"10.1175\/2007JTECHO574.1","article-title":"A composite view of surface signature and interior properties of nonlinear internal waves: Observations and applications","volume":"25","author":"Chang","year":"2008","journal-title":"J. Atmos. Ocean. Technol"},{"key":"ref_12","doi-asserted-by":"crossref","unstructured":"Da Silva, J.C.B., Magalhaes, J.M., Batista, M., Gostiaux, L., Gerkema, T., and New, A.L. (2012, January 22\u201327). The EUFAR Transnational Access Project A.NEW (Airborne Observations of Nonlinear Evolution of Internal Waves Generated by Internal Tidal Beams). Munich, Germany.","DOI":"10.1109\/IGARSS.2012.6351868"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"483","DOI":"10.1357\/002224006778715766","article-title":"Tidal fronts and their role in air-sea gas exchange","volume":"64","author":"Baschek","year":"2006","journal-title":"J. Mar. Res"},{"key":"ref_14","unstructured":"Baschek, B (2003). Air-Sea Gas Exchange in Tidal Fronts, Ph.D. Thesis, University of Victoria, Victoria, BC, Canada."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"53","DOI":"10.1017\/S002211208600246X","article-title":"Maximal two-layer exchange over a sill and through the combination of a sill and contraction with barotropic flow","volume":"164","author":"Farmer","year":"1986","journal-title":"J. Fluid Mech"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"513","DOI":"10.1098\/rspa.2001.0887","article-title":"Stratified flow over topography: Bifurcation fronts and transition to the uncontrolled state","volume":"458","author":"Armi","year":"2002","journal-title":"Proc. Roy. Soc. A"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"26","DOI":"10.2307\/1352707","article-title":"Tidal intrusion fronts","volume":"15","author":"Largier","year":"1992","journal-title":"Estuaries"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"431","DOI":"10.1007\/s10236-007-0114-5","article-title":"On the influence of stratification and tidal forcing upon mixing in sill regions","volume":"57","author":"Davies","year":"2007","journal-title":"Ocean Dyn"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1135","DOI":"10.1175\/1520-0485(2004)034<1135:TGTOTK>2.0.CO;2","article-title":"Tidally generated turbulence over the knight inlet sill","volume":"34","author":"Klymak","year":"2004","journal-title":"J. Phys. Oceanogr"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"1419","DOI":"10.1175\/2010JPO4435.1","article-title":"Upstream internal jumps in stratified sill flow: Observations of formation, evolution, and release","volume":"40","author":"Cummins","year":"2010","journal-title":"J. Phys. Oceangr"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"2156","DOI":"10.1029\/2000JC000504","article-title":"Observations and linear analysis of sill-generated internal tides and estuarine flow in Haro Strait","volume":"107","author":"Pawlowicz","year":"2002","journal-title":"J. Geophys. Res"},{"key":"ref_22","first-page":"17","article-title":"Generation of internal waves in the Strait of Georgia, British Columbia","volume":"23","author":"Gargett","year":"1976","journal-title":"Deep-Sea Res"},{"key":"ref_23","unstructured":"Wang, C (2009). Geophysical Observations of Nonlinear Internal Solitary-Like Waves in the Strait of Georgia, Ph.D. Thesis, University British Columbia, Vancouver, BC, Canada."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"30695","DOI":"10.1029\/1998JC900024","article-title":"Diagnosing vertical mixing in a two-layer exchange flow","volume":"103","author":"Pawlowicz","year":"1998","journal-title":"J. Geophys. Res"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"2107","DOI":"10.1080\/01431160500034086","article-title":"Simple and robust removal of sun glint for mapping shallow-water benthos","volume":"26","author":"Hedley","year":"2005","journal-title":"Int. J. Remote Sens"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"571","DOI":"10.1175\/1520-0442(2003)016<0571:BPOASF>2.0.CO;2","article-title":"Bulk parameterization of air-sea fluxes: Updates and verification for the COARE algorithm","volume":"16","author":"Fairall","year":"2003","journal-title":"J. Clim"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"6525","DOI":"10.1364\/AO.37.006525","article-title":"Influence of bubbles on scattering of light in the ocean","volume":"37","author":"Zhang","year":"1998","journal-title":"Appl. Opt"},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"485","DOI":"10.1016\/j.csr.2007.10.007","article-title":"Infrared imagery of \u2018breaking\u2019 internal waves","volume":"28","author":"Marmorino","year":"2007","journal-title":"Cont. Shelf Res"},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Aguilar, D.A., and Sutherland, B.R. Internal wave generation from rough topography. http:\/\/dx.doi.org\/10.1063\/1.2214538.","DOI":"10.1063\/1.2214538"},{"key":"ref_30","doi-asserted-by":"crossref","unstructured":"Talke, S.A., Horner-Devine, A.R., and Chickadel, C.C. (2010). Mixing layer dynamics in separated flow over an estuarine sill with variable stratification. J. Geophys. Res., 115.","DOI":"10.1029\/2009JC005467"},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Wang, B., Giddings, S.N., Fringer, O.B., Gross, E.S., Fong, D.A., and Monismith, S.G. (2011). Modeling and understanding turbulent mixing in a macrotidal salt wedge estuary. J. Geophys. Res., 116.","DOI":"10.1029\/2010JC006135"},{"key":"ref_32","unstructured":"Apel, J., and Jackson, C. (2004). SAR Marine User\u2019s Manual, US Department of Commerce."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"266","DOI":"10.3390\/rs1030266","article-title":"Deriving ocean surface drift using multiple SAR sensors","volume":"1","author":"Liu","year":"2009","journal-title":"Remote Sens"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"820","DOI":"10.1109\/TGRS.2009.2030885","article-title":"First analysis of TerraSAR-X along-track InSAR-derived current fields","volume":"48","author":"Romeiser","year":"2010","journal-title":"IEEE Trans. Geosci. Remote Sens"}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/5\/7\/3239\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T21:47:43Z","timestamp":1760219263000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/5\/7\/3239"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2013,7,4]]},"references-count":34,"journal-issue":{"issue":"7","published-online":{"date-parts":[[2013,7]]}},"alternative-id":["rs5073239"],"URL":"https:\/\/doi.org\/10.3390\/rs5073239","relation":{},"ISSN":["2072-4292"],"issn-type":[{"type":"electronic","value":"2072-4292"}],"subject":[],"published":{"date-parts":[[2013,7,4]]}}}