Jo Høkedal

• Kleiv, Øyvind; Folkestad, Are; Høkedal, Jo; Sørensen, Kai & Aas, Eyvind (2015). Estimation of upward radiances and reflectances at the surface of the sea from above-surface measurements. Ocean Science. ISSN 1812-0784. 11(5), s. 779–788. doi: 10.5194/os-11-779-2015. Fulltekst i vitenarkiv Vis sammendrag

  During 4 field days in the years 2009–2011, 22 data sets of measurements were collected in the inner Oslofjord, Norway. The data consist of recordings of spectral nadir radiances in air and water as well as spectral downward irradiance in air. The studied wavelengths are 351, 400, 413, 443, 490, 510, 560, 620, 665, 681, 709 and 754 nm. The water-leaving radiance and the reflected radiance at the sea surface have been obtained from the measured nadir radiances in air and water, where the latter radiance has been extrapolated upwards to the surface. For comparison we present a simpler and much faster method that determines the water-leaving and reflected radiances solely from above-surface measurements of upward nadir radiance and downward irradiance. This new method is based on an assumption about similarity in spectral shape of the radiance reflected at the surface, and it makes use of the small ratio between water-leaving and reflected radiances at 351 and 754 nm in the Oslofjord. A comparison between the quantities determined by the two mentioned methods shows that the average relative deviations between their results are less than or equal to 15 % for the reflected radiance, at the studied wavelengths. The average relative deviation of the water-leaving radiance at 560 nm is 24 %. These results are obtained for a cloudiness range of 1–8 oktas (12.5–100 %) and solar zenith angles between 37 and 51°. We consider these to be acceptable uncertainties for a first check of satellite products in the inner Oslofjord.

• Hovland, Erlend Kjeldsberg; Hancke, Kasper; Alver, Morten; Drinkwater, Kennneth F.; Høkedal, Jo & Johnsen, Geir [Vis alle 8 forfattere av denne artikkelen] (2014). Optical impact of an Emiliania huxleyi bloom in the frontal region of the Barents Sea. Journal of Marine Systems. ISSN 0924-7963. 130, s. 228–240. doi: 10.1016/j.jmarsys.2012.07.002.
• Aas, Eyvind; Høkedal, Jo & Sørensen, Kai (2014). Secchi depth in the Oslofjord-Skagerrak area: theory, experiments and relationships to other quantities. Ocean Science. ISSN 1812-0784. 10, s. 177–199. doi: 10.5194/os-10-177-2014. Fulltekst i vitenarkiv Vis sammendrag

  The Secchi depth and its relationships to other properties of the sea water in the Oslofjord–Skagerrak area have been investigated. White and black disks of different sizes have been applied, and the Secchi depth has been observed with the naked eye, through colour filters and with a water telescope. Spectral luminances and illuminances have been calculated from recordings of radiance and irradiance, and attenuation coefficients have been determined. A theoretical expression for the Secchi depth based on luminances has been tested against field observations, and it is found that the field results for the product of Secchi depth and attenuation coefficients are on average only 4% less than the predicted value for the white disk. For the Secchi depths observed through colour filters or for the black disk, the average field results are more than 30% smaller than the theoretical estimates. The reduction in the disk diameter from 30 to 10 cm should theoretically reduce the Secchi depths by 13–22%, while the field observations show an average reduction of 10–20%. Similarly we find from theory that the removal of sun glitter should increase the Secchi depth by 12%, while the observed increase is 14% on average for the white disk. Our overall conclusion is that the theoretical expression works well for the white disk, but less so for the colour filter observations and the black disk. Statistical relationships between Secchi depths and attenuation coefficients have been determined, and it is found that the root-mean-square errors relative to the mean value are smaller for the beam attenuation coefficients (12–24%, white disk) than for the vertical attenuation coefficients (16–65%, white disk). The depth of the 1% level of surface quanta irradiance (PAR) can be estimated with a relative root-mean-square error of 23% from observations of the white Secchi depth. Similar estimates of chlorophyll a and total suspended material will have rms errors in the range 40–90%. Our conclusion becomes that the Secchi depth observation is a very useful tool for checking the value and order of magnitude of other related quantities in the Oslofjord–Skagerrak area.

• Aas, Eyvind; Højerslev, Niels Kristian; Høkedal, Jo & Sørensen, Kai (2013). Optical water types of the Nordic Seas and adjacent areas. Oceanologia. ISSN 0078-3234. 55(2), s. 471–482. doi: 10.5697/oc.55-2.471. Fulltekst i vitenarkiv Vis sammendrag

  A new map of Jerlov's optical water types in the Nordic Seas and adjacent waters at 139 locations, as well as a table with statistical and geographical properties of the vertical attenuation coefficient of downward irradiance at 475 nm, are presented. The data analysis is based on 715 recordings at different stations, at latitudes between 54° and 82°N, and longitudes between 31°W and 49°E, obtained by different authors from May 1954 to August 2003. The results show that the Atlantic and Polar waters are typically of oceanic type II-III, although during algal blooms the optical conditions may change to coastal types 1, 3 and 5, which are also the most frequent types found in coastal areas.

• Okullo, Willy; Hamre, Børge; Frette, Øyvind; Stamnes, Jakob J.; Sørensen, Kai & Ssenyonga, Taddeo [Vis alle 9 forfattere av denne artikkelen] (2011). Validation of MERIS water quality products in Murchison Bay, Lake Victoria – Preliminary results. International Journal of Remote Sensing. ISSN 0143-1161. 32(19), s. 5541–5563. doi: 10.1080/01431161.2010.506895.
• Aas, Eyvind; Højerslev, Niels Kristian & Høkedal, Jo (2009). Conversion of sub-surface reflectances to above-surface MERIS reflectance. International Journal of Remote Sensing. ISSN 0143-1161. 30(21), s. 5767–5791. doi: 10.1080/01431160902744803.

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