Application of ocean colour index using the traditional Forel-Ule index in coastal waters Madihah Jafar‐-Sidik1, David George Bowers2, Joshua Willliam Griffiths2, Nurzaliah Ali1, Muhammad Ikhlas Zabidi1, and Saiyidah Munirah Mohd Shazali3
1. Borneo Marine Research Institute, Universiti Malaysia Sabah, Jalan UMS, 88400 Kota Kinabalu, Sabah, Malaysia
2. School of Ocean Sciences, Prifysgol Bangor University, Menai Bridge, Anglesey LL59 5AB, United Kingdom
3. Razak Faculty of Technology and Informatics, Universiti Teknologi Malaysia 54100 Kuala Lumpur, Malaysia
Abstract
Forel-Ule colour comparator index has been used to classify the colour of natural waters like seas, lakes and rivers since the end of the 19th century. The FU index scale includes 21 different colour ranging from indigo-blue to cola-brown representative as a gradient of natural colour classification globally and intensively. We test a method for calculating the colour of the sea, as measured on the traditional Forel-Ule (FU) scale, from satellite remote sensing measurements of reflectance. Forel-Ule colour scale numbers are calculated from 46 in situ radiometer profiles in the Mozambique channel and Irish Sea and compared to direct measurements of water colour made with the FU scale. There is generally good agreement (R2 -= -0.79, RMS difference -= -2.7) between observed and calculated values over the range FU2 (blue)-FU18 (greenish brown). The method is applied to map the colour, on the FU scale, of the Irish Sea using 242 in situ radiometer measurements. MODIS satellite data is used to calculate the long-term (2003-2014) average and seasonal variation of sea colour. The colour of the Irish Sea, from both in situ and satellite radiometry, shows patterns which are consistent with the known distribution and seasonal variation of suspended particles. A Forel-Ule colour scale map of western European waters shows that the most coloured waters are found in north-west European seas which are coloured by suspended sediments rather than phytoplankton blooms.
