Radiative characteristics of ice-covered fresh- and brackish-water bodies/Jaakattega kaetud merevee ja mageveekogude kiirguslikud karakteristikud

The structure and optics of ice and snow overlying bodies of water were studied in the years 2000-2003. The data were collected in the northern temperate region (nine Estonian and Finnish lakes and one brackish water site, Santala Bay, in the Gulf of Finland). In the present paper we describe the results concerning the radiative characteristics of the system "snow + ice cover on the water": albedo, attenuation of light, and planar and scalar irradiances through the ice. The basic data consist of irradiance measurements above and below ice cover for the PAR band of the solar spectrum (400-700 nm). Albedo varied across wide limits (0.20-0.70 for ice, 0.63-0.94 for snow), depending on the optical and physical properties of ice/snow and weather conditions. The vertically averaged light attenuation coefficient of the ice layer in the brackish waters of Santala Bay was higher than that in the lakes. The ratio of irradiance beneath the ice to incident irradiance increased 2.5-20 times after removing the snow, depending on the albedo and the thickness of ice and snow as well as on their optical properties. In the upper layer of water beneath the ice the ratio of planar to scalar quantum irradiances increased with depth (according to our earlier results obtained in summer this ratio decreased with increasing depth). Key words: ice optics, albedo, under-ice irradiance, euphotic zone. Aastatel 2000-2003 viidi labi rida toid veekogude jaakatte optiliste omaduste uurimiseks. Artiklis on kasitletud tulemusi, mis puudutavad susteemi "jaakate + lumekate" kiirguslikke karakteristikuid, milleks on albeedo, paikesekiirguse norgenemiskoefitsient, tasapinnaline ja skalaarne kiiritustihedus jaaaluses vees. Andmed on saadud kiiritustiheduse mootmiste teel spektripiirkonna 400-700 nm jaoks jaakatte peal ja all. Valitood toimusid subpolaarses regioonis (uheksa Eesti ja Soome jarve ning Santala laht Soomes). Talviste veekogude albeedo varieerus vahemikus 0,2-0,7 (jaapind) ja 0,63-0,94 (lumepind). Kiirguse vertikaalse norgenemiskoefitsiendi keskmistatud vaartused jaakihis soltusid vee soolsusest, olles Santala lahes suuremad kui jarvedes. Suhe "veealune kiiritustihedus jagatud pinnale langeva kiiritustihedusega" suurenes (2,5-20 korda), kui jaa pealt eemaldati lumi. Suhe tasapinnalise ja skalaarse kiiritustiheduse vahel suurenes sugavuse kasvades: see tulemus on vastupidine meie suvistel mootmistel saadule.