Field investigations of apparent optical properties of ice cover in Finnish and Estonian lakes in winter 2009/Jaakatte optiliste parameetrite mootmised Soome ja Eesti jarvedel 2009. aasta talvel

• Published in: Estonian journal of earth sciences 2011-03, Vol.60 (1), p.50
• Main Authors: Lei, Ruibo, Lepparanta, Matti, Erm, Ants, Jaatinen, Elina, Parn, Ove
• Format: Article
• Language: English
• Subjects: Environmental aspects; Ice; Lakes; Optical properties
• ISSN: 1736-4728
• DOI: 10.3176/earth.2011.1.05

A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February-April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74-0.92 in winter, going down to 0.18-0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14-25 [m.sup.-1]; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1-0.4 and 1-5 [m.sup.-1]. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light. Key words: lake ice, snow, optics, reflectance, transmittance, attenuation coefficient. Viiel Soome jarvel (Lovojarvi, Paajarvi, Iso Valkjarvi, Vanajavesi ja Vesijarvi) ning kahel Eesti jarvel (Peipsi ja Vortsjarv) viidi labi jaa ning lume optilisi mootmisi perioodil veebruarist aprillini 2009. Moodeti spektraalset kiiritustihedust jaakatte kohal ja all, samuti fotosunteetiliselt aktiivse kiirguse (PAR--400-700 nm) tiheduse ajalist muutlikkust jaakatte sees. Mootmised voimaldasid maarata jaakatte labipaistvust ja jaa ning lume norgenemis-ja tagasihajumiskoefitsiente eri perioodidel ning erinevatel tingimustel (jaa ja lume paksus, struktuur, tihedus, niiskus). Talvel oli oluline osa PAR-i joudmisel vette lumel, mille tagasihajumiskoefitsient kuundis vahemikku 0,74-0,92 ja norgenemiskoefitsient 14-25 [m.sup.-1], samal ajal kui parast lume eemaldamist varieerusid vastavad naitajad piirides 0,1-0,4 ning 1-5 [m.sup.-1]. Lumevaba jaa korral oli oluline osa ulemisel, lumijaa kihil, milles PAR-i norgenemiskoefitsient oli kuni kolm korda suurem kui allpoolses tardjaas. Aprillis, sulamisperioodil, langesid nii jaakatte tagasihajumiskoefitsient (0,18-0,22) kui ka norgenemiskoefitsient (tingituna lumijaa sulamisest), mistottu paranesid oluliselt ka valgustingimused jaa all. Mootmised naitasid, et jaakatte optilised omadused soltuvad oluliselt lume ja jaa stratigraafiast, seega jaakatet la