Mixing processes under seasonal ice cover in boreal lake Kuivajärvi

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dc.contributor Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta fi
dc.contributor University of Helsinki, Faculty of Science en
dc.contributor Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten sv
dc.contributor.author Ala-Könni, Joonatan
dc.date.issued 2019
dc.identifier.uri URN:NBN:fi:hulib-201912044059
dc.identifier.uri http://hdl.handle.net/10138/308229
dc.description.abstract Mixing processes under a seasonal ice cover in boreal lakes have received little attention from the physical limnological community. Even though the water is calm under the ice cover, many different phenomena are still able to cause mixing in the water column, which in turn affects the gas fluxes as well as physical and chemical properties of the water. Lakes in the boreal zone are very numerous. Understanding their behaviour helps us predict the effects of climate change in the boreal zone. In my thesis I present the various mechanisms that reign under the ice cover, and attempt to see these mechanisms in action in lake Kuivajärvi. Emphasis was placed on internal waves and the various components of the energy balance that can induce mixing. Data was collected with thermistor chains and a measurement raft between 24.1. – 3.5.2017. Two types of internal waves were observed during the ice-on season of 2016 – 2017. Short period barotropic seiches were observed during the whole ice-on season and transient long period baroclinic seiches were observed on two occasions. Other mixing processes seen in the lake were sediment heating in the dead of winter, penetrative convection caused by short wave radiation in the spring and diurnal stratification and mixing during spring caused by the daily heating and nightly cooling. Some mixing under the ice cover was found to depend on the meteorological conditions prevailing over the lake during the previous summer and just before the ice-on in late autumn, while others were more predictable. Long period internal waves and sediment heating are set in motion by meteorological conditions, while the spring mixing and overturn are more stable, due them being more a function of the orbital mechanics of our planet than the prevailing weather. Varying surface conditions of the lake ice cover make the measurement of especially the surface temperature complicated. Snow and ice are under a continuous metamorphosis due to the weather. This makes surface emissivity difficult to estimate, causing significant errors in the measurement of the outgoing longwave radiation. This in turn causes problems in defining the surface temperature from it. Also, the precipitation heat flux is difficult to estimate due to the lack of knowledge on the surface temperature. en
dc.language.iso eng
dc.publisher Helsingin yliopisto fi
dc.publisher University of Helsinki en
dc.publisher Helsingfors universitet sv
dc.title Mixing processes under seasonal ice cover in boreal lake Kuivajärvi en
dc.type.ontasot pro gradu -tutkielmat fi
dc.type.ontasot master's thesis en
dc.type.ontasot pro gradu-avhandlingar sv
dc.subject.discipline none und
dct.identifier.urn URN:NBN:fi:hulib-201912044059
dc.subject.specialization Hydrosfäärin geofysiikka fi
dc.subject.specialization Geophysics of the Hydrosphere en
dc.subject.specialization Hydrosfärens geofysik sv
dc.subject.degreeprogram Ilmakehätieteiden maisteriohjelma fi
dc.subject.degreeprogram Master's Programme in Atmospheric Sciences en
dc.subject.degreeprogram Magisterprogrammet i atmosfärsvetenskaper sv

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