Browsing by Subject "snow cover"

Sort by: Order: Results:

Now showing items 1-6 of 6
  • Aalto, Juha; Scherrer, Daniel; Lenoir, Jonathan; Guisan, Antoine; Luoto, Miska (2018)
    Soil temperature (ST) has a key role in Arctic ecosystem functioning and global environmental change. However, soil thermal conditions do not necessarily follow synoptic temperature variations. This is because local biogeophysical processes can lead to a pronounced soil-atmosphere thermal offset (Delta T) while altering the coupling (beta Tau) between ST and ambient air temperature (AAT). Here, we aim to uncover the spatiotemporal variation in these parameters and identify their main environmental drivers. By deploying a unique network of 322 temperature loggers and surveying biogeophysical processes across an Arctic landscape, we found that the spatial variation in Delta T during the AAT 0 period, Delta T was controlled by soil characteristics, vegetation and solar radiation (Delta T = -0.6 degrees C +/- 1.0 degrees C). Importantly, Delta T was not constant throughout the seasons reflecting the influence of beta Tau on the rate of local soil warming being stronger after (mean beta Tau = 0.8 +/- 0.1) than before (beta Tau = 0.2 +/- 0.2) snowmelt. Our results highlight the need for continuous microclimatic and local environmental monitoring, and suggest a potential for large buffering and non-uniform warming of snow-dominated Arctic ecosystems under projected temperature increase.
  • Hannula, Henna-Reetta (Ilmatieteen laitos - Finnish Meteorological Institute, 2022)
    Finnish Meteorological Institute Contributions 180
    Remote sensing of snow is a method to measure snow cover characteristics without direct physical contact with the target from airborne or space-borne platforms. Reliable estimates of snow cover extent and snow properties are vital for several applications including climate change research and weather and hydrological forecasting. Optical remote sensing methods detect the extent of snow cover based on its high reflectivity compared to other natural surfaces. A universal challenge for snow cover mapping is the high spatiotemporal variability of snow properties and heterogeneous landscapes such as the boreal forest biome. The optical satellite sensor’s footprint may extend from tens of meters to a kilometer; the signal measured by the sensor can simultaneously emerge from several target categories within individual satellite pixels. By use of spectral unmixing or inverse model-based methods, the fractional snow cover (FSC) within the satellite image pixel can be resolved from the recorded electromagnetic signal. However, these algorithms require knowledge of the spectral reflectance properties of the targets present within the satellite scene and the accuracy of snow cover maps is dependent on the feasibility of these spectral model parameters. On the other hand, abrupt changes in land cover types with large differences in their snow properties may be located within a single satellite image pixel and complicate the interpretation of the observations. Ground-based in-situ observations can be used to validate the snow parameters derived by indirect methods, but these data are affected by the chosen sampling. This doctoral thesis analyses laboratory-based spectral reflectance information on several boreal snow types for the purpose of the more accurate reflectance representation of snow in mapping method used for the detection of fractional snow cover. Multi-scale reflectance observations representing boreal spectral endmembers typically used in optical mapping of snow cover, are exploited in the thesis. In addition, to support the interpretation of remote sensing observations in boreal and tundra environments, extensive in-situ dataset of snow depth, snow water equivalent and snow density are exploited to characterize the snow variability and to assess the uncertainty and representativeness of these point-wise snow measurements applied for the validation of remote sensing observations. The overall goal is to advance knowledge about the spectral endmembers present in boreal landscape to improve the accuracy of the FSC estimates derived from the remote sensing observations and support better interpretation and validation of remote sensing observations over these heterogeneous landscapes. The main outcome from the work is that laboratory-controlled experiments that exclude disturbing factors present in field circumstances may provide more accurate representation of wet (melting) snow endmember reflectance for the FSC mapping method. The behavior of snow band reflectance is found to be insensitive to width and location differences between visible satellite sensor bands utilized in optical snow cover mapping which facilitates the use of various sensors for the construction of historical data records. The results also reveal the high deviation of snow reflectance due to heterogeneity in snow macro- and microstructural properties. The quantitative statistics of bulk snow properties show that areal averages derived from in-situ measurements and used to validate remote sensing observations are dependent on the measurement spacing and sample size especially over land covers with high absolute snow depth variability, such as barren lands in tundra. Applying similar sampling protocol (sample spacing and sample size) over boreal and tundra land cover types that represent very different snow characteristics will yield to non-equal representativeness of the areal mean values. The extensive datasets collected for this work demonstrate that observations measured at various scales can provide different view angle to the same challenge but at the same time any dataset individually cannot provide a full understanding of the target complexity. This work and the collected datasets directly facilitate further investigation of uncertainty in fractional snow cover maps retrieved by optical remote sensing and the interpretation of satellite observations in boreal and tundra landscapes.
  • Ruosteenoja, Kimmo; Räisänen, Jouni; Jylhä, Kirsti; Mäkelä, Hanna; Lehtonen, Ilari; Simola, Henriikka; Luomaranta, Anna; Weiher, Stefan (Ilmatieteen laitos, 2013)
    Raportteja - Rapporter - Reports 2013:4
    Tiivistelmä Mittaustiedot osoittavat, että vuoden keskilämpötila on noussut Suomessa viimeksi kuluneen noin sadan vuoden aikana lähes asteen. IPCC:n 4. arviointiraportin laadinnan yhteydessä tuotettujen maailmanlaajuisten ja alueellisten ilmastomalliajojen tuloksia tarkastelemalla voidaan nähdä, että lämpiäminen jatkuu tulevaisuudessakin. Nyt elettävä vuosikymmen (2011–2020) on oleva jo selvästi yli 90 % todennäköisyydellä vertailujaksoa 1971–2000 lämpimämpi ja noin 75 % todennäköisyydellä myös sitä sateisempi. Samalla kun ilmastomme lämpenee, talvet muuttuvat keskimäärin vähälumisemmiksi, kosteammiksi ja pilvisemmiksi. Vuosisadan loppuvuosikymmeninä auringonsäteilyä tulee talvisin maan pinnalle 10–15 % nykyistä vähemmän, ilman keskimääräinen suhteellinen kosteus kohoaa 2-3 prosenttiyksiköllä ja lumen vesiarvo putoaa Etelä- Suomessa jopa 70–80 %. Talvisin Itämeressä on jäätä ja maaperässä routaa nykyistä ohuemmalti. Ainakin maan eteläosat ovat tuolloin jo siirtyneet Köppenin luokittelujärjestelmän kylmätalvisesta D-ilmastovyöhykkeestä leutojen talvien C-vyöhykkeeseen. Ilmaston lämmetessä hellepäivien määrä saattaa jopa nelinkertaistua ennen vuosisadan loppua, ja jo lähivuosikymmeninä suurin osa mitattavista kuukausien keskilämpötiloista on nykyisiin ilmastollisiin vertailuarvoihin suhteutettuina normaalia korkeampia. Vertailuarvoja pitääkin ilmaston lämmetessä toistuvasti päivittää. Päivittämisestä huolimatta ne kuvaavat vääjäämättä mennyttä eivätkä juuri kyseisen ajankohdan ilmastoa, sillä vertailuarvot joudutaan aina laskemaan edellisten vuosikymmenien säähavaintotietojen perusteella. Ennätyksellisen korkeitten kuukausikeskilämpötilojen mittaamisen todennäköisyys kasvaa koko ajan. Rankkoja sateita ja kovia tuulia esiintyy jonkin verran nykyistä useammin, ja syksyllä ja talvella puhaltaa entistä useammin lännen ja lounaan suunnalta. Säät vaihtelevat edelleenkin päivästä ja vuodesta toiseen. Kovia pakkasia, runsaslumisia talvia ja alhaisia kuukausikeskilämpötiloja esiintyy jatkossakin, vaikka ne käyvät aikaa myöten yhä harvinaisemmiksi. Jotkut nykyiset voimassaolevat kuukausikeskilämpötilojen kylmyysennätykset saattavat silti vielä rikkoutua lähimpinä vuosikymmeninä. Kasvihuonekaasujen päästöt viime kädessä määräävät, kuinka voimakkaana ilmastonmuutos lopulta toteutuu. Epävarmuutta ilmastoennusteisiin aiheuttaa myös eri mallien tulosten poikkeaminen toisistaan ja ilmaston luonnollinen satunnainen vaihtelu vuosikymmenestä toiseen. Sammandrag Meteorologiska observationsdata visar, att den årliga medeltemperaturen i Finland har stigit nästan en grad under det senaste århundradet. I denna rapport har vi analyserat resultat från ett antal globala och regionala modelsimuleringar som producerats för IPCCs fjärde utvärderingsrapport. Enligt modelsimuleringarna fortsätter temperaturen att stiga även i framtiden. Redan det nuvarande decenniet (2011-2020) kommer antagligen att vara varmare (sannolikheten över 90 %) och regnigare (sannolikheten cirka 75 %) än referensperioden 1971-2000. I framtiden blir vintrarna snöfattigare, fuktigare och molnigare. Under perioden 2070-2099 fås enligt simuleringarna i genomsnitt 10-15 % mindre solstrålning om vintern, den relativa fuktigheten ökar med 2-3 procentenheter och i södra Finland sjunker snötäckets vatteninnehåll med upp till 70-80 %. Tjäle i marken och is i Östersjön blir mindre allmänna. Klimatzonerna förskjuts så att åtminstone de sydligaste delarna av landet kommer att ligga inom den tempererade C-zonen i stället för den nuvarande zonen D med kalla vintrar. Antalet varma sommardagar, då medeltemperaturen överstiger 20 grader, kan t.o.m. fyrdubblas innan slutet av detta århundrade. Redan under de närmaste decennierna kommer flertalet månader att ha medeltemperaturer över de nutida klimatologiska normalvärdena. Därför bör normalvärdena uppdateras återkommande. Trots uppdatering motsvarar dessa värden inte det aktuella klimatet vid en given tidpunkt, utan oundvikligen det gångna klimatet. Sannolikheten för rekordhöga temperaturer ökar med tiden. Kraftig nederbörd samt hårda vindar kan väntas att förekomma oftare än nuförtiden, och under höst och vinter ökar frekvensen av vindar från väst och syd. Väderförhållandena varierar från dag till dag även i framtiden. Sträng kyla, snörika vintrar och låga medeltemperaturer förekommer också i framtiden, även om de blir alltmer ovanliga med tiden. Trots detta är det möjligt att några gällande köldrekord kommer att brytas under de nästa årtiondena. I sista hand är det de framtida utsläppen av växthusgaserna som bestämmer hur stor klimatförändring blir. Vidare osäkerhet i klimatprognoserna förorsakas av skillnader emellan olika klimatmodeller, samt av de naturliga fluktuationerna i klimatet. Abstract Meteorological measurements indicate that the annual mean temperature in Finland has increased by nearly one degree during the past one hundred years. Global and regional climate model simulations prepared for the 4th Assessment Report of the IPCC reveal that warming will continue in the future. It is likely that during the ongoing decade (2011-2020) the mean temperature is already higher (with a probability of more than 90 %) and precipitation more abundant (with a probability of about 75 %) than in the reference period 1971-2000. Along with increasing temperatures, Finnish winters are becoming moister, darker and less snowy. In the last three decades of the present century, solar radiation in winter is estimated to decrease by 10-15 %, relative humidity to increase by 2-3 percentage points, and in southern Finland the snow water equivalent will probably be reduced by 70-80 %. There will be less frost in the ground, and the ice cover in the Baltic Sea will decrease. By the end of the century, the southern parts of Finland will transfer from the boreal D climate zone to the temperate C zone. The number of hot summer days with a mean temperature above 20°C may increase fourfold before the end of the century. Even in the next few decades, most monthly mean temperatures are expected to exceed the prevailing standard climate normals. Despite regular updating, in a warming climate these standard normals will always represent the observed past rather than the present climate. The probability of the occurrence of record-high temperatures will increase with time. Heavy precipitation and strong winds may occur somewhat more frequently than hitherto, and in autumn and winter winds will blow ever more often from the west and south. Weather conditions continue to vary in time in the future as well. Cold weather, snowy winters and low monthly mean temperatures will still occur, even though they will get less frequent with time. Some present lowtemperature records may still be broken during the next few decades. The magnitude of the climatic change is determined by future greenhouse gas emissions. Additional uncertainty is induced by the differences among the various climate models and by natural variability occurring in the climate system.
  • Karvonen, Juha (Helsingfors universitet, 2008)
    Experiments with outdoor viticulture were started in Southeast and Southwest Finland in the 1930s. Our rather short growing season and lack of suitable varieties have hindered professional extensive outdoor viticulture. The grapevine varieties bred for northern conditions and the forecasted prolongation of our growing season will likely lead to viticulture in Southern Finland within the next few decades. Soil temperature has an important influence on the survival and growth of the grapevine. Soil temperature is affected by air temperature, cultivation site, soil cultivation, vegetation, soil type and wintertime snow cover. The aim of my Master`s thesis was to measure soil temperatures of grapevine sites and, based on the reults, to estimate the optimal planting depth of a grapevine in Southern Finland. The effect of changes in air and soil temperatures on grapevine growth and development in Tuusula, Vehmersalmi and some Central European localities was also followed. Measurements revealed that soil temperature was at its lowest in March, when in Tuusula at a depth of 20 cm it decreased to -0.7ºC and at a depth of 60 cm to 2.0ºC. Compared with soil temperatures measured by the Finnish Meteorological Institute in other localities, the temperatures at a depth of 20 cm in Maaninka fell to -0.8ºC, in Juva to -0.3ºC and in Jokioinen to -1.6ºC and at a depth of 50 cm in Maaninka to 0.0ºC and in Jokioinen to -0.3ºC. In Tuusula, the annual average soil temperatures at a depth of 20 cm was 6.0ºC and at a depth of 60 cm 7.9ºC. In regression analysis, strong correlations (r2 = 0,497 - 0,684) were obtained between air temperatures measured at grapevine sites at a heigth of 150 cm, ground surface temperatures and soil temperatures measured at a depth of 20-60 cm. In the winter months of December, January, March and April, when the snow cover remained thin, the correlation between snow cover and soil temperatures was weak. The soil temperature during the coldest winter month at a depth of 20 cm fell to slightly below 0.0ºC, at a depth of 40 cm it remained at about 0ºC and at a depth of 60 cm it remained at 2ºC. Based on this, the depth of 40-60 cm can be regarded as the optimal planting depth for grapevines in Southern and Eastern Finland. At this depth, the freezing risk for roots in winter is minor, and in spring solar radiation quickly raises the soil temperature. In 2002-2007, the grapevine growing season had begun in Tuusula as a weeping at the earliest on April 24th. The buds began to swell and break earliest on May 1st. The flowering began earliest on June 16th and lasted for about two weeks. The earliest harvest began on September 14th. From the start of flowering to the start of harvest, the time elapsed was 75-92 days. Growth slackened as the soil temperatures fell and ceased altogether in September. In Central Europe, the weeping of the grapevines starts because of higher air and soil temperatures a couple of months earlier than in Southern Finland, but the flowering begins no more than one month and the harvest only 2-3 weeks earlier. The quicker growth and development in the north can be explained by the quicker warming of the air and soil, the longer days and the abundant supply of light in early summer.
  • Salminen, Miia (Finnish Meteorological Institute, 2017)
    Finnish Meteorological Institute Contributions 139
    Monitoring of snow cover in northern hemisphere is highly important for climate research and for operational activities, such as those related to hydrology and weather forecasting. The appearance and melting of seasonal snow cover dominate the hydrological and climatic patterns in the boreal and arctic regions. Spatial variability (in particular during the spring and autumn transition months) and long-term trends in global snow cover distribution are strongly interconnected to changes in Earth System (ES). Satellite data based estimates on snow cover extent are utilized e.g. in near-real-time hydrological forecasting, water resource management and to construct long-term Climate Data Records (CDRs) essential for climate research. Information on the quantitative reliability of snow cover monitoring is urgently needed by these different applications as the usefulness of satellite data based results is strongly dependent on the quality of the interpretation. This doctoral dissertation investigates the factors affecting the reliability of snow cover monitoring using optical satellite data and focuses on boreal regions (zone characterized by seasonal snow cover). Based on the analysis of different factors relevant to snow mapping performance, the work introduces a methodology to assess the uncertainty of snow cover extent estimates, focusing on the retrieval of fractional snow cover (within a pixel) during the spring melt period. The results demonstrate that optical remote sensing is well suited for determining snow extent in the melting season and that the characterizing the uncertainty in snow estimates facilitates the improvement of the snow mapping algorithms. The overall message is that using a versatile accuracy analysis it is possible to develop uncertainty estimates for the optical remote sensing of snow cover, which is a considerable advance in remote sensing. The results of this work can also be utilized in the development of other interpretation algorithms. This thesis consists of five articles predominantly dealing with quantitative data analysis, while the summary chapter synthesizes the results mainly in the algorithm accuracy point of view. The first four articles determine the reflectance characteristics essential for the forward and inverse modeling of boreal landscapes (forward model describes the observations as a function of the investigated variable). The effects of snow, snow-free ground and boreal forest canopy on the observed satellite scene reflectance are specified. The effects of all the error components are clarified in the fifth article and a novel experimental method to analyze and quantify the amount of uncertainty is presented. The five articles employ different remote sensing and ground truth data sets measured and/or analyzed for this research, covering the region of Finland and also applied to boreal forest region in northern Europe.
  • Deshpande, Purabi; Lehikoinen, Petteri; Thorogood, Rose; Lehikoinen, Aleksi (2022)
    Aim Abundances of animals vary according to species-specific habitat selection, but habitats are undergoing rapid change in response to anthropogenic alterations of land use and climate. The long-term decline of snowfall is one of the most dramatic abiotic changes in boreal regions, with potential to alter species communities and shape future ecosystems. However, the effects of snow cover on habitat-specific abundances remain unclear for many taxa. Here we explore whether long-term declines in snow cover affect the abundances of overwintering birds. Taxon Fifty bird species. Location Finland, Northern Europe. Methods We used generalized linear mixed models to analyse citizen-led monitoring data from 196 transects over a 32-year period to assess whether abundances of birds have changed in built-up areas, farmlands and forests, and whether these covary with warming temperatures and decreasing snow. We then explored if changes in abundance can be explained by body mass, migration strategy or feeding guilds of the species. Results Over the study period, the abundance of overwintering birds increased. This increase was most pronounced in farmlands (69.6%), where abundances were positively associated with decreasing snow depth. On the other hand, while abundances in built-up habitats (19.5%) decreased over the study period, they increased in periods of high snow depths. Finally, we found that the short-distance migration strategy explains changes in bird abundances with snow. In farmlands, ground feeding birds and heavier birds also show a positive trends in abundance with decreasing snow depths. Main conclusions Local snow conditions are driving habitat selection of birds in the winter; birds in farmlands were most responsive to a decrease in snow depth. Changing snow depths can affect bird movements across habitats in the winter, but also influence migratory patterns and range shifts of species.