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Polar View snow monitoring services provide daily mapping of the Snow Covered Area (SCA) in three geographical regions: 1. Central
Europe and Alps 2. Baltic
Sea Area 3.
Scandinavia Since 2010, the consolidated and combined maps of the regional products are also available via the Polar View Snow Service Portal. The Snow Water Equivalent (SWE) of Northern Eurasia, derived by the Finnish Meteorological Institute (FMI), which was previously supported by Polar View, is now provided within the European Space Agency (ESA) Data User Element (DUE) ‘GlobSnow’ project. The Polar View Snow Service Portal - Offering Common and Consolidated Snow Information The three geographically based services, described in the lower section, cover the specific needs of the mainly locally operating users and give consideration to the specific geographic aspects of the regions. The services use different sensors and dedicated algorithms and distribute the products in different ways. Within the continuing scope of Polar View, a consolidation of the products of the service partners, and a combined product (covering the complete pan-European service area) was launched. These products, characterized by lower spatial and temporal resolution is now publicly available on the Polar View Snow Service Portal. Common, consolidated information on the snow covered area fulfils the demands for a larger user group, e.g. the global change community and gives stronger recognition and publicity to Polar View. The concept of the portal was developed utilizing the following principles:
The new Snow Service Portal (launched Summer 2010) now gives access to recently enhanced 10 day composite products of each service with consolidated specifications and the derived ‘Combined Product’. In addition, the portal provides direct the link to the service providers and contact persons for the snow products. Service Description Using the inputs from each service provider and each service area, a multiday composite product, as common denominator product, was introduced. To achieve best conformity with user requirements and technical aspects, the specifications were chosen:
These specifications are also in analogy with projected GlobSnow and H-SAF products. For the products presented on the portal, the most common data format was selected (PNG & KML) to allow easy access, download and viewing using e.g. Google Earth. Methodology The available high resolution products from the partners, which up now were only delivered to directly involved users, serve as input for the 10d day composite product. According to the characteristics of each region and product algorithm, services and products are only available for a certain time span. On the one hand snow cover for the Alpine regions are of nearly half-year interest (starting November), on the other hand the snow melt processes in the Northern areas are of large interest in the spring to summer. Since for Central Europe also the irregularly snow fall and melt process in the low lands already in autumn are of interest, the long lasting snow coverage in Norway and Finland demands intense observation starting lately in April. In addition the low sun elevation for the areas north of ~ 55° N disables an earlier start of the local services. Due to this situation, the period of the single Polar View snow services shows large temporal spread. The generated 10d day composite product from each region serves as direct input for the additionally ‘Combined Product’. For this first available pan-European product, representing the overall snow situation (44° - 72° N), a number of overlaps between the original products had to be treated. For some cases, (e.g. Poland) where only small overlaps between the services occur, the consolidated product is calculated using mosaic techniques, using the information of the more assigned / closer service provider. In case of large key areas, like the mountainous areas of Scandinavia (located in Norway and Sweden), where two service providers (KSAT & SYKE) have outreaching coverage, a more sophisticated method is applied. Considering the fact that both services have different focus, the more specialised algorithm / service was chosen. Therefore, the mountainous areas of Sweden, Sweden and Norway (> 500m a.s.l.) were covered by results from the multi-sensoral & multi-temporal algorithm developed NR & NORUT, while the non-mountainous and forested areas were covered by results from the SYKE algorithm. To keep the original results from the different service providers (different algorithms), the user can obtain the map results from all sources within the portal for overlapping areas.
The lower mountainous regions in Central Europe are characterized by the frequent temperature changes causing the snow pack to accumulate and melt off several times each winter season. Rainfall combined with snowmelt is the most critical situation for the origin of floods. Within the Alps the snow is clearly of large interest, not only for hydrology. Remote Sensing provides the desired spatial characterizations of snow properties for the users, e.g. to control and update hydrological models. Service Description / Methodology This service provides frequent updates on snow cover relating to relevant information for flood forecasting and early warning in Central Europe. The snow-monitoring service consists of two different processing chains for optical and microwave data. The spatial resolution of both products is 1 km.
Service Validation and Quality The service has been validated through continuous application over 18 months for NOAA-AVHRR data. A service based on ENVISAT data was tested in the summer of 2004 and processing provided operationally in winter season 2004/2005. Products are regularly delivered within 1 hour after reception of NOAA-AVHRR data and 3 hours after ASAR data availability. For the winter season 2002/2003 the quality of the snow cover products was 95.6% based on validation with 14,000 measurements. Area of Interest and Key End Users The Snow Service Central Europe is available for all major catchments of Central Europe and the Alps. Product delivery and format is dependant on users requirements. Presently, the end users are the Flood Forecast Centers of Baden-Württemberg (www.hvz.baden-wuerttemberg.de) and Rhineland Palatine, (www.hochwasser-rlp.de) responsible for the Upper Rhine, Neckar, and Moselle.
The lowlands in Northern Europe and in the Baltic Sea drainage basin are characterized by several accumulations and melting periods until a permanent seasonal snow cover appears. In spring, the melting process may cause rapid changes on water storages, which leads to flooding. The boreal zone is characterized by a thick seasonal snow pack. As temporary water storage, snow is a valuable source of energy when a large volume of water is released during the melting process. On the other hand, sudden runoff raises a risk of flooding in certain areas. Service Description The snow service provided by the Finnish Environment Institute (SYKE) comprises mapping of regional fraction of snow covered area (SCA) in the Baltic Sea drainage area. This near-real-time service is based on daily Terra/MODIS imagery; an automated processing system is used to produce the SCA for 0.05º × 0.05º grid cells over the target area. The processing line includes radiometric and geometric corrections, cloud masking and the actual SCA estimation. Methodology SCA is estimated applying a semi-empirical reflectance model developed at SYKE. In this model, the reflectance from a target area is expressed as a function of SCA. Average effective forest canopy transmissivity or each grid cell and generally applicable reflectance values for wet snow, snow-free ground and dense forest canopy are applied as model parameters. Through transmissivity, the forest coverage for each unit area is individually determined. The key idea is that the effective transmissivity is estimated using the Earth observation data, and includes even using data similar to that employed in the actual SCA estimation. Neither auxiliary land cover data nor forest data is needed; only water mask is necessary. This type of approach enables operational snow mapping in an extensive and heterogeneous area such as the boreal forest zone. Service Validation The SCA product is validated against snow course network (including SCA observations) covering the whole Finland. A snow course is a 24 km long trail on the ground representing the different terrain and land cover types typical for the locality. Snow depth, snow density and SCA are observed in several points along the course. MODIS derived SCA maps from year 2003-2009 have been validated with these data. Area of Interest and Key End Users Polar View services will cover almost the whole of the Baltic Sea drainage basins. Hence also areas outside boreal zone are included. The algorithm is planned to work in areas where seasonal snow cover stays at least several weeks and then gradually melts off. The current End users (with SLA) are the modeling and assessment unit of SYKE (responsible for national hydrological forecasting for the entire Finland), Kemijoki ltd (hydropower company), Finnish Meteorological Institute (weather and safety centre), University of Lund and Estonian Environment Data Centre.
The service has been developed and demonstrated by Norut and NR within a series of projects. Most recently in EU FP 5 project EnviSnow, the service chain was demonstrated pre-operationally for hydropower companies in southern Norway and western Sweden. Under Polar View, the coverage area was expanded to cover the whole of Norway and Sweden. The primary information product generated is snow covered area (SCA),comprising the percentage of snow cover per pixel based on a multi-temporal mosaic of optical and SAR data. Snow maps are derived from Envisat ASAR and Terra MODIS data combined to generate the multi-sensor and multi-temporal snow map. Optical data provides the highest confidences, but are often hampered with cloud coverage. SAR data are independent of daylight and clouds, but are less accurate in determining the snow cover fraction and can only be used when wet snow is present. The final product delivered to the users is a mosaic combining the pixels with highest confidence from the time series of images obtained over the previous few days before the last acquisition (up to the previous 7 days). The service is automated and runs operationally at KSAT with manual inspection and quality assessment of the final products. The final product provides a percentage of snow cover per pixel based on a multi-temporal mosaic of optical and SAR data, and is produced by KSAT after each data acquisition during the snowmelt season (1 April - 1 July). The focus of the Scandinavian service is largely on commercial companies, working in the field of hydropower / water resources management.
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