Ground-based station network in arctic and subarctic Eurasia: An overview

The international Pan-Eurasian Experiment (PEEX) program addresses the full spectrum of problems related to climate change in Eurasian Northern latitudes. All PEEX activities rely on the bulk of high-quality observational data provided by the ground and marine stations, remote sensing and satellite tools. So far, no coordinated station network has ever existed in Eurasia, moreover, the current scope of relevant research remains largely unknown as no prior assessment has been done to date. This paper makes the first attempt to overview the existing ground station pool in the Arctic-Boreal region with the focus on Russia. The  geographical, climatic and ecosystem representativeness of the current stations is discussed, the gaps are identified and tentative station network developments are proposed.


INTRODUCTION
The Pan-Eurasian Experiment (PEEX) is a multifaceted, multidisciplinary program that aims to bring together climate change research, infrastructure management, societal initiatives and policy-making in order to understand and mitigate the effects of climate change over Eurasia [Kulmala et al. 2011b, Kulmala et al. 2015, Lappalainen et al. 2014]. PEEX addresses the critical problems, such as ecosystem shifts, infrastructure degradation and societal processes that are expectedto occur in the region.
The PEEX-labelled studies aim to elucidate the climate change processes at a high spatio-temporal resolution. Such a target can only be achieved by combining ground observations, remote sensing products and advanced modeling approaches . In this triad, the ground-based observations component is probably the most crucial element that largely determines the quality of the model forecasts. It is clear that in such a large and diverse region as Eurasia, only a very extensive ground-based observation network would yield satisfactory results. PEEX proposes the establishment of such a network as a part of its 2 nd focus area, PEEX Infrastructure , Kulmala et al. 2015, . Siberia (and Russia as a whole) is currently lacking a coordinated, coherent ground based atmosphere-ecosystem measurement network.
The most important task of PEEX Research Infrastructure is to initiate motion towards high level Pan-Eurasian Observation Networks, which is based on a hierarchical SMEARtype (Station for Measuring Ecosystem-Atmosphere Relations) integrated landatmosphere observation system. As the first step, maximal utilization of the existing ground infrastructure is planned. This measure will be superseded by upgrading the currently functioning sites or building new sites where necessary.
The potential PEEX ground-based research infrastructure is centered around the Flagship stations (supersites), supported by flux/ advanced and standard stations . The structure of the potential Flagship stations echoes that of the supersite SMEAR-II in Hyytiälä, Southern Finland, which is equipped for research into a broad spectrum of atmospheric physics, biogeochemistry and geophysics topics. Designed to address more specific and local processes, the Standard and Flux stations are supposed to provide a higher-definition view of the processes in different climates, biomes and vegetation communities. At least one Flagship station per each representative ecosystem or every1000-2500 km should be founded in order to ensure sufficient coverage . Worldwide, an optimal ground observation network would consist of about 20 supersites, 500 flux stations and 10000 standard stations , of which a large fraction would be situated within the PEEX domain, due to its great extent. However, in spite of the apparent scale of the problem, no attempt to summarize the existing groundbased observational site network for the PEEX region and identify the potential gaps has been attempted to date.
This study employs the ground-based observational infrastructure inventory conducted by the University of Helsinki together with the institutes of the Russian Academy of Sciences and Moscow State University. First, an overview of the existing facilities within Russia is presented, taking into account their geographical distribution, representation of ecosystems and climates. Then, the needs of development in the ground-based observational site network are discussed. Station infrastructure in China is discussed shortly as well.

MATERIALS AND METHODS
First, the information on measurement and research facilities was collected, processed and classified so that to allow systematic overview and demonstration. In particular, the information on the station location, ecosystem features, facilities and equipment was considered. See Table 1 for the list of specific items collected for each station.
The collected data was processed to derive further information on the stations. Special attention was paid to the presence of specific instrumentation such as aerosol measurement devices and eddy-covariance setups.
With the use of mapping tools, spatial distributions of the research facilities were investigated in relation to geographical areas, climates and vegetation zones. The Natural Earth (http://www.naturalearthdata.com/) satellite-derived spatial data were used to build the underlying land cover map. The elevations of the sites above sea level, where missing, were estimated from a world digital elevation map (https://asterweb.jpl. nasa.gov/gdem.asp).

Geographical distribution of the stations
The PEEX program addresses continentalscale processes, which necessitates the use of a wide observational network. The focus area covering Northern Europe, Russia and China encompasses several geographical regions and is highly heterogeneous is terms of data coverage. Fig. 1   is comparatively similar between the Russian, Chinese and West European stations. However, since many sites in Western Europe are situated in coastal areas, this region has a greater share of low-lying sites (height a.s.l. <100 m). China has rather equal shares of low-and high-elevation sites, having the largest number of sites above 1500m.

WMO-GAW, eddy-covariance and aerosol measurement sites
PEEX aims to promote the use of state-of-theart measurement techniques for aerosol and flux measurements. Aerosol measurement tools falling into this category include e.g. differential mobility particle sizers (DMPS), whereas the relevant flux measurement technique is eddy-covariance (EC). The sites in Russia equipped with aerosol or EC instrumentation are shown in Fig. 4

Representation of climates by the measurement network
The continent-wide extent of the PEEX domain means that it spans multiple climatic and vegetation zones. The PEEX target zone is mainly represented by the continental climates (the D group in Köppen-Geiger classification), but also the areas experiencing subtropical or steppe climate are found in the Black Sea region and along the Mongolian border. In terms of areal coverage, the dominating climates are continental Dfb, typical of Central and Eastern Europe and South-Western Siberia, and Dfc, a major Boreal climate variety ( Table 2).The absolute majority of measurement sites are found in these two climates, with small numbers in the Arctic coast tundra and the drier South Boreal climate zone Dwc.
A more detailed view is presented in Table 2, where the ecosystem dimension is added. Similar ecosystem types are found in different climates -for example, forests cover almost entire Russia except for the steppe and Arctic regions. The same can be said about the freshwater bodies and wetlands that are ample throughout the country. Although the population is sparse in large parts of Siberia and the Russian Far East, sizeable urban areas do occur everywhere, and they, too, frequently lack continuous observations. This analysis reveals the apparent lack of any measurements or monitoring in many ecosystem/climate pairs. While the major temperate, Boreal and arctic climates (Dwc, Dfb, Dfc, tundra) can be described as well represented in terms of ecosystem coverage, the rest are poorly covered by measurements. Mountainous areas in many climatic zones are represented as scantly. Many climates represented by small fractions of land do not host any measurement stations at all. It is particularly striking that the number of permanent wetland sites is incomparably smaller then the number of forest sites, in all climates. The steppe climate Bsk and one of the boreal types Dfd are also represented by a disproportionately small number of stations.

Identifying the development needs of the PEEX ground-based observational network
The vastness of the PEEX target region, Northern and Eastern Eurasia, is both an advantage and a challenge. On the one hand, the wide geographical extent of the PEEX program, both the East-West and North-South gradients can be well described on a continental scale. The multitude of climates and ecosystems encountered across these gradients gives the potential to construct a realistic, high-definition vision of the climate change-related processes. On the other hand, however, the challenge lies in the fact that most of the PEEX target region is represented by pristine areas, which are often difficult to reach.
Since Russia covers most of the PEEX domain, the development of instrumentation over its vast area is of primary importance. The current permanent measurement site distribution in Russia is explained, firstly, by the proximity to the major population centers and developed infrastructure, and, secondly, by high interest in certain major biomes, such as boreal forest and tundra. Unsurprisingly, as a result, some areas are represented better than the other. Nevertheless, even when a certain region does host an isolated measurement station, this cannot be regarded as sufficient coverage. For instance, there is only one peatland research station with an EC setup in the whole of West Siberia (Mukhrino Field Station), the region that, in principle, is characterized with a great variety of peatland landscapes.  Kulmala et al. 2015]. An example of such deficiency in EC and aerosol measurements was given in Fig. 4. Adopting the estimate by Hari et al. (2015) that about 500 flux stations should exist worldwide, the present number of Russian EC sites is far behind the optimum; the same pertains to the aerosol measurements. As of June 2015, no Russian sites were part of the GAW In-Situ Aerosol Network.
Specific station network development needs have to be identified. One can approach this problem by identifying the climate/ ecosystem groups that are currently missing continuous monitoring. This analysis reveals that many gaps remain (see Table 2). Ideally, each climate/ecosystem pair should be monitored by at least one Flux-level station, and preferably more than one as significant local variations are common. The existing Russian EC/aerosol sites may become the first Flux-level sites. In terms of Flagship stations, one per each climate in Table 2 (SMEAR-I-II-II-IV), Estonia (SMEAR-Järviselja) and China (SMEAR-Nanjing), and the ecosystem station network in China.

CONCLUSIONS
The success of the PEEX mission to provide the next generation solutions to the climate change problems directly depends on the quality and coverage of the ground station data. The ground measurement network in its current shape needs being upgraded and extended to the previously underrepresented ecosystems and climatic zones. To address these needs, we motivate the foundation of a network of Flagship, Flux and Standard stations that would cover the PEEX domain. The region in question features some of the most remote areas of the world, with harsh climate, low infrastructure development and sparse population. While the existing infrastructure provides a valuable basis, building a network complying with the criteria proposed by Hari et al. [2015] will require extensive efforts.
The initiation of the PEEX Observation Network -Preliminary Phase Program is the main approach to the infrastructure development at the moment. We envision that it should include the following practical actions: -to identify the ongoing measurement routines of the PEEX Preliminary phase ground stations; -to analyse the end-user requirements of the global and regional scale climate and air quality modelling communities in the PEEX domain; -to provide an outline for the PEEX labelled network , including the GEOGRAPHY. ENVIRONMENT. SUSTAINABILITY. 01 (09) 2016 measurement and data productarchiving -delivery requirements for each station category; -to identify the key gaps in the initial phase observational network including longterm observational activities within PEEX domain, in Europe, in China and globally; -to initialize harmonization of the observations in the PEEX network following e.g. the accepted practices from World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) programme or European observation networks; -to improve satellite observations over the PEEX domain of interest; -to develop methods and methodology for inter-platform comparisons between the ground based and satellite observations; -to establish a PEEX education program for instruction in measurement techniques and data analysis for both young scientists and technical experts.
These tasks require strong ties and international cooperation between the leading institutions, involving practical efforts to coordinate, harmonize and jointly manage the research infrastructure. The development of a coherent, extensive, continuous and comprehensive groundbased measurement site network thus poses a challenge for the scientific community and the governments.