| 1. |
- Halldin, Sven, et al.
(författare)
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NOPEX - a northern hemisphere climate processes land surface experiment
- 1998
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Ingår i: Journal of Hydrology. - 0022-1694 .- 1879-2707. ; 212-213, s. 172-187
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Tidskriftsartikel (refereegranskat)abstract
- The interface between land surfaces and the atmosphere is a key area in climate research, where lack of basic knowledge prevents us from reducing the considerable uncertainties about predicted changes. Boreal forests play an important, but not well known, role in the global hydrological and biogeochemical cycles. NOPEX (a NOrthern hemisphere climate Processes land surface EXperiment) is devoted to the study of land surface-atmosphere interaction in a northern European forest-dominated landscape. The main NOPEX region represents the southern edge of the boreal zone. It consists of a highly heterogeneous landscape, with forests, mires, agricultural land and lakes. A second study site, in northern Finland, representing the northern edge of the boreal zone, will be introduced into NOPEX in connection with its coming winter-time field activities. Field activities, dominating the initial phase of NOPEX, are conceived to strike a balance between the need to cover multi-annual observations and the resources required to carry out measurements covering all relevant spatial scales. The long-term data collection activities, the Continuous Climate Monitoring (CCM), form the backbone of the field programme. A suite of Concentrated Field Efforts (CFEs) covering periods of summer, spring and winter brings together scientists from more then 20 countries during month-long campaigns. CFEs have been carried out in May-June 1994 and April-July 1995. A third, winter-time CFE is planned for 1998-99. The System for Information in NOPEX (SINOP) is the database which forms a backbone for modelling and analysis work, dominating the second stage of NOPEX. A series of PhD courses are run in parallell to the research activities. Analysis and modelling are done in four interacting areas, including local-scale processes, meso-scale surface-atmosphere coupling and remote sensing techniques. The fourth area, regionalization methods, aims at bringing the previous three together in order to provide improved parameterization schemes for exchange of energy, momentum, water and CO2 between land and atmosphere in hydrological and meteorological models from the meso to the global scale.
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| 2. |
- Halldin, Sven, et al.
(författare)
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Energy, water and carbon exchange in a boreal forest landscape - NOPEX experiences
- 1999
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-9:SI, s. 5-29
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Forskningsöversikt (refereegranskat)abstract
- The role of the land surface in controlling climate is still underestimated and access to information from the boreal-forest zone is instrumental to improve this situation. This motivated the organisation of NOPEX (Northern hemisphere climate-Processes land-surface Experiment) in the southern part of the European boreal zone. This paper summarises results from NOPEX in its first phase, dealing with spring- and summertime conditions. Two concentrated field efforts (CFE1 on 27 May-23 June 1994, CFE2 on 18 April-14 July 1995) were carried out with coordinated measurements of energy, water, and CO2 budgets at 13 ground-based sites and at various airborne platforms. Flux aggregation was a central issue in the heterogeneous, patchy NOPEX landscape. It is shown that simple land-use-weighted averaging of fluxes from fields/forests/lakes agree well with regional fluxes. Momentum fluxes can be parameterised over the whole area with a roughness length of approximately 1.5 m, whereas fluxes of sensible heat and other scalars depend on the averaging scale, Local measurements of soil moisture can be classified and meaningful averages can be deduced with a 1 km resolution. Lakes play an important role and differs in both diurnal and annual cycles compared to the forests and fields. Multiannual data from an agricultural and a forest site has allowed quantification and modelling of seldom occuring phenomena. One unexpected result was that the Norunda Common forest acted as a source and not a sink of CO2. The successful completion of CFE1-2 and a pilot winter campaign (CFE3) will lead NOPEX into its final phase, devoted to wintertime processes. Measurements and model results reside in SINOP. the System for Information in NOPEX, open for NOPEX participants. Data from CFE1 and CFE2 are released on CD as an integrated part of this Special Issue. (C) 1999 Elsevier Science B.V. All rights reserved.
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| 3. |
- Lundberg, Angela, et al.
(författare)
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New approach to the measurement of interception evaporation
- 1997
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Ingår i: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 14:5, s. 1023-1035
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Tidskriftsartikel (refereegranskat)abstract
- Evaporation of water intercepted by vegetation represents an important (sometimes major) part of evapotranspiration in temperate regions. Interception evaporation is an important process where insufficient measurement techniques hamper progress in knowledge and modeling. An ideal technique to study the interception evaporation process should monitor intercepted mass (and its vertical distribution) and interception loss with high accuracy (0.1 mm) and time resolution (1 min), and give correct area estimates. The method should be inexpensive, require minor supervision during extended periods, and work in dense forests. Net precipitation techniques, in which interception evaporation is determined from the difference between gross precipitation (measured with funnels) and throughfall (measured with funnels, troughs, or plastic sheet net-rainfall gauges) fulfill many of the requirements but usually have a too-low accuracy and time resolution for process studies. Precipitation measurements are normally affected by distortion of the wind field around gauges as well as by adhesive and evaporative losses. Throughfall measurements with precipitation funnels, troughs, or plastic sheet net-rainfall gauges, manually emptied or combined with tipping buckets, usually have too-low accuracy and time resolution for process studies and are impaired by adhesive losses. A new loadcell-based system to determine interception evaporation from gross and net precipitation is presented. A weighing gauge with minimal wind loss is used for precipitation, and weighing troughs are used for throughfall measurements. The weighing troughs minimize adhesive-loss errors and react instantaneously. Preliminary results with the method confirm that it can be used for process studies with a high accuracy (0.1 mm) and a high time resolution (1 min).
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| 4. |
- Lundin, Lars-Christer, et al.
(författare)
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Continuous long-term measurements of soil-plant-atmosphere variables at a forest site
- 1999
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 53-73
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Tidskriftsartikel (refereegranskat)abstract
- It is a major challenge in modem science to decrease the uncertainty in predictions of global climate change. One of the largest uncertainties in present-day global climate models resides with the understanding of processes in the soil-vegetation-atmosphere-transfer (SVAT) system. Continuous, long-term data are needed in order to correctly quantify balances of water, energy and CO2 in this system and to correctly model it. It is the objective of this paper to demonstrate how a combined system of existing sensor, computer, and network technologies could be set up to provide continuous and reliable long-term SVAT-process data from a forested site under almost all environmental conditions. The Central Tower Site (CTS) system was set up in 1993-1994 in a 25 m high boreal forest growing on a highly heterogeneous till soil with a high content of stones and blocks. It has successfully monitored relevant states and fluxes in the system, such as atmospheric fluxes of momentum, heat, water vapour and CO2, atmospheric profiles of temperature, water vapour, CO2, short-and long-wave radiation, heat storage in soil and trees, sap-dow and a variety of ecophysiological properties, soil-water contents and tensions, and groundwater levels, rainfall and throughfall. System uptime has been more than 90% for most of its components during the first 5 years of operation. Results from the first 5 years of operation include e.g., budgets for energy, water and CO2, information on important but rarely occurring events such as evaporation from snow-covered canopies, and reactions of the forest to extreme drought. The carbon budget shows that the forest may be a sink of carbon although it is still growing. The completeness of the data has made it possible to test the internal consistency of SVAT models. The pioneering set-up at the CTS has been adopted by a large number of SVAT-monitoring sites around the world. Questions concerning tower maintenance, long-term calibration plans, maintenance of sensors and data-collection system, and continuous development of the computer network to keep it up to date are, however, only partly of interest as a research project in itself. It is thus difficult to get it funded from usual research-funding agencies. The full value of data generated by the CTS system can best be appreciated after a decade or more of continuous operation. Main uses of the data would be to evaluate how SVAT models handle the natural variability of climate conditions, quantification of water. carbon and energy budgets during various weather conditions, rind development of new parameterisation schemes in global and regional climate models.
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| 5. |
- Lundin, Lars-Christer, et al.
(författare)
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System of information in NOPEX : retrieval, use, and query of climate data
- 1999
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923 .- 1873-2240. ; 98-99, s. 31-51
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Tidskriftsartikel (refereegranskat)abstract
- The uncertainty in climate predictions caused by improper understanding of the role of the land-surface is underestimated and easy access to data from a series of landscape types around the globe would improve this. Such data exist from a series of large-scale land-surface experiments but access to them has been difficult. It is the objective of this paper to demonstrate how the System for Information in NOPEX (SINOP) could be set up to provide a combination of data archive and tool for executing various time-limited and long-term field activities. Setting up and running SINOP involved both technical and psychological issues. The major technical problems were associated with (i) the uninterrupted flow of large data volumes, (ii) data homogeneity, and (iii) the exploding technology development. The psychological and organisational problems were more difficult to tackle than the technical problems. Funding agencies assumed somebody else would take care of data archiving and documentation, academic organisations have difficulties to compete with the private market for database managers, many individual scientists were unwilling to deliver their datasets and, especially, unwilling to document them. It is suggested that changes in attitudes from scientists, academic organisations, and publishers are needed to give credit for the publication of good datasets and for the production of good documentation about them. CDs incorporating a subset of SINOP with well-documented datasets from NOPEX operations in 1994 and 1995 are published together with this NOPEX Special Issue. The CDs include climate variables, such as radiation, fluxes of heat, momentum, and water vapour, and various energy storage terms as well as hydrological variables from 13 sites within the central-Swedish NOPEX region, at the southern boundary of the boreal zone. The publication of these data is seen as a step towards giving data-set owners proper and citeable credit for their work.
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