| 1. |
- Gustafsson, David, et al.
(författare)
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Boreal forest surface parameterization in the ECMWF model - 1D test with NOPEX long-term data
- 2003
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Ingår i: Journal of applied meteorology (1988). - 0894-8763 .- 1520-0450. ; 42:1, s. 95-112
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Tidskriftsartikel (refereegranskat)abstract
- The objective of the present study was to assess the performance and recent improvements of the land surface scheme used operationally in the European Centre for Medium-Range Weather Forecasts (ECMWF) in a Scandinavian boreal forest climate/ecosystem. The previous (the 1999 scheme of P. Viterbo and A. K. Betts) and the new (Tiled ECMWF Surface Scheme for Exchange Processes over Land, TESSEL) surface schemes were validated by single-column runs against data from NOPEX (Northern Hemisphere Climate-Processes Land-Surface Experiment). Driving and validation datasets were prepared for a 3-yr period (1994-96). The new surface scheme, with separate surface energy balances for subgrid fractions (tiling), improved predictions of seasonal as well as diurnal variation in surface energy fluxes in comparison with the old scheme. Simulated wintertime evaporation improved significantly as a consequence of the introduced additional aerodynamic resistance for evaporation from snow lying under high vegetation. Simulated springtime evaporation also improved because the limitation of transpiration in frozen soils was now accounted for. However, downward sensible heat flux was still underestimated during winter, especially at nighttime, whereas soil temperatures were underestimated in winter and overestimated in summer. The new scheme also underestimated evaporation during dry periods in summer, whereas soil moisture was overestimated. Sensitivity tests showed that further improvements of simulated surface heat fluxes and soil temperatures could be obtained by calibration of parameters governing the coupling between the surface and the atmosphere and the ground heat flux, and parameters governing the water uptake by the vegetation. Model performance also improved when the seasonal variation in vegetation properties was included.
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| 2. |
- Mölder, Meelis, et al.
(författare)
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Energy balance closure of two bog surfaces in central Sweden
- 2002
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Typical bogs in the boreal forest zone can be characterised by hummock and hollow micro-topography and sparsely vegetated surfaces. Their energy balance has been studied much less than for other types of surface, i.e. fields and forests. Micrometeorological measurements were carried out in central Sweden at two bogs in different summer seasons. From the measured profiles of wind speed, air temperature and humidity, the turbulent sensible and latent heat fluxes were calculated according to the Monin-Obukhov similarity theory. The daytime sensible and latent heat fluxes were typically of similar size, with the latent heat fluxes still being slightly higher. Independent measurements of turbulent, radiative and ground heat fluxes allowed to consider the surface energy balance closure. During fair weather conditions, the net radiation exceeded the sum of turbulent and ground heat fluxes by up to 200 W m-2 when measurements with ground heat flux plates were used in the analysis. It is difficult to attribute this discrepancy to errors in turbulent fluxes, because the fetch was long enough (400 m or more). Also, the size and shape of the two bogs and the positions of the tower were different, but the discrepancies in the energy budget were very similar. It is, however, known that measurements with heat flux plates in the peat are problematic. The ground heat flux measured with plates was very low and was considered to be the most unreliable component of the surface energy balance. An alternative method from the literature, which used temperature measurements in the peat and at the surface but did not require any information on the soil thermal properties near the surface, was used for alternative ground-heat-flux calculations. The use of this method improved the closure of the surface energy balance, but an about 100 W m-2 large discrepancy still remained unexplained. A further improvement is expected when lateral heat exchange in the hummocks could be taken into account.
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| 3. |
- Mölder, Meelis, et al.
(författare)
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Excess resistance of bog surfaces in central Sweden
- 2002
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Ingår i: Agricultural and Forest Meteorology. - 1873-2240. ; 112:1, s. 23-30
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Tidskriftsartikel (refereegranskat)abstract
- Typical bogs in boreal forest zone can be characterized by hummock and hollow micro-topography and sparsely vegetated surfaces, thus, are quite distinct from other types of wetlands. Micrometeorological measurements were carried out in central Sweden at two bogs in different summer seasons. The data analysis aimed at obtaining good estimates for the roughness length z(ou) and the kB(-1) factor. The roughness length for wind speed (z(ou)) was estimated to be 2 cm. A good surface temperature estimate is crucial for kB-1 calculations. The surface temperature of such a heterogeneous surface showed high spatial variability, the differences between its minimum and maximum values reaching 10 K during daytime. However, an infrared thermometer averaging over a circle of a few meters still gave an acceptable result. The kB-1 factor was determined through the roughness sublayer Stanton number and the corresponding drag coefficient and was dependent on the roughness Reynolds number (Re-o). It took the form: kB(-1) = 1.58Re(o)(0.25) - 3.4. Within considerable scatter of data, a constant kB(-1) = 3.2 was also acceptable. This result lies in between the bluff-rough and vegetated surface cases.
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| 4. |
- Mölder, Meelis, et al.
(författare)
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Turbulence characteristics and dispersion in a forest - tests of Thomson's random-flight model
- 2004
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 127:3-4, s. 203-222
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Tidskriftsartikel (refereegranskat)abstract
- A dispersion experiment, where the tracer SF6 Was released from line sources, was carried out within a young pine forest. Line sources made of silicone tubes were installed at different heights and distances from a tower. Vertical concentration profiles were measured on the tower and the samples were analysed with a gas chromatograph. The measured concentration profiles were compared to those calculated using Thomson's random-flight model. To provide input information to the model on turbulence, the turbulence characteristics were measured at four different heights with sonic anemometers. Data are presented for standard deviations of the velocity components, the momentum flux, the Lagrangian time scale and the mean horizontal wind velocity, depending on height and thermal stability. The derived turbulence characteristics resemble previous results in the literature; but are still stand specific. Satisfactory agreement was obtained between the measured and simulated SF6 concentration profiles. However, the model clearly underestimates the concentrations in the upper part of the stand. The results should add more confidence to the continued use of Thomson's model and are useful for planning better dispersion experiments. (C) 2004 Elsevier B.V. All rights reserved.
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