One-dimensional Land Surface Model Research Articles

Variability of the surface energy balance in permafrost-underlain boreal forest

Abstract. Boreal forests in permafrost regions make up around one-third of the global forest cover and are an essential component of regional and global climate patterns. Further, climatic change can trigger extensive ecosystem shifts such as the partial disappearance of near-surface permafrost or changes to the vegetation structure and composition. Therefore, our aim is to understand how the interactions between the vegetation, permafrost and the atmosphere stabilize the forests and the underlying permafrost. Existing model setups are often static or are not able to capture important processes such as the vertical structure or the leaf physiological properties. There is a need for a physically based model with a robust radiative transfer scheme through the canopy. A one-dimensional land surface model (CryoGrid) is adapted for the application in vegetated areas by coupling a multilayer canopy model (CLM-ml v0; Community Land Model) and is used to reproduce the energy transfer and thermal regime at a study site (63.18946∘ N, 118.19596∘ E) in mixed boreal forest in eastern Siberia. An extensive comparison between measured and modeled energy balance variables reveals a satisfactory model performance justifying its application to investigate the thermal regime; surface energy balance; and the vertical exchange of radiation, heat and water in this complex ecosystem. We find that the forests exert a strong control on the thermal state of permafrost through changing the radiation balance and snow cover phenology. The forest cover alters the surface energy balance by inhibiting over 90 % of the solar radiation and suppressing turbulent heat fluxes. Additionally, our simulations reveal a surplus in longwave radiation trapped below the canopy, similar to a greenhouse, which leads to a magnitude in storage heat flux comparable to that simulated at the grassland site. Further, the end of season snow cover is 3 times greater at the forest site, and the onset of the snow-melting processes are delayed.

Dissimilarity of Scalar Transport in the Convective Boundary Layer in Inhomogeneous Landscapes

A land-surface model (LSM) is coupled with a large-eddy simulation (LES) model to investigate the vegetation-atmosphere exchange of heat, water vapour, and carbon dioxide (CO2) in heterogeneous landscapes. The dissimilarity of scalar transport in the lower convective boundary layer is quantified in several ways: eddy diffusivity, spatial structure of the scalar fields, and spatial and temporal variations in the surface fluxes of these scalars. The results show that eddy diffusivities differ among the three scalars, by up to 10–12%, in the surface layer; the difference is partly attributed to the influence of top-down diffusion. The turbulence-organized structures of CO2 bear more resemblance to those of water vapour than those of the potential temperature. The surface fluxes when coupled with the flow aloft show large spatial variations even with perfectly homogeneous surface conditions and constant solar radiation forcing across the horizontal simulation domain. In general, the surface sensible heat flux shows the greatest spatial and temporal variations, and the CO2 flux the least. Furthermore, our results show that the one-dimensional land-surface model scheme underestimates the surface heat flux by 3–8% and overestimates the water vapour and CO2 fluxes by 2–8% and 1–9%, respectively, as compared to the flux simulated with the coupled LES-LSM.

Tempo-spatial characteristics of energy budget and evapotranspiration in the eastern Siberia

Spatio-temporal variations in the energy budget and evapotranspiration (ET) in eastern Siberia were examined for a period of 19 years, from 1986 to 2004, for the Lena and Kolyma watersheds, which are underlain by permafrost. The energy budget and ET were calculated with a land surface model [Yamazaki, T., Yabuki, H., Ishii, Y., Ohta, T., Ohata, T., 2004. Water and energy exchanges at forests and a grassland in eastern Siberia evaluated using a one-dimensional land surface model. J. Hydrometeol. 5, 504–515] that considered vegetation, snow, and soil processes. Station data were used to compile gridded time series of forcing data. The spatial distribution of the energy budget showed that the latent heat flux ( Q E) was a significant term in the surface energy budget in forested regions, while the sensible heat ( Q H) was significant in the tundra region. A similar result was also obtained for the seasonal variation; in the Kolyma watershed, for instance, Q H dominated the seasonal energy budget, even during the growing season. In the Lena watershed, 53% of precipitation ( P G) was lost through ET, which occurred during a very sharply pronounced growing season. Transpiration ( E T) was a major component of ET in the Lena watershed, while in the Kolyma watershed, evaporation from the forest floor ( E S) was dominant. This suggested clear relations between the energy budget and the difference in land surface and climate. In the Lena and Kolyma watersheds, a time series of P G exhibited strong annual variations with an increasing trend during 1986–2004. ET also exhibited an increasing trend in the two watersheds. However, the interannual variation was weaker than that of P G, reflecting the influence of extensive permafrost in the area. The ET components also showed an increasing trend. In contrast, E S decreased during 1986–2004, the trend of E S thus being inversely correlated to that of P G. Interestingly, annual ET also exhibited an inverse correlation with P G in the Lena and Kolyma watersheds, although this relationship was weak. In contrast, ET displayed a strong correlation with air temperature ( T a) and the timing of the disappearance of snow.

Validation of A One-Dimensional Snow-Land Surface Model at the Sleepers River Watershed

A one-dimensional land surface model, based on conservations of heat and water substance inside the soil and snow, is presented. To validate the model, a stand-alone experiment is carried out with five years of meteorological and hydrological observations collected from the NOAA-ARS Cooperative Snow Research Project (1966–1974) at the Sleepers River watershed in Danville, Vermont, U.S.A. The numerical results show that the model is capable of reproducing the observed soil temperature at different depths during the winter as well as a rapid increase of soil temperature after snow melts in the spring. The model also simulates the density, temperature, thickness, and equivalent water depth of snow reasonably well. The numerical results are sensitive to the fresh snow density and the soil properties used in the model, which affect the heat exchange between the snowpack and the soil.

Water and Energy Exchanges at Forests and a Grassland in Eastern Siberia Evaluated Using a One-Dimensional Land Surface Model

Water and energy exchanges are evaluated for two larch forests, one pine forest, and one grassland area in eastern Siberia near Yakutsk using a one-dimensional land surface model. Diurnal and seasonal variations of fluxes are simulated reasonably with general stomatal parameters at all sites. In the grassland site, the Bowen ratio is 0.2 in midsummer; it is smaller than that in forest sites (about 1). Sensitivity tests indicate that leaf area should be given accurately along with total plant area index including stem and branch areas. If both plant and leaf areas are given, the outline of seasonal heat balance can be simulated using the same stomatal parameters for forests and grassland sites with the model. In the larch site on the left bank of the Lena River, although input precipitation varies widely from 82 to 236 mm year to year from 1998 through 2000, calculated total evapotranspiration varies only within a range of 50 mm around 238 mm in the larch site. Understory evapotranspiration contributes 37%‐44% to total evapotranspiration; interception is 15%‐21% of precipitation. Evapotranspiration normalized by potential evaporation is 0.37 for larch sites almost independent of year; for grassland it is 0.52. At some sites, evapotranspiration in the warm season exceeds precipitation, thereby implying either a warm-season depletion of water storage in the soil column (most likely melted water from the thawing of the soil) or a horizontal transport of subsurface melt water from neighboring areas, or both.

Parameterization of Albedo over Heterogeneous Surfaces in Coupled Land-Atmosphere Schemes for Environmental Modeling. Part I: Theoretical Background

In grid-based environmental models, the underlying surface consists of patches of solid and liquid parts and different plant communities, creating a very heterogeneous picture in the grid cell. In these cases, numerical modelers usually use a simple arithmetic average to determine the grid-cell albedo, a key variable in the parameterization of the land-surface radiative transfer over the grid cell. The object of this paper is to consider the assumptions for aggregating the albedo over a very heterogeneous surface where various surfaces occur at different heights, and, then propose a method for deriving a general expression for it. The suggested expression for the albedo is compared with the conventional approach, for the two-patches grid-cell with a simple geometrical distribution and different heights of its components. A numerical test is performed to compare the two approaches by numerical simulation of the evolution of the surface temperature over the particular grid-cell. Specifically, a one-dimensional land-surface model was applied to an isolated rocky grid-cell with a hole in the center; the model was forced with meteorological observations taken on July 17, 1999 in Philadelphia, PA.

A One-dimensional Land Surface Model Adaptable to Intensely Cold Regions and its Applications in Eastern Siberia.

A one-dimensional land surface model has been developed to estimate water and energy fluxes between the atmosphere and ground surfaces. The model includes three submodels; vegetation, snow cover, and soil. In order to adapt the model to intensely cold regions, the snow cover submodel can consider depth hoar by means of effective temperature gradient. This submodel can calculate profiles of snow temperature, density, and liquid water content using meteorological data inputs; the model structure is simple having snow layers with the same thickness. The ratio of fallen snow amount to rain amount is given as a function of wet-bulb temperature. The model has been applied to a GAME-Siberia site in the Central Yakutia, Russia. Since we do not have complete data sets in midwinter, the model has been validated through whole winter simulations at several routine stations first. Snow depth and snow-cover period are simulated reasonably. Next, fluxes have been estimated using 1998 data at the GAME plain taiga site, compared with the observed fluxes. Diurnal and seasonal changes of fluxes are simulated reasonably. The calculated snow cover disappearance date is earlier than the observed day.