Timothy G. F. Kittel

Coupled Atmosphere–Biophysics–Hydrology Models for Environmental Modeling

The formulation and implementation of LEAF-2, the Land Ecosystem‐Atmosphere Feedback model, which comprises the representation of land‐surface processes in the Regional Atmospheric Modeling System (RAMS), is described. LEAF-2 is a prognostic model for the temperature and water content of soil, snow cover, vegetation, and canopy air, and includes turbulent and radiative exchanges between these components and with the atmosphere. Subdivision of a RAMS surface grid cell into multiple areas of distinct land-use types is allowed, with each subgrid area, or patch, containing its own LEAF-2 model, and each patch interacts with the overlying atmospheric column with a weight proportional to its fractional area in the grid cell. A description is also given of TOPMODEL, a land hydrology model that represents surface and subsurface downslope lateral transport of groundwater. Details of the incorporation of a modified form of TOPMODEL into LEAF-2 are presented. Sensitivity tests of the coupled system are presented that demonstrate the potential importance of the patch representation and of lateral water transport in idealized model simulations. Independent studies that have applied LEAF-2 and verified its performance against observational data are cited. Linkage of RAMS and TOPMODEL through LEAF-2 creates a modeling system that can be used to explore the coupled atmosphere‐biophysical‐ hydrologic response to altered climate forcing at local watershed and regional basin scales.

Regional Analysis of the Central Great Plains

lobal-scale impacts of human activities are changing the way many ecologists define research problems. The new definitions entail a shift of focus from sites and site-specific experiments to regions and regional analyses. It is at the regional scale that interactions and impacts of large-scale processes, such as global warming, can be assessed and understood (Pastor and Post 1986, Rosswall et al. 1988). Furthermore, regions represent socioeconomic and political units whose behavior will both influence, and in turn be influenced by, global change. This shift in focus to the regional scale is accompanied by a new set of challenges that will require new research questions and methods. Most current knowledge about ecosystems has been generated from studies in

Evidence that local land use practices influence regional climate, vegetation, and stream flow patterns in adjacent natural areas

We present evidence that land use practices in the plains of Colorado influence regional climate and vegetation in adjacent natural areas in the Rocky Mountains in predictable ways. Mesoscale climate model simulations using the Colorado State University Regional Atmospheric Modelling System (RAMS) projected that modifications to natural vegetation in the plains, primarily due to agriculture and urbanization, could produce lower summer temperatures in the mountains. We corroborate the RAMS simulations with three independent sets of data: (i) climate records from 16 weather stations, which showed significant trends of decreasing July temperatures in recent decades; (ii) the distribution of seedlings of five dominant conifer species in Rocky Mountain National Park, Colorado, which suggested that cooler, wetter conditions occurred over roughly the same time period; and (iii) increased stream flow, normalized for changes in precipitation, during the summer months in four river basins, which also indicates cooler summer temperatures and lower transpiration at landscape scales. Combined, the mesoscale atmospheric/land-surface model, short-term trends in regional temperatures, forest distribution changes, and hydrology data indicate that the effects of land use practices on regional climate may overshadow larger-scale temperature changes commonly associated with observed increases in CO2 and other greenhouse gases.

Nonlinear Influence of Mesoscale Land Use on Weather and Climate

Abstract This paper demonstrates that the influence of mesoscale landscape spatial variability on the atmosphere must be parameterized (or explicitly modeled) in larger-scale atmospheric model simulations including general circulation models. The mesoscale fluxes of heat that result from this variability are shown to be of the same order of magnitude but with a different vertical structure than found for the turbulent fluxes. These conclusions are based on experiments in which no phase changes of water were permitted. When, for example, cumulus clouds organized in response to the landscape pattern develop, the mesoscale influence on larger-scale climate is likely to be even more important. To parameterize surface thermal inhomogeneities, the influence of landscape must be evaluated using spectral analysis or an equivalent procedure. For horizontal scales much less than the local Rossby radius, based on the results of Dalu and Pielke, the surface heat fluxes over the different land surfaces can be proporti...

Sensitivity of a general circulation model to global changes in leaf area index

Methods have recently become available for estimating the amount of leaf area at the surface of the Earth using satellite data. Also available are modeled estimates of what global leaf area patterns would look like should the vegetation be in equilibrium with current local climatic and soil conditions. The differences between the actual vegetation distribution and the potential vegetation distribution may reflect the impact of human activity on the Earths surface. To examine model sensitivity to changes in leaf area index (LAI), global distributions of maximum LAI were used as surface boundary conditions in the National Center for Atmospheric Research community climate model (NCAR CCM2) coupled with the biosphere atmosphere transfer scheme (BATS). Results from 10-year ensemble averages for the months of January and July indicate that the largest effects of the decreased LAI in the actual LAI simulation occur in the northern hemisphere winter at high latitudes despite the fact that direct LAI forcing is negligible in these regions at this time of year. This is possibly a result of LAI forcing in the tropics which has long-ranging effects in the winter of both hemispheres. An assessment of the Asian monsoon region for the month of July shows decreased latent heat flux from the surface, increased surface temperature, and decreased precipitation with the actual LAI distribution. While the statistical significance of the results has not been unambiguously established in these simulations, we suspect that an effect on modeled general circulation dynamics has occurred due to changes of maximum LAI suggesting that further attention needs to be paid to the accurate designation of vegetation parameters. The incorporation of concomitant changes in albedo, vegetation fractional coverage, and roughness length is suggested for further research.

Grassland biogeochemistry: Links to atmospheric processes

Regional modeling is an essential step in scaling plot measurements of biogeochemical cycling to global scales for use in coupled atmosphere-biosphere studies. We present a model of carbon and nitrogen biogeochemistry for the U.S. Central Grasslands region based on laboratory, field, and modeling studies. Model simulations of the geography of C and N biogeochemistry adequately fit observed data. Model results show geographic patterns of cycling rates and element storage to be a complex function of the interaction of climatic and soil properties. The model also includes regional trace gas simulation, providing a link between studies of atmospheric geochemistry and ecosystem function. The model simulates nitrogenous trace gas emission rates as a function of N turnover and indicates that they are variable across the grasslands. We studied effects of changing climate using information from a global climate model. Simulations showed that increases in temperature and associated changes in precipitation caused increases in decomposition and long-term emission of Co2 from grassland soils. Nutrient release associated with the loss of soil organic matter caused increases in net primary production, demonstrating that nutrient interactions are a major control over vegetation response to climate change.

Physiological Interactions Along Resource Gradients in a Tallgrass Prairie

Spatial variability in availability of resources that limit photosynthesis (water and N) leads to variation in rates of atmosphere-biosphere exchange. N content and allocation are canopy properties that link ecosystem, physiological, and biophysical pro- cesses and that vary in space at scales relevant to atmosphere-biosphere interaction. We studied landscape-scale variation in these and related canopy properties in Kansas Tallgrass Prairie (USA). The tallgrass ecosystem was suited to this investigation because primary production in the prairie is constrained by N availability. This work was designed to aid in interpretation and spatial extrapolation of gas exchange measurements made using aerodynamic techniques as part of FIFE (First ISLSCP Field Experiment), a NASA-sup- ported study. We collected data on spatial distribution of biomass, leaf area index (LAI), canopy N mass, N concentration ((N)), and gas exchange along topographic and manage- ment gradients. We also measured height distribution of N, light interception, and gas exchange within canopies as a function of position in the landscape. Substantial variation in biomass, LAI, N accumulation, and N allocation occurred over time, with topography, and as a result of previous burning. The vertical gradient of (N) and photosynthetic capacity within canopies were correlated, in space and time, with biomass and canopy light inter- ception. The gradients were steeper in high biomass sites than in low biomass sites. In addition, proportional N allocation to the upper layer increased with time (12% in June, 32% in August) as biomass increased. As nutrient uptake increased within the tallgrass landscape, biomass increased and light limitation in the lower canopy was induced. As this light limitation increased with increasing biomass, or with accumulation of dead vegetation, allocation of N to the upper canopy increased. Height distribution of photosynthetic ca- pacity paralleled within-canopy N allocation and light interception. As resource ratios (light, water, and nitrogen) varied in the landscape, so did rates of gas exchange. This work suggests that interactions between light extinction, N allocation, and photosynthesis that have been proposed for monospecific stands apply to the multispecies, but structurally simple, canopy of the tallgrass prairie. Models of plant performance based on evolutionary arguments may provide a powerful basis for spatial extrapolation of atmosphere-ecosystem exchange rates from sites to landscapes and larger regions.

Potential climatic impacts of vegetation change: A regional modeling study

The human species has been modifying the landscape long before the development of modern agrarian techniques. Much of the land area of the conterminous United States is currently used for agricultural production. In certain regions this change in vegetative cover from its natural state may have led to local climatic change. A regional climate version of the Colorado State University Regional Atmospheric Modeling System was used to assess the impact of a natural versus current vegetation distribution on the weather and climate of July 1989. The results indicate that coherent regions of substantial changes, of both positive and negative sign, in screen height temperature, humidity, wind speed, and precipitation are a possible consequence of land use change throughout the United States. The simulated changes in the screen height quantities were closely related to changes in the vegetation parameters of albedo, roughness length, leaf area index, and fractional coverage.

Modeled responses of terrestrial ecosystems to elevated atmospheric CO2: A comparison of simulations by the biogeochemistry models of the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP)

Abstract Although there is a great deal of information concerning responses to increases in atmospheric CO2 at the tissue and plant levels, there are substantially fewer studies that have investigated ecosystem-level responses in the context of integrated carbon, water, and nutrient cycles. Because our understanding of ecosystem responses to elevated CO2 is incomplete, modeling is a tool that can be used to investigate the role of plant and soil interactions in the response of terrestrial ecosystems to elevated CO2. In this study, we analyze the responses of net primary production (NPP) to doubled CO2 from 355 to 710 ppmv among three biogeochemistry models in the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United States, doubled atmospheric CO2 causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO2 and the mean annual temperature and annual precipitation of biomes or grid cells indicate that there are negative relationships between precipitation and the response of NPP to doubled CO2 for all three models. In contrast, there are different relationships between temperature and the response of NPP to doubled CO2 for the three models: there is a negative relationship in the responses of BIOME-BGC, no relationship in the responses of Century, and a positive relationship in the responses of TEM. In BIOME-BGC, the NPP response to doubled CO2 is controlled by the change in transpiration associated with reduced leaf conductance to water vapor. This change affects soil water, then leaf area development and, finally, NPP. In Century, the response of NPP to doubled CO2 is controlled by changes in decomposition rates associated with increased soil moisture that results from reduced evapotranspiration. This change affects nitrogen availability for plants, which influences NPP. In TEM, the NPP response to doubled CO2 is controlled by increased carboxylation which is modified by canopy conductance and the degree to which nitrogen constraints cause down-regulation of photosynthesis. The implementation of these different mechanisms has consequences for the spatial pattern of NPP responses, and represents, in part, conceptual uncertainty about controls over NPP responses. Progress in reducing these uncertainties requires research focused at the ecosystem level to understand how interactions between the carbon, nitrogen, and water cycles influence the response of NPP to elevated atmospheric CO2.

Simulation model for the effects of climate change on temperate grassland ecosystems.

Abstract We studied the responses of temperate grasslands to climate change using a grassland ecosystem model which simulates seasonal dynamics of shoots, roots, soil water, mycorrhizal fungi, saprophytic microbes, soil fauna, inorganic nitrogen, plant residues and soil organic matter. Forty-year simulations were made for several climate change scenarios. The model was driven with observed weather and with combinations of elevated atmospheric CO 2 , elevated temperature, and either increased or decreased precipitation. Precipitation and CO 2 level accounted for most of the variation among climate change treatments in the responses of soil, plants, animals and microbes. Elevated temperature extended the growing season but depressed photosynthesis in the summer, with little net effect on annual primary production. Doubling CO 2 (1) caused persistent increases in primary production, in spite of greater nitrogen limitation, and (2) led to greater storage of carbon in plant residues and soil organic matter. The increased carbon storage was not great enough to keep pace with the present rate of increase in atmospheric CO 2 .