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Atmospheric general circulation models used for climate simulation and weather forecasting require the fluxes of radiation, heat, water vapor, and momentum across the land-atmosphere interface to be specified. These fluxes are calculated by submodels called land surface parameterizations. Over the last 20 years, these parameterizations have evolved from simple, unrealistic schemes into credible representations of the global soil-vegetation-atmosphere transfer system as advances in plant physiological and hydrological research, advances in satellite data interpretation, and the results of large-scale field experiments have been exploited. Some modern schemes incorporate biogeochemical and ecological knowledge and, when coupled with advanced climate and ocean models, will be capable of modeling the biological and physical responses of the Earth system to global change, for example, increasing atmospheric carbon dioxide.

The formulation of a revised land surface parameterization for use within atmospheric general circulation models (GCMs) is presented. The model (SiB2) incorporates several significant improvements over the first version of the Simple Biosphere model (SiB) described in Sellers et al. The improvements can be summarized as follows: (i) incorporation of a realistic canopy photosynthesis–conductance model to describe the simultaneous transfer of CO2 and water vapor into and out of the vegetation, respectively; (ii) use of satellite data, as described in a companion paper, Part II, to describe the vegetation phenology; (iii) modification of the hydrological submodel to give better descriptions of baseflows and a more reliable calculation of interlayer exchanges within the soil profile; (iv) incorporation of a \"patchy\" snowmelt treatment, which prevents rapid thermal and surface reflectance transitions when the area-averaged snow cover is low and decreasing. To accommodate the changes in (i) and (ii) above, the original two-layer vegetation canopy structure of SiB1 has been reduced to a single layer in SiB2. The use of satellite data in SiB2 and the performance of SiB2 when coupled to a GCM are described in the two companion papers, Parts II and III.

The physiological response of terrestrial vegetation when directly exposed to an increase in atmospheric carbon dioxide (CO2) concentration could result in warming over the continents in addition to that due to the conventional CO2 \"greenhouse effect.\" Results from a coupled biosphere-atmosphere model (SiB2-GCM) indicate that, for doubled CO2 conditions, evapotranspiration will drop and air temperature will increase over the tropical continents, amplifying the changes resulting from atmospheric radiative effects. The range of responses in surface air temperature and terrestrial carbon uptake due to increased CO2 are projected to be inversely related in the tropics year-round and inversely related during the growing season elsewhere.

SiB2, the second-generation land-surface parameterization developed by Sellers et al., has been incorporated into the Colorado State University general circulation model and tested in multidecade simulations. The control run uses a \"bucket\" hydrology but employs the same surface albedo and surface roughness distributions as the SiB2 run. Results show that SiB2 leads to a general warming of the continents, as evidenced in the ground temperature, surface air temperature, and boundary-layer-mean potential temperature. The surface sensible heat flux increases and the latent heat flux decreases. This warming occurs virtually everywhere but is most spectacular over Siberia in winter. Precipitation generally decreases over land but increases in the monsoon regions, especially the Amazon basin in January and equatorial Africa and Southeast Asia in July. Evaporation decreases considerably, especially in dry regions such as the Sahara. The excess of precipitation over evaporation increases in the monsoon regions. The precipitable water (vertically integrated water vapor content) generally decreases over land but increases in the monsoon regions. The mixing ratio of the boundary-layer air decreases over nearly all continental areas, however, including the monsoon regions. The average (composite) maximum boundary-layer depth over the diurnal cycle increases in the monsoon regions, as does the average PBL turbulence kinetic energy. The average boundary-layer wind speed also increases over most continental regions. Groundwater content generally increases in rainy regions and decreases in dry regions, so that SiB2 has a tendency to increase its spatial variability. SiB2 leads to a general reduction of cloudiness over land. The net surface longwave cooling of the surface increases quite dramatically over land, in accordance with the increased surface temperatures and decreased cloudiness. The solar radiation absorbed at the ground also increases. SiB2 has modest effects on the simulated general circulation of the atmosphere. Its most important impacts on the model are to improve the simulations of surface temperature and snow cover and to enable the simulation of the net rate of terrestrial carbon assimilation.

Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land-Atmosphere Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5 degrees C) and a decrease in the annual evapotranspiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested areas in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspiration, indicating a reduction in the regional moisture convergence. There was also an increase in the length of the dry season in the southern half of the Amazon Basin, which could have serious implications for the reestablishment of the tropical forests following massive deforestation since rainforests only occur where the dry season is very short or nonexistent. An empirical bioclimatic scheme based on an integrated soil moisture stress index was used to derive the movement of the savanna-forest boundary in response to the simulated climate change produced by large-scale deforestation. The implications of possible climate changes in adjacent regions are discussed.

The radiative and physiological effects of doubled atmospheric carbon dioxide (CO2) on climate are investigated using a coupled biosphere-atmosphere model. Five 30-yr climate simulations, designed to assess the radiative and physiological effects of doubled CO2, were compared to a 30-yr control run. When the CO2 concentration was doubled for the vegetation physiological calculations only assuming no changes in vegetation biochemistry, the mean temperature increase over land was rather small (0.3 K) and was associated with a slight decrease in precipitation (-0.3%). In a second case, the vegetation was assumed to have adapted its biochemistry to a doubled CO2 (2 X CO2) atmosphere and this down regulation caused a 35% decrease in stomatal conductance and a 0.7-K increase in land surface temperature. The response of the terrestrial biosphere to radiative forcing alone--that is, a conventional greenhouse warming effect--revealed important interactions between the climate and the vegetation. Although the global mean photosynthesis exhibited no change, a slight stimulation was observed in the tropical regions, whereas in the northern latitudes photosynthesis and canopy conductance decreased as a result of high temperature stress during the growing season. This was associated with a temperature increase of more than 2 K greater in the northern latitudes than in the Tropics (4.0 K vs 1.7 K). These interactions also resulted in an asymmetry in the diurnal temperature cycle, especially in the Tropics where the nighttime temperature increase due to radiative forcing was about twice that of the daytime, an effect not discernible in the daily mean temperatures. The radiative forcing resulted in a mean temperature increase over land of 2.6 K and 7% increase in precipitation with the least effect in the Tropics. As the physiological effects were imposed along with the radiative effects, the overall temperature increase over land was 2.7 K but with a smaller difference (0.7 K) between the northern latitudes and the Tropics. The radiative forcing resulted in an increase in available energy at the earth's surface and, in the absence of physiological effects, the evapotranspiration increased. However, changes in the physiological control of evapotranspiration due to increased CO2 largely compensated for the radiative effects and reduced the evapotranspiration approximately to its control value.

Two 3-year (1979-1982) integrations were carried out with a version of the GLA GCM that contains the Simple Biosphere Model (SIB) for simulating land-atmosphere interactions. The control case used the usual SiB vegetation cover (comprising 12 vegetation types), while its twin, the deforestation case, imposed; scenario in which all tropical rainforests were entirely replaced by grassland. Except for this difference, all other initial and prescribed boundary conditions were kept identical in both integrations. An intercomparison of the integrations shows that tropical deforestation: decreases evapotranspiration and increases land surface outgoing longwave radiation and sensible heat flux thereby warming and drying the planetary boundary layer. This happens despite the reduced absorption of solar radiation due to higher surface albedo of the deforested land; produces significant and robust local as well as global climate changes. The local effect includes significant changes (mostly reductions) in precipitation and diabetic heating, while the large-scale effect is to weaken the Hadley circulation but invigorate the southern Ferrel cell, drawing larger air mass from the indirect polar cells; decreases the surface stress (drag force) owing to reduced surface roughness of deforested land, which in turn intensifies winds in the planetary boundary layer, thereby affecting the dynamic structure of moisture convergence. The simulated surface winds are about 70% stronger and are accompanied by significant changes in the power spectrum of the annual cycle of surface and PBL winds and precipitation. Our results broadly confirm several findings of recent tropical deforestation simulation experiments. In addition, some global-scale climatic influences of deforestation not identified in earlier studies are delineated.

A comprehensive series of global datasets for land–atmosphere models has been collected, formatted to a common grid, and released on a set of CD-ROMs. This paper describes the motivation for and the contents of the dataset. In June of 1992, an interdisciplinary earth science workshop was convened in Columbia, Maryland, to assess progress in land–atmosphere research, specifically in the areas of models, satellite data algorithms, and field experiments. At the workshop, representatives of the land–atmosphere modeling community defined a need for global datasets to prescribe boundary conditions, initialize state variables, and provide near-surface meteorological and radiative forcings for their models. The International Satellite Land Surface Climatology Project (ISLSCP), a part of the Global Energy and Water Cycle Experiment, worked with the Distributed Active Archive Center of the National Aeronautics and Space Administration Goddard Space Flight Center to bring the required datasets together in a usable format. The data have since been released on a collection of CD-ROMs. The datasets on the CD-ROMs are grouped under the following headings: vegetation; hydrology and soils; snow, ice, and oceans; radiation and clouds; and near-surface meteorology. All datasets cover the period 1987–88, and all but a few are spatially continuous over the earth's land surface. All have been mapped to a common 1° × 1° equal-angle grid. The temporal frequency for most of the datasets is monthly. A few of the near-surface meteorological parameters are available both as six-hourly values and as monthly means.